JP6669506B2 - 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.

  Lithium ion secondary batteries are used as power supplies for smartphones and personal computers, and also as power supplies for automobiles. For batteries used in these applications, researches aimed at improving various characteristics such as higher output, higher energy density, and improved cycle characteristics and rate characteristics have been repeated.

  For example, as a technique for attempting to improve characteristics of a lithium ion secondary battery, a technique of adding various compounds to an electrolyte has been proposed. For example, Patent Literature 1 discloses a technique in which lithium bisoxalate borate (LiBOB) is added to a non-aqueous electrolyte to suppress deterioration in charge / discharge cycle life characteristics and battery swelling when left at high temperatures. Patent Literature 2 also describes a technique for improving cycle characteristics and durability by adding LiBOB to a non-aqueous electrolyte. Patent Literature 3 discloses a technique of adding boric acid and LiBOB to improve charge / discharge cycle performance.

JP 2006-216378 A JP 2013-89445 A JP 2014-216127 A

  As described above, various studies have been made on the improvement of the battery characteristics. However, with the expansion of applications, batteries are required to have higher energy density. In order to increase the energy density of a battery, it is necessary to drive the battery under a high voltage. The present applicant has been consistently studying a battery provided with an electrolytic solution having a sulfonylimide anion compound.However, when the battery is driven under a high voltage, the electrolytic solution is decomposed at the positive electrode or the positive electrode active material is decomposed. There is a problem that the transition metal is eluted and the capacity and rate characteristics of the battery are reduced. The above prior art does not describe a solution to these problems. Further, these problems become particularly remarkable when the lithium ion secondary battery is stored at a high temperature.

  The present invention has been made in view of the circumstances as described above, and its purpose is to prevent decomposition of a sulfonylimide compound even when stored at high temperatures or when used under high voltage, It is an object of the present invention to provide a non-aqueous electrolyte capable of suppressing a decrease in capacity and rate characteristics as a battery, and a lithium ion secondary battery using the same.

The non-aqueous electrolyte of the present invention that can achieve the above object is represented by the general formula (1) [N (XSO ₂ ) (FSO ₂ )] ^- (wherein X is a fluorine atom and carbon number An alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms);
A lithium cation,
A compound represented by the following general formula (2):

(In the general formula (2), M is B or P, A ^{a +} is a metal ion, hydrogen ion, or onium ion, a is 1 to 3, b is 1 to 3, p is b / a, and m is 1 to 3. , N represents 0 to 4, q represents 0 or 1, R ¹ represents a C ₁ -C ₁₀ alkylene group or a C ₁ -C ₁₀ halogenated alkylene group, R ² represents fluorine or a C ₁ -C ₁₀ The fluorinated alkyl group, X ¹ and X ² each independently represent O or S.)
It is characterized by including the medium.

  The non-aqueous electrolyte preferably contains 0.01 to 5% by mass of the compound represented by the general formula (2), and the compound represented by the general formula (2) is a bisoxalyl borate salt. Is also preferred.

Further, the non-aqueous electrolyte contains at least one anion selected from the group consisting of an anion represented by the following general formulas (7) and (8) and a hexafluoroarsenate ion (AsF ₆ ⁻ ). May be.
[PF _l (C _m F _{2m + 1} ) _6-l ] ^- (0 ≦ l ≦ 6, 1 ≦ m ≦ 4) (7)
_{[BF n (C o F 2o} + 1) 4-n] - (0 ≦ n ≦ 4,1 ≦ o ≦ 4) (8)

  The non-aqueous electrolyte of the present invention is preferably used for a lithium ion secondary battery having a rated charging voltage of 4.2 V or more. The present invention also includes a lithium ion secondary battery provided with the non-aqueous electrolyte of the present invention.

  The nonaqueous electrolytic solution of the present invention can suppress the decomposition of a compound having a sulfonylimide anion under high pressure. Further, elution of the transition metal from the positive electrode can be suppressed. For this reason, the lithium ion secondary battery having the nonaqueous electrolyte of the present invention is effective at suppressing a decrease in capacity and rate characteristics after high-temperature storage at high voltage. Therefore, according to the non-aqueous electrolyte of the present invention, it is possible to provide a lithium ion secondary battery in which a decrease in capacity and rate characteristics is suppressed.

