JP2017120710A - Negative electrode material for secondary batteries, and nonaqueous electrolyte secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material which can decrease and hold down a silicon expansion coefficient and which can realize the increase in conductivity and satisfactory charge and discharge cycle characteristics.SOLUTION: Particles including amorphous or microcrystalline silicon as a negative electrode active substance are coated with a conductive DLC under a temperature condition of 500°C or lower to impart conductivity to the particles, which enables the decrease in the electric resistance of an electrode, and allows lithium ions to go into or out of the material smoothly. In addition, the amorphous or microcrystalline structure of the silicon can be retained and as such, a stress caused by the volume expansion and shrinkage during the time of charge/discharge is relaxed within a range of a yield stress. Further, a hard DLC film reduces the expansion during the time of charge. Therefore, the fine powdering or the increase in expansion coefficient, which accompanies the charge/discharge, can be suppressed at a high level, and satisfactory cycle characteristics can be retained.SELECTED DRAWING: Figure 1

Description

  TECHNICAL FIELD The present invention relates to a negative electrode material for a secondary battery comprising a silicon-containing negative electrode material for a lithium secondary battery and a conductive diamond-like carbon (DLC) as a coating, and a nonaqueous electrolyte secondary battery using the same. .

  In recent years, with the development and popularization of mobile tools, personal computers, electric motors, and temporary power storage devices, high-capacity energy sources have been demanded, and a typical example is a lithium secondary battery.

  Conventionally, Si (silicon) having a capacity (4200 mAh / g) more than 10 times that of a conventional graphite-based material (theoretical capacity is 372 mAh / g) has attracted attention as a negative electrode material for next-generation non-aqueous electrolyte secondary batteries. Yes.

On the other hand, silicon is known to be pulverized by expansion / contraction during charge / discharge. Since the yield stress of a metal is inversely proportional to the size of the crystallite, if the size of the silicon crystallite is made as small as possible and the stress of expansion / contraction associated with charge / discharge can be made less than the yield stress, the shape of the particles can be reduced. Maintaining and capacity deterioration due to pulverization is less likely to occur. Therefore, studies have been made on amorphous or microcrystalline silicon-containing alloy powders prepared by rapid solidification or mechanical alloying, and silicon oxide powder materials such as SiOx in which microcrystalline silicon is scattered in SiO _2. Yes.

  However, even if the pulverization can be suppressed with these materials, the conductivity of silicon is very low compared with the graphite used as the current negative electrode active material, and silicon itself is almost non-conductive as a battery material. Therefore, it is difficult to obtain sufficient conductivity for smooth charge / discharge even if it is a silicon alloy alloyed with a metal having high conductivity as well as its oxide. It becomes a factor of deterioration.

  In order to solve the above-mentioned problems, for example, a technique for compounding with conductive carbon (for example, Patent Document 1: Japanese Patent Laid-Open No. 2006-228640), 2) A current collector made of metal such as copper or nickel so as to cover silicon Technology for plating on top (for example, Patent Document 2: Japanese Patent Laid-Open No. 2006-236684), 3) Technology for forming a carbon film on the surface by CVD (for example, Patent Document 3: Japanese Patent Laid-Open No. 2009-212074A), 4) A technique for directly forming a silicon thin film on a current collector (for example, Patent Document 4: WO2004 / 109839) has been proposed.

  1) As used in the surface coating of general natural graphite, when mixed with conductive carbon by mixing and firing with softened pitch, it can be coated relatively uniformly, and the conductivity is improved. It is possible to increase and lower the surface resistance to the equivalent level of graphite, but in order to obtain conductivity that is effective for battery performance, it is necessary to carbonize the pitch by heat treatment at a temperature of 1000 ° C. or higher. is there. When silicon materials such as silicon oxide and low crystal alloys with low silicon crystallinity are combined with carbon in this way to suppress expansion, the crystallinity of silicon develops due to heat during firing. The stress may not be relaxed during charge and discharge, and silicon may crack and become fine powder, and may be isolated in the electrode, resulting in capacity deterioration.

  2) By forming a carbon film on the surface by thermal CVD, the same effect as the above 1 can be obtained, but the reaction temperature of thermal CVD is the lowest when acetylene is used as the carbon source at 600 ° C. to 750 ° C. In general, since a carbon film is formed at a temperature of 750 ° C. to 950 ° C. using methane or ethylene, it affects the crystal structure of silicon, which also causes a decrease in life.

  3) By applying metal plating to silicon on the current collector, conductivity can be imparted to the surface of silicon and electrode resistance can be reduced. Due to the expansion and contraction associated with, cracks may be generated between the plating layer and silicon, and the silicon particles may be electrically isolated, making it impossible to charge and discharge.

