KR20160088338A - Anode active material for lithium ion secondary battery, and manufacturing method therefor - Google Patents

Abstract

A lithium ion secondary battery, a lithium ion secondary battery, a lithium ion secondary battery, and a lithium ion secondary battery, wherein the lithium ion secondary battery includes a lithium ion secondary battery, a lithium ion secondary battery, and a lithium ion secondary battery. Provides a charging method. According to an aspect of the present invention for solving the above-mentioned problems, there is provided a silicon- And a carbon material covering the surface of the silicon-based active material fine particles.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an anode active material for a lithium ion secondary battery and a method for manufacturing the anode active material,

The present invention relates to a negative electrode active material for a lithium ion secondary battery, a method for manufacturing a negative electrode active material for a lithium ion secondary battery, and a charging method for a lithium ion secondary battery and a lithium ion secondary battery.

A carbon material such as graphite is mainly used as a cathode material of current lithium ion batteries. However, although the capacity per unit mass of graphite is close to the theoretical value and the high capacity using graphite is mainly composed of the improvement of the electrode density, the limit of high capacity has been reached. Research and development of silicon-based active materials as a next generation material having a higher discharge capacity than graphite is underway. However, the silicon-based active material has a problem in that cycle life is shortened because a large volume change is accompanied when the lithium-based active material is alloyed with lithium. In addition, the silicon-based active material can decompose the solvent upon contact with the solvent during the electrochemical reaction. The solvent decomposition product accumulates on the silicon-based active material, thereby interfering with the entry and exit of lithium ions into the silicon-based active material. Particularly, when the silicon-based active material is broken due to the volume change during charging / discharging and a new surface (surface on which the decomposition products of solvent decomposition is not present) appears, the new portion reacts with the solvent. And the corresponding reaction is increased by repeating the charge / discharge cycle. Even in this respect, the cycle life is shortened.

In relation to these problems, the technique disclosed in Non-Patent Document 1 makes fine particles of a silicon-based active material, specifically silicon particles, by pulverizing them, and coating the silicon fine particles with a carbon material. According to this technique, the absolute amount of the volume change is reduced by making the silicon particles into fine particles, while the contact between the silicon fine particles and the solvent is suppressed by the carbon material, so that the cycle life can be expected to be improved.

Journal of Power Sources 222 (2013) 400-409

However, the silicon fine particles have a problem of being easily oxidized in the atmosphere. The oxidized silicon microparticles, i.e., silicon oxide inevitably reacts with lithium ions upon initial charging. Therefore, irreversible capacity increases as the amount of silicon oxide increases. In the technique disclosed in the non-patent document 1, since the silicon particles were pulverized in the atmosphere, the silicon fine particles were exposed to the atmosphere during the pulverization. So this problem could not be solved.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a novel and improved lithium ion secondary battery capable of improving the cycle life of a lithium ion secondary battery while maintaining a high discharge capacity of the silicon- A method for manufacturing a negative electrode active material for a lithium ion secondary battery, a method for charging a lithium ion secondary battery, and a method for charging a lithium ion secondary battery.

According to an aspect of the present invention for solving the above problems, there is provided an anode active material for a lithium ion secondary battery comprising silicon-based active material fine particles and surface-modified fine particles having a carbon material covering the surface of the silicon-based active material fine particles.

Here, the total mass of the carbon material is preferably 0.35 to 3.5 mass% with respect to the total mass of the silicon-based active material fine particles and the carbon material.

According to this aspect, the cycle life is particularly improved when the covering amount of the carbon material becomes a value within the above range.

The silicon crab active material microparticles may be polycrystalline.

According to this aspect, the stress due to the expansion and shrinkage of the silicon-based active material can be alleviated by the space formed inside the polycrystal.

The average particle diameter of the silicon-based active material fine particles is preferably 200 nm or less.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is improved.

The thickness of the carbon material is preferably 2 to 20 nm.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is improved.

According to another aspect of the present invention for solving the above problems, there is provided a method for manufacturing a non-aqueous electrolyte, comprising: a first step of preparing a dispersion solution in which silicon-based active material fine particles are dispersed in a nonaqueous solvent by pulverizing a silicon- And a third step of preparing a negative electrode active material coated with a carbon material on the surface of the silicon-based active material fine particles by firing the mixed solution in an inert atmosphere to form a negative electrode for a lithium ion secondary battery A method of manufacturing an active material is provided.

According to this aspect, since the silicon-based active material fine particles are less likely to be exposed to the atmosphere in the first to third steps, a negative electrode active material having a small amount of silicon oxide can be produced. Further, since the negative electrode active material is covered with the carbon material, the silicon-based active material fine particles are hardly exposed to the air during the use of the negative electrode active material. Therefore, the irreversible capacity is reduced. In other words, a high discharge capacity of the silicon-based active material can be maintained. In addition, when the negative electrode active material is assembled into the lithium ion secondary battery, direct contact between the silicon-based active material fine particles and the solvent can be suppressed.

Here, the total mass of the carbon material is preferably 0.35 to 3.5 mass% with respect to the total mass of the silicon-based active material fine particles and the carbon material

According to this aspect, the cycle life is particularly improved when the covering amount of the carbon material becomes a value within the above range.

The average particle diameter of the silicon-based active material fine particles is preferably 200 nm or less.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is improved.

The thickness of the carbon material is preferably 2 to 20 nm.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is improved.

The organic substance may be at least one selected from the group consisting of hydroxy acids, monosaccharides and oligosaccharides.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is improved.

And the hydroxy acid may be at least one selected from the group consisting of citric acid and glycolic acid.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is improved.

And the nonaqueous solvent may be at least one selected from the group consisting of alcohols and ethers.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is improved.

The inert atmosphere may be composed of at least one selected from the group consisting of argon and nitrogen.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life is also improved.

The firing can be carried out within a temperature range of 500 to 800 ° C.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life is also improved.

According to another aspect of the present invention for solving the above-described problems, there is provided a lithium secondary battery comprising lithium-containing lithium-containing active material fine particles and fine carbon particles coated with surface-modified fine particles comprising carbonaceous material covering the surfaces of the silicon- An anode active material for an ion secondary battery is provided.

The total mass of the carbon material is preferably 0.35 to 3.5 mass% with respect to the total mass of the silicon-based active material fine particles and the carbon material

According to this aspect, since the silicon-based active material fine particles are coated with the carbon material, the silicon-based active material fine particles are less likely to be exposed to the atmosphere during the use of the surface-modified fine particles. Thus, the irreversible capacity of the negative electrode active material is reduced. In other words, a high discharge capacity of the silicon-based active material can be maintained. In addition, direct contact between the silicon-based active material fine particles and the solvent is suppressed. Therefore, the cycle life of the lithium ion secondary battery can be improved. Further, the surface-modified fine particles are introduced into hard carbon as they are complexed with hard carbon. Therefore, the stress caused by the expansion and shrinkage of the surface-modified fine particles is alleviated by the hard carbon, so that the cycle life is also improved. In addition, a large amount of surface modified fine particles can be introduced into the hard carbon, thereby improving the theoretical capacity of the negative electrode active material, that is, the discharge capacity.

The silicon-based active material fine particles may be in the form of a cylinder.

According to this aspect, it is possible to suppress the absolute amount of expansion and contraction of the silicon-based active material fine particles, particularly, the absolute amount of expansion and contraction in the direction (diameter direction) perpendicular to the axial direction. In this respect, the cycle life is also improved.

Here, the hard carbon is obtained by firing a starting material having a high system, and the total mass of the surface-modified fine particles is preferably 50 to 80 mass% with respect to the total mass of the hard carbon and the surface-modified fine particles.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

Also, the starting material of the high system may be any one or more selected from the group consisting of PVC, sucrose, gelatin and agar.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

The hard carbon may be obtained by firing a starting material of the liquid phase, and the total mass of the surface-modified fine particles is preferably 75 to 90 mass% with respect to the total mass of the hard carbon and the surface-modified fine particles.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

The starting material of the liquid system is preferably any one selected from the group consisting of resol type phenol resin and furfuryl alcohol.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

The silicon-based active material fine particles preferably have an axial length of 500 nm or less and a diameter of 50 nm or less.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

The thickness of the carbon material is preferably 2 to 20 nm.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

According to another aspect of the present invention, there is provided a method for producing a non-aqueous electrolyte, comprising: a first step of preparing a dispersion solution in which silicon-based active material fine particles are dispersed in a nonaqueous solvent by pulverizing a silicon- A second step of preparing a mixed solution by dissolving the surface-modified fine particles in a solvent; and a third step of producing the surface-modified fine particles coated with the carbon material on the surface of the silicon-based active material fine particles by firing the mixed solution in an inert atmosphere, And a fourth step of mixing the starting materials of carbon and firing in an inert atmosphere to produce a negative electrode active material.

The total mass of the carbon material is preferably 0.35 to 3.5 mass% with respect to the total mass of the silicon-based active material fine particles and the carbon material

According to this aspect, since the silicon-based active material fine particles are less likely to be exposed to the atmosphere in the first to third steps, the surface-modified fine particles having a small amount of silicon oxide can be produced. In addition, since the surface-modified fine particles are coated with a carbon material, the silicon-based active material fine particles are less likely to be exposed to the atmosphere during the use of the surface-modified fine particles. Thus, the irreversible capacity of the negative electrode active material is reduced. In other words, a high discharge capacity of the silicon-based active material can be maintained. In addition, direct contact between the silicon-based active material fine particles and the solvent is suppressed. Therefore, the cycle life of the lithium ion secondary battery can be improved. Further, the surface-modified fine particles are introduced into hard carbon by being complexed with hard carbon. Therefore, the stress caused by the expansion and shrinkage of the surface-modified fine particles is alleviated by the hard carbon, so that the cycle life is also improved. In addition, a large amount of surface modified fine particles can be introduced into the hard carbon, thereby improving the theoretical capacity of the negative electrode active material, that is, the discharge capacity. In addition, since the silicon-based active material fine particles are in the shape of a cylinder, the absolute amount of expansion and contraction, in particular, the absolute amount of expansion and contraction in the direction perpendicular to the axial direction (radial direction) can be suppressed. In this respect, the cycle life is also improved.

At least one of the inert atmosphere in the third step and the fourth step may be composed of at least one selected from the group consisting of argon and nitrogen.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

The firing of the fourth step may be performed within a temperature range of 500 to 800 ° C.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

According to another aspect of the present invention, there is provided a lithium ion secondary battery comprising the negative active material.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

Here, a metal lithium foil can be disposed on a part of the negative electrode current collector.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life is also improved.

According to another aspect of the present invention, there is provided a method of charging a lithium ion secondary battery, wherein the charging capacity of the lithium ion secondary battery is maintained at 70% or less of the theoretical capacity realized by the silicon-based active material fine particles.

According to this aspect, the irreversible capacity of the lithium ion secondary battery is reduced while the cycle life of the lithium ion secondary battery is improved.

As described above, according to the present invention, the cycle life of the lithium ion secondary battery can be improved while maintaining a high discharge capacity of the silicon-based active material.

1 is a side cross-sectional view schematically showing a configuration of a lithium ion secondary battery according to an embodiment of the present invention.
2 is a graph showing the correspondence relationship between the firing temperature, the initial efficiency, and the cycle life.
3 is a graph showing a correspondence relationship between C / Si ratio, initial efficiency, and cycle life.
4 is a graph showing the relationship between the Si average particle diameter and the cycle life.
5 is a TEM photograph of the negative electrode active material.
6 is a graph showing the theoretical capacity of the negative electrode active material of the present embodiment compared with that of the conventional negative electrode active material.
7 is a graph showing a comparison between the charging capacity and the non-charging capacity of the negative electrode active material of the present embodiment and the conventional negative electrode active material.
8 is a graph showing the correspondence relationship between the charging ratio of the silicon-based active material and the discharge capacity and cycle life of the lithium ion secondary battery.
9 is a graph showing the correspondence relationship between the firing temperature, the initial efficiency, and the cycle life in the fourth step.
10 is a graph showing a correspondence relationship between Si / hard carbon ratio and cycle life.
11 is a graph showing a correspondence relationship between Si / hard carbon ratio and cycle life.
12 is a graph showing a correspondence relationship between the discharge test environmental temperature and the discharge capacity.
13 is a graph showing the correspondence relationship between the firing temperature, the initial efficiency, and the cycle life in the third step.
14 is a graph showing the relationship between the C / Si ratio of the surface-modified fine particles, the initial efficiency, and the cycle life.
15 is an STEM photograph of the silicon-based active material fine particles according to this embodiment.
Fig. 16 is an enlarged view of Fig. 15. Fig.
17 is a TEM photograph of the silicon-based active material fine particles obtained in Comparative Example 2-1.
18 is a TEM photograph of the polycrystalline silicon-based active material fine particles.
19 is a TEM photograph of single crystal silicon based active material fine particles.
20 is a graph showing a correspondence relationship between the charging ratio of the silicon-based active material and the discharge capacity and cycle life of the lithium ion secondary battery.
21 is a graph showing the correspondence relationship between the firing temperature, the initial efficiency, and the cycle life in the fourth step.
22 is a graph showing a correspondence relationship between the Si / hard carbon ratio and the cycle life.
23 is a graph showing a correspondence relationship between the Si / hard carbon ratio and the cycle life.
24 is a graph showing a correspondence relationship between the discharge test environmental temperature and the discharge capacity.
25 is a graph showing the correspondence relationship between the firing temperature, the initial efficiency, and the cycle life in the third step.
26 is a graph showing the correspondence relationship between the C / Si ratio of the surface-modified fine particles, the initial efficiency, and the cycle life.
27 is a graph showing the correspondence relationship between the Si average particle diameter and the cycle life.
28 is a TEM photograph of the polycrystalline silicon-based active material fine particles.
29 is a TEM photograph of the single crystal silicon based active material fine particles.

Best Mode for Carrying Out the Invention Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, elements having substantially the same functional configuration in the present specification and drawings are denoted by the same reference numerals, and redundant description will be omitted.

(Upper limit on the negative electrode active material according to the present embodiment)

The inventor of the present invention has studied the background art of the present embodiment by diligent research, and thus has succeeded in overcoming the negative electrode active material according to the present embodiment. Here, the inventor of the present invention will now explain how the negative electrode active material according to the present embodiment has reached the top.

As described above, the silicon-based active material involves a large volume change when alloying with lithium. Therefore, when the silicon-based active material is used as a negative electrode active material of a lithium ion secondary battery, the negative active material layer expands and shrinks during charging and discharging, thereby shortening the cycle life of the lithium ion secondary battery. The silicon-based active material decomposes the solvent when it comes into contact with the solvent in the electrochemical reaction. The decomposition products of the solvent deposit on the silicon-based active material, thereby interfering with the entry and exit of lithium ions into the silicon-based active material. Particularly, when the silicon-based active material is cleaved by a change in volume during charging / discharging and a new surface (surface on which no decomposition products of solvent is formed) appears, the portion reacts with the solvent newly. As the charge / discharge cycle is repeated, the corresponding reaction is increased. Therefore, the cycle life is shortened also in this respect.

Here, the inventors of the present invention basically made a method of suppressing the volume change of the negative electrode active material layer including the silicon-based active material (reducing the absolute value of the volume change) by making the silicon-based active material into fine particles like the technique disclosed in the non-patent document 1.

However, as described above, there is a problem that the fine particles of the silicon-based active material are easily oxidized by oxygen in the atmosphere. The oxidized silicon-based active material, i.e., silicon oxide, irreversibly reacts with lithium ions during initial charging. Therefore, the larger the amount of silicon oxide, the larger the irreversible capacity. In the technique disclosed in the non-patent document 1, since the silicon-based active material was pulverized in the atmosphere, the silicon-based active material was exposed to the atmosphere during pulverization. Therefore, this problem could not be solved.

In order to suppress the oxidation of the silicon-based active material, the present inventor has found that the silicon-based active material is first pulverized in a wet process, specifically, pulverized in a non-aqueous solvent. Further, the present inventors have found that an organic material dissolved in a non-aqueous solvent is added to a nonaqueous solvent in which silicon-based active material fine particles are dispersed to prepare a mixed solution and the mixed solution is fired under an inert atmosphere. By this treatment, the silicon-based active material fine particles are coated with the carbon material. Therefore, the silicon-based active material fine particles coated with the carbon material are inhibited from oxidation by the carbon material. At the time of pulverization and firing, the silicon-based active material fine particles are present in a non-aqueous solvent and thus are not exposed to the atmosphere. Therefore, oxidation during pulverization and firing can also be suppressed. The coating method of the carbon material according to the present embodiment can be realized at a lower cost than the coating method disclosed in the non-patent document 1 (a method of coating a carbon material onto silicon fine particles by a chemical vapor deposition method using acetylene gas).

The present inventors have conceived that silicon-based active material fine particles coated with a carbon material are used as an anode active material for a lithium ion secondary battery. As a result, contact between the silicon fine particles and the solvent is suppressed by the carbon material.

