JP6324845B2 - Non-aqueous electrolyte and lithium ion secondary battery including the same - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte and a lithium ion secondary battery including the same.

  A battery such as a lithium ion secondary battery is used as a power source for a mobile phone, a personal computer, a power source for an automobile, and the like. In addition, in a battery used for such an application, research aimed at improving various characteristics such as high energy density and improvement of cycle characteristics and rate characteristics has been repeated.

For example, Patent Documents 1 to 4 disclose techniques in which a nitrile compound such as succinonitrile is used as an additive for a nonaqueous electrolytic solution. Patent Document 1 discloses a technique for suppressing deterioration of charge / discharge characteristics of a lithium secondary battery due to dissolution of a negative electrode current collector during overdischarge by accumulating dinitrile compounds and the like added to an electrolyte on the surface of the negative electrode current collector. In Patent Document 2, when a ternary positive electrode active material is used, a specific amount of halogenated ethylene carbonate and a specific amount of vinyl ethylene carbonate are used as an additive together with a specific amount of succinonitrile, A technique for suppressing a decrease in life characteristics due to a succinonitrile-derived film formed on the surface of the positive electrode is disclosed. In Patent Document 3, the general formula: M ⁺ [(ZY) ₂ N] ⁻ (M is a monovalent cation, Y is SO ₂ or CO, each substituent Z is independently a fluorine atom, or at least one polymerization) An organic salt which may contain a possible functional group and may be perfluorinated, and at least one of the substituents Z is a fluorine atom), an electrolyte salt, succinonitrile, etc. Is used as an additive to improve the low-temperature cycle characteristics of the battery, and Patent Document 4 describes a general formula; M ⁺ [(ZY) ₂ N] ⁻ (M, Y and Z is the same as above, and the compound represented by the formula (1) is an electrolyte salt, contains a chain carboxylic acid ester or a cyclic ether, and uses succinonitrile or the like as an additive to decompose the electrolyte or increase the viscosity of the electrolyte. And suppress In, it discloses that can improve cycle characteristics after high-temperature storage by a additive succinonitrile.

JP 2006-73513 A JP 2010-15968 A JP 2013-16456 A JP 2013-225388 A

  Thus, a technique for improving battery characteristics by adding a nitrile compound such as succinonitrile to a nonaqueous electrolytic solution has been proposed. However, when a lithium ion secondary battery equipped with a non-aqueous electrolyte containing succinonitrile or the like is operated under a high voltage condition (eg, 4.35 V or more), the decomposition reaction of succinonitrile proceeds on the surface of the positive electrode, The film derived from decomposition products grows on the surface of the positive electrode, which increases the internal resistance of the battery, lowers the initial capacity, and lowers the high-temperature storage characteristics and life characteristics of the lithium ion secondary battery. The present inventors have found out.

  The present invention has been made paying attention to the above-described circumstances, and its purpose is to increase the internal resistance of a battery when a lithium ion secondary battery is operated under a high voltage condition of 4.35V or higher. It is intended to provide a nonaqueous electrolytic solution capable of suppressing the deterioration of high temperature storage characteristics and life characteristics and a lithium ion secondary battery including the same.

The non-aqueous electrolyte solution of the present invention that has achieved the above object is the following general formula (1-1);
LiPF _a (C _m F _{2m + 1} ) _6-a (0 ≦ a ≦ 6, 1 ≦ m ≦ 2) (1-1)
And the following general formula (1-2);
LiBF _b (C _n F _{2n + 1} ) _4-b (0 ≦ b ≦ 4, 1 ≦ n ≦ 2) (1-2)
At least one electrolyte salt (1) selected from the group consisting of electrolyte salts represented by:
The following general formula (2);
(FSO ₂ ) (XSO ₂ ) NLi (2)
(In general formula (2), X represents a fluorine atom, a C1-C6 alkyl group, or a C1-C6 fluoroalkyl group.)
An electrolyte salt (2) represented by:
The following general formula (3);

(In general formula (3), R represents an alkylene group having 1 to 6 carbon atoms or an arylene group having 6 to 10 carbon atoms)

And a dinitrile compound represented by:
The electrolyte salt (2) is characterized by containing 5 mol% or more with respect to 100 mol% of the total amount of the electrolyte salt (1) and the electrolyte salt (2).