  4) When a silicon thin film is formed directly on the current collector, conductivity can be obtained only from the current collector side, and the resistance increases remarkably with the thickness of the electrode, and from the current collector due to expansion due to charge / discharge. If it is detached, there is an adverse effect that it is impossible to secure a conductive path.

  In order to prevent the negative electrode active material containing silicon from cracking or pulverizing due to expansion and contraction due to charge and discharge, it is useful to reduce the crystallite size and increase the yield stress. It is required to prevent crystallites from growing in the process of imparting properties. In particular, when silicon is alloyed with a transition metal or the like to form a negative electrode active material, since the melting point is lower than that of silicon alone, it is required not to cause silicon crystal growth.

  Accordingly, there is an urgent need to develop a negative electrode active material for a secondary battery that improves the conductivity of the secondary battery while suppressing the crystal growth of silicon and improving the battery performance of the secondary battery.

JP 2006-228640 A JP 2006-236684 A JP 2005-294079 A WO2004 / 109839

  The present inventors have found that the negative electrode material containing silicon has the technical advantage of including silicon, and considers the crystallinity of silicon to improve the function as a negative electrode material for secondary batteries. It was. The present inventors have also found that the temperature in the step of imparting conductivity is lowered (preferably 500 ° C. or lower) in order to suppress the crystal growth of silicon. The present invention has been made based on these findings.

The present invention is a negative electrode material for a lithium secondary battery,
Comprising at least silicon and conductive diamond-like carbon (DLC);
The conductive DLC is a coating of a part or all of the negative electrode material for a lithium secondary battery containing the silicon,
In the X-ray diffraction pattern of the negative electrode material for a lithium secondary battery, the size of the crystallite obtained from the half width of the (111) diffraction line of the silicon is 20 nm or less. It is a negative electrode material.

In another aspect of the present invention, a method for producing a negative electrode material for a lithium secondary battery, comprising:
At least silicon,
In a manufacturing method, comprising covering or adhering conductive DLC to a part or all of the negative electrode material for lithium secondary battery containing silicon at a low temperature (preferably a temperature of 500 ° C. or less). is there.

  According to the secondary battery using the negative electrode material for the secondary battery according to the present invention, the silicon-containing negative electrode active material and the silicon-containing negative electrode active material are formed by covering the silicon-containing particles as the negative electrode active material with the conductive DLC. It becomes possible to reduce the electrical resistance of the electrode. In addition, in silicon-containing particles, silicon maintains an amorphous or microcrystalline structure and relieves stress caused by volume expansion and contraction during charge / discharge within the range of yield stress, thereby suppressing pulverization associated with charge / discharge at a high level. It becomes possible to do. As a result, it is possible to maintain good cycle characteristics without the silicon utilization rate being lowered by repeated charge and discharge.

  According to the present invention, all or part of silicon particles containing a silicon phase having a silicon crystallite size of 20 nm or less is coated with conductive DLC formed at a low temperature (preferably a temperature of 500 ° C. or less). By doing so, high conductivity can be imparted without increasing the size of the silicon crystallites. The improved conductivity facilitates the insertion and extraction of lithium ions into and from silicon-containing active materials, and the amorphous or microcrystalline structure is maintained after coating, so that silicon expands during charge and discharge. Since shrinkage can be suppressed within the range of yield stress, good charge / discharge cycle characteristics can be obtained. Furthermore, since the hard DLC film also reduces expansion during charging, the electrode expansion coefficient is also reduced.

FIG. 1 is a diagram showing a Raman spectrum of a secondary electron negative electrode material according to the present invention.

[Definition]
(Raman spectroscopy)
Raman spectroscopy is an analytical method that can illuminate a material with monochromatic light, measure the Raman scattered light scattered from the material, and obtain structural information such as compound identification and crystallinity and orientation. . Graphite, which is often used for investigating the structure of carbon materials, especially sp2 hybrid orbitals, shows a sharp peak called the G band at 1580 cm ⁻¹ , and when the graphite structure is disturbed, the G band peak expands slightly. At the same time, a peak called D band appears at 1350 cm ⁻¹ .
Since the sp3 hybrid orbit of the complete diamond structure appears as a sharp peak at 1330 cm ⁻¹ , this peak may be observed as a shoulder band in the D band. The ratio I (D) / I (G) between the D band and the G band is an index for evaluating the degree of graphitization, and is also an index of conductivity in connection with it. In the case of the DLC film, it is known that carbons of the sp3 structure and the sp2 structure are included, and it is possible to know whether the insulating property is close to diamond or the conductivity derived from graphite by the ratio of these.
The 1330 cm ^-1 peak derived from the complete diamond structure is weakened, absorbed by the D band peak, and conductivity appears when the G band peak intensity I (G) begins to exceed the D band peak intensity I (D). Furthermore, the graphite structure develops as I (D) / I (G) decreases. Raman spectroscopy can be measured using a LabRam HREvolution Raman spectrometer manufactured by HORIBA, and was actually used in the present invention.