Since the silicon-based active material itself has low conductivity, it is difficult to introduce lithium ions into the negative electrode active material layer (the layer including the negative electrode active material) when the silicon-based active material is used as the negative electrode active material as it is. However, in the present embodiment, the specific surface area of the negative electrode active material layer is improved by making the silicon-based active material into fine particles, and the surface of the silicon-based active material is coated with a carbon material to improve the conductivity of the negative electrode active material. Therefore, it is easy to introduce lithium ions into the negative electrode active material layer. According to the present embodiment as described above, the cycle life of the lithium ion secondary battery can be improved at a low cost while maintaining a high discharge capacity of the silicon-based active material.

(Configuration of Lithium Ion Secondary Battery)

First, the configuration of the lithium ion secondary battery 10 according to the present embodiment will be described based on FIG.

The lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, and a separator layer 40. The charging reached voltage (redox potential) of the lithium ion secondary battery 10 is, for example, 4.3 V (vs. Li / Li ⁺ ) to 5.0 V or less, particularly 4.5 V or more and 5.0 V or less. The shape of the lithium ion secondary battery 10 is not particularly limited. That is, the lithium ion secondary battery 10 may be any of a cylindrical, square, laminate, button, and the like.

The positive electrode 20 has a current collector 21 and a positive electrode active material layer 22. The current collector 21 may be any conductor, and may be made of, for example, aluminum, stainless steel, nickel coated steel, or the like.

The positive electrode active material layer 22 may include at least a positive electrode active material and further include a conductive agent and a binder. The positive electrode active material is, for example, a solid solution oxide containing lithium, but is not particularly limited as long as it is a material capable of occluding and releasing lithium ions electrochemically. The solid solution oxides include Li _a Mn _x Co _y Ni _z O ₂ (1.150? A? 1.430, 0.45? _X? 0.6, 0.10? Y? 0.15, 0.20? Z? 0.28), LiMn _x Co _y Ni _z O ₂ (0.3? X? 0.85, 0.10? Y? 0.3, 0.10? Z? 0.3) and LiMn _1.5 Ni _0.5 O ₄ .

The conductive agent is not particularly limited as long as it is carbon black such as Ketjen black, acetylene black, natural graphite, artificial graphite and the like, but it is for increasing the conductivity of the positive electrode.

The binder is not particularly limited as long as it binds the positive electrode active material and the conductive agent onto the current collector 21. Examples of the binder include polyimide, polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, but are not limited to, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, cellulose nitrate and the like. However, Is adhered on the collector 21 without any particular limitation.

The cathode 30 includes a current collector 31 and a negative electrode active material layer 32. The current collector 31 is a conductor, and is preferably electrochemically stable and does not dissolve or is difficult to alloy with lithium, and is preferably made of copper, stainless steel, nickel plated steel, or the like.

The negative electrode active material layer 32 includes a negative electrode active material and a binder. The negative active material is a silicon-based active material fine particle coated with a carbon material. Here, the silicon-based active material is a material containing silicon (atom) and capable of electrochemically intercalating and deintercalating lithium ions. Examples of the silicon active material include microparticles of silicon alone and microparticles of silicon compounds. The silicon compound is not particularly limited as long as it is used as a negative electrode active material of a lithium ion secondary battery. Examples of the silicon compound include silicon alloys and the like. Examples of the silicon alloys include Si-Ti-Ni alloys and Si-Al-Fe alloys.

The average particle size of the silicon-based active material fine particles is preferably as small as possible. When the silicon-based active material fine particles are alloyed with lithium by the cathode 30, the absolute value of the expansion per one particle is small as the silicon-based active material fine particle diameter is small. Therefore, expansion of the cathode 30 can be suppressed. Specifically, the average particle diameter of the silicon-based active material fine particles is preferably 200 nm or less, more preferably 100 nm or less. The cycle life is particularly improved when the average particle diameter of the silicon-based active material fine particles falls within these ranges. The lower limit of the average particle size of the silicon-based active material fine particles is not particularly limited, but is preferably 50 to 60 nm, for example. That is, in the wet pulverization described later, for example, by setting the pulverization time to 30 hours or more, it is possible to set the average particle diameter to about 50 to 60 nm.

Here, the average particle size of the silicon-based active material fine particles is a diameter of the silicon-based active material fine particles in a spherical shape, and can be measured by, for example, a laser diffraction / scattering particle size distribution measuring device. In this case, the particle size distribution of the silicon-based active material fine particles is measured by a laser diffraction / scattering type particle size distribution measuring device and the arithmetic mean value of the silicon-based active material fine particles is calculated as the average particle size based on the particle size distribution.

The carbon material is formed on the surface of the silicon-based active material fine particles by baking the organic material together with the silicon-based active material fine particles. The carbon material includes carbon as a main component. The thickness of the carbon material is not particularly limited, but is preferably in the range of 2 to 20 nm. When the thickness of the carbon material exceeds 20 nm, lithium out of the silicon-based active material may be inhibited. When the thickness of the carbon material is less than 2 nm, the carbon material is less likely to inhibit direct contact between the silicon-based active material and the electrolyte. The thickness of the carbon material can be measured by direct observation in TEM or argon etching by XPS. Fig. 5 shows an example of a TEM image. Region A represents a silicon-based active material, and Region B represents a carbon material.

The total amount of the carbon material to the coating amount of the carbon material, that is, the total mass of the silicon-based active material fine particles and the carbon material is 0.35 to 3.5 mass%. This is because the cycle life is particularly improved when the covering amount of the carbon material becomes a value within the above range as shown in Examples to be described later. The covering amount of the carbon material can be measured by, for example, a differential thermal analysis apparatus. The preferable coating amount of the carbon material is 0.7 to 1.75 mass%.

In the negative electrode active material of the present embodiment, the silicon-based active material fine particles are covered with the carbon material, so that the silicon-based active material fine particles are hardly exposed to the atmosphere. Therefore, oxidation of the silicon-based active material fine particles is suppressed. As a result, the initial irreversible capacity decreases. In addition, since direct contact between the silicon-based active material fine particles and the electrolyte is suppressed, decomposition of the solvent is suppressed. Thus, the cycle life is improved.

The binder may be the same as the binder constituting the positive electrode active material layer 22. When coating the negative electrode active material layer 32 on the current collector 31, carboxymethylcellulose (hereinafter referred to as CMC) as a thickening agent can be used in a proportion of not less than 1/10 of the mass of the binder and not more than the same mass. The content of the binder including the thickener is preferably 1% by mass or more and 10% by mass or less based on the total mass of the negative electrode material mixture. When the content of the binder containing the thickener is within these ranges, the battery characteristics are particularly improved.

The separator layer 40 includes a separator and an electrolytic solution. The separator is not particularly limited, and any separator may be used as long as it is used as a separator of a lithium ion secondary battery. As the separator, it is preferable to use a porous film or nonwoven fabric exhibiting excellent high rate discharge performance singly or in combination. Examples of the resin constituting the separator include polyolefin resins represented by polyethylene, polypropylene and the like, polyethylene terephthalate and polybutylene terephthalate (PVDF), vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetra A vinylidene fluoride-tetrafluoroethylene copolymer, a vinylidene fluoride-trifluoroethylene copolymer, a vinylidene fluoride-fluoroethylene copolymer, a vinylidene fluoride-hexafluoroacetone copolymer, a vinylidene fluoride- Ethylene copolymers, vinylidene fluoride-propylene copolymers, Vinylidene fluoride copolymer, vinylidene fluoride-trifluoro propylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene copolymer, tetrafluoroethylene copolymer, And the like.

The nonaqueous electrolytic solution may be the same as the nonaqueous electrolytic solution used in the lithium secondary battery from the past without particular limitation. The non-aqueous electrolyte has a composition containing an electrolyte salt in a non-aqueous solvent. Examples of the non-aqueous solvent include cyclic carbonic ester esters such as propylene carbonate, ethylene carbonate, ethylene carbonate, chloroethylene carbonate, and vinylene carbonate. Cyclic esters such as γ-butyrolactone and γ-valerolactone; alicyclic esters such as dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate; Chain esters such as methyl formate, methyl acetate and butyric acid methyl, tetrahydrofuran or its derivatives, 1, 3-dioxane, Dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyl diglyme, and the like. Ether; nitriles such as acetonitrile and benzonitrile; dioxolane or derivatives thereof; ethylene sulfide, sulfolane, sultone, sultone or a derivative thereof, or a mixture of two or more thereof, but is not limited thereto.

In the electrolyte salt as, for _{example, LiClO 4, LiBF 4, LiAsF} 6, LiPF 6, LiPF 6-x (C n F2 n + 1) x [ stage, 1 <x <6, n = 1 or2], LiSCN, LiBr, (K), such as Li, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN, _{salt, LiCF 3 SO 3, LiN (} CF 3 SO 2) 2, LiN (C 2 F 5 SO 2) 2, LiN (CF 3 SO 2) (C 4 F 9 SO 2), LiC (CF 3 SO 2) _{3, LiC (C 2 F 5} SO 2) 3, (CH 3) 4 NBF 4, (CH 3) 4 NBr, (C 2 H 5) 4 NClO 4, (C 2 H 5) 4 NI, (C 3 _{H 7) 4 NBr, (n} -C 4 H 9) 4 NClO 4, (n-C 4 H 9) 4 NI, (C 2 H 5) 4 N-maleate, (C 2 H 5) 4 N-benzoate (C ₂ H ₅ ) ₄ N-phthalate, stearyl sulfonic acid lithium, octyl sulfonic acid, and dodecyl benzene sulphonic acid. Salts, etc. These ionic compounds may be used singly or in combination of two or more. The concentration of the electrolyte salt may be the same as that of a non-aqueous electrolyte used as a conventional lithium secondary battery, and is not particularly limited. In the present embodiment, a non-aqueous electrolyte containing a suitable lithium compound (electrolyte salt) at a concentration of about 0.8 to 1.5 mol / L can be used.

However, various additives may be added to the non-aqueous electrolyte. Examples of such an additive include a negative electrode active additive, a positive electrode active additive, an ester additive, a carbonic ester additive, a sulfate ester additive, a phosphate ester additive, a borate ester additive, an acid anhydride additive, Additives, and the like. Any one of them may be added to the nonaqueous electrolyte solution or a plurality of kinds of additives may be added to the nonaqueous electrolyte solution.

(Manufacturing Method of Lithium Ion Secondary Battery)

Next, a method of manufacturing the lithium ion secondary battery 10 will be described. The positive electrode 20 is made as follows. First, a slurry is formed by dispersing a mixture of the positive electrode active material, the conductive agent and the binder in the above ratio in a solvent (for example, N-methyl-2-pyrrolidone). Next, the slurry is formed (for example, coated) on the current collector 21 and dried to form the positive electrode active material layer 22. The coating method is not particularly limited. As the coating method, for example, a knife coater method, a gravure coater method, or the like can be considered. Each of the following coating processes is carried out in the same manner. Then, the positive electrode active material layer 22 is pressed to a density within the above range with a press machine. Thus, the positive electrode 20 is fabricated.

The cathode 30 is fabricated as follows. First, a method of manufacturing the negative electrode active material will be described. The manufacturing method of the negative electrode active material is largely divided into the following three processes. In the first step, a silicon-based active material is pulverized in a non-aqueous solvent to prepare a dispersion solution in which silicon-based active material fine particles are dispersed in a non-aqueous solvent. That is, in the first step, the silicon-based active material is wet pulverized to make the silicon-based active material fine. According to this method, oxidation of the silicon-based active material is suppressed at the time of pulverization.

As the non-aqueous solvent used for the wet pulverization in the first step, it is possible to suppress the re-aggregation of the silicon-based active material fine particles without reacting with the silicon-based active material, thereby being industrially economical and capable of dissolving the carbon source- . Specifically, alcohols such as ethanol and methanol, and ethers are preferred. These nonaqueous solvents may be used alone or in combination of plural kinds.

The apparatus used for the wet milling in the first step is not particularly limited, and for example, a ball mill apparatus using a medium, a planetary ball mill apparatus, a bead mill apparatus, or the like can be used. It is preferable to use an electrochemically inactive ceramic material, such as zirconium oxide or alumina, in order to minimize the influence of impurities on the media or device container. In order to finely grind the silicon-based active material as fine as possible, the media diameter (diameter) such as zirconium oxide or alumina is preferably small and preferably 50 to 100 μm or less.

In the second step, a mixed solution is prepared by dissolving an organic substance dissolved in a non-aqueous solvent in a dispersion solution. The organic material usable in the second step is preferably a material which is soluble in a non-aqueous solvent and has a high carbon content. By using such an organic material, it is easy to form a dense carbon material film. The content of carbon as a specific content is preferably 10.7 to 44.1 mass% with respect to the total mass of the organic material. The oxygen content is preferably in the range of 16.7 to 63.1 mass% with respect to the total mass of the organic material. Specific examples of the organic substance include, for example, a hydroxy acid, a monosaccharide, and an oligosaccharide.

Examples of the hydroxyl acid include citric acid and glycolic acid. Examples of the monosaccharide include erythritol, D-erythrulose, D-erythrose, D-threose, D-arabinose, , L-arabinose, D-xylulose, L-xylulose, D-xylose, D-ricinose D-lyxose, Llyxose, D-ribulose, D-idose, D-quinovose, glucaracic acid, D-digitoxose, D-cymarose, 2-deoxy-D-glucose, L-fucose, D D-psicose, D-fructose, L-rhamnose, L-Iduronic acid, D-glucuronic acid ) And the like. Examples of oligosaccharides include isomaltose, rutinose, and the like. Any one of these organic substances may be used, and a plurality of types may be used in combination.

The amount of the organic substance to be added is preferably 10 mass% or less, more preferably 5 mass% or less, and further preferably 2 mass% or less, based on the mass of the dispersion solution. The lower limit is not particularly limited as long as the parameters required for the carbon material are satisfied.

In the third step, the carbon material is coated on the surface of the silicon-based active material fine particles by firing the mixed solution in an inert atmosphere. Thus, an anode active material is produced.

Here, the inert atmosphere can be implemented by, for example, argon or nitrogen. These gases can be used alone or in combination. Regarding the firing temperature, the temperature at which the carbonization of the organic material proceeds is preferably at least 550 캜. The upper limit temperature is preferably not higher than 800 DEG C at which SiC is not generated by the reaction of silicon and carbon. The optimum temperature differs depending on the type of the organic material used for the firing. The negative electrode active material is produced by the above process. As described above, the silicon-based active material fine particles are not exposed to the air because they are present in the non-aqueous solvent in the second to third steps. Therefore, the oxidation during firing is also suppressed. The coating method of the carbon material according to the present embodiment can be realized at a lower cost than the coating method disclosed in the non-patent document 1 (a method of coating a carbon material onto silicon fine particles by a chemical vapor deposition method using acetylene gas).

Next, the same process as that of the positive electrode 20 is performed. That is, a mixture of a negative electrode active material and a binder is dispersed in a solvent (for example, N-methyl-2-pyrrolidone) to form a slurry. Subsequently, the slurry is formed (for example, coated) on the current collector 31 and dried to form the negative electrode active material layer 32. Then, the negative electrode active material layer 32 is pressed by a press machine. Thus, the cathode 30 is fabricated.

Subsequently, the separator is placed between the positive electrode 20 and the negative electrode 30 to produce an electrode structure. Next, the electrode structure is processed into a desired shape (e.g., cylindrical shape, angled shape, laminate shape, button shape, etc.) and inserted into the container of the corresponding shape. Then, the electrolytic solution of the above composition is injected into the container to impregnate the electrolyte into each pore in the separator. Thus, a lithium ion secondary battery is fabricated.

(Example 1-1)

In Example 1-1, the negative electrode active material was produced by the following steps.

(First step)

Silicon particles having an average particle diameter of 25 mu m were added in a blend of 10 mass% with respect to the total mass of ethanol and silicon particles, and then mixed with zirconium oxide beads having a diameter (particle diameter) of 50 mu m, (Labstar) LMZ015. The average particle diameter of the single silicon particles was measured using a laser diffraction / scattering type particle size distribution analyzer LA-920 manufactured by Horiba, Ltd. Specifically, the particle size distribution was measured using the apparatus and an arithmetic average of the particle sizes was calculated based on the particle size distribution.

The charged amount of the zirconium oxide beads was 80 vol% relative to the volume of the container. Then, the stirrer (Agitator) was driven at 12m / s for 24 hours. Thus, a dispersion solution in which silicon-based active material fine particles (silicon fine particles) were dispersed was prepared. The average particle diameter of the silicon-based active material fine particles was measured by the same method as that of the silicon particles, but the average particle diameter was 110 nm.

(Second Step)

2 g of citric acid (2 mass% based on the total mass of the dispersion solution) was added to 100 g of the dispersion solution prepared in the first step to prepare a mixed solution.

