JP2017084739A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery arranged so that the corrosion of an aluminum positive electrode current collector is suppressed even if a nonaqueous electrolyte solution contains an imide salt.SOLUTION: A lithium ion secondary battery according to the present invention comprises: a positive electrode; a negative electrode; and a nonaqueous electrolyte solution. The nonaqueous electrolyte solution contains LiPF, and an imide salt expressed by the following general formula (1): (XSO)(FSO)NLi(where X represents a fluorine atom, an alkyl group with 1-6C, or a fluoroalkyl group with 1-6C). The positive electrode has a current collector composed of a piece of aluminum foil of 97.0-99.0% in purity.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery containing a non-aqueous electrolyte.

  In recent years, since lithium ion secondary batteries have high energy density, they have been used as power supplies for mobile communication devices, power supplies for portable information terminals, etc., and the market has grown rapidly with the spread of these terminals. Various studies have been conducted for the purpose of improving the property, improving the cycle characteristics and energy density.

  In recent years, attention has been focused on nonaqueous electrolytic solutions containing an imide salt such as LiFSI as an electrolyte salt having ion conductivity and thermal stability superior to those of the prior art. However, non-aqueous electrolytes containing imide salts cause corrosion in the aluminum foil (hereinafter sometimes referred to as “aluminum current collector”) used for the positive electrode current collector of lithium ion secondary batteries, so that the battery characteristics are maintained. It was difficult to do. In recent years, reducing the manufacturing cost of lithium ion secondary batteries has become an important theme, and it has been required to balance productivity and performance.

  In the case of using a nonaqueous electrolytic solution containing an imide salt, a technique for suppressing corrosion of an aluminum current collector by adding an additive has been proposed. For example, Patent Document 1 discloses a technique of adding a sulfonic acid ester and / or a sulfone compound to a nonaqueous electrolytic solution. Patent Document 2 discloses a technique of adding a predetermined borate compound and / or boron compound and a carbonate-based solvent to a non-aqueous electrolyte. Further, Patent Document 3 discloses a technique for adding a fluorine-containing carbonate to a non-aqueous electrolyte.

JP 2014-13704 A JP 2014-22291 A JP 2014-203748 A

  The present invention has been made paying attention to the above-described circumstances, and the object thereof is to have good productivity even when the non-aqueous electrolyte contains an imide salt, and to collect an aluminum positive electrode current collector. An object of the present invention is to provide a lithium ion secondary battery in which body corrosion is suppressed.

The present invention that has solved the above problems is a lithium ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte includes LiPF ₆ and general formula (1): (XSO ₂ ) an imide salt represented by (FSO ₂ ) N ^— Li ⁺ (wherein X represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms) The current collector of the positive electrode is an aluminum foil having a purity of 97.0% or more and 99.0% or less, which may be referred to as “imide salt (1)”.

In the present invention, it is also a preferred embodiment that the concentration of LiPF ₆ in the non-aqueous electrolyte is 0.1 mol / L or more and 1.4 mol / L or less.

  In the present invention, it is also a preferred embodiment that the concentration of the imide salt in the non-aqueous electrolyte is 0.1 mol / L or more and 1.4 mol / L or less.

Since the lithium ion secondary battery of the present invention uses a nonaqueous electrolytic solution containing LiFP ₆ and an imide salt (1) and an aluminum foil having a predetermined purity as a positive electrode current collector, Corrosion can be suppressed. Therefore, according to the present invention, various characteristics such as cycle characteristics, rate characteristics, thermal stability, and specific capacity (hereinafter referred to as these characteristics) while having good productivity and suppressing corrosion of the aluminum current collector. Can be collectively referred to as “battery characteristics”).

LiPF ₆ is known to form an AlF ₃ passive film on the surface of the aluminum current collector, and to suppress the corrosion of the aluminum current collector. Even when LiPF ₆ was added, it was difficult to suppress corrosion of the aluminum current collector. In particular, when the voltage was high, the aluminum current collector was easily corroded.

  Therefore, the present inventor has repeatedly investigated the corrosion inhibition of the aluminum current collector in the lithium ion secondary battery including a non-aqueous electrolyte containing an imide salt as an electrolyte salt. As a result, it was found that corrosion of the aluminum current collector can be suppressed by using an aluminum foil having a predetermined purity for the current collector.