(X-ray diffraction measurement: X-Ray Diffraction: measurement of XRD)
XRD is a diffraction that occurs when X-rays are scattered and interfered by electrons around the atoms when the sample is irradiated with X-rays (black condition: 2 dsin θ = nλ: d is the distance between the two surfaces, The angle formed is θ, an arbitrary integer n, and the wavelength λ of X-rays), and this makes it possible to identify the constituent components, determine their quantification, specify the crystal size, crystallinity, etc. .
In the present invention, it is confirmed by XRD that the crystallite size of silicon in the silicon alloy and silicon oxide particles is not changed before and after the formation of the DLC film.

(Crystallite)
A crystallite means a maximum aggregate of particles regarded as a single crystal, and one particle is composed of a plurality of crystallites.
(Crystallite size)
The crystallite size (size) is measured by a diffraction device using X-rays, for example, half width and Scherrer formula [D (Å) = K * λ / (β * cos θ): formula Where K is a constant, λ is the wavelength of the X-ray, β is the broadening of the diffraction line depending on the size of the crystallite, and θ is introduced into the diffraction angle 2θ / θ].
In the present invention, the size of silicon crystallites was calculated from the peak half-value width at a diffraction angle 2θ = 28.4 ° corresponding to the (111) crystal plane of silicon.

[Anode material for secondary battery]
The negative electrode material for a secondary battery according to the present invention is a negative electrode material for a lithium secondary battery, wherein the conductive DLC is present as a coating (attachment) of a part or all of the negative electrode material for a lithium secondary battery containing silicon. In the X-ray diffraction pattern, the crystallite size obtained from the half width of the silicon (111) diffraction line is 20 nm or less.

  In the present invention, a part or all of the negative electrode material (surface) for a lithium secondary battery containing silicon, for example, a powder surface containing amorphous or microcrystalline silicon is not changed in crystallinity of silicon. In addition, since the conductive DLC is coated or adhered, good battery conductivity can be imparted.

  In the present invention, the crystallite size obtained from the half width of the (111) diffraction line of silicon is 20 nm or less. As a result, it is possible to suppress the deterioration of the battery life due to the expansion and contraction of silicon accompanying charging and discharging. The X-ray diffraction pattern is as described in the above definition.

  According to the aspect of the present invention, no crystal growth of silicon occurs, and in the X-ray diffraction pattern of the negative electrode material containing silicon, the size of the crystallite obtained from the half width of the (111) diffraction line of silicon is DLC. It does not change before and after the generation.

  The negative electrode material for a secondary battery according to the present invention includes a silicon phase having a crystallite size of 20 nm or less, and can improve the electrochemical performance of both of them by coating (adhesion) with conductive DLC. it can. That is, according to the present invention that employs these technical matters, since expansion and contraction associated with charge and discharge are performed within the range of yield stress, pulverization does not occur, and lithium ion insertion / desorption is performed by imparting conductivity. By performing the separation smoothly, good life characteristics can be obtained.

(Silicon material)
The negative electrode material according to the present invention comprises at least silicon. In addition to silicon alloys and silicon oxides (SiOx), silicon can be used as a raw material such as amorphous silicon powder, silicon nanofibers, silicon nanowires, and silicon / graphite composites. Two or more silicon raw materials may be mixed and used.

(Conductive DLC: Diamond-like carbon)
According to the present invention, the conductive DLC is employed as a coating and a deposit without changing the size of the silicon crystallites in the silicon-containing particles before and after the coating (deposition). The conductive DLC is preferably produced at a low temperature, preferably 500 ° C. or less, more preferably 350 ° C. or less.
DLC (DLC may be a plate, foil, film, layer, etc., regardless of its shape) is coated or adhered to a silicon substance, preferably a silicon alloy or oxide particles containing silicon. All or only a part of the particles may be coated.

According to a preferred embodiment of the present invention, the spectrum of G band derived from SP2 structures present in the wavelength region of 1580 cm ^-1 in the DLC Raman spectrum, the spectrum of the D band derived SP3 structure present in a wavelength region of 1350 cm ^-1 Are overlapped, the half-width of the D band obtained by peak separation is 150 cm ⁻¹ or more, and the peak intensity I (D) of the D band and the peak intensity I (G) of the G band The ratio [I (D) / I (G)] is 1.0 or less. When the half band width of the D band is 150 cm ⁻¹ or less, the hardness inherently possessed by the DLC film cannot be obtained, so that the effect of suppressing the expansion is reduced, and when I (D) / I (G) is 1.0 or more, charge / discharge is performed. In this case, sufficient conductivity cannot be obtained.