  The electrolyte salt (2) contained in the nonaqueous electrolytic solution of the present invention is a compound having a fluorine atom, an alkyl group having 1 to 3 carbon atoms, or a fluoroalkyl group having 1 to 3 carbon atoms as X in the general formula (2). Preferably there is.

In the nonaqueous electrolytic solution of the present invention, the content of the electrolyte salt (2) is 10 mol% to 90 mol% with respect to 100 mol% of the total amount of the electrolyte salt (1) and the electrolyte salt (2). preferable.
The present invention also includes a lithium ion secondary battery provided with the above non-aqueous electrolyte.

  According to the non-aqueous electrolyte of the present invention, even when a lithium ion secondary battery is operated under a high voltage condition, it is difficult for the internal resistance of the battery to increase, and the deterioration of high temperature storage characteristics and life characteristics is suppressed. can do. Therefore, according to the nonaqueous electrolytic solution of the present invention, it is possible to provide a lithium ion secondary battery in which the deterioration of the characteristics is suppressed.

It is a figure which shows the result (remaining capacity) of Experimental example 1. It is a figure which shows the result (recovery capacity) of Experimental example 1. It is a figure which shows the result (life characteristic) of the example 2 of an experiment.

1. Non-aqueous electrolyte The non-aqueous electrolyte of the present invention is the following general formula (1-1);
LiPF _a (C _m F _{2m + 1} ) _6-a (0 ≦ a ≦ 6, 1 ≦ m ≦ 2) (1-1)
And the following general formula (1-2);
LiBF _b (C _n F _{2n + 1} ) _4-b (0 ≦ b ≦ 4, 1 ≦ n ≦ 2) (1-2)
At least one electrolyte salt (1) selected from the group consisting of electrolyte salts represented by:
The following general formula (2);
(FSO ₂ ) (XSO ₂ ) NLi (2)
(In general formula (2), X represents a fluorine atom, a C1-C6 alkyl group, or a C1-C6 fluoroalkyl group.)
An electrolyte salt (2) represented by:
The following general formula (3);

(In general formula (3), R represents an alkylene group having 1 to 6 carbon atoms or an arylene group having 6 to 10 carbon atoms)

And a dinitrile compound represented by the following (hereinafter sometimes referred to as dinitrile compound (3)), and
The electrolyte salt (2) is characterized by containing 5 mol% or more with respect to 100 mol% of the total amount of the electrolyte salt (1) and the electrolyte salt (2).

  According to the non-aqueous electrolyte having the above-described configuration, the present inventors hardly increase the internal resistance of the lithium ion secondary battery even when operated under high voltage conditions, and have high-temperature storage characteristics and lifetime. The present inventors have found that the deterioration of characteristics can be suppressed and completed the present invention.

The present inventors consider the reason why the above-mentioned problem is solved by the nonaqueous electrolytic solution of the present invention as follows. The non-aqueous electrolyte of the present invention contains electrolyte salts (1) and (2) in addition to the dinitrile compound represented by the general formula (3), and the electrolyte salt (2) represented by the general formula (2) However, by decomposing on the surface of the positive electrode or coordinating with the surface of the positive electrode, a film that can withstand the operation of the lithium ion secondary battery under high voltage (not easily decomposed) is formed on the surface of the positive electrode. This coating makes it difficult for the dinitrile compound (3) to be in direct contact with the surface of the positive electrode, and the decomposition reaction of the electrolyte solution (solvent, electrolyte salt, etc.) on the surface of the positive electrode is suppressed. As a result, it is considered that even when the lithium ion secondary battery is operated under a high voltage condition, an increase in the internal resistance of the battery is suppressed and excellent battery performance is exhibited.
First, the electrolyte salt contained in the nonaqueous electrolytic solution of the present invention will be described.

1-1. Electrolyte salt (1)
The nonaqueous electrolytic solution of the present invention has a general formula (1-1); LiPF _a (C _m F _{2m + 1} ) _6-a (0 ≦ a ≦ 6, 1 ≦ m ≦ 2), and a general formula (1- 2); at least one electrolyte salt selected from the group consisting of electrolyte salts represented by LiBF _b (C _n F _{2n + 1} ) _4-b (0 ≦ b ≦ 4, 1 ≦ n ≦ 2) (1) including. By using these electrolyte salts, corrosion of the positive electrode current collector caused by the electrolyte salt (2) is suppressed.