The present inventor conducted an experiment on the corrosivity of a current collector using aluminum foil of various purity using a non-aqueous electrolyte containing an imide salt and LiPF _{6. The} corrosivity is affected by the purity of the aluminum foil. I found out. Specifically, it was found that the higher the purity of the aluminum foil, the less likely it is to corrode. Therefore, it is effective to suppress corrosion by using an aluminum foil having a purity as high as possible for the non-aqueous electrolyte.

  However, the aluminum foil used for the current collector is usually subjected to a rolling process to be processed into a thin film of about 10 to 30 μm. However, the higher the purity of the aluminum foil due to work hardening during rolling, the more hard and brittle it becomes. In order to increase the energy density per unit volume of the battery, the positive electrode needs to be pressed at a high pressure to increase the positive electrode density. Under such circumstances, in a positive electrode using a high-purity aluminum foil, processing defects such as cracking of the aluminum foil are frequently generated in the process of producing a battery wound cell, resulting in a decrease in battery productivity. A new problem occurred. Accordingly, as a result of studying the purity, defect occurrence rate, and corrosivity of the aluminum foil, the present inventor has found that by controlling the purity of the aluminum foil within a predetermined range, it is possible to achieve both a corrosion inhibiting effect and productivity. Invented.

  Hereinafter, the lithium ion secondary battery of the present invention will be described.

1. Lithium Ion Secondary Battery The lithium ion secondary battery of the present invention is characterized in that it includes a positive electrode and a negative electrode, and has a nonaqueous electrolytic solution as an electrolytic solution. More specifically, a separator is provided between the positive electrode and the negative electrode, and the non-aqueous electrolyte is contained in the outer case together with the positive electrode, the negative electrode, and the like in a state of being impregnated in the separator.

  The shape of the lithium ion secondary battery according to the present invention is not particularly limited, and any conventionally known shape such as a cylindrical shape, a square shape, a laminate shape, a coin shape, and a large size can be used. Further, when used as a high voltage power source (several tens of volts to several hundreds of volts) for mounting on an electric vehicle, a hybrid electric vehicle, etc., a battery module configured by connecting individual batteries in series can be used. .

2. The positive electrode is a positive electrode active material composition containing a positive electrode active material, a conductive additive, a binder, a dispersion solvent and the like supported on a positive electrode current collector, and is usually formed into a sheet shape. Yes.

  Examples of the method for producing the positive electrode include a method in which the positive electrode active material composition is applied to the positive electrode current collector by a doctor blade method or the like, or the positive electrode current collector is immersed in the positive electrode active material composition and then dried; A method in which a sheet obtained by kneading, molding and drying an active material composition is bonded to a positive electrode current collector via a conductive adhesive, pressed and dried; a positive electrode active material composition to which a liquid lubricant is added Examples include a method of applying or casting on an electric body to form a desired shape, removing the liquid lubricant, and then stretching in a uniaxial or multiaxial direction.

2-1. Positive Electrode Current Collector In the present invention, a conductive aluminum foil (hereinafter referred to as “aluminum foil”) such as aluminum or an aluminum alloy is used as a material for the positive electrode current collector. If the purity of the aluminum foil is low, the battery characteristics deteriorate due to corrosion due to the imide salt contained in the non-aqueous electrolyte. The purity of the aluminum foil is 97.0% or more, preferably 97.2% or more, more preferably 97.3% or more. From the viewpoint of inhibiting corrosion, the higher the purity of the aluminum foil, the better. However, if the purity becomes too high, defects such as cracks occur in the aluminum foil as described above, and the productivity decreases. Therefore, the purity of the aluminum foil is 99.0% or less, preferably less than 99.0%, more preferably 98.8% or less, still more preferably 98.7% or less, and still more preferably 98.5% or less.

  The different elements contained in the aluminum foil are not particularly limited, and examples thereof include various known alloy elements such as Si, Fe, Cu, Mn, Mg, Zn, and Ti. Further, the content of each different element is not particularly limited, and is within a range specified in relation to the purity of the aluminum foil.

  The material of the aluminum foil that satisfies the above purity is not particularly limited, and examples thereof include alloy symbols 5005, 8021, 3003, and 30004 defined in JIS H4000 (2006).

  In addition, the thickness of aluminum foil is not specifically limited, A normal thickness (for example, 10-30 micrometers) may be sufficient.

2-2. Positive Electrode Active Material As the positive electrode active material, any known positive electrode active material used in lithium ion secondary batteries may be used as long as it can absorb and release lithium ions.