  According to a preferred embodiment of the present invention, the thickness of the coating by DLC is 5 nm or more and 500 nm or less, preferably 10 nm or more and 50 nm or less. If the DLC coating is too thin, sufficient conductivity will not be exhibited, and if it is too thick, it will take time to insert and remove lithium ions, and the proportion of silicon in the particles will be relatively small. Capacity will drop.

  According to a preferred embodiment of the present invention, the specific resistance of DLC contained in the negative electrode material is 50 mΩ · cm or less, preferably 20 mΩ · cm or less.

(Volume cumulative particle size distribution)
According to a preferred aspect of the present invention, the 50% diameter of the volume cumulative particle size distribution of the negative electrode material is 1 μm or more and 10 μm or less. The 90% diameter of the volume cumulative particle size distribution of the negative electrode material is 30 μm or less, and preferably 20 μm or less. The measurement of 50% diameter and 90% diameter of the volume cumulative particle size distribution is obtained, for example, by the cumulative frequency when measured after dispersing for 3 minutes by built-in ultrasonic waves using a laser diffraction particle size distribution measuring device manufactured by Nikkiso Co., Ltd. Can be done.

[Method for producing negative electrode material for secondary battery]
(Preparation of silicon material)
As described in the negative electrode material for secondary batteries, the silicon-containing raw material used in the present invention is amorphous silicon powder, silicon nanofiber, silicon nanowire, silicon / graphite, in addition to silicon alloy and silicon oxide (SiOx). A complex or the like can be used. Two or more silicon raw materials may be mixed and used. The size of the particles is adjusted by pulverization, granulation or the like so that these raw materials have the preferable volume particle size distribution.

(Conductive DLC formation)
As a method for forming DLC, there are plasma chemical vapor deposition (CVD), plasma ion implantation, ion plating, ion beam evaporation, sputtering, and the like. In the present invention, inductive coupling type is used. It is preferable to use a plasma ion implantation film forming method. In the present invention, a plasma ion assist film forming apparatus manufactured by Plasma Ion Assist can be used.

  In either method, the sp2 structure and the sp3 structure are mixed, and the ratio of sp2 and sp3 varies depending on the carbon source, film formation temperature, additives, and other manufacturing conditions, and the conductivity also varies accordingly. The specific resistance of the conductive DLC is about 10 to 100 mΩ · cm, which is 6 digits or more lower than the specific resistance of silicon. Moreover, since DLC is a hard film, the effect which suppresses the expansion | swelling accompanying charging / discharging of the negative electrode active material containing silicon is also acquired.

In general, DLC is insulative because it mainly has an sp3 structure, but conductivity is imparted when nitrogen or boron ions are implanted during film formation.
In a more preferred embodiment of the present invention, the DLC is preferably formed in a low temperature region where the crystallinity of silicon does not change, for example, a temperature of 500 ° C. or lower. In this respect, DLC that can be formed (attached) at 500 ° C. or less is optimal, and among these, plasma ion implantation that can impart conductivity by ion implantation is a particularly preferable method in the present invention.

  According to the aspect of the present invention, the negative electrode material containing silicon preferably has a crystallite size of 20 nm or less. Examples include amorphous silicon, silicon nanofibers, silicon alloys, silicon oxides such as SiOx, and silicon / graphite composites. Silicon alloys have a melting point that is several hundred degrees lower than silicon alone or silicon oxide. In many cases, silicon crystals grow at the ambient temperature in the conventional conductivity imparting step. However, according to the present invention, since DLC is formed (film formation) at a low temperature, preferably 500 ° C. or less, conductivity can be imparted without causing crystal growth.

[Anode for secondary battery]
In this invention, the negative electrode for lithium secondary batteries provided with the negative electrode material for secondary batteries by this invention can be proposed.

[Secondary battery]
The present invention proposes a lithium secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, wherein the negative electrode is a negative electrode for a secondary battery according to the present invention.

  Generally, a lithium secondary battery includes a positive electrode made of a positive electrode material and a positive electrode current collector, a negative electrode made of a negative electrode material and a negative electrode current collector, and a separator capable of conducting lithium ions by blocking electronic conduction between the positive electrode and the negative electrode. An organic electrolyte containing lithium salt for conducting lithium ions is injected into the gap between the electrode and the separator material.

