WO2015107822A1 - Lithium ion secondary battery - Google Patents

Abstract

A cathode (1) is a cathode active material layer containing a cathode active material represented by Li_xM_yP_1-zSi_zO₄ (M being a combination of Fe and Mn or a combination of Fe and/or Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al, and Y, 0≤x≤2, 0.8≤y≤1.2, and 0<z≤1), and an anode (2) is an anode active material layer containing an anode active material comprising carbon having a specific surface area of 2.5-5.0 m²/g.

Description

Lithium ion secondary battery

The present invention relates to a lithium ion secondary battery. More specifically, the present invention relates to a lithium ion secondary battery having excellent cycle characteristics particularly in a low temperature environment.

As a secondary battery, a lithium ion secondary battery has been put into practical use and is widely spread. Furthermore, in recent years, lithium ion secondary batteries are attracting attention not only as small-sized batteries for portable electronic devices, but also as large-capacity devices for in-vehicle use and power storage. A lithium ion secondary battery has a positive electrode, a negative electrode, an electrolyte, and a separator as its main components. The positive electrode includes a positive electrode active material, and the negative electrode includes a negative electrode active material.

In general, a layered transition metal oxide typified by LiCoO ₂ is used as the positive electrode active material (for example, Japanese Patent Laid-Open Publication No. 2002-270160: Patent Document 1). However, the layered transition metal oxide easily causes oxygen desorption at a relatively low temperature of about 150 ° C. in a fully charged state, and the thermal desorption reaction of the battery can occur due to the oxygen desorption. Therefore, the battery having such a positive electrode active material may cause accidents such as heat generation and ignition.

For this reason, lithium iron phosphate (LiFePO ₄ ) having a stable olivine structure that does not release oxygen when abnormal and has a lower olivine structure than LiCoO ₂ is expected (for example, Japanese Patent Application Laid-Open No. 2008-2008). No.-10316 ”: Japanese Patent Application Laid-Open No. 2011-71017: Japanese Patent Application Laid-Open No. 2011-71017: Japanese Patent Application Laid-Open No. 2011-71017.

It is known that LiFePO ₄ has a large volume change rate of about 7% between insertion and desorption of Li, and capacity deterioration is caused by repeating charge and discharge cycles. For this reason, a technology for suppressing capacity deterioration by partially replacing Fe and P constituting LiFePO ₄ with other elements has been reported (Japanese Patent Laid-Open Publication No. 2012-18836): Patent Document 4 ).

Japanese Patent Publication “JP 2002-270160 A” Japanese Published Patent Publication “Japanese Patent Laid-Open No. 2008-10316” Japanese Patent Publication “Japanese Unexamined Patent Publication No. 2011-71017” Japanese Patent Publication “JP 2012-18836 A”

In the above-mentioned Patent Document 4, capacity deterioration can be suppressed to some extent. However, the lithium ion secondary battery is desired to be used as a large-capacity device. In such a device, it is necessary to suppress the capacity deterioration even when the environmental temperature varies greatly. In particular, since the capacity deterioration when the charge / discharge cycle is repeated at a low temperature is larger than that at room temperature, it is desired to suppress the capacity deterioration at a low temperature.

An object of the present invention is to provide a nonaqueous electrolyte secondary battery excellent in cycle characteristics under a low temperature environment.

A lithium ion secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode,
The positive electrode has the following general formula (1)
_{Li x M y P 1-z} Si z O 4 (1)
(Wherein M is a combination of one or both of Fe and Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al and Y, or Fe and A positive electrode active material layer including a positive electrode active material that is a combination with Mn and represented by 0 ≦ x ≦ 2, 0.8 ≦ y ≦ 1.2, and 0 <z ≦ 1)
The negative electrode is a negative electrode active material layer containing a negative electrode active material made of carbon having a specific surface area of 2.5 to 5.0 m ² / g.

According to the present invention, it is possible to suppress capacity deterioration due to repeated charge / discharge cycles particularly at low temperatures.

It is the schematic of a lithium ion secondary battery, (a) is an example of the schematic diagram of a structural member, (b) is a figure which shows an example of the whole schematic diagram of a lithium ion secondary battery. It is a graph which shows the relationship between the specific surface area of a negative electrode active material, and a return capacity maintenance factor. (A) is a graph which shows the relationship between charging time and a cell voltage, (b) is a graph which shows the relationship between charging time and a positive electrode potential.

When the inventors of the present invention repeat the charge / discharge cycle at a low temperature, the lithium ion secondary battery having a deteriorated capacity is
(1) A film mainly composed of excess lithium is formed on the negative electrode surface;
(2) It is observed that dendrite is formed on the negative electrode surface. The inventors examined this observation result, and found that a specific combination of the positive electrode active material and the negative electrode active material was particularly effective in suppressing the deterioration of the capacity, and reached the present invention.

Hereinafter, the present invention will be described in detail.

The lithium ion secondary battery of the present invention can suppress capacity deterioration due to repeated charge / discharge cycles particularly at low temperatures. Here, low temperature means 0 degreeC or less.

The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode.

(A) Positive electrode A positive electrode is a positive electrode active material layer containing a positive electrode active material.