Specifically, lithium cobalt acid, lithium nickel acid, lithium manganese _{acid, LiNi 1-xy Co x Mn} y O 2 or _{LiNi 1-xy Co x Al y} O 2 (0 ≦ x ≦ 1,0 ≦ y ≦ 1 ), A transition metal oxide such as a ternary oxide represented by Li _x Ni _y Mn _(2-y) O ₄ (0.9 ≦ x ≦ 1.1, 0 <y <1) Lithium nickel manganate, compounds having an olivine structure such as LiAPO ₄ (A = Fe, Mn, Ni, Co), and solid solution materials incorporating a plurality of transition metals (electrochemically inert layered Li ₂ MnO ₃ and Electrolytically active layered LiM ″ O [M ″ = transition metal such as Co and Ni]) and the like can be cited as the positive electrode active material. These positive electrode active materials may be used alone or in combination.

2-3. Conductive aid Examples of the conductive aid include acetylene black, carbon black, graphite, metal powder material, single-walled carbon nanotube, multi-walled carbon nanotube, and vapor grown carbon fiber.

2-4. Binders As binders, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide resins such as polyamideimide; polyethylene, polypropylene and the like Polyolefin resin; poly (meth) acrylic resin; polyacrylic acid; cellulose resin such as carboxymethylcellulose; These binders may be used alone or in combination of two or more. These binders may be dissolved in a solvent at the time of use or dispersed in a solvent.

  The blending amounts of the conductive auxiliary agent and the binder can be appropriately adjusted in consideration of the intended use of the battery (emphasis on output, importance on energy, etc.), ion conductivity, and the like.

In producing the positive electrode, examples of the solvent used in the positive electrode active material composition include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetone, ethanol, ethyl acetate, and water. These solvents may be used in combination. The amount of the solvent used is not particularly limited, and may be appropriately determined according to the production method and the material to be used.
3. Negative electrode A negative electrode is a negative electrode current collector comprising a negative electrode active material, a dispersion solvent, a binder, and a conductive auxiliary agent, if necessary. Molded.

3-1. Negative Electrode Current Collector As a material for the negative electrode current collector, a conductive metal such as copper, iron, nickel, silver, stainless steel (SUS) can be used. Among these, copper is preferable because it can be easily processed into a thin film.

3-2. Negative electrode active material As the negative electrode active material, a conventionally known negative electrode active material used in lithium secondary batteries can be used as long as it can occlude and release lithium ions. Specifically, graphite materials such as artificial graphite and natural graphite, mesophase fired bodies made from coal / petroleum pitch, carbon materials such as non-graphitizable carbon, Si, Si alloys, Si-based negative electrode materials such as SiO, Sn An Sn-based negative electrode material such as an alloy, or a lithium alloy such as lithium metal or a lithium-aluminum alloy can be used.

  As a manufacturing method of the negative electrode, a method similar to the manufacturing method of the positive electrode can be adopted. Further, the same conductive assistant, binder and material dispersing solvent as used in the production of the negative electrode are used.

4). Non-aqueous electrolyte The lithium ion secondary battery of the present invention has an electrolyte salt as a general formula (1): (XSO ₂ ) (FSO ₂ ) N ^- Li ⁺ (wherein X is a fluorine atom, having 1 to 6 carbon atoms) A non-aqueous electrolyte containing an imide salt represented by an alkyl group or a C1-C6 fluoroalkyl group) and LiPF ₆ is used.

4-1. Imide salt (1)
The non-aqueous electrolyte of the present invention contains an imide salt represented by the general formula (1): (XSO ₂ ) (FSO ₂ ) N ^— Li ⁺ . Even when the non-aqueous electrolyte of the present invention contains the imide salt (1), corrosion of the aluminum current collector is difficult to occur. A lithium ion secondary battery having good battery characteristics can be provided.

  In said formula (1), X represents a fluorine atom, a C1-C6 alkyl group, or a C1-C6 fluoroalkyl group independently. The alkyl group is preferably a linear alkyl group, and examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. The fluoroalkyl group may be linear, branched, cyclic, or a combination thereof. The fluoroalkyl group may be any group in which part of the hydrogen atoms bonded to the carbon atom is replaced with a fluorine atom. Specific examples of the fluoroalkyl group include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, a pentafluoroethyl group, a fluoropropyl group, a fluoropentyl group, and a fluorohexyl group. Groups and the like. X is preferably a fluorine atom, a trifluoromethyl group, or a pentafluoroethyl group.