Examples of the electrolyte salt represented by the general formula (1-1) (hereinafter sometimes referred to as the electrolyte salt (1-1)) include LiPF ₆ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiPF ₃ (C ₃ F ₇ ) ₃ , LiPF ₃ (C ₄ F ₉ ) _{3 and the} like are preferable. More preferred are LiPF ₆ and LiPF ₃ (C ₂ F ₅ ) ₃ , and more preferred is LiPF ₆ .

Examples of the electrolyte salt represented by the general formula (1-2) (hereinafter sometimes referred to as the electrolyte salt (1-2)) include LiBF ₄ , LiBF (CF ₃ ) ₃ , and LiBF (C ₂ F ₅ ) _3. , LiBF (C ₃ F ₇ ) _{3 and the} like are preferable, LiBF ₄ and LiBF (CF ₃ ) ₃ are more preferable, and LiBF ₄ is more preferable.

Electrolyte salt (1) may be used individually by 1 type, and may be used in combination of 2 or more type. Preferred electrolyte salts include LiPF ₆ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiBF _4, LiBF (CF ₃ ) ₃ , more preferably LiPF ₆ , LiPF ₃ (C ₂ F ₅ ) ₃ , More preferred is LiPF ₆ .

  The concentration of the electrolyte salt (1) in the nonaqueous electrolytic solution is preferably 0.05 mol / L or more, more preferably 0.1 mol / L or more, further preferably 0.2 mol / L or more, The saturated concentration or less is preferable, more preferably 2.5 mol / L or less, and still more preferably 2.0 mol / L or less. If the concentration of the electrolyte salt (1) in the non-aqueous electrolyte is too high, the viscosity of the non-aqueous electrolyte may increase and the ionic conductivity may decrease. On the other hand, if the concentration is too low, it may be difficult to obtain desired ionic conductivity.

1-2. Electrolyte salt (2)
The electrolyte salt (2) according to the present invention is represented by the general formula (2); (FSO ₂ ) (XSO ₂ ) NLi. In General Formula (2), X represents a fluorine atom, a C1-C6 alkyl group, or a C1-C6 fluoroalkyl group. The alkyl group having 1 to 6 carbon atoms is preferably a linear or branched alkyl group, and more preferably a linear alkyl group. Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include those in which some or all of the hydrogen atoms of the alkyl group having 1 to 6 carbon atoms are substituted with fluorine atoms. Examples thereof include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, and a pentafluoroethyl group. Among these, a fluorine atom, a C1-C3 alkyl group, and a C1-C3 fluoroalkyl group are preferable as X.

  Specific examples of the electrolyte salt (2) include lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (methylsulfonyl) imide, lithium (fluorosulfonyl) (ethylsulfonyl) imide, lithium (fluorosulfonyl) (propyl) Examples include sulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide, lithium (fluorosulfonyl) (heptafluoropropylsulfonyl) imide, and the like. Preferred are lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide, lithium (fluorosulfonyl) (heptafluoropropylsulfonyl) imide More preferred are lithium bis (fluorosulfonyl) imide and lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide.

  In the nonaqueous electrolytic solution of the present invention, one type of electrolyte salt (2) may be contained alone, or two or more types of electrolyte salt (2) may be contained. As the electrolyte salt (2), a commercially available product may be used, or a product synthesized by a conventionally known method may be used.

  The nonaqueous electrolytic solution of the present invention contains 5 mol% or more of the electrolyte salt (2) with respect to 100 mol% of the total amount of the electrolyte salt (1) and the electrolyte salt (2). Preferably it is at least 7 mol%, more preferably at least 10 mol%, even more preferably at least 15 mol%, preferably at most 90 mol%, more preferably at most 85 mol%, still more preferably at most 80 mol%. It is. If the content of the electrolyte salt (2) in the non-aqueous electrolyte is too large, the viscosity of the non-aqueous electrolyte may increase and the ionic conductivity may decrease, and the positive electrode collector due to the electrolyte salt (2). There is a risk of corrosion of the electric body. On the other hand, if the content of the electrolyte salt (2) is too small, a coating film is not sufficiently formed on the surface of the positive electrode, and decomposition of the electrolytic solution on the surface of the positive electrode may be difficult to be suppressed.