(Third step)

The mixed solution prepared in the second step was transferred to a crucible made of ceramic and installed in a glass tube made of quartz. After the argon gas was flowed into the tube furnace at a flow rate of 500 cm &lt; ³ &gt; / min, the inside of the tube was heated to 600 DEG C at a rate of 2.5 DEG C / min. Thereafter, the inside of the tube furnace was maintained at 600 DEG C for 5 hours, and the furnace was taken out after the furnace was cooled to room temperature.

Then, a sample in the crucible was crushed in an agate mortar to prepare an anode active material. The amount of the carbon material covering the silicon-based active material was calculated as follows. In other words, the weight loss of citric acid was measured using EXSTAR 6000, a differential thermal analyzer manufactured by Seiko Instruments Co., Ltd., and the carbonation rate of citric acid was calculated from the result to be 3.57 mass%. From the results, the mass% of the total mass of the silicon-based active material fine particles and the carbon material of the carbon material was calculated to be 0.7% by mass.

In addition, the surface of the negative electrode active material was analyzed by an X-ray photoelectron spectroscopic analyzer AXIS-ULTRA-DLD manufactured by Kratos while performing argon etching to analyze the elements in the thickness direction of the negative electrode active material. Thus, the thickness of the carbon material was measured. As a result, the thickness of the carbon material was measured to be 3 nm. Further, the surface of the negative electrode active material was analyzed using a JEOL-2010 FEF manufactured by JEON TRANSMISSION ELECTRON MICROSCOPE (TEM). The carbon material had a thickness of 2 to 10 nm.

(Cathode)

40 parts by mass of the negative electrode active material and 45 parts by mass of graphite were added to a polyimide solution obtained by mixing and dissolving N-methyl-2-pyrrolidone in 15 parts by mass of polyimide. Subsequently, this mixed solution was mixed with a mortar mortar to prepare a slurry. Subsequently, this slurry was applied to a copper foil having a thickness of 10 mu m by using an applicator. Then the slurry at room temperature under argon flow (500cm ³ / min), to 350 ℃ at a speed of 2.5 ℃ / min, and was heat-treated (dried) for 30 minutes at this temperature. Thus, a negative electrode active material layer, that is, a negative electrode was produced. After the pre-press density of the negative electrode active material layer is 0.8g / cm ³ and the press is 1.1g / cm ^3, the thickness was press transition 40μm.

Next, a lithium foil was prepared as a counter electrode, and a lithium foil and a negative electrode were laminated through a polyethylene separator having a thickness of 20 m to produce an electrode structure. Subsequently, a 2032 coin battery was produced by adding 0.1 cm &lt; ³ &gt; of a 1M LiPF6EC / DEC (1: 1) electrolyte to the electrode structure.

(Measurement of initial efficiency and cycle life)

This battery was subjected to a charge-discharge test using a BTS2010 charge / discharge tester manufactured by Index Co., Ltd. 0.05 C at the first cycle to the second cycle, 0.1 C at the third cycle to the fourth cycle, and then to 0.5 C for the next cycle. Charging was carried out with a constant current constant voltage and discharging with a constant current. The voltage range was 0.02 to 1.5 V and the test temperature was 25 ° C.

The initial efficiency and cycle life of the lithium ion secondary battery were measured. The initial efficiency was defined as the charge / discharge efficiency (discharge capacity / charge capacity) of the first cycle and the cycle life was determined as the number of cycles when the discharge capacity was less than 90% by charge / discharge at 0.5C.

(Example 1-2)

The procedure of Example 1-1 was repeated except that isomaltose of an oligosaccharide was used as an organic substance.

(Example 1-3)

Treated in the same manner as in Example 1-1, except that erythritol, which is a monosaccharide, was used as an organic substance.

(Comparative Example 1-1)

The same process as in Example 1-1 was carried out except that the silicon-based active material fine particles prepared in the first step of Example 1-1 were used as the negative electrode active material.

(evaluation)

The initial efficiency and cycle life of Examples 1-1 to 3 and Comparative Example 1-1 are shown in Table 1 in comparison. However, in Table 1, the initial efficiency and cycle life of Examples 1-1 to 1-3 are standardized by Comparative Example 1-1.

Initial efficiency Cycle life Example 1-1 / Comparative Example 1-1 1.16 2.7 Example 1-2 / Comparative Example 1-1 1.18 2.5 Example 1-3 / Comparative Example 1-1 1.13 2.8

As shown in Table 1, the initial efficiency of Example 1-1 (silicon-based active material microparticles coated with a carbon material) was lower than that of Comparative Example 1-1 (silicon-based active material fine particles were not coated with carbon material) % And the cycle life was improved 2.7 times. The reason why the initial efficiency is improved is that in Embodiment 1-1, the silicon-based active material fine particles are covered with the carbon material, so that direct contact between the silicon-based active material fine particles and oxygen in the atmosphere is suppressed. That is, in Example 1-1, generation of silicon oxide which is a main factor of the initial irreversible capacity was suppressed. The reason why the cycle life is improved is thought to be that the direct contact between the silicon-based active material fine particles and the solvent is suppressed by the carbon material

In comparison between Examples 1-2 and 1-3 and Example 1-1, the initial efficiency of Example 1-2 was better than that of Example 1-1. The reason for this is considered to be that the carbon content (C = 42 mass%) in isomaltose is higher than the carbon content (C = 37.5 mass%) of citric acid, so that a more dense carbon film is formed. The cycle life of Example 1-3 was better than that of Example 1-1. The reason for this is considered that the oxidation rate of the silicon-based active material fine particles is further suppressed because the oxygen content (O = 51 mass%) of isomaltose is lower than the oxygen content (O = 58 mass%) of citric acid.

(Examples 1-4)

Next, the inventor conducted Example 4 to confirm the correspondence relationship between the firing temperature and the characteristics of the lithium ion secondary battery. Example 1-4 was the same as Example 1-1 except that the firing temperature in the third step of Example 1-1 was varied within the range of 400 to 900 占 폚. The results are shown in Fig. The horizontal axis of FIG. 2 represents the firing temperature and the vertical axis represents the initial efficiency and cycle life. However, the initial efficiency and the cycle life show a value standardized as Comparative Example 1-1.

2, the initial efficiency was higher than that of Comparative Example 1-1 between 400 and 900 ° C, and the cycle life was higher than that of Comparative Example 1-1 between 550 and 800 ° C. Therefore, the firing temperature is preferably 550 to 800 ° C. The reason why the cycle life and the initial efficiency are lowered when the firing temperature is lower than 400 占 폚 is that the carbonization of the organic material is difficult to proceed at a low firing temperature and the coating amount of the carbon material is lowered. In addition, when the firing temperature is 900 ° C, the initial efficiency and cycle life are considerably lowered. The reason for this may be adverse effects due to the generation of electrochemically inactive SiC. That is, by reacting the silicon-based active material with fine particles, the reactivity with carbon becomes high, and SiC is partially generated at 900 ° C.

(Example 1-5)

The inventors of the present invention conducted Examples 1-5 in order to confirm the correspondence relationship between the amount of the carbon material covered, that is, the mass% of the carbon material with respect to the total mass of the silicon-based active material fine particles and the carbon material, and the characteristics of the lithium ion secondary battery . Specifically, the treatment was carried out in the same manner as in Example 1-1, except that the coating amount of the carbon material was varied within the range of 0.07 to 6.7 mass% by varying the addition amount of citric acid in the second step of Example 1-1. The results are shown in Fig. The abscissa of FIG. 3 represents the coating amount of the carbon material (the mass% of the carbon material with respect to the total mass of the silicon-based active material fine particles and the carbon material and the C / Si ratio), and the ordinate represents the initial efficiency and the cycle life. However, the initial efficiency and the cycle life show a value standardized as Comparative Example 1-1.

According to FIG. 3, when the coating amount of the carbon material is 0.35 to 3.5 mass%, the cycle life and the initial efficiency become better than those of Comparative Example 1-1, and the coating amount of the carbon material becomes 0.7 to 1.75 mass% The efficiency is maximized. The reason why the cycle life and the initial efficiency are lowered when the coating amount of the carbon material exceeds 3.5% by mass is considered to be due to the structure of the carbon material. That is, the carbon material covering the silicon-based active material fine particles is presumed to have a graphite structure. However, since this structure is immature (more amorphous portions are detailed), when the covering amount of the carbon material increases, Entrance into the particulate matter is rather hindered. When the coating amount of the carbon material is 0.7 to 1.75 mass%, the cycle life and the initial efficiency are maximized. When the coating amount of the carbon material is within this range, it can be considered that the carbon material is hard to inhibit the movement of lithium ions and that the oxidation inhibiting effect of the silicon-based active material fine particles by the carbon material is maximized.

(Examples 1-6)

The present inventors conducted Examples 1-6 to confirm the correspondence relationship between the average particle diameter of the silicon-based active material fine particles and the characteristics of the lithium ion secondary battery. Specifically, the same treatment as in Example 1-1 was conducted except that the average particle diameter of the silicon-based active material fine particles was varied within a range of 100 to 900 nm by adjusting the milling time in the first step of Example 1-1. The results are shown in Fig. The abscissa of FIG. 4 represents the average particle diameter of the silicon-based active material fine particles and the ordinate represents the cycle life. The short cycle life shows a value standardized as Comparative Example 1-1.

According to Fig. 4, the cycle life is prolonged as the average particle diameter is smaller, but the cycle life is improved at any particle size than the comparative example. In addition, the cycle life varied sharply, especially at 200 nm. In the case of the graph shape, it is estimated that it will be maximized at 100 nm or less. Accordingly, the average particle diameter is preferably 200 nm or less, more preferably 100 nm or less.

However, when the wet grinding method of Example 1-1 is used, re-agglomeration and particle grinding are difficult, so that it takes much time to grind to 100 nm or less. The average particle size can be selected industrially in terms of cost and performance.

(Example 1-7)

The present inventors conducted the following Examples 1-7 in order to confirm that the same effects were obtained in other silicon-based active materials. Specifically, the same process as in Example 1-1 was performed except that the silicon of Example 1-1 was replaced with a silicon alloy (Si: Al: Fe = 55: 29: 16 (mass ratio)). As a result, the initial efficiency and cycle life were as high as those of Example 1-1.

As described above, according to this embodiment, since the silicon-based active material fine particles are less likely to be exposed to the atmosphere in the first to third steps, a negative active material having a small amount of silicon oxide can be produced. Further, since the negative electrode active material is coated with a carbon material, it is difficult for the silicon-based active material fine particles to be exposed to the atmosphere during use of the negative electrode active material. Therefore, the irreversible capacity is reduced. In other words, a high discharge capacity of the silicon-based active material can be maintained. Further, when the negative electrode active material is placed in the lithium ion secondary battery, the direct contact between the silicon-based active material fine particles and the solvent is suppressed. Also, the first to third steps can be implemented at a lower cost than the technique disclosed in the non-patent document 1. [ Therefore, the cycle life of the lithium ion secondary battery can be improved at a low cost.

In this case, the average particle size of the silicon-based active material fine particles can be made to be 200 nm or less. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The carbon material may have a thickness of 2 to 20 nm. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The organic material may be at least one selected from the group consisting of hydroxy acids, monosaccharides and oligosaccharides. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

Further, the hydroxy acid may be at least one selected from the group consisting of citric acid and glycolic acid. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The nonaqueous solvent may be at least one selected from the group consisting of alcohols and ethers. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The inert atmosphere may be composed of at least one selected from the group consisting of argon and nitrogen. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The firing can be carried out within a temperature range of 550 to 800 ° C. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

(Upper limit on the negative electrode active material according to another embodiment)

Here, in order to inhibit the oxidation of the silicon-based active material, the inventor of the present invention first grinds the silicon-based active material in a wet state, specifically, grinding in a non-aqueous solvent. Further, the present inventors have found that an organic material dissolved in a non-aqueous solvent is added to a nonaqueous solvent in which silicon-based active material fine particles are dispersed to prepare a mixed solution and the mixed solution is fired under an inert atmosphere. By this treatment, the silicon-based active material fine particles are coated with the carbon material. Therefore, the silicon-based active material fine particles coated with the carbon material (hereinafter referred to as &quot; surface-modified fine particles &quot;) are inhibited from being oxidized by the carbon material. Also, at the time of pulverization and firing, the silicon-based active material fine particles are present in a non-aqueous solvent and thus are hardly exposed to the atmosphere. Therefore, oxidation during pulverization and firing is also suppressed. The method of coating a carbon material according to the present embodiment can be realized at a lower cost than the coating method disclosed in Non-patent Document 1 (a method of coating a carbon material onto silicon fine particles by a chemical vapor deposition method using acetylene gas).

Further, the present inventors have studied to further suppress the change in the volume of the negative electrode active material layer. The silicon-based active material also has a problem of rapid deterioration when the battery is repeatedly charged until full charge. The present inventor has also studied this problem.

As a result, the present inventors paid attention to hard carbon. Hard carbon has a structure in which skeleton particles having the same layer structure as graphite are randomly arranged, and a large number of gaps are formed between the skeleton particles. Therefore, in the hard carbon, a space capable of absorbing the stress due to the expansion and contraction of the surface-modified fine particles, more specifically, a plurality of interstices between the skeleton particles and a gap between the skeleton particles are formed. Therefore, a large amount of surface-modified fine particles can be introduced into the space in the hard carbon by complexing the hard carbon and the surface-modified fine particles (specifically, mixing and baking). Therefore, the volume change of the negative electrode active material can be further suppressed and the theoretical capacity of the negative electrode active material can be improved by using the surface-modified fine particles and the hard carbon complexed with the surface-modified fine particles as the negative electrode active material.

Further, the present inventors have studied to suppress the volume change of the silicon-based active material fine particles themselves. As a result, the present inventors paid attention to silicon type silicon-based active material fine particles as silicon-based active material fine particles. The columnar silicon-based active material particles have a length (that is, a diameter) in a direction perpendicular to the axial direction is shorter than an axial direction. For this reason, the present inventors thought that the absolute amount of change in the volume of the negative electrode active material can be reduced by using the columnar silicon-based active material particles as the negative electrode active material. The present inventors have further improved the cycle life when the surface-modified fine particles are prepared by using silicon-based silicon-based active material fine particles and the surface-modified fine particles and the hard carbon are used to prepare the negative electrode active material. The details will be described by way of example.

6 shows the theoretical capacity of an anode active material composed of a mixture of silicon-based active material fine particles and hard carbon at a mass ratio of 7: 3, the theoretical capacity of an anode active material mixed with silicon-based active material fine particles and graphite at a mass ratio of 2: 8, And the theoretical capacity of the negative electrode active material. In this example, the silicon-based active material fine particles are silicon fine particles. The bar graph corresponding to the silicon-based active material represents the theoretical capacity realized by the silicon-based active material in the theoretical capacity of the entire negative electrode active material. The same is true for bar graphs corresponding to graphite and hard carbon. Since hard carbon can introduce a large amount of fine particles of silicon, the anode active material composed of silicon-based active material fine particles and hard carbon is very large in theoretical capacity.

The graphite has a charge potential characteristic in which charging is started for the first time when the charge potential is raised to a potential close to the lithium potential. Therefore, when a mixture of silicon-based active material fine particles and graphite is used as a negative electrode active material of a lithium ion secondary battery, the silicon-based active material fine particles are already in a fully charged state when charging of graphite is started. Therefore, the silicon-based active material fine particles deteriorate rapidly.

On the other hand, hard carbon has a charge potential characteristic in which the charge capacity gradually increases with an increase in charge potential. Therefore, when the surface-modified fine particles and the hard carbon complexed with the surface-modified fine particles are used as the negative electrode active material of the lithium ion secondary battery, the filling potential of the negative electrode active material, in particular, the charging capacity of the silicon- .

Fig. 7 shows a comparison of the charging capacities of the respective negative electrode active materials shown in Fig. As shown in FIG. 7, the negative electrode active material obtained by compounding the silicon-based active material fine particles and the hard carbon at a mass ratio of 7: 3 was filled up to the theoretical capacity of the negative electrode active material in which the silicon-based active material fine particles and graphite were mixed at a mass ratio of 2: Charging capacity remains. That is, the silicon-based active material fine particles are not fully charged. Therefore, the deterioration of the silicon-based active material is suppressed and the cycle life is improved.

Since the silicon-based active material itself has low conductivity, it is difficult to introduce lithium ions into the negative electrode active material layer (the layer including the negative electrode active material) when the silicon-based active material is used as the negative electrode active material as it is. However, in the present embodiment, the specific surface area of the negative electrode active material layer is improved by making the silicon-based active material into fine particles, and the surface of the silicon-based active material is coated with a carbon material to improve the conductivity of the negative electrode active material. Further, by compounding the surface-modified fine particles with hard carbon, the conductivity of the negative electrode active material is further improved. Therefore, it is easy to introduce lithium ions into the negative electrode active material layer.