  The imide salt (1) is preferably lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, or lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide, more preferably lithium. Bisfluorosulfonylimide.

  The imide salt (1) may be used alone or in combination of two or more. As the imide salt (1), a commercially available product may be used, or a product synthesized by a conventionally known method may be used.

In the present invention, it is preferable to appropriately adjust the imide salt (1) concentration and the LiPF ₆ concentration in order to suppress corrosion of the aluminum current collector having the predetermined aluminum purity.

  The concentration of the imide salt (1) in the non-aqueous electrolyte is not particularly limited. However, if the concentration is too low, there is a possibility that desired battery characteristics cannot be obtained, such as reduced ionic conductivity and reduced rate characteristics. Therefore, it is preferably at least 0.1 mol / L, more preferably at least 0.2 mol / L, even more preferably at least 0.3 mol / L, even more preferably at least 0.4 mol / L, most preferably at least 0.5 mol / L. That's it. On the other hand, if the concentration of the imide salt (1) in the non-aqueous electrolyte becomes too high, the aluminum current collector may corrode, and is preferably 1.4 mol / L or less, more preferably 1.3 mol / L or less. More preferably, it is 1.2 mol / L or less, and most preferably 1.0 mol / L or less.

4-2. LiPF ₆ (lithium hexafluorophosphate)
The nonaqueous electrolytic solution of the present invention contains LiPF ₆ as an electrolyte salt. LiPF ₆ is an electrolyte salt that has an effect of suppressing corrosion of aluminum by forming a passive film on the surface of the aluminum current collector. The concentration of LiPF ₆ is not particularly limited, but is preferably 0.1 mol / L or more, more preferably 0.2 mol / L or more, in order to suppress corrosion of the aluminum current collector in the presence of the imide salt (1). Preferably it is 0.4 mol / L or more. On the other hand, if the LiPF ₆ concentration becomes too high, the ionic conductivity may decrease due to an increase in viscosity and the battery characteristics may deteriorate. Therefore, it is preferably 1.4 mol / L or less, more preferably 1.0 mol / L or less. More preferably, it is 0.8 mol / L or less.

4-3. Other Electrolyte Salts The nonaqueous electrolytic solution of the present invention may contain an electrolyte salt different from the LiPF ₆ and the imide salt (1) (hereinafter sometimes referred to as “other electrolyte salt”) as an electrolyte salt. Alternatively, only LiPF ₆ and the imide salt (1) may be used. Other electrolyte salts are not particularly limited, and any conventionally known electrolyte salt used in the electrolyte solution of a lithium ion secondary battery can be used. By using another electrolyte salt in combination, corrosion of aluminum caused by the imide salt (1) can be further suppressed.

The other electrolyte salt is represented by the general formula (2): M′PF _a (CmF _{2m + 1} ) _5-a (where 0 ≦ a ≦ 5, 1 ≦ m ≦ 2; hereinafter, electrolyte salt (2) General formula (3): M′BFb (CnF _{2n + 1} ) _4-b (where 0 ≦ b ≦ 4, 1 ≦ n ≦ 2; hereinafter referred to as electrolyte salt (3)) ), General formula (4): M′AsF ₆ (hereinafter referred to as electrolyte salt (4)), and general formula (5): M′SbF ₆ (hereinafter referred to as electrolyte salt (5)). It is preferable that it is at least one compound selected from the group consisting of a salt compound containing fluorine represented by: In formulas (2) to (5), M ′ represents an alkali metal ion. Examples of the alkali metal ion include lithium ion, sodium ion, potassium ion, rubidium ion, and cesium ion, preferably lithium ion, sodium ion, and potassium ion.

The fluorine-containing salt compound, preferably _{LiPF 3 (C 2 F 5)} 3, LiBF 4, LiBF (CF 3) 3, LiAsF 6, LiSbF 6, more preferably LiBF _4, LiAsF _6, more preferably LiBF ₄ It is. Other electrolyte salts may be used alone or in combination of two or more of the fluorine-containing salt compounds exemplified above.

  As other electrolyte salts, commercially available products may be used, or those synthesized by a conventionally known method may be used.