  Further, the nonaqueous electrolytic solution of the present invention has a total concentration of 0.2 mol / L or more and a saturated concentration of all the electrolyte salts including the electrolyte salts (1) and (2) contained in the nonaqueous electrolytic solution. It is preferable to use it in the following range. More preferably 0.4 mol / L or more, still more preferably 0.8 mol / L or more, more preferably 3.0 mol / L or less, still more preferably 2.0 mol / L or less, and still more preferably Is 1.5 mol / L or less.

  The concentration of the electrolyte salt (2) in the nonaqueous electrolytic solution is preferably 0.02 mol / L or more, more preferably 0.04 mol / L or more, further preferably 0.08 mol / L or more, It is preferable that it is 2.7 mol / L or less, More preferably, it is 1.8 mol / L or less, More preferably, it is 1.4 mol / L or less. If the concentration of the electrolyte salt (2) is too high, the positive electrode current collector may be corroded. On the other hand, if the concentration of the electrolyte salt (2) is too low, a coating film is not sufficiently formed on the surface of the positive electrode. There is a possibility that the decomposition of the electrolyte solution generated on the surface may not be sufficiently suppressed.

1-3. Other Electrolyte Salts The nonaqueous electrolytic solution of the present invention may contain other electrolyte salts different from the electrolyte salts (1) and (2). As such other electrolyte salt, a conventionally known electrolyte salt can be used, and examples thereof include trifluoromethanesulfonate ion, perchlorate ion, fluoroarsenate ion, AlCl ₄ ⁻ , C [(CN) ₃ ] ⁻ , N [(CN) ₂ ] ⁻ , C [(CF ₃ SO ₂ ) ₃ ] ⁻ , SbF ₆ ^−, dicyanotriazolate ion (DCTA) and the like, such as a salt of an inorganic cation or an organic cation. Examples include electrolyte salts.
Other electrolyte salts may be used in a range where all electrolyte salt concentrations in the non-aqueous electrolyte are within the above range.

1-4. Dinitrile compound (3)
The nonaqueous electrolytic solution of the present invention contains a dinitrile compound represented by the following general formula (3).

In general formula (3), R represents an alkylene group having 1 to 6 carbon atoms or an arylene group having 6 to 10 carbon atoms.

  The dinitrile compound (3) is decomposed during operation of the lithium ion secondary battery, and the decomposition product forms a film on the surface of the positive electrode. This coating has an effect of suppressing the oxidative decomposition of the electrolyte and additives contained in the electrolytic solution on the surface of the positive electrode active material. Further, the dinitrile compound (3) has an effect of suppressing elution of copper when exposed to an overdischarged state by coordinating with a copper current collector used for a negative electrode of a lithium ion secondary battery.

  The alkylene group may have any of linear, branched, and cyclic structures, and may have two or more of these structures. A linear alkylene group is preferred. Carbon number of an alkylene group becomes like this. Preferably it is 1-4, More preferably, it is 1-3.

  Specific alkylene groups include methylene group, ethylene group, n-propylene group, iso-propylene group, n-butylene group, iso-butylene group, sec-butylene group, tert-butylene group, pentylene group, hexylene group, Examples include a cyclohexylene group. Among these, a methylene group, an ethylene group, and an n-propylene group are preferable, and a methylene group and an ethylene group are more preferable.

  Examples of the arylene group include a phenylene group, a (2,4-, 2,5-) tolylene group, and a benzylene group. A phenylene group and a (2,4-, 2,5-) tolylene group are preferable, and a phenylene group is more preferable.