(I) Cathode Active Material The cathode active material is represented by the following general formula (1)
_{Li x M y P 1-z} Si z O 4 (1)
It is represented by

In general formula (1), M is
(I) a combination of one or both of Fe and Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al and Y, or (ii) a combination of Fe and Mn It is a combination. For example, Fe / Co, Fe / Ni, Fe / Zr, Fe / Sn, Fe / Al, Fe / Y, Mn / Co, Mn / Ni, Mn / Zr, Mn / Sn, Mn / Al, Mn / Y, Fe / Mn is mentioned.

X can take a range of 0 ≦ x ≦ 2, y can take a range of 0.8 ≦ y ≦ 1.2, and z can take a range of 0 <z ≦ 1. When x is larger than 2, when M is selected from the above metals, a large amount of impurities may be generated. When y is smaller than 0.8, many impurities may be generated when M is selected from the above metals, and when M is selected from the above metals, many impurities may be generated. When z is 0, the material volume change during the charge / discharge reaction may increase. x is in the range of 0.5 ≦ x ≦ 1.5 (when the battery is discharged), y is in the range of 0.9 ≦ y ≦ 1.1, and z is in the range of 0.01 ≦ z ≦ 0.25. It is preferable. If x, y, and z are out of this range, the average particle diameter of the particles may not be controlled within an appropriate range when the positive electrode active material is produced by a general production method (produced in the discharged state of the battery). x is in the range of 0.9 ≦ x ≦ 1.1 (when the battery is discharged), y is in the range of 0.95 ≦ y ≦ 1.05, and z is in the range of 0.025 ≦ z ≦ 0.05. It is more preferable. When x, y, and z are out of this range, the average particle diameter of the particles may not be controlled within an appropriate range when the positive electrode active material is manufactured by a solid phase method (manufactured in a battery discharge state). The ratio of the plurality of elements constituting M is not particularly limited as long as the sum is within the range that y can take.

As a more specific positive electrode active material,
Li _x (Fe _1-a Co _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
_{Li x (Fe 1-a Ni} a) y P 1-z Si z O 4 (0.9 ≦ x ≦ 1.1,0.01 ≦ a ≦ 0.125,0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Fe _1-a Zr _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.025 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Fe _1-a Sn _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
_{Li x (Fe 1-a Al} a) y P 1-z Si z O 4 (0.9 ≦ x ≦ 1.1,0.01 ≦ a ≦ 0.125,0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Fe _1−a Y _a ) _y P _{1−z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Mn _1-a Co _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Mn _1-a Ni _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Mn _1-a Zr _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Mn _1-a Sn _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Mn _1-a Al _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Mn _1−a Y _a ) _y P _{1−z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Li _x (Fe _1-a Mn _a ) _y P _{1-z S} iZ O ₄ (0.9 ≦ x ≦ 1.1, 0.01 ≦ a ≦ 0.125, 0.95 ≦ y ≦ 1.05, 0.01 ≦ z ≦ 0.25)
Etc. However, x is a value in the discharge state of a battery.

(A) Method for producing positive electrode active material The positive electrode active material uses a combination of carbonate, hydroxide, chloride, sulfate, acetate, oxide, oxalate, nitrate, etc. of each element as a raw material. Can be manufactured. As the production method, methods such as a firing method, a solid phase method, a sol-gel method, a melt quench method, a mechanochemical method, a coprecipitation method, a hydrothermal method, and a spray pyrolysis method can be used. Among these methods, a firing method under an inert atmosphere (for example, a nitrogen atmosphere) (firing conditions are 400 to 800 ° C. for 1 to 48 hours) is convenient.

(B) Others The surface of the positive electrode active material may be coated with carbon in order to improve conductivity. The coating may be on the entire surface of the positive electrode active material or a part thereof.

The proportion of carbon to be coated is preferably in the range of 1 to 30 parts by weight with respect to 100 parts by weight of the positive electrode active material. When the amount is less than 1 part by weight, the effect of covering carbon may not be sufficiently obtained. When the amount is more than 30 parts by weight, the capacity of the battery may be lowered in order to inhibit the diffusion of lithium at the interface between the positive electrode active material and the electrolytic solution. A more desirable ratio is in the range of 1.5 to 15 parts by weight. Within this range, it is easy to control the amount of the conductive material included in the positive electrode active material to an appropriate amount when the positive electrode is manufactured.

The carbon coating method is not particularly limited, and a known method can be used. For example, the raw material of the positive electrode active material may be mixed with a compound serving as a carbon source, and the resulting mixture may be coated by firing in an inert atmosphere. As the compound serving as the carbon source, it is necessary to use a compound that does not prevent the raw material from changing to the positive electrode active material. Examples of such compounds include saccharides such as sucrose and fructose, glycols such as polyethylene glycol, fats such as lauric acid, pitch, and tar.

(II) Configuration Other than Positive Electrode Active Material The positive electrode may contain a conductive material, a binder, and a current collector in addition to the positive electrode active material. The positive electrode can be produced by a known method such as applying a paste obtained by mixing an active material with water or an organic solvent, optionally together with a conductive material and a binder, to a current collector.

Binders (binders) include polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluoro rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, Nitrocellulose, acrylic resin, carboxymethylcellulose and the like can be used.

As the conductive material, acetylene black, carbon, graphite, natural graphite, artificial graphite, needle coke, or the like can be used.

Among the artificial graphite of the conductive material, VGCF (registered trademark) is a high crystallinity of graphite, has high electron _{conductivity, Li x M y P 1-} z Si z O 4 obtained by replacing a part element of the present application The contact resistance between the positive electrode active materials or the contact resistance between the positive electrode active material and the current collector can be reduced.