And propylene carbonate may be used as a solvent of the electrolyte by using the anode active material in which the surface-modified fine particles and the hard carbon are combined. When the negative electrode active material is graphite, it is difficult to apply propylene carbonate as a solvent of the electrolytic solution because propylene carbonate is easily decomposed on graphite. For this reason, in a lithium ion secondary battery using graphite as the negative electrode active material, solid electrolyte ethylene carbonate mixed with a low boiling point solvent is used as a solvent for the electrolyte solution at room temperature. Therefore, in such a lithium ion secondary battery, solidification is likely to occur at an extremely low temperature (for example, -20 占 폚 or less). Further, the lithium ion secondary battery in which the electrolyte is solidified can not function as a battery because its conductivity is lost.

On the other hand, in the case of hard carbon, the decomposition reaction of propylene carbonate is difficult to occur. Therefore, propylene carbonate can be used as a solvent of the electrolyte by using the anode active material in which the surface-modified fine particles and the hard carbon are combined. And propylene carbonate is liquid at room temperature and hard to solidify even at very low temperature. Therefore, the lithium ion secondary battery in which propylene carbonate is used as a solvent for the electrolyte can hardly solidify the electrolyte solution even in a very low temperature environment, so that conductivity can be maintained. That is, the lithium ion secondary battery can function as a battery. Such low-temperature characteristics are an important requirement in a wide range of application environments such as electric trains.

As a result of the above examination, the present inventor has found that a negative electrode active material in which surface modified fine particles and hard carbon are combined is applied to a lithium ion secondary battery. Further, the present inventors made the silicon-based active material fine particles constituting the surface-modified fine particles into a cylindrical shape. According to this embodiment, the cycle life of the lithium ion secondary battery can be improved at a low cost while maintaining a high discharge capacity of the silicon-based active material. Further, the lithium ion secondary battery of the present embodiment can be used under a wider temperature range.

(Configuration of Lithium Ion Secondary Battery)

The configuration of the lithium ion secondary battery 10 according to the present embodiment will be described based on FIG. 1 again, but the description overlapping with the above description is omitted.

The cathode 30 includes a current collector 31 and a negative electrode active material layer 32. The current collector 31 is a conductor, and is preferably a material which does not dissolve electrochemically stably or is difficult to alloy with lithium, and is made of, for example, copper, stainless steel, nickel-plated steel or the like. It is preferable to dispose a metal lithium foil on a portion of the current collector 31 where the electrode collector terminal is welded, for example, on a portion not facing the positive electrode active material layer 22, and the like. By arranging the metal lithium foil, lithium is precharged to silicon after the liquid electrolyte is poured. Thus, the initial irreversible capacity can be reduced. For example, when the lithium ion battery 10 is a coin battery, metallic lithium may be attached to the current collector 31 at the back of the negative electrode active material layer 32. In the case where the lithium ion battery 10 is a cylindrical battery or a prismatic battery, a metal lithium foil may be attached to the unoccupied portion of the negative electrode active material layer 32 of the current collector 31.

The silicon-based active material fine particles are in the shape of a cylinder. Therefore, the silicon-based active material fine particles can suppress the absolute amount of volume change, particularly the absolute amount of volume change in the direction perpendicular to the axial direction (radial direction). Therefore, the cycle life is also improved in this respect. Figs. 15 and 16 show silicon-based active material fine particles X1 as one example of cylindrical silicon-based active material fine particles. 15 and 16 are STEM photographs of silicon-based active material fine particles (X1). 16 is an enlarged view of Fig. The present inventors obtained STEM observation of silicon-based active material fine particles (X1) from all angles, and as a result, the same photograph as in FIG. 15 was obtained. That is, in any of the STEM photographs, a substantially rectangular silicon-based active material fine particle (X1) as shown in Fig. As a result, the silicon-based active material fine particles (X1) can be estimated as a cylindrical shape. In Fig. 17, silicon-based active material fine particles X2 having generally spheroidal shape as silicon-based active material fine particles having a shape other than a columnar shape are shown. 17 is a TEM photograph of the silicon-based active material fine particles (X2). The present inventors obtained a TEM observation of the silicon-based active material fine particles (X2) from all angles, and as a result, the same photograph as in Fig. 17 was obtained. That is, in any TEM photograph, the elliptically shaped silicon-based active material fine particles (X2) shown in Fig. 17 appeared. As a result, the silicon-based active material fine particles (X2) can be estimated to have a generally spheroidal shape. 17, the grid (Y) of the sample stage is also shown.

The silicon-based active material fine particles are preferably polycrystalline. When silicon-based active material microcrystals are polycrystals, the silicon-based active material microparticles have a plurality of single crystal particles (domains) and a space is formed between the domains. Therefore, the stress caused by the volume change of the silicon-based active material microparticles is mitigated by this space. In this respect, the cycle life is also improved.

Here, as a method for distinguishing between microcrystalline silicon-based active material particles and monocrystalline silicon-based active material fine particles, TEM observation or XRD analysis can be mentioned. 18 shows a TEM photograph of the polycrystalline silicon-based active material fine particles. 19 shows TEM photographs of the monocrystalline silicon-based active material fine particles. As shown in Figs. 18 and 19, the polycrystalline silicon-based active material fine particles have a plurality of single crystal particles therein. On the other hand, the monocrystalline silicon-based active material microparticles are composed of a single single crystal grain. However, the magnification in FIG. 18 is 800,000 times, and the magnification in FIG. 19 is 400,000 times.

In addition, the peak shape of XRD differs according to the difference in structure. For example, when XRD analysis of a polycrystalline silicon-based active material and a monocrystalline silicon-based active material at a speed of 3 ^° / min and a range of 5 ^° to 90 ^° , the half width of the polycrystalline main peak (corresponding to the (111) plane) is in the range of 0.13 ^° ± 0.005 ^° and the value in the half-width of the main peak is a single crystal 0.11 ^о is a value in the range of ± 0.005 ^о. It is possible to distinguish between microcrystalline silicon-based active material particles and monocrystalline silicon-based active material particles by TEM observation or XRD analysis.

The silicon-based active material fine particles are preferably as small as possible, while the aspect ratio (axial length / diameter) is preferably as large as possible. The smaller the silicon-based active material fine particles and the larger the aspect ratio, the smaller the absolute value of the volume change when the silicon-based active material fine particles are alloyed with lithium. Therefore, the volume change of the cathode 30 can be suppressed. Specifically, the silicon-based active material fine particles preferably have an axial length of 500 nm or less and a diameter of 50 nm or less. The axial length is 350 nm or less and the diameter is more preferably 30 nm or less. The cycle life is particularly improved when the axial length and diameter of the silicon-based active material microparticles fall within these ranges. There is no particular limitation on the axial length and the lower limit of the diameter of the silicon-based active material fine particles. However, it is preferable that the lower limit of the axial length is 50 nm and the lower limit of the diameter is 10 nm in consideration of the capability of the apparatus for crushing the silicon-based active material and the cost increase due to the increase of the processing time. As shown in the examples described later, by increasing the treatment time (milling time) by the bead mill, the axial length and diameter of the silicon-based active material fine particles can be reduced. Here, the axial length and diameter of the silicon-based active material fine particles can be measured by STEM observation of the silicon-based active material fine particles.

The carbon material is formed on the surface of the silicon-based active material fine particles by baking the organic material together with the silicon-based active material fine particles. The carbon material includes carbon as a main component. The thickness of the carbon material is not particularly limited, but is preferably in the range of 2 to 20 nm. When the thickness of the carbon material exceeds 20 nm, lithium out of the silicon-based active material may be inhibited. When the thickness of the carbon material is less than 2 nm, the carbon material is less likely to inhibit direct contact between the silicon-based active material and the electrolyte. However, the thickness of the carbon material can be measured by direct observation in TEM or argon etching by XPS.

The total mass of the carbon material with respect to the coating amount of the carbon material, that is, the total mass of the silicon-based active material fine particles and the carbon material is 0.35 to 3.5 mass%. This is because the cycle life is particularly improved when the covering amount of the carbon material becomes a value within this range, as shown in the following embodiments. However, the coating amount of the carbon material can be measured by, for example, a differential thermal analysis apparatus. The preferable covering amount of the carbon material is 0.7 to 1.75 mass%.

In the surface-modified fine particles of the present embodiment, since the silicon-based active material fine particles are coated with the carbon material, the silicon-based active material fine particles are hardly exposed to the atmosphere. Therefore, oxidation of the silicon-based active material fine particles is suppressed. As a result, the initial irreversible capacity decreases. In addition, since direct contact between the silicon-based active material fine particles and the electrolyte is suppressed, decomposition of the solvent is suppressed. Thus, the cycle life is improved.

The cycle life is improved when the mass ratio of the surface-modified fine particles and the hard carbon is within a specific range. This range depends on the starting material of the hard carbon. For example, when the hard carbon is obtained by firing a starting material of a high system, the range of the mass ratio in which the cycle life is improved is the surface modified fine particles: hard carbon = 50:50 to 80:20. That is, the total mass of the surface-modified fine particles is preferably 50 to 80 mass% with respect to the total mass of the hard carbon and the surface-modified fine particles. Examples of the starting materials of the high system include PVC, sucrose, gelatin and agar. Any one of these starting materials may be used, and a plurality of types may be used.

On the other hand, when the hard carbon is obtained by firing the starting material of the liquid system, the range of the mass ratio in which the cycle life is improved is the surface modified fine particles: hard carbon = 75:25 to 90:10. That is, the total mass of the surface-modified microparticles is preferably 75 to 90 mass% with respect to the total mass of the hard carbon and the surface-modified microparticles. Examples of the starting materials of the liquid system include resole type phenol resin and furfuryl alcohol. Any one of these starting materials may be used, and a plurality of types may be used.

Therefore, the ratio of the hard carbon obtained as the starting material of the high system occupied in the negative electrode active material is larger than that of the hard carbon obtained as the starting material of the liquid system. That is, when hard carbon is obtained from a high-starting material, more hard carbon is needed to improve cycle life. The reasons for this are as follows. That is, it is difficult for the starting material of the high system to be mixed with the surface-modified fine particles than the starting material of the liquid system. Therefore, the hard carbon obtained as a starting material of a high system has lower homogeneity than the hard carbon obtained as a starting material of the liquid system, and hardly comes into contact with the surface modified fine particles. For example, the contact area per unit mass is small. Therefore, it can be considered that when hard carbon is obtained from a starting material of high system, more hard carbon is required to improve the properties as the negative active material. It is also conceivable that the carbonation rate of the starting material (the mass ratio of carbon atoms to the total mass of the starting material) as a secondary factor influencing the mass ratio range, which improves cycle life. For example, a phenol resin in a resole type has a carbonization rate of about 28% by mass, while a PVC content is 10% by mass. The larger the carbonization rate of the starting material is, the larger the volume density of the hard carbon becomes, and the mass of the hard carbon necessary for improving the cycle life is reduced.

The surface modified fine particles are complexed with hard carbon. Specifically, the surface-modified fine particles are introduced into the spaces in the hard carbon, that is, between the interstices of the skeleton particles and the interstices between the skeletons. The hybridization of the hard carbon and the surface modifying fine particles is carried out by mixing the hard carbon starting material and the surface modifying fine particles and firing the mixture. The details will be described later.

The nonaqueous electrolyte solution may be the same as the nonaqueous electrolyte solution used in the lithium secondary battery from the past without any particular limitation. The non-aqueous electrolyte has a composition containing an electrolyte salt in a non-aqueous solvent. Examples of the non-aqueous solvent include cyclic carbonate esters such as propylene carbonate, ethylene carbonate, ethylene carbonate, chloroethylene carbonate, and vinylene carbonate cyclic esters such as γ-butyrolactone and γ-valerolactone; cyclic esters such as dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and the like; Chain esters such as methyl formate, methyl acetate and butyric acid methyl, tetrahydrofuran or its derivatives, dioxane (dioxane) ), 1, 4-dioxane, 1, 2-dimethoxyethane, 1, 4-dibutoxyethane, methyl diglyme Ether, nitriles such as acetonitrile and benzonitrile, dioxolane or derivatives thereof, ethylene sulfide, sulfolane, sultone, sultone or a derivative thereof, or a mixture of two or more thereof, but the present invention is not limited thereto. However, propylene carbonate can be suitably used in the present embodiment.

(Manufacturing Method of Lithium Ion Secondary Battery)

Next, a method of manufacturing the lithium ion secondary battery 10 will be described. The positive electrode 20 is made as follows. First, the slurry is formed by dispersing the mixture of the positive electrode active material, the conductive agent and the binder in the above ratio in a solvent (for example, N-methyl-2-pyrrolidone) (For example, by coating) and drying the mixture to form the positive electrode active material layer 22. [ The single coating method is not particularly limited. As the coating method, for example, a knife coater method, a gravure coater method, or the like can be considered. Each of the following coating processes is carried out in the same manner. Next, the positive electrode active material layer 22 is pressed to have a density within the above range by a press machine. Thus, the positive electrode 20 is fabricated.

The cathode 30 is fabricated as follows. First, a method of manufacturing the negative electrode active material will be described. The manufacturing method of the negative electrode active material is divided into the following four processes. In the first step, a silicon-based active material is pulverized in a non-aqueous solvent to prepare a dispersion solution in which silicon-based active material fine particles are dispersed in a non-aqueous solvent. That is, in the first step, the silicon-based active material is wet pulverized to make the silicon-based active material fine. According to this method, oxidation of the silicon-based active material is suppressed at the time of pulverization.

As the non-aqueous solvent used for the wet pulverization in the first step, it is possible to suppress the re-aggregation of the silicon-based active material fine particles without reacting with the silicon-based active material, and industrially economical, while dissolving the carbon source organic matter in the second step . Specifically, alcohols such as ethanol and methanol, and ethers are preferred. These non-aqueous solvents may be used alone or in combination of plural kinds thereof.

As the apparatus used for wet milling in the first step, a bead mill apparatus using media can be used. Particularly preferable examples of the bead mill apparatus are MSC mills (for example, MSC100 mill) manufactured by Coke Co., Ltd., Japan. Such a bead mill apparatus can perform the grinding of the silicon-based active material mildly as compared with other bead mill apparatus, ball mill apparatus and the like. Specifically, in the MSC mill, the silicon-based active material is pulverized for a longer time such as other bead mill devices. Therefore, the silicon-based active material fine particles in a columnar shape can be easily produced. It is also important to employ as small beads as possible in order to obtain fine particles. After the treatment, it is necessary to separate the beads and the pulverized material. In the mills other than the MSC mill manufactured by Japan Coke Industries Co., Ltd., these were separated using a screen method or a gap method. However, these methods cause particles to re-agglomerate and become clogged. For this reason, pulverization to the extent necessary for the present invention could not be performed in other mills. On the other hand, the MSC mill uses a centrifugal rotor to separate beads and pulverized material by centrifugal force. This eliminates the bead diameter limit that can be used in MSC mills. Therefore, the MSC mill is capable of finer grinding. Further, in the MSC mill, fine pulverization can be performed while avoiding contact with the atmosphere of the object to be milled, and oxidation of the material can be suppressed. For example, silicon-based active material microparticles according to the present embodiment can be produced by subjecting a silicon-based active material to fine pulverization with an SC100 mill, followed by micro-milling with an MSC100 mill. However, by lengthening the fine grinding time by the MSC100 mill, the axial length and diameter of the silicon-based active material fine particles can be reduced.

It is preferable to use an electrochemically inactive ceramic material such as zirconium oxide or alumina in order to reduce the influence of impurities as much as possible on the container of the medium or apparatus. In order to finely grind the silicon-based active material as fine as possible, the diameter (diameter) of the medium such as zirconium oxide or alumina is preferably as small as possible and preferably about 50 to 100 mu m.

In the second step, a mixed solution is prepared by dissolving an organic substance dissolved in a non-aqueous solvent in a dispersion solution. The organic material usable in the second step is preferably a material which is soluble in a nonaqueous solvent and has a high carbon content (carbonization ratio). This is because it is easy to form a dense carbon material film by using such an organic material. The specific content of carbon is preferably 10.7 to 44.1 mass% with respect to the total mass of the organic material. The content of oxygen is preferably in the range of 16.7 to 63.1 mass% with respect to the total mass of the organic material. Specific examples of the organic substance include, for example, a hydroxy acid, a monosaccharide, and an oligosaccharide.

Examples of the hydroxyl acid include citric acid and glycolic acid. Examples of the monosaccharide include erythritol, D-erythrulose, D-erythrose, D-threose, D-arabinose, L-arabinose, D-xylulose, L-xylulose L-xylulose, D-xylose, D-lyxose, Llyxose, D-ribulose, D- idose, D-quinovose, glucaricacid, D-digitoxose, D-cymarose, 2-deoxy D-glucose D-glucose, L-fucose, D-psicose, D-fructose, L-rhamnose, L- L-iduronic acid, D-glucuronic acid, and the like. Examples of oligosaccharides include isomaltose, rutinose, and the like. Any one of these organic substances may be used, and a plurality of types may be used in combination.