  Specific examples of the dinitrile compound (3) include malononitrile, succinonitrile, glutaronitrile, adiponitrile, pimonitrile, suberonitrile, 2-methylmalononitrile, dimethylmalononitrile, 2,3-dimethylsuccinodinitrile, tetramethylsc Chain dinitrile compounds such as sinonitrile and 2-methylglutaronitrile; 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, 1,4-dicyanonaphthalene, 2,4-dicyanotoluene And aromatic dinitrile compounds such as Among these, a chain dinitrile compound is preferable, succinonitrile, glutaronitrile, and adiponitrile are more preferable, and succinonitrile and adiponitrile are more preferable from the viewpoint of availability and ease of preparation of the nonaqueous electrolytic solution. In addition, a dinitrile compound (3) may be used individually by 1 type, and may be used in combination of 2 or more type.

  The dinitrile compound (3) is 100% by mass in the non-aqueous electrolyte (electrolyte salt (1), (2), other electrolyte salt optionally used, solvent described later, and optionally used additive). The same applies hereinafter), preferably 0.02% by mass or more, more preferably 0.05% by mass or more, still more preferably 0.1% by mass or more, and 5.0% by mass or less. More preferably, it is 4.0 mass% or less, More preferably, it is 3.0 mass% or less. Even if the dinitrile compound (3) is used in a large amount, an effect commensurate with the amount of use cannot be obtained, but rather the battery performance is lowered because the film is excessively formed on the positive electrode surface and the internal resistance of the battery is increased. There is. On the other hand, if the concentration of the dinitrile compound (3) is too low, the effect derived from the dinitrile compound may be difficult to obtain.

1-5. Solvent The nonaqueous electrolytic solution of the present invention may contain a solvent. Solvents that can be used in the nonaqueous electrolytic solution of the present invention can dissolve and disperse the electrolyte salts (1), (2), the dinitrile compound (3), and other electrolyte salts and additives that are optionally used. There are no particular limitations as long as it is a non-aqueous solvent, a polymer used in place of the solvent, a medium such as a polymer gel, and the like, and any conventionally known solvent used in lithium ion secondary batteries can be used.

  As a non-aqueous solvent, a solvent having a large dielectric constant, high solubility of the electrolyte salts (1), (2) and the dinitrile compound (3), a boiling point of 60 ° C. or higher, and a wide electrochemical stability range Is preferred. 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 a polymer or polymer gel is used instead of a solvent, the following method may be employed. That is, a method in which a solution in which an electrolyte salt or the like is dissolved in the above-mentioned non-aqueous solvent is dropped onto a polymer formed by a conventionally known method to impregnate and carry the electrolyte salt and the non-aqueous solvent; A method in which a polymer and an electrolyte salt, etc. are melted and mixed at a temperature, and then formed into a film and impregnated with a non-aqueous solvent (hereinafter referred to as a gel electrolyte); A method of forming a film by casting or coating and then volatilizing an organic solvent; melting a polymer and an electrolyte salt, etc. at a temperature higher than the melting point of the polymer, mixing and molding (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.

1-6. Other Components The nonaqueous electrolytic solution of the present invention may contain an additive for the purpose of improving various characteristics of the lithium ion secondary battery.
As additives, cyclic carbonates having unsaturated bonds such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), methyl vinylene carbonate (MVC), ethyl vinylene carbonate (EVC); fluoroethylene carbonate, trifluoropropylene carbonate, Carbonate compounds such as phenylethylene carbonate and erythritan carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetra Carboxylic anhydrides such as carboxylic dianhydride and phenyl succinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methane sulfone Sulfur-containing compounds such as methyl, busulfan, sulfolane, sulfolene, dimethylsulfone, tetramethylthiuram monosulfide, trimethylene glycol sulfate; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2- Nitrogen-containing compounds such as oxazolidinone, 1,3-dimethyl-2-imidazolidinone and N-methylsuccinimide; phosphates such as monofluorophosphate and difluorophosphate; saturated carbonization such as heptane, octane and cycloheptane A hydrogen compound; and the like.

  The additive is preferably used in a concentration range of 0.1% by mass to 10% by mass in the non-aqueous electrolyte of the present invention (more preferably 0.2% by mass to 8% by mass, and even more preferably 0%). .3 mass% to 5 mass%). 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.

2. Lithium ion secondary battery The lithium ion secondary battery of the present invention is a positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, And a non-aqueous electrolyte. 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 lithium ion secondary battery of the present invention includes the above-described nonaqueous electrolytic solution of the present invention.