(Negative electrode)
The negative electrode is produced, for example, by applying a mixture of a negative electrode material (negative electrode active material), a conductive agent and a binder onto a negative electrode current collector, and then drying. If necessary, a filler can be further added to the mixture. The negative electrode material (negative electrode active material) is a negative electrode material for a secondary battery according to the present invention.

(Binder for negative electrode)
The binder is a component that promotes the bonding of the material and the conductive agent and the bonding of the material to the current collector. Usually, the binder is added at 0.5 to 20% by mass based on the total weight of the mixture including the negative electrode material.
Examples of the binder include styrene butadiene rubber (SBR), polyacrylic acid, polyimide, polyvinylidene fluoride, and polyacrylonitrile (polypropylene). polyvinylidene fluoride-co-hexafluoropropylene, PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluo)
), poly (vinylidene fluoride-trichloroethylene), poly (vinylidene fluoride-co-chloroethylene) Polyvinylpyrrolidone, Polyvinylacetate
e), ethylene vinyl acetate copolymer (polyethylene-co-vinyl acetate), polyethylene oxide (polyethylene oxide), cellulose acetate (cellulose acetate), cellulose acetate butyrate (cellulose acetate butyrate), cellulose acetate propionate (celloate acetate) propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose. one selected from the group consisting of rose), pullulan, carboxymethyl cellulose (CMC), and acrylonitrile-styrene-butadiene copolymer, or two of them A mixture of two or more species can be used, but the present invention is not particularly limited thereto, and various types of binder polymers can be used.

(Conductive material)
Although the conductivity of the negative electrode material containing silicon of the present invention is given by DLC, a conductive material may be added separately at the time of manufacturing the electrode. The conductive agent is usually added at 0.1 to 50% by mass based on the total weight of the mixture including the negative electrode material. When particles containing silicon are used for the negative electrode active material, the conductivity is lower than that of graphite, but battery characteristics equivalent to those of a graphite electrode can be obtained by appropriately selecting a conductive material. Such a conductive agent is, for example, graphite such as natural graphite or artificial graphite; carbon black, acetylene black, ketjen black (trade name), graphene, carbon nanotube, Carbon nanofibers, channel blacks, furnace blacks, lamp blacks, thermal blacks and other carbon blacks, conductive fibers such as carbon fibers and metal fibers, fluorocarbons, aluminum, nickel powders and other metal powders; zinc oxide, potassium titanate, etc. Examples thereof include conductive metal oxides such as conductive whiskers and titanium oxide; and conductive materials such as polyphenylene derivatives. In particular, the fibrous conductive material, when particles containing silicon are used as the negative electrode active material, maintains a conductive path between the active materials or between the active material and the current collector even by expansion and contraction due to charge and discharge. It is particularly preferable because it has a structure that does not easily fall off.

(Negative electrode current collector)
The current collector is manufactured with a thickness of 3 to 50 μm. Such a current collector is not limited as long as it does not induce a chemical change in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like is used. The negative electrode current collector can form fine irregularities on the surface to increase the adhesion of the positive electrode material, and various forms such as films, sheets, wheels, nets, porous bodies, foams, and nonwoven fabrics are possible. It is.

(Positive electrode)
The positive electrode is produced, for example, by applying a mixture of a positive electrode material, a conductive agent and a binder on a positive electrode current collector and then drying. If necessary, a filler can be further added to the mixture.

<Positive electrode active material>
As the positive electrode active material, a lithium-containing transition metal oxide can be desirably used. For example, Li _x CoO ₂ (0.5 <x <1.3), Li _x NiO ₂ (0.5 <x <1.3) Li _x MnO ₂ (0.5 <x <1.3), Li _x Mn ₂ O ₄ (0.5 <x <1.3), Li _x (Ni _a Co _b Mn _c ) O ₂ (0. 5 <x <1.3, 0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1), Li _x Ni _1-y Co _y O ₂ (0.5 <x <1. 3,0 <y <1), Li x Co 1-y Mn y O 2 (0.5 <x <1.3,0 ≦ y <1), Li x Ni 1-y Mn y O 2 (0. 5 <x <1.3, 0 ≦ y <1), Li _x (Ni _a Co _b Mn _c ) O ₄ (0.5 <x <1.3, 0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), Li x Mn 2-z Ni z O 4 (0.5 <x <1.3 0 <z <2), Li x Mn 2-z Co z O 4 (0.5 <x <1.3,0 <z <2), Li x CoPO 4 (0.5 <x <1.3) And any one selected from the group consisting of Li _x FePO ₄ (0.5 <x <1.3), or a mixture of two or more thereof, wherein the lithium-containing transition metal oxide is It can also be coated with a metal such as aluminum (Al) or a metal oxide. In addition to the lithium-containing transition metal oxide (oxide), a sulfide, a selenide, a halide, and the like can also be used.