When the non-aqueous electrolyte contains the other electrolyte salt, it is desirable to add so as to obtain a desired effect. The other electrolyte salt in the non-aqueous electrolyte is not particularly limited as long as the total concentration of LiPF ₆ and the imide salt (1) is used in the range of the saturation concentration or less, but if the concentration is too low, Since sufficient effects may not be obtained, it is preferably 0.05 mol / L or more, more preferably 0.1 mol / L or more. On the other hand, if the concentration of the other electrolyte salt is too high, the ionic conductivity may decrease due to an increase in viscosity. Therefore, it is preferably 1.0 mol / L or less, more preferably 0.7 mol / L or less, and still more preferably 0. .5 mol / L or less.

4-4. Solvent The nonaqueous electrolytic solution of the present invention contains a solvent for dissolving the electrolyte salt and the additive. The solvent that can be used in the nonaqueous electrolytic solution of the present invention is not particularly limited as long as it can dissolve and disperse the electrolyte salt and the additive, and a nonaqueous solvent, a polymer used in place of the solvent, Any conventionally known solvent used for batteries, such as a medium such as a polymer gel, can be used.

  As the non-aqueous solvent, a solvent having a large dielectric constant, high solubility of the electrolyte salt, a boiling point of 60 ° C. or higher, and a wide electrochemical stability range is preferable. More preferably, it is an organic solvent (non-aqueous solvent) having a low water content. Examples of such organic solvents include ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,6-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, Ethers such as 1,4-dioxane and 1,3-dioxolane; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate (ethyl methyl carbonate), diethyl carbonate (diethyl carbonate), diphenyl carbonate and methyl phenyl carbonate; Ethylene (ethylene carbonate), propylene carbonate (propylene carbonate), 2,3-dimethylethylene carbonate, butylene carbonate, vinylene carbonate, 2-vinyl Cyclic carbonates such as ethylene acid; aromatic carboxylic acid esters such as methyl benzoate and ethyl benzoate; lactones such as γ-butyrolactone, γ-valerolactone, and δ-valerolactone; trimethyl phosphate, ethyl phosphate Phosphate esters such as dimethyl, diethylmethyl phosphate, triethyl phosphate; acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, isobutyronitrile Nitriles such as dimethylsulfone, ethylmethylsulfone, diethylsulfone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, and the like; aromatic nitriles such as benzonitrile and tolunitrile; nitromethane, 1,3 − Methyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro -2 (1H) - pyrimidinone, can be mentioned 3-methyl-2-oxazolidinone and the like.

  Among these, carbonate esters (carbonate solvents) such as chain carbonate esters and cyclic carbonate esters, lactones, and ethers are preferable. Dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, γ -Butyrolactone, γ-valerolactone, and the like are more preferable, and carbonate solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate are more preferable. The said non-aqueous solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

  When the polymer or polymer gel is used in place of a solvent, the following method may be employed. That is, a method in which a solution in which an electrolyte salt is dissolved in the above-mentioned non-aqueous solvent is dropped onto a polymer formed by a conventionally known method so that the electrolyte salt and the non-aqueous solvent are impregnated and supported; In this method, the polymer and the electrolyte salt are melted and mixed, and then formed into a film and impregnated with a non-aqueous solvent (hereinafter referred to as a gel electrolyte); a non-aqueous electrolyte solution in which the electrolyte salt is previously dissolved in an organic solvent and the polymer After mixing, the film is formed by the casting method or coating method, and the organic solvent is volatilized; the polymer and the electrolyte salt are melted at a temperature equal to or higher than the melting point of the polymer, and mixed to form (the intrinsic polymer electrolyte) );

  Polymers used in place of solvents include polyether polymers such as polyethylene oxide (PEO) and polypropylene oxide which are homopolymers or copolymers of epoxy compounds (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.) , Methacrylic polymers such as polymethyl methacrylate (PMMA), nitrile polymers such as polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), fluoropolymers such as polyvinylidene fluoride-hexafluoropropylene, and copolymers thereof Etc.

4-5. Other Additives The non-aqueous electrolyte solution according to the present invention aims to improve various characteristics such as improvement of cycle characteristics and safety of lithium ion secondary batteries in addition to the electrolyte salt, additives, and solvents. An additive may be contained.