The amount of the organic substance to be added is preferably 10 mass% or less, more preferably 5 mass% or less, and further preferably 2 mass% or less, based on the mass of the dispersion solution. The lower limit is not particularly limited as long as it satisfies the parameters required for the carbon material.

In the third step, the carbon material is coated on the surface of the silicon-based active material fine particles by firing the mixed solution in an inert atmosphere. Thereby, the surface-modified fine particles are produced.

Here, the inert atmosphere can be implemented by, for example, argon, nitrogen, etc. These gases can be used singly or in combination. With regard to the firing temperature, 500 ° C or more is preferable considering the temperature at which carbonization of the organic material proceeds. The upper limit temperature is preferably 800 DEG C or less at which SiC is not generated by the reaction of silicon and carbon. The optimum temperature differs depending on the type of the organic material used for the firing. The surface-modified fine particles are produced by the above process. As described above, in the second to third steps, the silicon-based active material fine particles are not exposed to the air because they are present in the non-aqueous solvent. Therefore, the oxidation during firing is also suppressed. Further, the coating method of the carbon material according to the present embodiment can be realized at a lower cost than the coating method disclosed in the non-patent document 1 (a method of coating a carbon material onto silicon fine particles by a chemical vapor deposition method using acetylene gas).

In the fourth step, the negative electrode active material is produced by compounding the hard carbon and the surface-modified fine particles. The hybridization of the hard carbon and the surface modifying fine particles is carried out by mixing the hard carbon starting material and the surface modifying fine particles and firing the mixture.

Examples of starting materials for hard carbon include phenol resin, PVC, furfurylalcohol, sucrose, gelatin, and agar. Any one of these starting materials may be used, and a plurality of kinds of starting materials may be used.

On the other hand, when the hard carbon is obtained by baking the starting material of the liquid system, the range of the mass ratio in which the cycle life is improved is the surface modified fine particles: hard carbon = 75:25 to 90:10. That is, the total mass of the surface-modified microparticles is preferably 75 to 90 mass% with respect to the total mass of the hard carbon and the surface-modified microparticles. Examples of the starting materials of the liquid system include resole type phenol resin and furfuryl alcohol.

There is no particular limitation on the mixing method, and it may be either wet or dry. The firing is preferably carried out in an inert atmosphere. The inert atmosphere can be implemented by, for example, argon or nitrogen. These gases may be used alone or in combination. Regarding the firing temperature, it is preferably 500 ° C or more in consideration of the temperature at which hard carbon is produced. The upper limit temperature is preferably 800 占 폚 or less at which SiC is not generated by the reaction of silicon and carbon. The optimum temperature depends on the kind of starting material used in the calcination.

Next, the same process as that of the positive electrode 20 is performed. That is, a mixture of the negative electrode active material and the binder is dispersed in a solvent (for example, N-methyl-2-pyrrolidone) to form a slurry. Then, the slurry is formed (for example, coated) on the current collector 31 and dried to form the negative electrode active material layer 32. Subsequently, the negative electrode active material layer 32 is pressed by a press machine. Thus, the cathode 30 is fabricated.

Next, a separator is sandwiched between the positive electrode 20 and the negative electrode 30 to fabricate an electrode structure. Next, the electrode structure is processed into a desired shape (e.g., a cylindrical shape, a square shape, a laminate shape, a button shape, or the like) and inserted into the container of the corresponding shape. Subsequently, an electrolyte solution having the composition described above is injected into the container to impregnate each pore in the separator with an electrolytic solution. Thus, a lithium ion secondary battery is fabricated.

(Charging method of lithium ion secondary battery)

As described above, in the lithium ion secondary battery 10 according to the present embodiment, a large amount of silicon-based active material fine particles can be used for the negative electrode active material, so that the theoretical capacity of the negative electrode active material is extremely high. In this embodiment, when the lithium ion secondary battery is charged, the charging capacity of the lithium ion secondary battery is maintained at 70% or less of the theoretical capacity realized by the silicon-based active material fine particles, that is, the full capacity of the silicon-based active material fine particles. As a result, deterioration of the silicon-based active material fine particles is suppressed, and the cycle life is improved.

(Example 2-1)

In Example 2-1, a negative electrode active material was produced by the following steps.

(First step)

As the silicon particles, metal silicon particles (SIE04PB) manufactured by Kohsundo Chemical Co., Ltd. were prepared. The average particle diameter of the silicon particles was 30 mu m. The particle diameter of the silicon particles was measured by using a laser diffraction / scattering particle size distribution analyzer LA-950 manufactured by Horiba, Inc., which is a diameter when the silicon particles are regarded as spheres. Specifically, the particle size distribution was measured using this apparatus, and the arithmetic average of the particle sizes was calculated based on the particle size distribution. The average particle diameter described below was also measured by this method.

The silicon particles were added to ethanol in an amount of 10% by mass based on the total mass of the ethanol and the silicon particles, and then introduced into SC100 mill manufactured by Japan Cox Industrial Co., Ltd., together with zirconium oxide beads having a diameter (particle diameter) of 500 탆.

The charged amount of the zirconium oxide beads was 60 volume% with respect to the container volume. The agitator was driven at a speed of 13 m / s for 6 hours. Thus, an ethanol dispersion of silicon particles was obtained. The zirconium oxide beads were removed from the ethanol dispersion, and the ethanol dispersion after removing the zirconium oxide beads was introduced into MSC100 mill manufactured by Japan Coke Co., Ltd. together with zirconium oxide beads having a diameter of 50 mu m. The amount of the zirconium oxide beads was set at 55% by volume with respect to the volume of the container. Then, the stirrer was driven at a speed of 13 m / s for 20 hours. Thus, a dispersion solution in which silicon-based active material fine particles (silicon fine particles) were dispersed was prepared. The above silicon-based active material fine particles were observed with a STEMHD-2700 manufactured by Hitachi High-Technologies Corporation, and the STEM photograph shown in Fig. 15 was obtained. From the STEM photographs shown in FIG. 15, about 20 samples were extracted and the axial length of the sample was measured. As a result, the axial length was distributed in the range of 130 to 330 nm. On the other hand, each sample was magnified and observed as shown in FIG. 16, and the diameter of the sample was measured. As a result, the maximum diameter was 30 nm.

(Second Step)

2 g of citric acid (2 mass% based on the total mass of the dispersion solution) was added to 100 g of the dispersion solution prepared in the first step to prepare a mixed solution.

(Third step)

The mixed solution prepared in the second step was transferred to a crucible made of ceramic and installed in a glass tube made of quartz. The inside of the tube was filled with argon gas at a flow rate of 500 cm &lt; ³ &gt; / min, and the inside of the tube was heated to 600 DEG C at a rate of 2.5 DEG C / min. Thereafter, the inside of the tube furnace was maintained at 600 DEG C for 5 hours to naturally cool, and after reaching room temperature, the crucible was taken out.

Subsequently, the sample in the crucible was pulverized in an agate mortar to prepare surface-modified fine particles. The amount of the carbon material covering the silicon-based active material is 2.4% by mass, calculated on the basis of the carbonization rate obtained as a result of the above-described calcination test in citric acid. On the other hand, the weight loss of citric acid was measured by using a differential thermal analyzer EXSTAR6000 manufactured by Seiko Instruments Co., Ltd. As a result, the covering amount of the carbon material was measured and found to be 2.3% by mass. As a result, it was confirmed that the two values were almost the same value.

The surface of the negative electrode active material was analyzed by STEMHD-2700 manufactured by Hitachi High-Technologies Corporation. The carbon material had a thickness of 3 to 8 nm.

(Fourth step)

Next, the surface-modified fine particles and the PVC were weighed so that the mass ratio of the surface-modified fine particles to the hard carbon was 65:35, and the mixture was uniformly mixed with menno induction. Subsequently, the resulting mixture was transferred to a crucible made of ceramic and placed in a glass tube made of quartz. The inside of the tube was heated to 600 DEG C at a rate of 2.5 DEG C / min after argon gas was flowed at a rate of 500 cm &lt; ³ &gt; / min. Thereafter, the inside of the tube furnace was maintained at 600 ° C for 3 hours to naturally cool, and after reaching room temperature, the crucible was taken out. Then, the sample in the crucible was crushed by MeNOU induction. By this, the surface modified fine particles and the hard carbon were combined. That is, an anode active material was prepared.

(Preparation of negative electrode)

75 parts by mass of the negative electrode active material and 10 parts by mass of carbon black were added to a polyimide solution obtained by mixing and dissolving 15 parts by mass of polyimide by addition of N-methyl-2-pyrrolidone to prepare a mixed solution. Then, the mixed solution was mixed with agate mortar to prepare a slurry. The slurry was applied to a copper foil having a thickness of 10 mu m using an applicator. Then, the slurry was heated to 350 DEG C under an argon flow (500 cm &lt; ³ &gt; / min) and heat treated (dried) at this temperature for 30 minutes. Thus, a negative electrode active material layer, that is, a negative electrode was produced.

Next, a lithium foil was adhered to the copper foil as a counter electrode, and the counter electrode and the cathode were laminated through a polyethylene separator having a thickness of 20 mu m to produce an electrode structure. Then, a 2032 coin battery was produced by adding 0.1 cm &lt; ³ &gt; of 1M-LiPF6EC / DEC (1: 1 volume ratio) electrolyte to the electrode structure.

(Measurement of initial efficiency and cycle life)

This battery was subjected to a charge-discharge test using a BTS2010 charge / discharge tester manufactured by Index Co., Ltd. 0.05 C at the second cycle from the first cycle, 0.1 C at the fourth cycle from the third cycle, and then charged and discharged at 0.5 C thereafter. Charging was carried out under a constant current constant voltage and discharging was carried out under a constant current. The charging capacity was 1000 mAh / g, the discharge end voltage was 1.5 V, and the test temperature was 25 ° C.

The initial discharge capacity and cycle life of the lithium ion secondary battery were measured. When the initial efficiency is set to the charge / discharge efficiency (discharge capacity / charge capacity) of the first cycle and the cycle life is less than 90% of the discharge capacity of the fifth cycle by charge / discharge at 0.5C did.

(Example 2-2)

The treatment was carried out in the same manner as in Example 2-1 except that the treatment time of the MSC100 mill of Example 2-1 was changed to 10 hours. The obtained silicon-based active material fine particles had an axial length of 300 to 480 nm and a maximum diameter of 48 nm.

(Example 2-3)

The treatment was carried out in the same manner as in Example 2-1 except that the treatment time of the MSC100 mill of Example 2-1 was changed to 5 hours. The obtained silicon-based active material fine particles had an axial length of 550 to 700 nm and a maximum diameter of 65 nm.

(Example 2-4)

The treatment was carried out in the same manner as in Example 2-1 except that the treatment time of the MSC100 mill of Example 2-1 was changed to 1 hour. The obtained silicon-based active material fine particles had an axial length of 720 to 850 nm and a maximum diameter of 110 nm.

(Example 2-5)

As the silicon particles, monoclinic silicon particles (Lot 3775061) manufactured by Kohsundo Kagaku Kogyo Co., Ltd. were prepared. After the analysis of silicon particles in a measurement condition of a speed ^о 3 / minute, range of 5 ~ 90 ^о XRD, the full width at half maximum of the main peak was 0.1072 ^о. The average particle diameter of the silicon particles was 48 mu m. In Example 2-5, the same process as in Example 2-1 was carried out except that the monocrystalline silicon particles were used in place of the metal silicon particles of Example 2-1. The obtained silicon-based active material fine particles had an axial length of 150 to 290 nm and a maximum diameter of 32 nm. Then, the surface-modified fine particles according to Example 2-5 were observed using a transmission electron microscope (TEM) JEOL-2010 FEF manufactured by Japan Electronics Co., Ltd. As a result, a TEM photograph shown in Fig. 19 was obtained.

(Example 2-6)

The procedure of Example 2-5 was repeated except that the MSC100 mill treatment time of Example 2-5 was changed to 5 hours. The obtained silicon-based active material fine particles had an axial length of 570 to 680 nm and a maximum diameter of 60 nm.

(Example 2-7)

As the silicon particles, polycrystalline silicon particles (Lot 3775062) manufactured by Kohsundo Chemical Co., Ltd. were prepared. Speed 3 ^о / minute for silicon particles, range 5 to 90 as a result of XRD analysis of the measurement conditions ^о, the full width at half maximum of the main peak was 0.1317 ^о. The average particle diameter of the silicon particles was 53 mu m. In Example 2-7, the same processes as in Example 2-1 were carried out except that the above-mentioned polycrystalline silicon particles were used in place of the metal silicon particles of Example 2-1. The obtained silicon-based active material fine particles had an axial length of 100 to 210 nm and a maximum diameter of 25 nm. Further, the surface-modified fine particles according to Example 7 were observed using a transmission electron microscope (TEM) JEOL-2010 FEF manufactured by Japan Electronics Co., Ltd., and TEM images shown in Fig. 18 were obtained.

(Example 2-8)

The procedure of Example 2-7 was repeated except that the MSC100 mill treatment time of Example 2-7 was changed to 5 hours. The obtained silicon-based active material microparticles had an axial length of 520 to 640 nm and a maximum diameter of 53 nm.

(Comparative Example 2-1)

The procedure of Example 2-1 was repeated except that the first step of Example 2-1 was changed as follows. That is, the silicon particles (metal silicon particles) of Example 2-1 were added to ethanol in a blend ratio of 10% by mass with respect to the total mass of ethanol and silicon particles, and were mixed with a zirconium oxide ball having a diameter of 5 mm P-7, and operated at a peripheral speed of 800 rpm for 5 hours. The amount of zirconium oxide balls to be charged was 50 volume% with respect to the volume of the container. Thus, a dispersion solution in which silicon-based active material fine particles (silicon fine particles) were dispersed was prepared. The silicon-based active material fine particles were observed with a transmission electron microscope (TEM) JEOL-2010 FEF manufactured by Japan Electronics Co., Ltd., and TEM images shown in Fig. 17 were obtained. The silicon-based active material fine particles in the TEM photograph were formed in a substantially ellipsoidal shape and clearly different in shape from the cylindrical shape. In addition, the maximum length of the silicon-based active material fine particles in the long axis direction and the maximum short axis direction length in the TEM photographs were 114 nm and 78 nm, respectively. Therefore, the aspect ratio of these lengths is clearly different from this embodiment. In other words, no silicon-based active material fine particles as in this example were observed.

(evaluation)

Table 2 shows the initial efficiency and cycle life in Examples 2-1 to 2-8 and Comparative Example 2-1. However, in Table 2, the initial efficiency and cycle life of Examples 2-1 to 2-8 are standardized by Example 2-4 and Comparative Example 2-1.

Initial efficiency Cycle life Example 2-1 / Comparative Example 2-1 1.04 2.15 Example 2-4 / Comparative Example 2-1 1.01 1.34 Example 2-1 / Example 2-4 1.03 1.6 Example 2-2 / Example 2-4 1.05 1.35 Example 2-3 / Example 2-4 1.01 1.07 Example 2-6 / Comparative Example 2-1 1.01 1.5 Example 2-8 / Comparative Example 2-1 1.01 1.54 Examples 2-5 / Examples 2-6 1.04 1.55 Examples 2-7 / Examples 2-8 1.07 1.75 Examples 2-7 / Examples 2-5 1.03 1.15

In Example 2-1, columnar silicon-based active material fine particles were identified, but in Comparative Example 2-1, no columnar silicon-based active material fine particles were found. As a result, the initial efficiency was 4%, the cycle life was 115%, and Example 2-1 was higher than Comparative Example 2-1. This is presumed to be due to the shape. That is, the silicon-based active material fine particles of Example 2-1 were smaller in size than the silicon-based active material fine particles of Comparative Example 2-1, so that the absolute amount of volume change accompanied by the alloying and detachment of lithium was small. That is, the stress caused by the volume change accompanied by the alloying and disconnection with lithium is alleviated. Particularly, the volume change in the radial direction is reduced. This improves cycle life characteristics. Furthermore, since the diameter is small, alloying of lithium into the silicon-based active material fine particles and a releasing reaction from the silicon-based active material fine particles are apt to occur. For this reason, the initial efficiency is considered to be improved. The same result was obtained in other embodiments.