(Binder for positive electrode)
The positive electrode binder is a component that promotes the binding of the active substance and the conductive agent, and the binding of the active substance to the current collector. Usually, the binder is added at 1 to 50% by weight based on the total weight of the mixture containing the positive electrode active material. For example, polyvinylidene fluoride, polyvinyl alcohol, polyimide, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene copolymer (EPDM), sulfone EPDM, styrene butylene rubber, fluororubber, various copolymers and the like.

<Positive electrode current collector>
The positive electrode current collector is manufactured with a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and has high conductivity. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel that has been surface-treated with carbon, nickel, titanium, silver, or the like is used. The positive electrode current collector can increase the adhesion of the positive electrode material by forming fine irregularities on the surface, and various forms such as films, sheets, wheels, nets, porous bodies, foams, and nonwoven fabrics are possible. It is.

(Conductive material for positive electrode)
Although the thing similar to what was described in the negative electrode can be used, it is not limited to this.

(Separator)
The separator is an insulating thin film that is interposed between the positive electrode and the negative electrode and has high ion permeability and mechanical strength. Generally, the separator has a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm.
Examples of such separators include porous polymer films such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. Porous polymer films produced from various polyolefin polymers can be used alone or in layers, or ordinary porous nonwoven fabrics such as nonwoven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc. can be used. However, it is not particularly limited to these. Alternatively, a porous organic-inorganic coating layer containing a mixture of inorganic particles and a binder polymer may be included on at least one surface of a porous polymer film or a porous nonwoven fabric. The binder is located in a part or all of the inorganic particles and functions to connect and fix the inorganic particles.

(Nonaqueous electrolyte)
In the non-aqueous electrolyte used in the present invention, the lithium salt that can be included as the non-aqueous electrolyte can be used without limitation, such as those normally used for electrolytes for lithium secondary batteries, for example, the anion of the lithium salt F ⁻ , Cl ⁻ , Br ⁻ , I ⁻ , NO ₃ ⁻ , N (CN) ₂ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ , PF ₆ ⁻ , (CF ₃ ) ₂ PF ₄ ⁻ , (CF ₃ ) ₃ PF ₃ ⁻ , (CF ₃ ) ₄ PF ₂ ⁻ , (CF ₃ ) ₅ PF ⁻ , (CF ₃ ) ₆ P ⁻ , CF ₃ SO ₃ ⁻ , CF ₃ CF ₂ SO ₃ ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (FSO ₂ ) ₂ N ⁻ , CF ₃ CF ₂ (CF ₃ ) ₂ CO ⁻ , (CF ₃ SO ₂ ) ₂ CH ⁻ , (SF ₅ ) ₃ C ⁻ , (CF ₃ SO ₂ ) _{3 ^{C -, CF 3 (CF 2}} ) 7 SO 3 -, CF 3 CO 2 -, CH 3 CO 2 -, SCN -, and _{(CF 3 CF 2 SO 2)} 2 N - group consisting of Any one selected.

In the present invention, as the organic solvent contained in the non-aqueous electrolyte, those usually used can be used without limitation, and typically, fluoro-ethylene carbonate (FEC) propionate ester (propionate ester). , More specifically, methyl propionate, ethyl propionate, propyl propionate and butyl propionate, propylene carbonate, PC , Ethylene carbonate (EC), diethyl carbonate (d Ethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone , Polyethylene sulfite, and any one selected from the group consisting of tetrahydrofuran, or a mixture of two or more thereof can be used. In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are high-viscosity organic solvents, have a high dielectric constant, and can be desirably used because they dissociate lithium salts in the electrolyte well. Also, by using such cyclic carbonate with low viscosity and low dielectric constant linear carbonate such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, a non-aqueous electrolyte having high electrical conductivity can be made. Can be used more desirably.
Optionally, the non-aqueous electrolyte used in the present invention may further include an additive such as an overcharge inhibitor contained in a normal non-aqueous electrolyte.

(Manufacturing)
The secondary battery according to the present invention can be manufactured by inserting a porous separator between a positive electrode and a negative electrode and introducing a nonaqueous electrolyte by a usual method. The secondary battery according to the present invention is used regardless of the outer shape, such as a cylindrical type, a square type, or a pouch type battery.

  The content of the present invention will be described using the following examples, but the scope of the present invention should not be construed as being limited to these examples. In addition, these examples realize preferred aspects of the present invention, and those skilled in the art will understand all aspects of the present invention based on the contents of these examples and the technical matters disclosed in this specification. Can be easily recognized and its contents can be easily implemented.