As the current collector, foamed (porous) metal having continuous pores, metal formed in a honeycomb shape, sintered metal, expanded metal, non-woven fabric, plate, foil, perforated plate, perforated foil, etc. are used. be able to.

As the organic solvent, N-methyl-2-pyrrolidone, toluene, cyclohexane, dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc. are used. be able to.

The thickness of the positive electrode active material layer is preferably about 0.01 to 20 mm. If it is too thick, the conductivity is lowered, and if it is too thin, the capacity per unit area is lowered. In addition, you may compress the positive electrode active material layer obtained by application | coating and drying with a roller press etc. in order to raise the packing density of an active material.

(B) Negative electrode A negative electrode is a negative electrode active material layer containing a negative electrode active material.

Carbon having a specific surface area of 2.5 to 5.0 m ² / g is used for the negative electrode active material. When the specific surface area is smaller than 2.5 m ² / g, for example, the lithium occlusion rate in the negative electrode becomes slow during low-temperature charging such as 0 ° C., and lithium may precipitate in a dendrite shape on the negative electrode surface. When larger than 5.0 m < ² > / g, side reaction with electrolyte increases and the charging / discharging efficiency of a battery may fall. The specific surface area is preferably 2.6 to 4.5 m ² / g. If the specific surface area is within this range, the reactivity with the electrolyte in the case of abnormal heat generation such that the battery temperature becomes, for example, 60 ° C. or more is lowered, and the high temperature storage stability of the battery is improved. Furthermore, the specific surface area is more preferably 2.8 to 4.3 m ² / g because it is not necessary to use a binder more than necessary during the production of the negative electrode. The specific surface area is a value measured by a BET method based on low-temperature low-humidity physical adsorption of an inert gas such as nitrogen gas.

The negative electrode active material is not particularly limited as long as it has carbon having the above specific surface area, and a known material can be used. Examples of such carbon include carbon such as natural or artificial graphite in the form of particles (scale-like, lump-like, fibrous, whisker-like, spherical, potato-like, pulverized particle-like, etc.). In particular, spherical and potato-like particles are in point contact with each other even when the electrode is pressed, and it is easy to obtain the effect of the specific surface area of the negative electrode active material.

Examples of artificial graphite include graphite obtained by graphitizing mesocarbon microbeads, mesophase pitch powder, isotropic pitch powder, and the like. Also, graphite particles having amorphous carbon attached to the surface can be used. Among these, natural graphite is more preferable because it is inexpensive, close to the redox potential of lithium, and can constitute a high energy density battery.

Furthermore, with respect to a material in which at least a part of the surface of the natural or artificial graphite particles is covered with crystalline carbon lower than that of the graphite, Li dendrite precipitation is unlikely to occur particularly when charging at a low temperature such as 0 ° C. The reason for this is that when Li ions are inserted into the graphite particles during charging, the carbon layer with low crystallinity existing on the surface relaxes the process from solvated Li ions to bare Li ions. It is considered that Li ions are suppressed from becoming Li metal on the surface of the graphite particles at low temperatures.

The average particle diameter of the negative electrode active material made of carbon is preferably in the range of 0.1 to 75 μm for producing carbon having the specific surface area, and is in the range of 5 to 25 μm for the following reason. preferable. When the average particle diameter is smaller than 5 μm, the gap between the particles in the negative electrode active material layer becomes narrow, and the number of Li ions in the electrolyte solution in the vicinity of the negative electrode active material particles is small. The effect at a low temperature such as 0 ° C., which is good, is not sufficient. Further, when the average particle diameter is larger than 25 μm, the number of contacts between the negative electrode active material particles is decreased, the resistance of the electrode is increased, and the effect of suppressing Li dendrite precipitation at low temperatures may be reduced. .

The above average particle diameter means a value at which the cumulative degree of particle volume is 50%, and is a value measured using a laser diffraction / scattering particle size distribution measuring apparatus (LMS-2000e manufactured by Seishin Enterprise Co., Ltd.).

Lithium transition metal oxide (eg, Li ₄ Ti ₅ O ₁₂ ), lithium transition metal nitride, transition metal oxide, silicon oxide, or the like can be used in combination with the carbon.

The negative electrode active material layer surface facing the separator of the negative electrode active material layer preferably has a larger area than the positive electrode active material layer surface facing the separator of the positive electrode active material layer. By having a large area, lithium ions from the positive electrode active material layer can be prevented from precipitating in a dendrite state during charging. That is, lithium ions that have come out of the positive electrode active material layer during charging move toward the negative electrode active material layer, but cannot reach the negative electrode active material layer when the negative electrode active material layer is equal to or smaller than the positive electrode active material layer. As a result, the possibility of depositing in a dendrite state increases. This possibility occurs even when the voltage can be kept high during low-temperature charging. Here, the area of the negative electrode active material layer surface is more preferably 1% or more than the surface area of the positive electrode active material layer surface, and more preferably in the range of 3 to 15%. This configuration is effective because the substitutional positive electrode active material is fast in supplying lithium to the negative electrode particularly at low temperatures.

The ratio of the negative electrode capacity to the positive electrode capacity at 25 ° C. (negative electrode capacity / positive electrode capacity) is preferably 1.3 or more, and more preferably in the range of 1.3 to 1.6. If the ratio is less than 1.3, the safety of the battery when it is overcharged may be reduced. If the ratio is greater than 1.6, the amount of the negative electrode with respect to the positive electrode inside the battery becomes excessive, and the energy density of the battery May be damaged.