  Examples of additives include carbonate compounds such as phenylethylene carbonate and erythritan carbonate; 1,3-propane sultone, 1,4-butane sultone, 1,5-pentane sultone, 1,4-hexane sultone, 4,6- Sulfonic esters such as heptane sultone, methyl methanesulfonate, methyl benzenesulfonate, methyl trifluoromethanesulfonate; sulfones such as sulfolane, 3-methylsulfolane, ethylmethylsulfone, diphenylsulfone, bis (4-fluorophenyl) sulfone Compound: succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinate Carboxylic anhydrides such as acid anhydrides; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methyls Nitrogen-containing compounds such as succinimide; phosphates such as monofluorophosphate and difluorophosphate; hydrocarbon compounds such as heptane, octane and cycloheptane;

  The other additive is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and further preferably 0.3% by mass or more in 100% by mass of the non-aqueous electrolyte of the present invention. Preferably, it is 10 mass% or less, More preferably, it is 8 mass% or less, More preferably, it is 5 mass% or less. When the amount of the additive used is too small, it may be difficult to obtain an effect derived from the additive.On the other hand, even if another additive is used in a large amount, it is difficult to obtain an effect commensurate with the added amount. There is a possibility that the viscosity of the non-aqueous electrolyte increases and the conductivity decreases.

Incidentally, the nonaqueous The electrolyte solution 100 mass%, the aforementioned imide salt (1), the sum of all components contained LiPF _6, other electrolyte salt, solvent, and optionally additives used, in a non-aqueous electrolyte solution Means.

5. Separator The separator is disposed so as to separate the positive electrode and the negative electrode. The separator is not particularly limited, and any conventionally known separator can be used in the present invention. Specific examples of the separator include a porous sheet (for example, a polyolefin microporous separator and a cellulose separator) made of a polymer that can absorb and hold a nonaqueous electrolyte solution, a nonwoven fabric separator, a porous metal body, and the like. It is done. Among these, a polyolefin-based microporous separator is preferable because it has a property of being chemically stable to an organic solvent.

  Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

  Examples of the material of the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass, and the like. Or it can be mixed and used.

6). Battery exterior materials Battery elements equipped with a positive electrode, negative electrode, separator, non-aqueous electrolyte, etc. are housed in battery exterior materials to protect the battery elements from external impacts and environmental degradation when using lithium ion secondary batteries. The In the present invention, the material of the battery exterior material is not particularly limited, and any conventionally known exterior material can be used.

7). Discharge voltage The higher the average discharge voltage, the higher the energy density of the battery. The average discharge voltage of the lithium ion secondary battery of the present invention is preferably 3.4 V or higher, more preferably 3.45 V or higher, still more preferably 3.5 V or higher. On the other hand, since the aluminum current collector tends to corrode when the average discharge voltage increases, it is preferably 4.0 V or less, more preferably 3.9 V or less, and even more preferably 3.8 V or less. In addition, the average discharge voltage in this invention is a value measured with a charging / discharging apparatus.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

1. Preparation of non-aqueous electrolyte Lithium bis (fluorosulfonyl) imide in a non-aqueous solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (MEC) are mixed at 3: 7 (volume ratio) to 1.0 mol / L (Manufactured by Nippon Shokubai Co., Ltd .: hereinafter referred to as “LiFSI”) and LiPF ₆ (manufactured by Kishida Chemical Co., Ltd.) are dissolved so as to be 0.2 mol / L, and a non-aqueous electrolyte (A ) Was prepared. Further, in the same manner as described above, LiFSI and LiPF ₆ were dissolved in a nonaqueous solvent so as to be 0.6 mol / L, and a nonaqueous electrolyte solution (B) was prepared.

2. Fabrication of laminated lithium ion secondary battery

2-1.
Method for measuring purity of aluminum foil The aluminum foil used in the above aluminum current collector is immersed and dissolved in a mixture of a 69% nitric acid aqueous solution and an 85% phosphoric acid aqueous solution so that nitric acid: phosphoric acid = 1: 4. Then, it was diluted with ultrapure water and calculated by quantifying the amount of aluminum with an ICP emission spectrometer.

2-2. Preparation of positive electrode sheet A positive electrode active material (LiCoO ₂ ), a conductive additive (acetylene black, AB), and a binder (polyvinylidene fluoride, PVdF) were mixed at a mass ratio of 100: 3: 3, and this was mixed with N— The positive electrode mixture slurry dispersed in methylpyrrolidone was applied to an aluminum current collector and dried. Subsequently, the sheet | seat apply | coated and dried similarly also to the back surface was produced by compressing with a roll press machine so that a density might be 3.8 or more.