(Preparation of negative electrode material)
[Example 1]
(1) A negative electrode material was prepared according to the following procedure.
1) Preparation of silicon raw material Silicon oxide powder was prepared as a silicon raw material. This is a commercial product, and SiOx having an average particle diameter of 5 μm was used. Moreover, the peak of diffraction angle 2θ = 28.4 ° corresponding to the (111) crystal plane of silicon in the X-ray diffraction pattern of SiOx was not observed, and it was confirmed that silicon was amorphous.

2) Preparation of silicon alloy powder Silicon (Si), chromium (Cr), and titanium (Ti) are prepared, and SiCrTi alloy powder (Si / Cr / Ti = 74/14/13: mass%) is prepared by gas atomization. Obtained. Further, in order to facilitate the amorphization of silicon in the next mechanical alloying process, sieving was performed so that the maximum diameter was 40 μm or less. Add 1 wt% stearic acid as an auxiliary agent to the SiCrTi alloy powder, put it into a vibration mill container together with a steel ball with a diameter of 15 mm to fill 80% of the vibration mill container, replace with nitrogen gas, and change the vibration frequency. The mechanical alloying treatment was performed at 1200 cpm for 24 hours. Thereafter, the particle size was adjusted by airflow classification so that the average particle size became 3 μm.
The XRD diffraction line of the obtained silicon alloy powder was measured, and it was confirmed that the (111) diffraction line peak derived from crystalline silicon did not appear. This shows that the silicon in the silicon alloy powder has an amorphous structure.

3) Conductive diamond-like carbon (DLC) coating Conductive diamond-like carbon (DLC) coating is performed on the obtained silicon alloy particles. Using a plasma ion implantation film forming apparatus of Plasma Ion Assist Co., Ltd., which can develop conductivity by converting hydrocarbon gas into plasma and implanting carbon ions, nitrogen ions and boron ions simultaneously. A DLC film having an average thickness of 50 nm was formed.
Raman spectra of the powder obtained, a broad D band with a peak at 1350 cm ^-1, a broad G band with a peak at 1580 cm ^-1 is observed partially overlap, show them in Figure 1 when peak separation Such a spectrum was obtained. At this time, the half band width of the D band was 274 cm ⁻¹ , and the half band width of the G band was 112 cm ⁻¹ .
When the specific resistance of the DLC film formed on the Kapton tape placed in the immediate vicinity of the silicon alloy powder during the DLC coating formation was measured with a Loresta manufactured by Mitsubishi Chemical Analytech, it was 15 mΩ · cm. Moreover, the change of the particle size of the silicon alloy after electroconductive DLC coating | cover was the range of the measurement error of the particle size distribution meter, and it confirmed that there was almost no change.

(2) Production of Electrode and Battery The negative electrode active material was prepared by mixing the prepared silicon alloy powder and graphite having an average particle size of 15 μm in a weight ratio of 10:90.
Negative electrode active material 95wt%, carbon black 1wt% as conductive material, SBR 2wt% as binder, CMC 2wt% as thickener, solid content concentration adjusted to pure viscosity with pure water The resulting slurry is applied to a copper foil of 20 μm thickness to a thickness of about 100 μm, vacuum dried at 120 degrees, pressed, and punched into a circle with a diameter of 13 mm to produce a negative electrode having an electrode density of 1.7 g / cc. did. A 2016 type coin cell using a negative electrode punched with 0.3 mm-thick metal lithium as a counter electrode and an electrolytic solution in which ethylene carbonate and diethyl carbonate are mixed at a ratio of 3: 7 and 1 mol of LiPF ₆ is dissolved. Was made.

[Example 2]
A coin cell was produced in the same manner as in Example 1 except that SiO powder having an average particle diameter of 5 μm was used instead of the silicon alloy powder.

[Comparative Example 1]
A coin cell was produced in the same manner as in Example 1 except that the silicon alloy powder of Example 1 was not coated with conductive DLC.

[Comparative Example 2]
A coin cell was produced in the same manner as in Example 2 except that the SiO powder of Example 2 was not coated with conductive DLC.
[Comparative Example 3]
Instead of coating the conductive DLC on the silicon alloy powder of Example 1, the same procedure as in Example 1 was performed except that carbon coating was performed at 700 ° C. by chemical vapor deposition (CVD) using acetylene as a source gas. A coin cell was prepared.