The negative electrode can be produced by a known method. Specifically, it can be manufactured in the same manner as described in the method for manufacturing the positive electrode. That is, a known method such as applying a paste obtained by mixing a negative electrode active material with an organic solvent together with a known binder and a known conductive material described in the preparation method of the positive electrode to a current collector. Can be produced.

(C) Separator Examples of the separator include porous materials and nonwoven fabrics. As a material for the separator, a material that does not dissolve or swell in an organic solvent contained in the electrolyte is preferable. Specific examples include organic materials such as polyester polymers, polyolefin polymers (for example, polyethylene and polypropylene), ether polymers, and inorganic materials such as glass.

Among these, non-woven fabrics such as polyethylene and polypropylene which are synthetic resins are preferable from the viewpoint of quality stability. Some of these synthetic resin nonwoven fabrics have a function in which the separator is dissolved by heat when the battery abnormally generates heat, and a function of blocking between the positive electrode and the negative electrode is added. From the viewpoint of safety, these are also preferable. Can be used for The thickness of the separator is not particularly limited as long as it can hold a necessary amount of electrolyte and can prevent a short circuit between the positive electrode and the negative electrode, and is usually about 0.01 to 0.1 mm. The thickness is preferably about 0.015 to 0.05 mm. If the thickness of the separator is less than 0.015 mm, the lithium dendrite deposit may break through the separator when the battery is overcharged, causing an internal short circuit of the battery. If the thickness is greater than 0.05 mm, the separator volume inside the battery is increased. There is a risk of increasing the energy density of the battery. The porosity of the separator is preferably 30 to 90%, and it is particularly 45 to 65% that does not affect Li dendrite precipitation even at a low temperature of 0 ° C.

The separator surface facing the negative electrode active material layer of the separator preferably has a larger area than the negative electrode active material layer surface. At the time of charging / discharging at a low temperature, the movement speed of lithium ions becomes slow, so that the amount of lithium ions tends to be smaller than the amount of negative electrode active material. Here, if the area of the separator is increased, the amount of lithium ions retained tends to increase. Considering these tendencies, the reaction efficiency of the negative electrode active material with lithium ions can be further improved by making the area of the separator larger than that of the negative electrode active material layer. The area of the separator surface is more preferably 1% or more larger than the surface of the negative electrode active material layer, and still more preferably in the range of 2 to 7%. If the area of the separator surface is smaller than 2% of the surface of the negative electrode active material layer, there is a risk that the separator contracts and the battery is short-circuited during abnormal heat generation such that the battery becomes higher than the softening temperature of the separator. If it is larger, the volume of the separator occupying the inside of the battery increases, which may impair the energy density of the battery.

(D) Others (I) Nonaqueous electrolyte A lithium ion secondary battery usually includes a nonaqueous electrolyte. As the non-aqueous electrolyte, for example, an organic electrolyte, a gel electrolyte, a polymer solid electrolyte, an inorganic solid electrolyte, a molten salt, or the like can be used. Of these, the use of an organic electrolyte is common.

Examples of the organic solvent constituting the organic electrolyte include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC) and butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate, and dipropyl carbonate. Chain carbonates such as γ-butyrolactone (GBL), lactones such as γ-valerolactone, furans such as tetrahydrofuran and 2-methyltetrahydrofuran, diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxy Examples include ethers such as ethane, ethoxymethoxyethane, dioxane, dimethyl sulfoxide, sulfolane, methyl sulfolane, acetonitrile, methyl formate, and methyl acetate. Use one or more of these in combination. Can do.

Moreover, since cyclic carbonates such as PC, EC and butylene carbonate are high-boiling solvents, they are suitable as a solvent to be mixed with GBL.

Examples of the electrolyte salt constituting the organic electrolyte include lithium borofluoride (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium trifluoroacetate (LiCF ₃ COO) ), Lithium salts such as lithium bis (trifluoromethanesulfone) imide (LiN (CF ₃ SO ₂ ) ₂ ), and one or more of these may be used in combination. The salt concentration of the electrolytic solution is preferably 0.5 to 3 mol / l. In particular, 0.8 to 1.5 mol / l is preferable in order to ensure the amount of lithium ions in the negative electrode during low-temperature charging.

(II) Battery container Various battery containers used for conventionally known lithium ion secondary batteries can also be used for the battery container, and there is no particular limitation. For example, a battery container such as a cylindrical type, a square type, or a film type may be used.

(E) Manufacturing method of lithium ion secondary battery A lithium ion secondary battery can be manufactured as follows, for example.

First, a positive electrode, a negative electrode, and a separator sandwiched between them are placed in a battery container in the form of a laminate. The laminate may have, for example, a strip-like planar shape. Moreover, when producing a cylindrical or flat battery, the laminate may be wound. The laminated body put in a battery container may consist of a some positive electrode, a negative electrode, and a separator.

Next, usually, after the positive electrode and the negative electrode are connected to the external conductive terminal of the battery, the battery container is sealed in order to block the positive electrode, the negative electrode and the separator from the outside air. In the case of a cylindrical battery, the sealing method is generally a method in which a lid having a resin packing is fitted into the opening of the battery container and the battery container and the lid are caulked. In the case of a square battery, a method of attaching a lid called a metallic sealing plate to the opening and performing welding can be used. In addition to these methods, a method of sealing with a binder and a method of fixing with a bolt via a gasket can also be used. Furthermore, a method of sealing with a laminate film in which a thermoplastic resin is attached to a metal foil can also be used. An opening for injecting electrolyte is usually provided at the time of sealing.