As shown in Examples 2-2 to 4, when the treatment time of MSC100 was changed, various kinds of silicon-based active material fine particles were obtained. As a result, the initial efficiency was 1% in Example 2-3 (length is 550 to 700 nm, maximum diameter is 65 nm) with respect to Example 2-4 (length of 720 to 850 nm, maximum diameter of 110 nm) The cycle characteristics were improved by 7%. On the other hand, in Example 2-2 (length of 300 to 480 nm, maximum diameter of 48 nm), the initial efficiency was 5% and the cycle characteristic was improved to 35%. Further Example 2-1 (length of 130 to 330 nm, The maximum value of the diameter was 30 nm), the initial efficiency was 3% and the cycle characteristic was improved by 60%. According to Examples 2-1 to 4, it was found that the length in the axial direction is preferably 500 nm or less, and the diameter is preferably 50 nm or less.

However, it is presumed that the improvement in the initial efficiency of Examples 2-1 to 2-3 was attributed to the fact that the movement of lithium ions in the silicon-based active material fine particles accompanying charging and discharging was smoothed by making the silicon-based active material fine particles finer. As for the cycle characteristics, the smaller the diameter, the smaller the amount of change in diameter due to alloying and separation with lithium. Therefore, it is assumed that the stress caused by the change in volume due to alloying and separation with lithium is alleviated.

Example 2-6 (length: 570 to 680 nm, maximum diameter: 60 nm), in which the length and diameter were long in Example 2-5 (length of 150 to 290 nm, maximum diameter of 32 nm) using single crystal silicon, , The initial efficiency was improved by 4% and the cycle life was improved by 55%. The initial efficiency was 7% as compared with Example 2-8 (length is 520 to 640 nm, maximum diameter is 53 nm), polycrystalline silicon of Example 2-7 (length is 100 to 210 nm, maximum diameter is 25 nm) Life span improved by 75%. In comparison between Example 2-5 and Example 2-7 for the same processing time in single crystal silicon and polycrystalline silicon, the length of the polycrystalline silicon is short and the diameter is also small. As a result, the characteristics of Example 2-7 were better than those of Example 2-5. Therefore, it can be seen that the polycrystalline silicon-based active material fine particles are superior to the silicon-based active material fine particles.

However, the following reason can be considered as the reason why the polycrystalline silicon-based active material fine particles are superior to the silicon-based active material fine particles of single crystal. That is, as shown in FIGS. 18 and 19, the polycrystalline silicon-based active material fine particles have a plurality of single crystal domains (domains) therein and spaces are formed between the domains. And each domain is alloyed with lithium upon charging to expand, but it is conceivable that the stress of this expansion is caused by the space between the domains. In the case of monocrystalline silicon-based active material microparticles, a single macro domain is alloyed with lithium, whereas in microcrystalline silicon-based active material microdomains, many domains are alloyed with lithium. For this reason, in the polycrystalline silicon-based active material fine particles, expansion due to alloying with lithium itself is dispersed, so that expansion stress can be considered to be alleviated. For this reason, the polycrystalline silicon-based active material fine particles are superior to the silicon-based active material fine particles of single crystal.

(Example 2-9)

The present inventors conducted the following Examples 2-9 to 13 and Comparative Examples 2-2 to 7 to confirm the effect of the carbon material and the hard carbon on the lithium ion secondary battery. Example 2-9 was treated in the same manner as in Example 2-1 except that a resole type phenol resin was used instead of PVC.

(Example 2-10)

The procedure of Example 2-1 was repeated except that sucrose was used instead of PVC.

(Examples 2-11)

The procedure of Example 2-1 was repeated except that gelatin was used instead of PVC.

(Examples 2-12)

The procedure of Example 2-1 was repeated except that agar was used instead of PVC.

(Examples 2-13)

The procedure of Example 2-1 was repeated except that furfuryl alcohol was used instead of PVC.

(Comparative Example 2-2)

The procedure of Example 2-1 was repeated except that the fourth step was performed as follows. That is, the silicon-based active material fine particles and artificial graphite were weighed so that the mass ratio of the silicon-based active material fine particles prepared in the first step to artificial graphite was 75:25, and these were uniformly mixed with menno induction. This mixture was used as an anode active material.

(Comparative Example 2-3)

The procedure of Comparative Example 2-1 was repeated except that a resole type phenol resin was used instead of PVC.

(Comparative Example 2-4)

The procedure of Comparative Example 2-1 was repeated except that sucrose was used instead of PVC.

(Comparative Example 2-5)

The same treatment as in Comparative Example 2-1 was carried out except that gelatin was used instead of PVC.

(Comparative Example 2-6)

Except that agar was used instead of PVC.

(Comparative Example 2-7)

The same treatment as in Comparative Example 2-1 was carried out except that furfuryl alcohol was used instead of PVC.

(evaluation)

The initial efficiency and cycle life of Examples 2-1 and 2-9 to 13 and Comparative Examples 2-2 to 7 are shown in Table 3 in comparison. However, in Table 3, the initial efficiency and cycle life of Examples 2-1 and 2-9 to 13 are standardized in Comparative Examples 2-2 to 7.

Initial efficiency Cycle life Example 2-1 / Comparative Example 2-2 1.21 2.4 Examples 2-9 / Comparative Examples 2-3 1.11 1.6 Example 2-10 / Comparative Example 2-4 1.05 1.4 Example 2-11 / Comparative Example 2-5 1.04 1.35 Examples 2-12 / Comparative Examples 2-6 1.04 1.35 Example 2-13 / Comparative Example 2-7 1.04 1.5

As shown in Table 3, the properties of Examples 2-1 and 2-9 to 13 were better than those of Comparative Examples 2-2 to 7. In Example 2-1, since the surface-modified fine particles are combined with the hard carbon, expansion due to hard carbon can be alleviated and the conductivity can be secured. On the other hand, graphite of Comparative Example 2-2 can not be said to absorb the stress due to its expansion when the electrode is expanded by silicon alloyed with lithium. Therefore, it can be said that the properties of Example 2-1 are improved compared with those of Comparative Example 2-2 for these reasons.

In Examples 2-9 to 13, composites were prepared using various carbon sources such as phenol resin instead of PVC in Example 2-1, and compared with 2-7 in Comparative Example 2-3. As a result, the same effect as in Example 2-1 using PVC was shown.

(Examples 2-14)

The present inventors conducted Examples 2-14 to confirm the correspondence relationship between the charging capacity, the discharge capacity and the cycle life of the lithium ion secondary battery. Specifically, evaluation was conducted in the same manner as in Example 2-1 except that the charging capacity of Example 2-1 was changed from 20 to 100% of the theoretical capacity realized by the silicon-based active material. The results are shown in Fig. 8 represents the charging ratio of the silicon-based active material (the charging capacity of the lithium ion secondary cell / the theoretical capacity realized by the silicon-based active material) and the vertical axis represents the discharging capacity and the cycle life of the first cycle. The short discharge capacity was expressed by normalizing the discharge capacity when the filling ratio of the silicon-based active material was 20%. The cycle life was expressed by normalizing the cycle life to 1 when the filling ratio of the silicon-based active material was 100%.

According to FIG. 8, when the charging capacity is controlled as compared with the case where the silicon-based active material is completely charged (the charging ratio of the silicon-based active material = 100%), the cycle life is improved. In particular, when the charging ratio of the silicon-based active material is 70% or less, the cycle life is greatly improved. As described above in Example 2-1, the discharge capacity is maintained at a higher level than that of the artificial graphite, and therefore, from the viewpoint of both the high discharge capacity (that is, the reduction of the irreversible capacity) and the long cycle life, It can be seen that the shape is excellent.

(Examples 2-15)

And the firing temperature in the fourth step was changed within the range of 400 to 900 占 폚. The results are shown in Fig. The abscissa of FIG. 9 represents the calcination temperature and the ordinate represents the initial efficiency and cycle life. However, the initial efficiency and the cycle life were expressed by normalizing the value when the firing temperature was 400 占 폚.

According to FIG. 9, both the initial efficiency and the cycle life were higher between 500 and 800 ° C than at 400 ° C. It can be considered that the carbonization of the carbon source of the hard carbon is hardly progressed when the temperature is lowered. In addition, the initial efficiency and cycle life at 900 ° C are considerably lowered, but this can be attributed to adverse effects due to the formation of electrochemically inert SiC. In other words, Si is produced at 900 ° C due to increased reactivity with carbon by making silicon finer.

(Examples 2-16)

The inventors of the present invention have found that there is a corresponding relationship between the mass ratio of the surface-modified fine particles and the hard carbon and the cycle life, and that the corresponding relationship varies depending on the starting material of hard carbon, Examples 2-16 and 2-17 . In Example 2-16, the same processes as in Example 2-1 were carried out except that the mass ratio of the surface-modified fine particles to the hard carbon was changed in the range of 40: 60 to 100: 0 in the fourth step. The results are shown in Fig. In Fig. 10, the abscissa axis represents the mass% of the surface-modified fine particles (mass% relative to the total mass of the surface-modified fine particles and hard carbon), and the ordinate axis represents the cycle life. The short cycle life was expressed by normalizing the value when the mass% of the surface-modified fine particles was 100%.

According to FIG. 10, when the mass ratio of the surface-modified fine particles to the hard carbon is in the range of 50:50 to 80:20, the cycle life is increased. Specifically, the cycle life for this range was 1.8 times larger than the cycle life when the mass% of the surface-modified fine particles was 100%.

(Example 2-17)

In Example 2-17, the fourth step was treated in the same manner as in Example 2-9 except that the mass ratio of the surface-modified fine particles to the hard carbon was changed within a range of 40:60 to 100: 0. The results are shown in Fig. 11, the horizontal axis represents the mass% of the surface-modified microparticles (mass% relative to the total mass of the surface-modified microparticles and the hard carbon), and the vertical axis represents the cycle life. The short cycle life was expressed by normalizing the value when the mass% of the surface-modified fine particles became 100%.

According to Fig. 11, when the mass ratio of the surface-modified fine particles to the hard carbon is in the range of 75:25 to 90:10, the cycle life is increased. Specifically, the cycle life for this range was 1.8 times larger than the cycle life when the mass% of the surface-modified fine particles was 100%.

According to Examples 2-16 and 2-17, it can be seen that there is a range in which the cycle life is improved in the mass ratio of the surface-modified fine particles and the hard carbon. As the proportion of hard carbon increases, the content of the surface-modified fine particles decreases. As a result, the theoretical capacity realized according to the silicon-based active material fine particles is lowered. Therefore, when the charged electricity quantity is controlled to 1000 mAh / g by the cycle life test, the charging ratio of the silicon-containing active material increases. That is, the load on the silicon-based active material increases. This accelerates the deterioration of the silicon-based active material fine particles and shortens the cycle life. On the other hand, if the ratio of the surface-modified fine particles is increased, it is difficult to alleviate the volume change of the silicon-based active material fine particles accompanied with charge and discharge by hard carbon, and the conductivity of the electrode is lowered. As a result, the life characteristics are degraded. Therefore, there is a range in which the cycle life is improved in the mass ratio of the surface-modified fine particles and the hard carbon. Further, it can be seen from Examples 2-16 and 2-17 that the range of the mass ratio in which the cycle life is improved is different depending on the starting material of hard carbon.

(Examples 2-18)

A battery was produced in the same manner as in Example 2-1 except that the electrolyte solution of Example 2-1 was changed to 1M-LiPF6PC / DEC (1: 1 volume ratio). In Example 2-18, the charge of the first cycle was set at 25 캜 in the same manner as in Example 2-1, and the environmental temperature of the battery was changed from 0 to -20 and maintained for 6 hours. Thereafter, a discharge test of 0.5 C was carried out at the environmental temperature. The results are shown in Fig. In Fig. 12, the discharge capacity at 25 DEG C was normalized to 100.

As shown in Fig. 12, in Example 2-18 using PC, the discharge capacity at a low temperature was significantly increased as compared with the cell of Example 2-1 using EC. It can be said that this is because it is not frozen even at a temperature of -20 캜 in a PC having a melting point of -55 캜, and migration of lithium ions is hardly inhibited. On the other hand, in the case of EC, since the melting point is 37 ° C, it is mixed with DEC having a low boiling point, so that it is liquid at room temperature but solidifies by lowering the temperature, and mobility of lithium ion is inhibited and discharge can not be performed. The availability of this PC can be a great advantage for applications such as electric trains because it can lead to the expansion of the available low temperature zone.

(Example 2-19)

Next, the present inventor conducted Example 2-19 in order to confirm the correspondence relationship between the firing temperature in the third step and the characteristics of the lithium ion secondary battery. In Example 2-19, the treatment was carried out in the same manner as in Example 2-1 except that the firing temperature in the third step of Example 2-1 was varied within the range of 400 to 900 占 폚. The results are shown in Fig. The horizontal axis in Fig. 13 represents the firing temperature, and the vertical axis represents the initial efficiency and the cycle life. However, the initial efficiency and the cycle life show the values standardized in the following Comparative Examples 2-8.

(Comparative Example 2-8)

The procedure of Example 2-19 was repeated except that the second and third steps of Example 2-1 were not carried out. That is, in Comparative Example 8, the surface of the silicon-based active material fine particles was not coated with the carbon material.

13, the initial efficiency was higher than that of Comparative Example 2-8 between 500 and 800 ° C, and the cycle life was higher than that of Comparative Example 8 between 500 and 800 ° C. Therefore, the firing temperature is preferably 500 to 800 ° C. The reason why the cycle life and the initial efficiency are lowered when the firing temperature is less than 500 deg. C is that the carbonization of the organic material is difficult to proceed at a low firing temperature and the coating amount of the carbon material is lowered. When the firing temperature is 900 占 폚, the initial efficiency and the cycle life are considerably lowered. The reason for this may be adverse effects due to the generation of electrochemically inactive SiC. That is, as the silicon-based active material is made into fine particles, the reactivity with carbon becomes high, and it can be considered that SiC is partially formed even at 900 ° C.

(Example 2-20)

The present inventors also conducted Example 2-20 to confirm the correspondence relationship between the amount of the carbon material covered, that is, the mass% of the carbon material with respect to the total mass of the silicon-based active material fine particles and the carbon material, and the characteristics of the lithium ion secondary battery did. Specifically, the treatment was carried out in the same manner as in Example 2-1 except that the coating amount of the carbon material was varied within the range of 0.07 to 6.7 mass% by varying the addition amount of citric acid in the second step of Example 2-1 . The results are shown in Fig. The abscissa of FIG. 14 represents the covering amount of the carbon material (mass% of the carbon material with respect to the total mass of the silicon-based active material fine particles and the carbon material, C / Si ratio), and the vertical axis represents the initial efficiency and cycle life. However, the initial efficiency and cycle life show the values standardized in Comparative Example 2-8.

According to Fig. 14, when the coating amount of the carbon material becomes 0.35 to 3.5 mass%, the cycle life and the initial efficiency become better than those of Comparative Example 2-8, and the coating amount of the carbon material becomes 0.7 to 1.75 mass% Efficiency is maximized. The reason why the cycle life and the initial efficiency are lowered when the coating amount of the carbon material exceeds 3.5% by mass is considered to be due to the structure of the carbon material. That is, the carbon material covering the silicon-based active material fine particles is presumed to have a graphite structure. However, since this structure is immature (more amorphous portions are detailed), when the covering amount of the carbon material increases, And it is rather inhibited. When the covering amount of the carbon material is 0.7 to 1.75 mass%, the cycle life and the initial efficiency are maximized. It can be considered that when the coating amount of the carbon material falls within this range, the carbon material is hard to inhibit the movement of lithium ions, and at the same time, the oxidation inhibiting effect of the silicon-based active material fine particles by the carbon material is maximized.

(Example 2-21)

The present inventors conducted the following Example 2-21 in order to confirm that the same effect is obtained also in other silicon-based active materials. Specifically, the same processes as in Example 2-1 were carried out except that the metal silicon of Example 2-1 was replaced with a polycrystalline silicon alloy (Si: Al: Fe = 55: 29: 16 (mass ratio)). As a result, initial efficiency and cycle life of the same degree as in Example 2-1 were obtained.

As described above, according to the present embodiment, the silicon-based active material fine particles are less likely to be exposed to the atmosphere in the first to third steps, and the surface-modified fine particles having a small amount of silicon oxide can be produced. Further, since the surface-modified fine particles are covered with the carbon material, the silicon-based active material fine particles are less likely to be exposed to the air during use of the surface-modified fine particles. Thus, the irreversible capacity of the negative electrode active material is reduced. In other words, it is possible to maintain a high discharge capacity of the silicon-based active material (i.e., realize a high initial efficiency). And the direct contact between the silicon-based active material fine particles and the solvent is suppressed. Therefore, the cycle life of the lithium ion secondary battery can be improved. Further, the surface-modified fine particles are introduced into hard carbon by being complexed with hard carbon. Therefore, the stress caused by the volume change of the surface-modified fine particles is alleviated by the hard carbon, and the cycle life is also improved in this respect. In addition, a large amount of surface modified fine particles can be introduced into the hard carbon, thereby improving the theoretical capacity of the negative electrode active material, that is, the discharge capacity.