2-3. Preparation of negative electrode sheet A negative electrode active material (graphite), a thickener (CMC), and a binder (SBR) were mixed at a mass ratio of 100: 1.1: 1.8, and this was mixed with water. The obtained negative electrode mixture slurry was prepared. The charge capacity of the positive electrode at 4.2 V charge is calculated, and the negative electrode mixture slurry is made of copper foil (negative electrode collector) so that the negative electrode lithium ion storage capacity / positive electrode charge capacity ratio is 1.1 or more and less than 1.2. And then dried. Next, after applying and drying in the same manner on the back surface, a negative electrode sheet was prepared by compressing with a roll press.

2-4. Fabrication of a laminated lithium ion secondary battery After welding a metal tab to the current collector exposed portion of the positive electrode sheet and the negative electrode sheet that were not coated with the active material, the positive electrode sheet and the negative electrode sheet, A wound body was produced by winding in a configuration sandwiching a polyethylene separator. A wound body was sandwiched between two aluminum laminate films to produce a lithium ion secondary battery cell. In the produced battery cell, 10 battery cells filled with the non-aqueous electrolyte (A) and 10 battery cells filled with the non-aqueous electrolyte (B) are sealed in a vacuum state, and the laminate type has a capacity of 1 Ah. A lithium ion secondary battery was produced.

  About the obtained lamination type lithium ion secondary battery, it charged to 3.7V by the amount of electric currents 0.5C in the environment of a temperature of 25 degreeC using the charging / discharging test device (ACD-01 by Asuka Electronics Co., Ltd.). Then, after leaving it for 48 hours, the cell was opened and sealed again in a vacuum state. Next, after charging to 4.2 V at a current amount of 0.5 C, a 48-hour energization test was performed in the constant voltage mode.

Corrosion evaluation test The battery after the constant voltage application test was disassembled and the positive electrode sheet was taken out, and the exposed portion of the aluminum current collector (welded portion of the positive electrode metal tab) was digital microscope (magnification: 1000 times, field of view: 3 mm × 3 mm, Observation by Keyence Co., Ltd.) and confirmed the presence or absence of pitting corrosion due to corrosion. The case where pitting corrosion was observed on the aluminum foil was evaluated as “Yes”, and the case where pitting corrosion was not observed was evaluated as “None”. The results are shown in Table 1.

Confirmation of defective rate of positive electrode sheet The battery after the constant voltage application test was disassembled, the positive electrode sheet was taken out, and the presence or absence of cracks in the wound part of the aluminum current collector was visually confirmed. The case where the positive electrode active material layer peeled off and the aluminum current collector was cracked or chipped was regarded as defective. Table 1 shows the number of defects in 20 pieces.

  As shown in Table 1, no. In Nos. 7 and 8, corrosion occurred in the aluminum current collector. On the other hand, corrosion did not occur when the purity of the aluminum foil was 97.0% or more, but when the purity was higher than 99.0%, the defective ratio was increased and the productivity was remarkably deteriorated. No. in which the purity of the aluminum foil is within the range specified by the present invention (97.0% to 99.0%). In Nos. 3 to 6, corrosion was suppressed and the defect rate was low. Especially the invention example (No. 4-6) in case the purity of aluminum foil is 98.5% or less also has a defect number of zero. Compared with 1 and 2, productivity improved remarkably.

  From the above results, it is considered that when the purity of the aluminum foil is too high, the aluminum foil becomes hard and brittle due to work hardening and the like, and it becomes easy to cause work cracks and the defect rate is increased. On the other hand, when the purity of the aluminum foil is low, it is considered that the aluminum current collector is corroded due to the imide salt (1). On the other hand, if the purity of the aluminum foil is adjusted within a predetermined range, the occurrence of defects due to work hardening and the like can be suppressed, and the corrosion of the aluminum current collector can also be suppressed. I knew it was possible.

Claims (3)

A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The non-aqueous electrolyte is
LiPF ₆ and
Including an imide salt represented by the following general formula (1),
(XSO ₂ ) (FSO ₂ ) N ⁻ Li ⁺ (1)
(In the formula, X represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms.)
The positive electrode current collector is an aluminum foil having a purity of 97.0% or more and 99.0% or less. 2. The lithium ion secondary battery according to claim 1, wherein the concentration of LiPF ₆ in the non-aqueous electrolyte is 0.1 mol / L or more and 1.4 mol / L or less.   The lithium ion secondary battery according to claim 1, wherein a concentration of the imide salt is 0.1 mol / L or more and 1.4 mol / L or less.