Furthermore, a lithium ion secondary battery can be obtained by injecting a nonaqueous electrolyte from the opening and then sealing the opening.

[Summary]
The lithium ion secondary battery according to aspect 1 of the present invention is
A positive electrode (1), a negative electrode (2), and a separator (9) between the positive electrode and the negative electrode,
The positive electrode has the following general formula (1)
_{Li x M y P 1-z} Si z O 4 (1)
(Wherein M is a combination of one or both of Fe and Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al and Y, or Fe and A positive electrode active material layer including a positive electrode active material that is a combination with Mn and represented by 0 ≦ x ≦ 2, 0.8 ≦ y ≦ 1.2, and 0 <z ≦ 1)
The negative electrode is a negative electrode active material layer containing a negative electrode active material made of carbon having a specific surface area of 2.5 to 5.0 m ² / g.

According to the above configuration, capacity deterioration due to repeated charge / discharge cycles at a particularly low temperature can be suppressed.

Furthermore, the lithium ion secondary battery which concerns on aspect 2 of this invention is the aspect 1,
The negative electrode active material layer surface of the negative electrode active material layer facing the separator may have a larger area than the positive electrode active material layer surface of the positive electrode active material layer facing the separator.

According to the above configuration, when the negative electrode active material layer surface facing the separator of the negative electrode active material layer has a larger area than the positive electrode active material layer surface facing the separator of the positive electrode active material layer, capacity degradation due to repeated charge / discharge cycles Can be further suppressed. Moreover, the lithium ion secondary battery has a good balance of Li ion insertion / desorption at the negative electrode with respect to Li ion desorption / insertion at the positive electrode during low temperature charge / discharge of the lithium ion secondary battery, and the input / output characteristics of the lithium ion secondary battery at low temperature It can be improved.

Furthermore, the lithium ion secondary battery which concerns on aspect 3 of this invention is the aspect 1 or 2,
The separator surface facing the negative electrode of the separator may have a larger area than the negative electrode active material layer surface facing the separator of the negative electrode active material layer.

According to the above configuration, when the separator surface facing the negative electrode of the separator has a larger area than the negative electrode active material layer surface facing the separator of the negative electrode active material layer, capacity deterioration due to repeated charge / discharge cycles can be further suppressed. Moreover, when the lithium ion secondary battery is overcharged, abnormal heat generation of the battery can be delayed.

Furthermore, the lithium ion secondary battery according to Aspect 4 of the present invention is any one of Aspects 1 to 3,
The M may be a combination of Fe and Zr.

According to the above configuration, when M is a combination of Fe and Zr, capacity deterioration due to repeated charge / discharge cycles can be further suppressed.

Furthermore, a lithium ion secondary battery according to Aspect 5 of the present invention is any one of Aspects 1 to 4,
The z may be 0.01 ≦ z ≦ 0.25.

According to the above configuration, when z is 0.01 ≦ z ≦ 0.25, capacity deterioration due to repeated charge / discharge cycles can be further suppressed. Moreover, since the average particle diameter of the positive electrode active material is controlled, the press effect at the time of producing the positive electrode is increased, and the electrode density can be improved. As a result, the volume energy density of the lithium ion secondary battery can be improved.

Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to the following examples. The reagents used in the examples were special grade reagents manufactured by Kishida Chemical Co. unless otherwise specified.

(Examples 1 to 3 and Comparative Examples 1 to 4)
[Synthesis of positive electrode active material A1]
LiFe _1-a Zr _a P _1-z Si _z O ₄ (x = 1, y = 1) (hereinafter, substituted positive electrode active material A1) was synthesized by the following procedure.

The starting materials are LiCH ₃ COO as a lithium source, FeC ₂ O ₄ .2H ₂ O as an iron source, ZrO (CH ₃ COO) ₂ as a zirconium source, (NH ₄ ) ₂ HPO ₄ as a phosphorus source, and SiO ₂ as a silicon source. used. Each of the above-mentioned substances such that LiCH ₃ COO as a lithium source is 0.6599 g and Li: Fe: Zr: P: Si is in a molar ratio of 1: 0.948: 0.052: 0.948: 0.052. Was weighed. These were mixed well in an agate mortar. This mixture was pulverized and mixed using a planetary ball mill. The ball mill conditions were a rotation speed of 400 rpm, a rotation time of 1 hour, a ball made of zirconia having a diameter of 10 mm, and a mill pot made of zirconia. 15% by weight of sucrose with respect to the obtained powder was dissolved in an aqueous solution, and the obtained powder was mixed, mixed well in an agate mortar, and dried at 60 ° C. The obtained powder is put into a quartz crucible, fired in a nitrogen atmosphere with a firing temperature of 550 ° C., a firing time of 12 hours, a heating / cooling rate of 200 ° C./h, and a particle size in the range of 0.4 to 80 μm by classification. And substitution system positive electrode active material A1 which is single phase powder with an average particle diameter of 15 micrometers was synthesize | combined. In the obtained substituted positive electrode active material A1, the Zr substitution amount a was 0.05, the Si substitution amount z was 0.05, and the lattice constant was (a, b, c) = (10.328,6. 008, 4.694). The substitution amounts a and z were obtained by a calibration curve method using an ICP mass spectrometer (ICP-MS 7500cs manufactured by Agilent Technologies).