In addition, since the silicon-based active material is in the shape of a cylinder, the absolute amount of the volume change, particularly, the absolute amount of volume change in the diameter direction can be reduced. In this embodiment, cycle life is also improved in this respect.

Here, the hard carbon is obtained by firing a starting material of a high system, and the total mass of the surface-modified fine particles is preferably 50 to 80 mass% with respect to the total mass of the hard carbon and the surface-modified fine particles. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life of the lithium ion secondary battery is improved.

Also, the starting material of the high system may be any one or more selected from the group consisting of PVC, sucrose, gelatin and agar. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life of the lithium ion secondary battery is improved.

Further, the hard carbon is obtained by firing the starting material of the liquid system, and the total mass of the surface-modified fine particles is preferably 75 to 90 mass% with respect to the total mass of the hard carbon and the surface-modified fine particles. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life of the lithium ion secondary battery is improved.

In addition, the average particle diameter of the silicon-based active material fine particles may be 200 nm or less. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

Further, the carbon material may have a thickness of 2 to 20 nm. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

At least one of the inert atmosphere in the third step and the fourth step may be composed of at least one selected from the group consisting of argon and nitrogen. In this case, irreversible capacity of the lithium ion secondary battery is reduced and cycle life is also improved do.

In addition, the firing in the fourth step can be carried out within a temperature range of 500 to 800 ° C. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

Further, a metal lithium foil can be disposed on a part of the negative electrode current collector, for example, a portion where the electrode current collector terminal is welded, and a portion not facing the positive electrode active material layer. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

In addition, the charging capacity of the lithium ion secondary battery can be maintained at 70% or less of the theoretical capacity realized by the silicon-based active material fine particles. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The organic material may be at least one selected from the group consisting of hydroxy acids, monosaccharides and oligosaccharides. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

And the hydroxy acid may be at least one selected from the group consisting of citric acid and glycolic acid. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

Also, the non-aqueous solvent may be at least one selected from the group consisting of alcohols and ethers. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The firing in the third step can be carried out within a temperature range of 500 to 800 ° C. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

(Upper limit on the negative electrode active material according to another embodiment)

The present inventors have found that a negative electrode active material obtained by compounding surface-modified fine particles and hard carbon is applied to a lithium ion secondary battery. The present inventors have also made the silicon-based active material microparticles constituting the surface-modified microparticles as polycrystals. According to this embodiment, the cycle life of the lithium ion secondary battery can be improved at a low cost while maintaining a high discharge capacity of the silicon-based active material. The lithium ion secondary battery of the present embodiment can be used in a wider temperature range.

(Configuration of Lithium Ion Secondary Battery)

Examples of the method for distinguishing between the polycrystalline silicon-based active material fine particles and the monocrystalline silicon-based active material fine particles include TEM observation or XRD analysis. 28 shows a TEM photograph of the polycrystalline silicon-based active material fine particles. 29 shows TEM photographs of the monocrystalline silicon-based active material fine particles. As shown in FIGS. 28 and 29, the polycrystalline silicon-based active material fine particles have a plurality of single crystal particles therein. In contrast, the monocrystalline silicon-based active material fine particles are composed of a single single crystal particle. The magnification in FIG. 28 is 800,000 times, and the magnification in FIG. 29 is 400,000 times.

In addition, differences in peak shapes of XRD can be seen due to these structural differences. For example, speed 3 ^о / minute, range of 5 to the half-width of 90 ^о a polycrystalline silicon active material and the single crystal silicon-based active material (corresponding to the (111) plane), a polycrystalline main peak when parsing XRD is 0.13 ^о ± 0.005 ^о and a full width at half maximum in the range of the single-crystal main peak is 0.11 ^о is a value in the range of ± 0.005 ^о. By observing on the TEM or XRD analysis, it is possible to distinguish the polycrystalline silicon-based active material fine particles and the monocrystalline silicon-based active material fine particles.

The average particle size of the silicon-based active material fine particles is preferably as small as possible. When the silicon-based active material fine particles are alloyed with lithium in the cathode 30, the absolute value of the expansion per one particle is small, in which the silicon-based active material fine particles have a small particle diameter. Therefore, expansion of the cathode 30 can be suppressed. Specifically, the average particle diameter of the silicon-based active material fine particles is preferably 200 nm or less, more preferably 100 nm or less. The cycle life is particularly improved when the average particle diameter of the silicon-based active material microparticles is within these ranges. The lower limit of the average particle diameter of the silicon-based active material fine particles is not particularly limited, but is preferably 50 to 60 nm, for example. That is, in the wet grinding described later, for example, by setting the grinding time to 30 hours or more, it is possible to set the average particle size to about 50 to 60 nm.

Here, the average particle diameter of the silicon-based active material fine particles is a diameter when the silicon-based active material fine particles are regarded as spheres, and can be measured by, for example, a laser diffraction / scattering particle size distribution measuring device. In this case, the particle size distribution of the silicon-based active material fine particles is measured by a laser diffraction / scattering type particle size distribution measuring device, and the arithmetic mean value of the particle size of the silicon-based active material fine particles is calculated as the average particle size based on the particle size distribution.

(Example 3-1)

In Example 3-1, the negative electrode active material was produced by the following steps.

(First step)

As the silicon particles, polycrystalline silicon particles (Lot 3775062) manufactured by Kohsundo Chemical Co., Ltd. were prepared. Speed 3 ^о / minute for silicon particles, range 5 to 90 as a result of XRD analysis of the measurement conditions ^о, the full width at half maximum of the main peak was 0.1317 ^о.

The average particle size of the silicon particles was 53 mu m. The average particle diameter of the silicon particles was measured using a laser diffraction / scattering type particle size distribution analyzer LA-950 manufactured by Horiba, Inc. Specifically, the particle size distribution was measured using this apparatus, and the arithmetic average of the particle sizes was calculated based on the particle size distribution. The average particle size described below was also measured by this method.

The silicon fine particles were added to ethanol in an amount of 10% by mass based on the total mass of ethanol and silicon fine particles, and then mixed with zirconium oxide beads having a diameter (particle diameter) of 50 袖 m with Leversta Mini (manufactured by Ashizawa Finetech) Labstar) into LMZ015.

The charged amount of the zirconium oxide beads was 80 vol% relative to the volume of the container. Then, the agitator was driven at a speed of 12 m / s for 24 hours. Thus, a dispersion solution in which silicon-based active material fine particles (silicon fine particles) were dispersed was prepared. The average particle diameter of the silicon-based active material fine particles was measured by the same method as that of the silicon fine particles, and the average particle diameter was 97 nm.

(Second Step)

2 g of citric acid (2 mass% based on the total mass of the dispersion solution) was added to 100 g of the dispersion solution prepared in the first step to prepare a mixed solution.

(Third step)

The mixed solution prepared in the second step was transferred to a crucible made of ceramic and installed in a glass tube made of quartz. The inside of the tube was filled with argon gas at a flow rate of 500 cm &lt; ³ &gt; / min to sufficiently replace the tube, and then the inside of the tube was heated to 600 DEG C at a rate of 2.5 DEG C / min. Thereafter, the inside of the tube furnace was naturally cooled at 600 ° C for 5 hours, and after reaching room temperature, the crucible was taken out.

Subsequently, the sample in the crucible was pulverized in an agate mortar to prepare surface-modified fine particles. The coating amount of the carbon material on the silicon-based active material is 2.4% by mass calculated from the carbonation rate obtained from the result of the above firing test in the citric acid group. On the other hand, the weight loss of citric acid was measured using EXSTAR6000, a differential thermal analyzer manufactured by Seiko Instruments Co., Ltd., and the amount of coating of the carbon material was measured from the result of the measurement. As a result, it was 2.3 mass%. Therefore, it is confirmed that they are almost the same value.

Further, the surface of the negative electrode active material was analyzed by using a transmission electron microscope (TEM) JEOL-2010 FEF manufactured by Japan Electronics Co., Ltd. The carbon material had a thickness of 3 to 8 nm. Further, when the surface-modified fine particles were observed under the same microscope, a TEM photograph shown in FIG. 28 was obtained.

(Fourth step)

Next, the surface-modified fine particles and PVC were weighed so that the mass ratio of the surface-modified fine particles to the hard carbon was 65:35, and the mixture was uniformly mixed with MeNO induced. Then, the mixture obtained by the mixing was transferred to a crucible made of ceramic and placed in a glass tube made of quartz. The inside of the tube was heated to 600 DEG C at a rate of 2.5 DEG C / min after argon gas was flowed at a rate of 500 cm &lt; ³ &gt; / min. Thereafter, the inside of the tube furnace was naturally cooled while being maintained at 600 DEG C for 3 hours, and after reaching room temperature, the crucible was taken out. Then, the samples in the crucible were crushed by MeNOU induction. By this, the surface modified fine particles and the hard carbon were combined. That is, an anode active material was prepared.

(Preparation of negative electrode)

75 parts by mass of the negative electrode active material and 10 parts by mass of carbon black were added to a polyimide solution obtained by mixing and dissolving 15 parts by mass of polyimide by addition of N-methyl-2-pyrrolidone to prepare a mixed solution. Then, this mixed solution was mixed with agate mortar to prepare a slurry. The slurry was applied to a copper foil having a thickness of 10 mu m using an applicator. Then, the slurry was heated to 350 DEG C at a rate of 2.5 DEG C / min under an argon flow (500 cm &lt; ³ &gt; / min) and heat treated (dried) at this temperature for 30 minutes. Thus, a negative electrode active material layer, that is, a negative electrode was produced.

Next, a lithium foil was adhered to the copper foil as a counter electrode, and the counter electrode and the cathode were laminated through a polyethylene separator having a thickness of 20 mu m to produce an electrode structure. Then, a 2032 coin battery was produced by adding 0.1 cm &lt; ³ &gt; of 1M-LiPF6EC / DEC (1: 1 volume ratio) electrolyte to the electrode structure.

(Measurement of initial efficiency and cycle life)

This battery was subjected to a charge-discharge test using a BTS2010 charge / discharge tester manufactured by Index Co., Ltd. 0.05 C at the second cycle from the first cycle, 0.1 C at the fourth cycle from the third cycle, and then charged and discharged at 0.5 C thereafter. Charging was carried out under a constant current constant voltage and discharging was carried out under a constant current. The charging capacity was 1000 mAh / g, the discharge end voltage was 1.5 V, and the test temperature was 25 ° C.

The discharge capacity, initial efficiency and cycle life of the lithium ion secondary battery were measured. Here, the initial efficiency was defined as the number of cycles when the discharge capacity (discharge capacity / charge capacity) of the first cycle was set to be less than 90% of the discharge capacity of the fifth cycle of the discharge capacity as the cycle life was 0.5 C or less.

(Example 3-2)

The procedure of Example 3-1 was repeated except that a resole type phenol resin was used instead of PVC.

(Example 3-3)

The procedure of Example 3-1 was repeated except that sucrose was used instead of PVC.

(Example 3-4)

The same procedures as in Example 3-1 were repeated except that gelatin was used instead of PVC.

(Example 3-5)

The procedure of Example 3-1 was repeated except that agar was used instead of PVC.

(Example 3-6)

The procedure of Example 3-1 was repeated except that furfuryl alcohol was used instead of PVC.

(Comparative Example 3-1)

As the silicon particles, monoclinic silicon particles (Lot 3775061) manufactured by Kohsundo Kagaku Kogyo Co., Ltd. were prepared. After the analysis of silicon particles in a measurement condition of a speed ^о 3 / minute, range of 5 ~ 90 ^о XRD, the full width at half maximum of the main peak was 0.1072 ^о. The average particle diameter of the silicon particles was 48 mu m. In Comparative Example 3-1, the same processing as in Example 3-1 was carried out except that the monocrystalline silicon particles were used instead of the polycrystalline silicon particles of Example 3-1. The surface-modified microparticles of Comparative Example 3-1 were observed under the same microscope, and TEM images shown in Fig. 29 were obtained.

(Comparative Example 3-2)

The procedure of Example 3-1 was repeated except that the fourth step was performed as follows. That is, the silicon-based active material fine particles and artificial graphite were weighed so that the mass ratio of the silicon-based active material fine particles produced in the first step to the artificial graphite was 75:25, and these were homogeneously mixed with menno induction. This mixture was used as an anode active material.

(Comparative Example 3-3)

The procedure of Comparative Example 3-1 was repeated except that a resole type phenol resin was used instead of PVC.

(Comparative Example 3-4)

The procedure of Comparative Example 3-1 was repeated except that sucrose was used instead of PVC.

(Comparative Example 3-5)

The same process as in Comparative Example 3-1 was carried out except that gelatin was used instead of PVC.

(Comparative Example 3-6)

The same procedure as in Comparative Example 3-1 was repeated except that agar was used instead of PVC.

(Comparative Example 3-7)

The same treatment as in Comparative Example 3-1 was conducted except that furfuryl alcohol was used instead of PVC.

(evaluation)

The initial efficiency and cycle life of Examples 3-1 to 6 and Comparative Examples 3-1 to 7 are shown in Table 4 in comparison. In Table 4, the initial efficiency and cycle life of Examples 3-1 to 6 are shown as standardized in Comparative Examples 3-1 to 7.

Initial efficiency Cycle life Example 3-1 / Comparative Example 3-1 1.08 1.65 Example 3-1 / Comparative Example 3-2 1.18 2.5 Example 3-2 / Comparative Example 3-3 1.10 1.5 Example 3-3 / Comparative Example 3-4 1.05 1.35 Example 3-4 / Comparative Example 3-5 1.04 1.25 Example 3-5 / Comparative Example 3-6 1.05 1.34 Example 3-6 / Comparative Example 3-7 1.03 1.4

As shown in Table 4, in Example 3-1 using polycrystalline silicon, the initial efficiency could be improved by 8% and the cycle life could be improved by 65% as compared with Comparative Example 3-1 using single crystal silicon. As shown in FIGS. 28 and 29, the polycrystalline silicon-based active material fine particles have a plurality of single crystal domains (domains) therein and a space is formed between the domains. Then, each domain is alloyed with lithium at the time of charging and expands, but it can be considered that the stress of expansion is relaxed by the interdomain space. In a single-crystal silicon-based active material, a single macroscopic domain is alloyed with lithium at the time of charging, but a plurality of minute domains are alloyed with lithium in the polycrystalline silicon-based active material. For this reason, in the polycrystalline silicon-based active material, the expansion due to alloying with lithium is dispersed by itself, and therefore, it can be considered that the stress of expansion is alleviated. Further, in Example 3-1, since the surface-modified fine particles are complexed with hard carbon, expansion due to hard carbon and relaxation of conductivity can be expected. For this reason, it can be said that the characteristics of Example 3-1 are improved compared with those of Comparative Example 3-1.

In addition, Example 3-1 was able to improve the initial efficiency by 18% and the lifetime by 2.5 times as compared with Comparative Example 3-2 (the negative electrode active material was a composite material of a polycrystalline silicon-based active material and graphite). It can be considered that the graphite of Comparative Example 3-2 can not absorb the stress due to its expansion when the electrode is expanded by silicon alloyed with lithium. Therefore, it can be said that the cycle deterioration is increased in Comparative Example 3-2.

In Examples 3-2 to 6, composites were prepared using various carbon sources such as phenol resin instead of PVC in Example 3-1, and compared with Comparative Examples 3-3 to 7. As a result, the same effect as in Example 3-1 using PVC was shown, and the initial efficiency was improved by 3 to 10% and the cycle life was improved by 25 to 50%.

(Example 3-7)

The present inventors conducted Examples 3-7 to confirm the correspondence relationship between the charging capacity, the discharge capacity and the cycle life of the lithium ion secondary battery. Specifically, the charging capacity of Example 3-1 was evaluated in the same manner as in Example 3-1 except that the charging capacity was changed from 20 to 100% of the theoretical capacity realized by the silicon-based active material. The results are shown in Fig. 20 represents the charging ratio of the silicon-based active material (the charging capacity of the lithium ion secondary cell / the theoretical capacity realized by the silicon-based active material), and the vertical axis represents the discharging capacity and the cycle life of the first cycle. The short discharge capacity was expressed by normalizing the discharge capacity when the filling ratio of the silicon-based active material was 20%. The cycle life is expressed by normalizing the cycle life to 1 when the charging ratio of the silicon-based active material is 100%.

According to FIG. 20, the cycle life is improved when the charging capacity is controlled as compared with the case where the silicon-based active material is completely charged (the charging ratio of the silicon-based active material = 100%). In particular, when the charging ratio of the silicon-based active material is 70% or less, the cycle life is greatly improved. As described in the above-mentioned Example 3-1, since the discharge capacity is maintained at a higher level than that of the artificial graphite, the present embodiment is advantageous in terms of both a high discharge capacity (that is, a reduction in irreversible capacity) It can be seen that it is excellent.