<Evaluation Test 1: Charge / Discharge Cycle Test>
About the coin cell (secondary battery) of an Example and a comparative example, 40 cycle charging / discharging was repeated at the 0.5C current rate. The capacity retention rate was calculated by dividing the discharge capacity at the 40th cycle by the discharge capacity at the first cycle and multiplying by 100. Moreover, the test was completed in the charge state of the 41st cycle, the coin cell was disassembled in a dry atmosphere with a dew point of −50 ° C., and the electrode thickness was measured. From this thickness, the electrode expansion coefficient was calculated by subtracting the thickness of the electrode before charging, dividing by the thickness of the electrode before charging, and multiplying by 100.

〔Evaluation results〕
For Examples and Comparative Examples, the size of silicon crystallites calculated from the half width of silicon (111) diffraction peak of X-ray diffraction line before and after conductive DLC coating or carbon coating produced at low temperature The capacity retention rate and the electrode expansion rate are shown in Table 1.
In Examples 1 and 2, no peak was observed before and after the conductive DLC coating, and silicon maintained an amorphous structure. In Comparative Example 3, it was confirmed that the crystallite size was increased.
Moreover, the capacity | capacitance maintenance factor became high compared with the comparative example 1 which the silicon containing alloy of Example 1 did not carry out conductive DLC coating. This is because conductivity is imparted, and lithium ions can be smoothly inserted and removed from silicon.
Furthermore, there is no difference in the expansion coefficient between Example 1 and Comparative Example 1, and it is shown that the silicon amorphous structure is maintained before and after the DLC coating, so that no pulverization or accompanying increase in voids in the electrode occurs. Yes.
With respect to the SiO of Example 2 and Comparative Example 2, the same result as that of the silicon alloy was obtained. In Comparative Example 3, the capacity retention rate of Comparative Example 1 was improved compared to the result before DLC coating, but the expansion rate increased. This is thought to be due to the increase in the crystallite size of silicon.

〔Comprehensive evaluation〕
According to the present invention, a powder containing amorphous or microcrystalline silicon is coated with or partially adhered to the powder surface with a conductive DLC film generated at a low temperature (500 ° C. or lower) without changing the crystallinity of silicon. As a result, it is possible to provide conductivity that facilitates insertion and removal of lithium ions without increasing the crystallite size of silicon, so that good cycle characteristics can be obtained and expansion coefficient is also suppressed. We were able to. By maintaining the amorphous or microcrystalline structure of silicon before DLC coating, the stress due to volume expansion / contraction during charge / discharge can be alleviated within the range of yield stress. This is because the expansion during charging is reduced. By suppressing the pulverization, the utilization rate of silicon is not lowered by repeated charge and discharge, and better cycle characteristics can be maintained together with the provision of conductivity.

Claims (8)

A negative electrode material for a lithium secondary battery,
Comprising at least silicon and conductive diamond-like carbon (DLC);
The conductive DLC is a coating of a part or all of the negative electrode material for a lithium secondary battery containing the silicon,
In the X-ray diffraction pattern of the negative electrode material for a lithium secondary battery, the size of the crystallite obtained from the half width of the (111) diffraction line of the silicon is 20 nm or less. Negative electrode material.   2. The negative electrode material for a secondary battery according to claim 1, wherein the negative electrode material for a lithium secondary battery is a silicon alloy containing a transition metal or an oxide containing silicon. In the Raman spectrum analysis of a conductive DLC film of the negative electrode material for the secondary battery, the spectrum of G band derived from SP2 structure present in a wavelength region of 1580 cm ^-1, from SP3 structure present in a wavelength region of 1350 cm ^-1 Part of the spectrum of the D band overlaps, the half band width of the D band obtained by peak separation is 150 cm ⁻¹ or more, the peak intensity I (D) of the D band and the peak intensity I (G) of the G band The negative electrode material for a secondary battery according to claim 1, wherein the ratio I (D) / I (G) is <1.0.   The negative electrode material for a secondary battery according to claim 1, wherein a specific resistance of the DLD film of the negative electrode material is 50 mΩ · cm or less. The DLD film has a thickness of 5 nm or more and 500 nm or less,
The negative electrode material for secondary batteries according to claim 1.   6. The X-ray diffraction pattern before and after coating of the negative electrode material with a DLC film has no change in the half-value width of a peak attributed to a (111) diffraction line of silicon. Negative electrode material for secondary batteries.   The negative electrode material for a secondary battery according to any one of claims 1 to 6, wherein the conductive DLC film according to claim 1 is produced by a plasma ion implantation film forming method. A non-aqueous electrolyte lithium secondary battery,
Comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator;
The said negative electrode is equipped with the negative electrode material for secondary batteries as described in any one of Claims 1-7, The nonaqueous electrolyte lithium secondary battery characterized by the above-mentioned.