The average particle size of the single-phase powder means a value at which the cumulative volume of the particles is 50%, and is a value measured using a laser diffraction / scattering type particle size distribution analyzer (LMS-2000e manufactured by Seishin Enterprise Co., Ltd.). is there.

Also, the lattice constant was obtained by the following procedure.

The substitutional positive electrode active material A1 was pulverized in an agate mortar, and a powder X-ray diffraction pattern was obtained using an X-ray analyzer MiniFlexII manufactured by Rigaku Corporation. The measurement conditions were set at a voltage of 30 kV, a current of 15 mA, a divergence slit of 1.25 °, a light receiving slit of 0.3 mm, a scattering slit of 1.25 °, a range of 2θ of 10 ° to 90 °, and a step of 0.02 ° The measurement time for each step was adjusted so that the peak intensity was 800-1500. Next, regarding the obtained powder X-ray diffraction pattern, “RIETA-FP” (F. Izumi and K. Momma, “Three-dimensional visualization in powder diffraction,” Solid State Phenom., 130, 15-20 (2007) ), An ins file is created with the parameters shown in Table 1 as initial values, a structure analysis is performed by Rietveld analysis using “DD3.bat”, and each parameter is read from the “.lst” file. The lattice constant was determined (S value (degree of convergence) was 1.1 to 1.3).

* In Table 1, * means allocation by the molar ratio obtained from ICP mass spectrometry. In the case of the substitutional positive electrode active material A1, the values are 0.95, 0.05, 0.95, and 0.05 from the top.

[Synthesis of positive electrode active material A2]
Except that the starting material is weighed so that the molar ratio of Li: Fe: Zr: P: Si is 1: 1: 0: 1: 0, LiFePO ₄ (x = 1, y = 1, z = 0) (hereinafter, unsubstituted positive electrode active material A2), a single-phase powder having a particle size in the range of 0.4 to 80 μm and an average particle size of 15 μm was synthesized. The lattice constant was (a, b, c) = (10.325, 6.004, 4.683).

[Production of positive electrode]
The positive electrode active material A1 or A2, acetylene black B, acrylic resin C and carboxymethyl cellulose D are weighed so that the positive electrode active material: B: C: D = 100: 4: 6: 1.2 (weight ratio). The mixture was stirred and mixed at room temperature using a Fillmix 40-40 type (manufactured by Primics) to obtain an aqueous positive electrode manufacturing paste. This paste was applied onto a rolled aluminum foil (thickness: 20 μm) as a current collector using a die coater so that the coating amount of the positive electrode active material per side was 15 to 17 mg / cm ² . The obtained coating film was dried in air at 100 ° C. for 30 minutes and pressed to obtain a positive electrode (coating surface size: 28 mm (vertical) × 28 mm (horizontal)) described in Table 2 below.

[Manufacture of negative electrode]
Any one of graphites (negative electrode active materials) L1 to L6 having different specific surface areas, styrene butadiene rubber M, and carboxymethyl cellulose N are weighed so that graphite: M: N = 98: 2: 1 (weight ratio). An aqueous negative electrode production paste was obtained by stirring and kneading at room temperature using an axial planetary mixer (manufactured by Primics). This paste was applied onto a rolled copper foil (thickness: 10 μm) as a current collector using a die coater so that the coating amount of graphite per side was 9 to 12 mg / cm ² . The obtained coating film was dried in air at 100 ° C. for 30 minutes and pressed to obtain a negative electrode (coating surface size: 30 mm (vertical) × 30 mm (horizontal)) shown in Table 2 below.

L1 is spherical natural graphite having an average particle diameter of 18.7 μm, L2 is potato-shaped artificial graphite having an average particle diameter of 5.6 μm, L3 is spherical natural graphite having an average particle diameter of 12.3 μm, and L4 is an average particle diameter. Was 21.8 μm spherical artificial graphite, L5 was spherical artificial graphite having an average particle diameter of 18.0 μm, and L6 was spherical natural graphite having an average particle diameter of 13.1 μm. The average particle diameter of graphite means a value at which the cumulative volume of particles is 50%, and is a value measured using a laser diffraction / scattering type particle size distribution measuring apparatus (LMS-2000e manufactured by Seishin Enterprise Co., Ltd.). The shape was observed by SEM.

[Manufacture of lithium ion secondary batteries]
The lithium ion secondary battery shown in FIG. 1B was manufactured by the procedure shown in FIG.

The positive electrode 1 and the negative electrode 2 were dried under reduced pressure at 130 ° C. for 24 hours, and then placed in a glow box in a dry Ar atmosphere. Next, an aluminum tab lead 4 with an adhesive film 3 was ultrasonically welded to the positive electrode 1, and a nickel tab lead 6 with an adhesive film 5 was ultrasonically welded to the negative electrode 2. In the glow box, a polyethylene microporous film (size: 31 mm (length) × 31 mm (width), thickness 25 μm, porosity 55%) was loaded as a separator 9 so that the coated surface 7 of the negative electrode 2 was hidden. Furthermore, a laminate was obtained by superposing the positive electrode 1 on the separator 9 so that the coating surface 8 overlapped the center of the separator 9. Further, the laminate was sandwiched between the aluminum laminate films 11 and 12, and the three sides of the aluminum laminate films 11 and 12 were thermally welded so that the adhesive films 3 and 5 of the tab leads 4 and 6 were sandwiched (13 is a heat fusion part). From one side of the unwelded, an electrolytic solution in which LiPF ₆ was dissolved was poured into a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1: 2 so as to be 1 mol / l. After the injection, the last side of the aluminum laminate bag was heat-sealed under a reduced pressure of 10 kPa to obtain a laminated lithium ion secondary battery (cell) 14 of the single cell 10. The injection amount of the electrolyte was appropriately determined according to the thickness of the positive electrode and the negative electrode used in the battery, and was set to an amount that allowed the electrolyte to sufficiently permeate the positive electrode, the negative electrode, and the separator.