2-1. Positive electrode The positive electrode is formed by supporting a positive electrode mixture containing a positive electrode active material, a conductive additive, a binder and the like on a positive electrode current collector, and is usually formed into a sheet shape.

  The method for producing the positive electrode is not particularly limited. For example, (i) a positive electrode active material composition in which a positive electrode mixture is dissolved or dispersed in a dispersion solvent is applied to a positive electrode current collector by a doctor blade method or the like. A method of immersing the current collector in the positive electrode active material composition and then drying; (ii) bonding a sheet obtained by kneading and drying the positive electrode active material composition to the positive electrode current collector through a conductive adhesive (Iii) A positive electrode active material composition added with a liquid lubricant is applied or cast onto a positive electrode current collector to form a desired shape, and then the liquid lubricant is removed. Then, the method of extending | stretching to a uniaxial or multiaxial direction; etc. are mentioned. Moreover, you may pressurize the positive mix layer after drying as needed. Thereby, the adhesive strength with the positive electrode current collector is increased, and the electrode density is also increased.

  The material used for the positive electrode current collector, the positive electrode active material, the conductive auxiliary agent, the binder and the positive electrode active material composition (solvent for dispersing or dissolving the positive electrode mixture) is not particularly limited. Use it. For example, each material described in JP 2014-13704 A can be used.

  The amount of the positive electrode active material used is preferably 75 parts by mass or more and 99 parts by mass or less with respect to 100 parts by mass of the positive electrode mixture, more preferably 85 parts by mass or more, and further preferably 90 parts by mass or more. Yes, preferably 98 parts by mass or less, more preferably 97 parts by mass or less.

  In the case of using a conductive additive, the content of the conductive additive in the positive electrode mixture is preferably 0.1% by mass to 10% by mass with respect to 100% by mass of the positive electrode mixture (more preferably Preferably 0.5% by mass to 10% by mass, more preferably 1% by mass to 10% by mass). If the amount of the conductive assistant is too small, the conductivity is extremely deteriorated and the load characteristics and the discharge capacity may be deteriorated. On the other hand, when the amount is too large, the bulk density of the positive electrode mixture layer is increased, and the content of the binder needs to be further increased.

In the case of using the binder, the content of the binder in the positive electrode mixture is preferably 0.1% by mass to 10% by mass with respect to 100% by mass of the positive electrode mixture (more preferably 0.5% by mass). % To 9% by mass, more preferably 1% to 8% by mass). If the amount of the binder is too small, good adhesion cannot be obtained, and the positive electrode active material and the conductive additive may be detached from the current collector. On the other hand, if the amount is too large, the internal resistance may be increased, and the battery characteristics may be adversely affected.
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.

2-2. Negative electrode The negative electrode is formed by supporting a negative electrode active material, a binder, and, if necessary, a negative electrode mixture containing a conductive auxiliary agent on a negative electrode current collector, and is usually formed into a sheet shape.
As a manufacturing method of the negative electrode, a method similar to the manufacturing method of the positive electrode can be employed. In addition, the same conductive auxiliary agent, binder, and material dispersing solvent as used in the positive electrode are used in the production of the negative electrode.

  As the material of the negative electrode current collector and the negative electrode active material, a conventionally known negative electrode active material may be used. For example, each material described in JP-A-2014-13704 can be used.

2-3. Separator The separator is disposed so as to separate the positive electrode and the negative electrode. There is no restriction | limiting in particular in a separator, In this invention, all the conventionally well-known separators can be used, For example, each material as described in Unexamined-Japanese-Patent No. 2014-13704 can be used.

2-4. 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.

  The shape of the lithium ion secondary battery according to the present invention is not particularly limited, and any conventionally known shape can be used as the shape of the lithium ion secondary battery, such as a cylindrical shape, a square shape, a laminate shape, a coin shape, and a large size. Can do. 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. .

  The rated charging voltage of the lithium ion secondary battery of the present invention is not particularly limited, but is preferably 4.35 V or more. The effect of the present invention is particularly remarkable when used at a voltage of 4.35 V or higher. More preferably, it is 4.4 V or more. The higher the rated charge voltage, the higher the energy density, but if it is too high, it may be difficult to ensure safety and long-term reliability of battery characteristics. Therefore, the rated charging voltage is preferably 4.6 V or less. More preferably, it is 4.5 V or less.