(Example 3-8)

And the firing temperature in the fourth step was changed within the range of 400 to 900 占 폚. The results are shown in Fig. The abscissa of FIG. 21 represents the calcination temperature and the ordinate represents the initial efficiency and cycle life. However, the initial efficiency and the cycle life were normalized to a value of 1 when the firing temperature was 400 캜.

According to FIG. 21, both the initial efficiency and the cycle life were higher between 500 and 800 ° C than at 400 ° C. If the temperature is lowered, it can be considered that the carbonization progress of the hard carbon carbon source is difficult to proceed. In addition, the initial efficiency and cycle life are greatly reduced at 900 DEG C, but this can be attributed to adverse effects due to the generation of electrochemically inert SiC. In other words, it is considered that SiC was produced at 900 ° C due to increased reactivity with carbon by making silicon finer.

(Example 3-9)

The inventors of the present invention have found that there is a corresponding relationship between the mass ratio of the surface-modified fine particles and the hard carbon and the cycle life, and that the corresponding relationship varies depending on the starting material of the hard carbon, 10. In Example 3-9, the fourth step was treated in the same manner as in Example 3-1 except that the mass ratio of the surface-modified fine particles to the hard carbon was changed within the range of 40: 60 to 100: 0. The results are shown in Fig. The abscissa of FIG. 22 represents the mass% of the surface-modified microparticles (mass% with respect to the total mass of the surface-modified microparticles and hard carbon), and the ordinate represents the cycle life. The short cycle life was expressed by normalizing the value when the mass% of the surface-modified fine particles was 100%.

According to Fig. 22, when the mass ratio of the surface-modified fine particles to the hard carbon is in the range of 50:50 to 80:20, the cycle life is increased. Specifically, the cycle life for this range was 1.8 times larger than the cycle life when the mass% of the surface-modified fine particles was 100%.

(Example 3-10)

In Example 3-10, the same processes as in Example 3-2 were carried out except that the mass ratio of the surface-modified fine particles to the hard carbon was changed within a range of 40: 60 to 100: 0 in the fourth step. The results are shown in Fig. The abscissa of FIG. 23 represents the mass% of the surface-modified microparticles (mass% with respect to the total mass of the surface-modified microparticles and the hard carbon), and the ordinate represents the cycle life. The short cycle life was expressed by normalizing the value when the mass% of the surface-modified fine particles was 100%.

According to FIG. 23, when the mass ratio of the surface-modified fine particles to the hard carbon is in the range of 75:25 to 90:10, the cycle life is increased. Specifically, the cycle life for this range was 1.8 times larger than the cycle life when the mass% of the surface-modified fine particles was 100%.

According to Examples 3-9 and 3-10, it can be seen that there is a range in which the cycle life is improved in the mass ratio of the surface-modified fine particles and the hard carbon. As the proportion of hard carbon increases, the content of the surface-modified fine particles decreases. As a result, the theoretical capacity realized according to the silicon-based active material fine particles is lowered. Therefore, when the charged electricity quantity is controlled to 1000 mAh / g by the cycle life test, the charging ratio of the silicon-containing active material increases. That is, the load of the silicon-based active material increases. Therefore, the deterioration of the silicon-based active material fine particles is accelerated and the cycle life is shortened. On the other hand, if the proportion of the surface-modified fine particles is increased, it is difficult to alleviate the expansion and contraction of the silicon-based active material particles due to charging and discharging with hard carbon, and the conductivity of the electrode is lowered. As a result, the life characteristics are degraded. Therefore, there is a range in which the cycle life is improved in the mass ratio of the surface-modified fine particles and the hard carbon. Further, it was also found from Examples 3-9 and 3-10 that the range of the mass ratio in which the cycle life is improved depends on the starting material of hard carbon.

(Examples 3-11)

A battery was produced in the same manner as in Example 3-1 except that the electrolytic solution of Example 3-1 was changed to 1M-LiPF6PC / DEC (1: 1 volume ratio). In Example 3-11, the charging of the first cycle was carried out at 25 ° C in the same manner as in Example 3-1, and the environmental temperature of the battery was changed from 0 to -20 and maintained for 6 hours. Thereafter, a discharge test of 0.5 C was carried out at the environmental temperature. The results are shown in Fig. In Fig. 24, the discharge capacity at 25 DEG C was normalized to 100.

As shown in Fig. 24, in Example 3-11 using PC, it was found that the discharge capacity at low temperature was greatly increased as compared with the battery of Example 3-1 using EC. It is considered that this is because the melting point does not freeze even at -20 占 폚 in the PC of -55 占 폚, and the movement of the lithium ion is not disturbed. On the other hand, in the case of EC, since it has a melting point of 37 ° C, it is mixed with DEC having a low boiling point, and it is believed that the liquid becomes solid as the liquid or the temperature decreases at room temperature and the mobility of lithium ion is lowered. The ability to use this PC can be a great advantage for applications in applications such as electric trains, since it leads to the expansion of available low-temperature zones.

(Examples 3-12)

Next, the inventors conducted Examples 3-12 to confirm the correspondence relationship between the firing temperature in the third step and the characteristics of the lithium ion secondary battery. In Example 3-12, the third step of Example 3-1 was treated in the same manner as in Example 3-1 except that the firing temperature was varied within the range of 400 to 900 占 폚. The results are shown in Fig. 25, the horizontal axis represents the firing temperature and the vertical axis represents the initial efficiency and cycle life. However, the initial efficiency and the cycle life are values standardized by the following Comparative Examples 3-8.

(Comparative Example 3-8)

The procedure of Example 3-12 was repeated except that the second and third steps of Example 3-1 were not carried out. That is, in Comparative Example 3-8, the surface of the silicon-based active material fine particles was not coated with the carbon material.

25, the initial efficiency was higher than that of Comparative Example 3-8 at 500 to 800 ° C, and the cycle life was higher than that of Comparative Example 3-8 at 500 to 800 ° C. Therefore, the firing temperature is preferably 500 to 800 ° C. The reason why the cycle life and the initial efficiency are lowered when the firing temperature is less than 500 ° C is that the carbonization of the organic material is difficult to proceed at a low firing temperature and the coating amount of the carbon material is lowered. When the firing temperature is 900 占 폚, the initial efficiency and the cycle life are considerably lowered. The reason for this is considered to be adverse effects due to the formation of electrochemically inactive SiC. That is, as the silicon-based active material is made into fine particles, reactivity with carbon becomes higher, and SiC is partially generated even at 900 ° C.

(Examples 3-13)

Furthermore, the inventors of the present invention conducted Examples 3-13 to confirm the correspondence relationship between the amount of the carbon material covered, that is, the mass% of the carbon material with respect to the total mass of the silicon-based active material fine particles and the carbon material, and the characteristics of the lithium ion secondary battery . Specifically, the treatment was carried out in the same manner as in Example 3-1 except that the coating amount of the carbon material was varied within the range of 0.07 to 6.7 mass% by varying the amount of citric acid added to the second step of Example 3-1. The results are shown in Fig. The abscissa of FIG. 26 represents the coating amount of the carbon material (mass% of the carbon material with respect to the total mass of silicon-based active material fine particles and carbon material, C / Si ratio), and the vertical axis represents the initial efficiency and cycle life. However, the initial efficiency and the cycle life are values standardized in Comparative Example 3-8.

According to Fig. 26, when the coating amount of the carbon material is 0.35 to 3.5 mass%, the cycle life and the initial efficiency become better than those of Comparative Example 3-8 and the coating amount of the carbon material becomes 0.7 to 1.75 mass% The efficiency is maximized. The reason why the cycle life and the initial efficiency are lowered when the coating amount of the carbon material exceeds 3.5% by mass is due to the structure of the carbon material. That is, it is presumed that the carbon material covering the silicon-based active material fine particles has a graphite structure. However, since this structure is immature (more amorphous portions are detailed), when the covering amount of the carbon material increases, This is because entrance into or out of the active material fine particles is rather inhibited. Also, when the coating amount of the carbon material is 0.7 to 1.75 mass%, the cycle life and the initial efficiency are maximized. When the covering amount of the carbon material is within this range, it is difficult for the carbon material to inhibit the movement of the lithium ion, and at the same time, the oxidation inhibiting effect of the silicon-based active material fine particles by the carbon material is maximized.

(Examples 3-14)

The inventors of the present invention conducted Examples 3-14 to confirm the correspondence relationship between the average particle diameter of the silicon-based active material fine particles and the characteristics of the lithium ion secondary battery. Specifically, the same procedure as in Example 3-1 was conducted except that the average particle size of the silicon-based active material fine particles was varied within a range of 100 to 900 nm by adjusting the milling time for the first step of Example 3-1. The results are shown in Fig. The abscissa of FIG. 27 represents the average particle diameter of the silicon-based active material fine particles and the ordinate represents the cycle life. The short cycle life shows the value normalized in Comparative Example 3-8.

According to FIG. 27, although the cycle life is prolonged as the average particle diameter is smaller, the cycle life is improved at all particle diameters rather than the comparative example. In addition, the cycle life varied sharply, especially at 200 nm. It was also estimated that the shape of the graph was maximized at 100 nm or less. Accordingly, the average particle diameter is preferably 200 nm or less, more preferably 100 nm or less. However, when the wet grinding method of Example 3-1 is used, re-agglomeration and particle grinding are difficult, and it takes much time to grind to 100 nm or less. The average particle size can be selected industrially in terms of cost and performance.

(Examples 3-15)

The inventors of the present invention conducted the following Examples 3-15 in order to confirm that the same effects were obtained in other silicon-based active materials. Specifically, the procedure of Example 3-1 was repeated except that the silicon of Example 3-1 was replaced with a polycrystalline silicon alloy (Si: Al: Fe = 55: 29: 16 (mass ratio)). As a result, initial efficiency and cycle life of the same degree as in Example 3-1 were obtained.

As described above, according to the present embodiment, since the silicon-based active material fine particles are hardly exposed to the atmosphere in the first to third steps, the surface-modified fine particles having a small amount of silicon oxide can be produced. Further, since the surface-modified fine particles are covered with the carbon material, the silicon-based active material fine particles are less likely to be exposed to the air during use of the surface-modified fine particles. Thus, the irreversible capacity of the negative electrode active material is reduced. In other words, it is possible to maintain the high discharge capacity of the silicon-based active material (i.e., realize high initial efficiency). In addition, direct contact between the silicon-based active material fine particles and the solvent is suppressed. Therefore, the cycle life of the lithium ion secondary battery can be improved. Further, the surface-modified fine particles are introduced into the hard carbon by being combined with the hard carbon. Therefore, the stress caused by the expansion and shrinkage of the surface-modified fine particles is alleviated by the hard carbon, so that the cycle life is also improved. In addition, a large amount of surface-modified fine particles can be introduced into the hard carbon, thereby improving the theoretical capacity of the negative electrode active material, that is, the discharge capacity.

The silicon-based active material is polycrystalline, and thus has a plurality of single crystal particles (domains), and spaces are formed between the domains. Therefore, the stress caused by the expansion and contraction of the silicon-based active material is mitigated by these spaces. In this respect, the cycle life is also improved.

Here, the hard carbon is obtained by firing a starting material of a high system, and the total mass of the surface-modified fine particles is preferably 50 to 80 mass% with respect to the total mass of the hard carbon and the surface-modified fine particles. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life of the lithium ion secondary battery is improved.

Also, the starting material of the high system may be any one or more selected from the group consisting of PVC, sucrose, gelatin and agar. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life of the lithium ion secondary battery is improved.

The hard carbon may be obtained by firing the starting material of the liquid system, and the total mass of the surface-modified fine particles is preferably 75 to 90 mass% with respect to the total mass of the hard carbon and the surface-modified fine particles. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life of the lithium ion secondary battery is improved.

In addition, the average particle diameter of the silicon-based active material fine particles may be 200 nm or less. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

Further, the carbon material may have a thickness of 2 to 20 nm. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

At least one of the inert atmosphere in the third step and the fourth step may be composed of at least one selected from the group consisting of argon and nitrogen. In this case, irreversible capacity of the lithium ion secondary battery is reduced and cycle life is also improved do.

In addition, the firing in the fourth step can be carried out within a temperature range of 500 to 800 ° C. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

Further, a part of the negative electrode current collector may be provided with a metal lithium foil, for example, at a portion where the electrode current collector terminal is welded to a portion not opposed to the positive electrode active material layer. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

In addition, the charging capacity of the lithium ion secondary battery can be maintained at 70% or less of the theoretical capacity realized by the silicon-based active material fine particles. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The organic material may be at least one selected from the group consisting of hydroxy acids, monosaccharides and oligosaccharides. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

And the hydroxy acid may be at least one selected from the group consisting of citric acid and glycolic acid. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

Also, the non-aqueous solvent may be at least one selected from the group consisting of alcohols and ethers. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

The firing in the third step can be carried out within a temperature range of 500 to 800 ° C. In this case, the irreversible capacity of the lithium ion secondary battery is reduced and the cycle life is also improved.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents. It should be understood as belonging.

[Description of Symbols]

10: Lithium ion secondary battery 20: Positive electrode

21: collector 22: positive electrode active material layer

30: cathode 31: collector

32: anode active material layer

Claims (24)

Silicon-based active material fine particles; And
And a surface-modified fine particle having a carbon material covering the surface of the silicon-based active material fine particles. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the carbon material is 0.35 to 3.5 mass% with respect to the total mass of the silicon-based active material fine particles and the carbon material. The negative electrode active material for a lithium ion secondary battery according to claim 1, further comprising a hard carbon compounded with the surface-modified fine particles. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the silicon-based active material fine particles are polycrystalline. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the silicon-based active material fine particles have an average particle size of 200 nm or less. The method according to claim 1,
Wherein the silicon-based active material fine particles have an axial length of 500 nm or less and a diameter of 50 nm or less. 3. The method according to claim 1 or 2,
Wherein the carbon material has a thickness of 2 to 20 nm. The method of claim 3,
The hard carbon can be obtained by firing a high-order starting material,
Wherein the total mass of the surface-modified microparticles is 50 to 80 mass% with respect to the total mass of the hard carbon and the surface-modified microparticles. 9. The method of claim 8,
Wherein the solid starting material is at least one selected from the group consisting of PVC, sucrose, gelatin, and agar. The method of claim 3,
The hard carbon can be obtained by firing a starting material of a liquid system,
Wherein the total mass of the surface-modified microparticles is 75 to 90 mass% with respect to the total mass of the hard carbon and the surface-modified microparticles. 11. The method of claim 10,
Wherein the starting material of the liquid system is at least one selected from the group consisting of resole-type phenol resin and furfuryl alcohol. A first step of preparing a dispersion solution in which silicon-based active material fine particles are dispersed in a non-aqueous solvent by pulverizing a silicon-based active material in a non-aqueous solvent;
A second step of preparing a mixed solution by dissolving an organic material dissolved in the non-aqueous solvent in the dispersion solution;
And a third step of producing the surface-modified fine particles coated with the carbon material on the surface of the silicon-based active material fine particles by firing the mixed solution in an inert atmosphere. 13. The method of claim 12,
Further comprising a fourth step of preparing a negative electrode active material by mixing the surface-modified fine particles and a starting material of hard carbon, and firing the mixture in an inert atmosphere to produce a negative electrode active material. 13. The method of manufacturing a negative electrode active material for a lithium ion secondary battery according to claim 12, wherein the milling is performed with a bead mill. 13. The method of claim 12, wherein the total mass of the carbonaceous material is 0.35 to 3.5 mass% with respect to the total mass of the silicon-based active material fine particles and the carbonaceous material. 13. The method of claim 12, wherein the organic material is at least one selected from the group consisting of hydroxy acids, monosaccharides, and oligosaccharides. 17. The method of claim 16,
Wherein the hydroxy acid is at least one selected from the group consisting of citric acid and glycolic acid. 13. The method of claim 12,
Wherein the non-aqueous solvent is at least one selected from the group consisting of alcohols and ethers. 13. The method of claim 12,
Wherein the inert atmosphere is formed of at least one selected from the group consisting of argon and nitrogen. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; 13. The method of claim 12,
Wherein the calcination is performed at a temperature in the range of 550 to 800 &lt; RTI ID = 0.0 &gt; C. &Lt; / RTI &gt; The method according to claim 12 or 13,
The method of manufacturing an anode active material for a lithium ion secondary battery according to claim 8, wherein at least one of the inert atmosphere in the third step and the fourth step is formed of at least one selected from the group consisting of argon and nitrogen. 14. The method of claim 13,
Wherein the calcination of the fourth step is performed at a temperature in the range of 500 to 800 ° C. 4. A method for producing a negative active material for a lithium ion secondary battery according to claim 1, A lithium ion secondary battery comprising the negative electrode active material according to any one of claims 1 to 19. 23. A method for charging a lithium ion secondary battery according to claim 23, wherein the charging capacity of the lithium ion secondary battery is maintained at 70% or less of the theoretical capacity realized by the silicon-based active material fine particles.

Images