The negative electrode active material layer surface area−positive electrode active material layer surface area / positive electrode active material layer surface area × 100 of these batteries was 14.8 (%), (separator surface area−negative electrode active material layer surface area) / negative electrode active material layer surface. The area × 100 was 6.8 (%).

[Measurement method of return capacity maintenance rate]
Under a 0 ° C. environment, charging and discharging were repeated twice for the laminated cell under CC-CV charging conditions (3.6 V cut, 0.01 C cut) at 0.1 C and CC discharging conditions (2.0 V cut). . Next, charge and discharge were repeated 98 times under CC-CV charge conditions at 1.0 C (3.6 V cut, 0.01 C cut) and CC discharge conditions at 1.0 C (2.0 V cut). Thereafter, the battery was charged and discharged by 0.1 C under the same conditions as the first time (1 cy) and the second time again. The charge / discharge capacity of each time was measured, and the ratio (101 cy ÷ 1 cy × 100) of the 101st (101 cy) charge / discharge capacity to the first time was calculated as the return capacity maintenance rate (%).

Table 2 shows the results obtained. Further, FIG. 2 shows the relationship between the specific surface area and the return capacity retention rate of the negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 3 in Table 2.

From Examples 1 to 3 and Comparative Examples 1 to 3 in Table 2 and FIG. 2, the return capacity at 0 ° C. is obtained when the specific surface area of the negative electrode active material is in the range of 2.5 to 5.0 m ² / g. It can be seen that the maintenance rate is kept high. When the ratio is less than 2.5, it can be seen that the retention rate decreases due to precipitation of dendrites. When larger than 5.0, reaction with electrolyte solution arises, Therefore It turns out that a maintenance factor falls because coating (SEI film | membrane) formation progresses on the negative electrode surface excessively.

From Examples 1 to 3 and Comparative Example 4, even when the specific surface area of the negative electrode active material is in the range of 2.5 to 5.0 m ² / g, the return capacity retention rate is kept high when the non-substituted positive electrode active material is used. It can be seen that when a substitutional positive electrode active material is used, it can be kept high. This means that the substitutional positive electrode active material is useful for charging and discharging at a low temperature.

(Examples 4 to 7)
A laminated cell was obtained in the same manner as in Example 2 except that the coating amount of the positive electrode and the negative electrode was changed. The obtained results are shown in Table 3.

In Table 3, the return capacity retention rate after 100 cycles means the ratio measured in the same manner as in Examples 1 to 3 above.

Also, the return capacity maintenance rate after 200 cycles was measured as follows. First, after performing the 101st charge / discharge in the same manner as in Examples 1 to 3 above, CC-CV charge conditions at 1.0 C (3.6 V cut, 0.01 C cut) and CC at 1.0 C Charging / discharging was repeated 99 times under discharge conditions (2.0 V cut). Thereafter, the same 0.1 C charge / discharge as in the first and second times was performed again. The charge / discharge capacity at each time was measured, and the ratio (201 cy ÷ 1 cy × 100) of the 201st (201 cy) charge / discharge capacity to the first time was calculated as the return capacity maintenance rate (%) after 200 cycles.

Furthermore, the capacity of the positive electrode (or negative electrode) was obtained as follows. First, an electrolytic solution in which LiPF ₆ was dissolved in a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1: 2 so as to be 1 mol / l was put in a 50 ml reagent bottle. Next, a positive electrode (or a negative electrode), a counter electrode made of lithium, and a reference electrode are installed so that they are not in contact with each other after the lead wire is added thereto, so that the beaker cell (lithium ion secondary battery) is installed. ) This beaker cell has a positive electrode active current obtained when a constant current charge / discharge of 0.1 C is performed at 25 ° C. in a potential range of 2 to 4 V (0 to 2.5 V for the negative electrode) with respect to the potential of lithium. The charge / discharge capacity per gram of the material (negative electrode active material) was measured, and the product of the amount applied to the positive electrode (or negative electrode) was used as the capacity of the positive electrode (or negative electrode).

In Examples 4 to 7, the return capacity retention rate after 100 cycles at 0 ° C. with respect to the negative electrode capacity / positive electrode capacity was 60% or more, and both were maintained well. The return capacity retention rate after 200 cycles was slightly lower in Example 4. In the case of using a negative electrode active material having a specific surface area of 2.5 to 5.0 m ² / g in the substitutional active material A1, in order to obtain a cell having a high return capacity retention ratio (60% or more) in a low temperature cycle, It was found that the negative electrode capacity / positive electrode capacity was preferably larger than 1.3, more preferably 1.4 or more. Further, when the negative electrode capacity / positive electrode capacity was 1.6 or more, the return capacity retention ratio was high and almost constant.