  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 solution In a non-aqueous solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., Table 1 shows the electrolyte (1)), lithium bis (fluorosulfonyl) imide (LiFSI, manufactured by Nippon Shokubai Co., Ltd., electrolyte (2)), and succinonitrile (manufactured by Kishida Chemical Co., Ltd., dinitrile compound (3)). Non-aqueous electrolytes 1 to 3 were prepared by dissolving to the indicated concentration.

2. Production of Laminate Type Lithium Ion Secondary Battery LiCoO ₂ as a positive electrode active material, acetylene black as a conductive auxiliary agent, PVdF (polyvinylidene fluoride) as a binder, positive electrode active material: conductive auxiliary agent: binder = 93: A positive electrode mixture slurry mixed at a mass ratio of 4: 3 and dispersed in NMP (N-methylpyrrolidone) is coated on both sides of an aluminum foil (positive electrode current collector), dried and compressed to produce a positive electrode sheet. did.

  Artificial graphite (trade name “MAG-E” manufactured by Hitachi Chemical Co., Ltd.) as the negative electrode active material, carbon black as the conductive additive, and CMC (carboxymethylcellulose) and SBR (styrene-butadiene rubber) as the binder (binder) Was mixed in a ratio of negative electrode active material: conductive auxiliary agent: binder = 100: 1.1: 1.5 (mass ratio), and this was mixed with pure water to prepare a slurry solution. This negative electrode mixture slurry was coated on one side on a copper foil (negative electrode current collector), dried and compressed to prepare a negative electrode sheet.

The active material coated surface of the positive electrode sheet (electrode area 12 cm ² (3 cm × 4 cm)) and the active material coated surface of the negative electrode sheet (electrode area 13.44 cm ² (3.2 cm × 4.2 cm)) are opposed to each other. A polyethylene separator (film thickness 20 μm) was sandwiched between them. The laminate laminated in the order of the negative electrode sheet, the separator, and the positive electrode sheet is sandwiched between two aluminum laminates, and the inside of the aluminum laminate film is filled with 0.7 mL of non-aqueous electrolyte 1 and sealed in a vacuum state, and the capacity is 32 mAh. A laminate type lithium ion secondary battery 1 was produced.
Laminated lithium ion secondary batteries 2 and 3 were produced in the same manner as described above except that the nonaqueous electrolytic solution 2 or 3 was used.

  The laminate type lithium ion secondary batteries 1 to 3 were charged for 60 minutes at a current value of 0.2 C using a charge / discharge test apparatus (“ACD-01” manufactured by Asuka Electronics Co., Ltd.), and then at 25 ° C. for 3 days. Then, the battery was further charged at a constant current and a constant voltage for 5 hours up to 4.4 V at a current value of 0.5 C. After 10 minutes of rest, the battery was discharged to 2.75 V at a current value of 0.2C. After discharging, the aluminum laminate film was opened for degassing and resealed in vacuum to complete laminated lithium ion secondary batteries 1 to 3.

3. Battery Evaluation Experimental Example 1: High temperature storage characteristics Each laminate obtained by using non-aqueous electrolytes 1 to 3 at 25 ° C. using a charge / discharge test apparatus (“ACD-01” manufactured by Asuka Electronics Co., Ltd.) The type lithium ion secondary battery was charged at a current value of 0.5 C at a constant voltage of 4.4 V until the current dropped to 0.02 C, and then discharged at a constant current of 0.2 C to 2.75 V.

  Next, each laminated lithium ion secondary battery was subjected to constant current / constant voltage charging for 5 hours at 25 ° C. until the voltage reached 4.4 V at a charging rate of 0.5 C, and then the laminated lithium ion secondary battery. The battery was stored at 60 ° C. for 2 weeks. The laminated lithium ion secondary battery after storage was returned to 25 ° C., and after 2 hours had elapsed, discharge was performed at a discharge rate of 0.2 C until the voltage reached 2.75 V. The discharge capacity at this time was defined as the remaining capacity after high-temperature storage. The results are shown in FIG.

  After charging the laminate type lithium ion secondary battery again under the above conditions (0.5C current value, 4.4V constant voltage, charging until 0.02C current), then 0.2C to 2.75V A constant current discharge was performed. The capacity at this time was defined as a recovery capacity. The results are shown in FIG.

  1 and 2, both the remaining capacity and the recovery capacity of the batteries of Examples 1 and 2 provided with the non-aqueous electrolytes 1 and 2 containing the electrolyte salt (2) and the dinitrile compound (3) are the electrolyte salt (2). The value was higher than that of the battery of Comparative Example 1 that did not contain. From this result, by using the dinitrile compound (3) together with the electrolyte salt (2), it is possible to suppress an increase in the internal resistance of the battery even when stored in a high temperature (60 ° C.) environment. As a result, a high voltage (4. 4V) It can be seen that even when operated under the conditions, it is possible to suppress a decrease in the remaining capacity and the recovery capacity.

Experimental Example 2: Lifetime characteristics For the laminated lithium ion secondary batteries 1 to 3, a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.) was used, and the temperature was 4C at a charge rate of 1C. .4V constant current constant voltage charging is performed until the amount of current reaches 0.02C, discharging is performed at a discharge rate of 1C until the voltage reaches 2.75V, and a charging / discharging pause time of 10 minutes is provided for each charging / discharging. A life characteristic test was conducted. The discharge capacity in each cycle is shown in FIG.

From FIG. 3, the battery of Comparative Example 1 had a lower initial capacity and inferior life characteristics compared to Examples 1 and 2. Since the non-aqueous electrolyte used in Comparative Example 1 did not contain the electrolyte salt (2), the inner surface of the battery was decomposed by the decomposition of the dinitrile compound (3) and the growth of the coating derived from the decomposition product on the positive electrode surface. This is thought to be due to an increase in resistance. On the other hand, from the results of Examples 1 and 2, the combined use of the electrolyte salt (2) and the dinitrile compound (3) reduces the initial capacity derived from the dinitrile compound (3) and deteriorates the life characteristics. It can be seen that it can be suppressed.
When a lithium ion secondary battery equipped with an electrolytic solution using the dinitrile compound (3) is operated under high voltage conditions (4.35 V or higher), the components of the electrolytic solution including the dinitrile compound (3) There was a problem that the growth of the coating derived from the decomposition reaction and the decomposition product proceeded on the surface of the positive electrode, and the performance of the lithium ion secondary battery was deteriorated. As described above, the electrolyte together with the dinitrile compound (3) According to the non-aqueous electrolyte of the present invention using the salt (2), the above problem is solved by protecting the surface of the positive electrode. As a result, a lithium ion secondary battery having excellent high-temperature storage characteristics and life characteristics can be obtained. Can be provided.

Claims (3)

The following general formula (1-1);
LiPF _a (C _m F _{2m + 1} ) _6-a (0 ≦ a ≦ 6, 1 ≦ m ≦ 2) (1-1)
And the following general formula (1-2);
LiBF _b (C _n F _{2n + 1} ) _4-b (0 ≦ b ≦ 4, 1 ≦ n ≦ 2) (1-2)
At least one electrolyte salt (1) selected from the group consisting of electrolyte salts represented by:
The following general formula (2);
(FSO ₂ ) (XSO ₂ ) NLi (2)
(In general formula (2), X represents a fluorine atom, a C1-C6 alkyl group, or a C1-C6 fluoroalkyl group.)
An electrolyte salt (2) represented by:
The following general formula (3);

(In general formula (3), R represents an alkylene group having 1 to 6 carbon atoms or an arylene group having 6 to 10 carbon atoms)
And a dinitrile compound represented by:
A nonaqueous electrolytic solution, wherein the content of the electrolyte salt (2) is 10 mol% to 90 mol% with respect to 100 mol% of the total amount of the electrolyte salt (1) and the electrolyte salt (2).   The non-aqueous solution according to claim 1, wherein the electrolyte salt (2) is a compound having a fluorine atom, an alkyl group having 1 to 3 carbon atoms, or a fluoroalkyl group having 1 to 3 carbon atoms as X in the general formula (2). Electrolytic solution. Lithium-ion secondary battery, comprising a non-aqueous electrolyte according to claim 1 or 2.