(Example 8)
Example 2 is an example showing that the negative electrode L2 has an optimum specific surface area when the substitutional positive electrode active material A1 is used in Examples 1 to 3. On the other hand, Comparative Example 4 is an example in which the negative electrode L2 having a specific surface area that was optimal for the substituted positive electrode active material A1 and the unsubstituted positive electrode active material A2 were combined. From comparison between Example 2 and Comparative Example 4 in Table 2, Example 2 using the substituted positive electrode active material A1 shows superior characteristics. This result suggests the advantage of using a substitutional positive electrode active material, particularly at low temperatures.

The reason why the substitution positive electrode active material is superior was verified by a tripotential beaker cell using graphite L2 for the negative electrode and A1 or A2 for the positive electrode. The beaker cell was obtained as follows. First, an electrolytic solution in which LiPF ₆ was dissolved to a concentration of 1 mol / l in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1: 2 was placed in a 50 ml reagent bottle. Next, the positive electrode and the negative electrode, together with a reference electrode made of lithium, are added so that they are not in contact with each other after the lead wire is added thereto, thereby obtaining a beaker cell (lithium ion secondary battery). It was. In this beaker cell, the positive electrode potential (with respect to Li) can be detected simultaneously while controlling the cell voltage (between the positive electrode and the negative electrode).

Measure the cell voltage and positive electrode potential (cell temperature 25 ° C.) over time of charging time, and the results are shown in FIG. 3 (a) and (b). In these figures, the solid line represents the case of using the substituted positive electrode active material A1, and the dotted line represents the case of using the non-substituted positive electrode active material A2.

3 (a), it can be seen that the cell voltage changes at a similar level with respect to the charging time. On the other hand, from FIG. 3B, it can be seen that the positive electrode potential is clearly higher in the substituted positive electrode active material A1 than in the unsubstituted positive electrode active material A2. Here, since the cell voltage is the potential difference between the positive electrode and the negative electrode, it is possible to calculate that the negative electrode potential has a higher value in the case of using the substituted positive electrode active material A1 than in the non-substituted positive electrode active material A2. . This tendency is considered to be the same not only in the beaker cell but also in the low temperature cycle evaluation in the laminate cell. When the negative electrode potential is high, it is possible to suppress the precipitation of Li dendride on the negative electrode surface. Therefore, it is considered that good cycle characteristics are obtained even in the laminate cell using the substitutional positive electrode active material A1.

In addition, the transition of the potential during charging depends on the properties of the positive electrode active material itself, in particular, whether or not it is a substitution system. That is, based on the results of Examples 1 to 3 and Comparative Examples 1 to 4 shown in Table 2, the negative electrode whose specific surface area is 2.5 to 5.0 m ² / g is a substituted positive electrode active material. It is considered that the laminate cell to be used is also effective.

(Examples 9 to 11)
(1) The coated surface (positive electrode active material layer surface) size of the positive electrode is 150 mm (vertical) × 225 mm (horizontal),
(2) The negative electrode coating surface (negative electrode active material layer surface) size is 151 mm (vertical) × 226 mm (horizontal),
(3) The separator surface size of a polyethylene microporous membrane as a separator is 152 mm (vertical) × 227 mm (horizontal), thickness 25 μm, porosity 55%,
(4) The laminated body is formed into a multilayer laminated structure comprising 88 positive electrodes, 176 separators, and 89 negative electrodes so as to be negative electrode / separator / positive electrode / separator / negative electrode /.
Batteries of Examples 9 to 11 were produced in the same manner as Examples 1 to 3 except for changing each. The obtained batteries were evaluated in the same manner as in Examples 1 to 3. As a result, a return capacity retention rate of 60% or more was obtained.

In addition, (negative electrode active material layer surface area−positive electrode active material layer surface area) / positive electrode active material layer surface area × 100 of these batteries is 1.1 (%), (separator surface area−negative electrode active material layer surface area) / negative electrode active The material layer surface area × 100 was 1.1 (%).

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention. Furthermore, a new technical feature can be formed by combining the technical means disclosed in each embodiment.

The present invention can be widely applied to all lithium ion secondary batteries.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 and 5 Adhesive film 4 and 6 Tab lead 7 and 8 Coating surface 9 Separator 10 Single cell 11 and 12 Aluminum laminated film 13 Thermal fusion part 14 Lithium ion secondary battery

Claims (5)

1. Including a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode,
   The positive electrode has the following general formula (1)
   _{Li x M y P 1-z} Si z O 4 (1)
   (Wherein M is a combination of one or both of Fe and Mn and at least one element selected from the group consisting of Co, Ni, Zr, Sn, Al and Y, or Fe and A positive electrode active material layer including a positive electrode active material that is a combination with Mn and represented by 0 ≦ x ≦ 2, 0.8 ≦ y ≦ 1.2, and 0 <z ≦ 1)
   A lithium ion secondary battery, wherein the negative electrode is a negative electrode active material layer including a negative electrode active material made of carbon having a specific surface area of 2.5 to 5.0 m ² / g.
2. The lithium ion secondary battery according to claim 1, wherein a surface of the negative electrode active material layer facing the separator of the negative electrode active material layer has a larger area than a surface of the positive electrode active material layer facing the separator of the positive electrode active material layer.
3. The lithium ion secondary battery according to claim 1 or 2, wherein a separator surface of the separator facing the negative electrode has a larger area than a surface of the negative electrode active material layer facing the separator.
4. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the M is a combination of Fe and Zr.
5. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the z is 0.01≤z≤0.25.

Images