1. Non-Aqueous Electrolyte The non-aqueous electrolyte of the present invention is represented by the general formula (1): [N (XSO ₂ ) (FSO ₂ )] ⁻ (In the general formula (1), X represents a fluorine atom and a carbon number of 1 to 1.) An alkyl group of 6 or a fluoroalkyl group of 1 to 6 carbon atoms) (hereinafter referred to as sulfonylimide anion (1)), a lithium cation, and a compound represented by the above general formula (2). It is characterized in that it contains the compound to be prepared.

1-1. Sulfonylimide anion (1)
The sulfonylimide anion (1) according to the present invention is represented by the general formula (1): [N (XSO ₂ ) (FSO ₂ )] ⁻ . In the general formula (1), 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 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. As a C1-C6 fluoroalkyl group, the thing in which some or all of the hydrogen atom which a C1-C6 alkyl group has is substituted by the fluorine atom is mentioned. Specific examples include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, and a pentafluoroethyl group. Among them, a fluorine atom, a trifluoromethyl group and a pentafluoroethyl group are preferable as X.

  Specific examples of the sulfonylimide anion (1) include bis (fluorosulfonyl) imide anion, (fluorosulfonyl) (methylsulfonyl) imide anion, (fluorosulfonyl) (ethylsulfonyl) imide anion, and (fluorosulfonyl) (tri (Fluoromethylsulfonyl) imide anion, (fluorosulfonyl) (pentafluoroethylsulfonyl) imide anion, and the like. Preferred are a bis (fluorosulfonyl) imide anion, a (fluorosulfonyl) (trifluoromethylsulfonyl) imide anion and a (fluorosulfonyl) (pentafluoroethylsulfonyl) imide anion, more preferably a bis (fluorosulfonyl) imide anion, Fluorosulfonyl) (trifluoromethylsulfonyl) imide anion.

  The non-aqueous electrolyte of the present invention may contain one kind of sulfonylimide anion (1) alone, or may contain two or more kinds of sulfonylimide anions (1).

  In the non-aqueous electrolyte of the present invention, the sulfonylimide anion (1) is dissociated from the cation, but before being dispersed and dissolved in the non-aqueous electrolyte, the state of the salt ion-bonded to the cation is present. Exists in. The electrolyte solution of the present invention may contain other cations as long as the lithium cation is essential, and may be any of inorganic cations and organic cations other than lithium.

  The sulfonylimide compound containing the sulfonylimide anion (1) can function as an electrolyte salt in the nonaqueous electrolytic solution of the present invention, or can function as an additive for improving the performance of a battery. It is possible. That is, when the sulfonylimide compound containing the sulfonylimide anion (1) is, for example, a lithium salt, it can function as an electrolyte salt of a lithium battery electrolyte.

Examples of the inorganic cation include monovalent inorganic cations such as Li ⁺ , Na ⁺ , K ⁺ , Cs ⁺ , and Pb ⁺ ; Mg ²⁺ , Ca ²⁺ , Zn ²⁺ , Pd ²⁺ , Sn ²⁺ , and Hg ²⁺ , Rh ²⁺ , Cu ²⁺ , Be ²⁺ , Sr ²⁺ , Ba ^{2+ and the} like; and trivalent inorganic cations such as Ga ³⁺ . Among these, alkali metal cations and alkaline earth metal cations are preferable, and Li ⁺ , Na ⁺ , Mg ^2+, and Ca ²⁺ are more preferable because they have a small ionic radius and are easily used in batteries and the like, and Li ⁺ is particularly preferable.

As the organic cation, general formula (3): L ⁺ -R _s (where L represents C, Si, N, P, S or O, R is the same or different organic group, and is bonded to each other. S represents the number of R bonded to L and is 3 or 4. In addition, s is a value determined by the valence of the element L and the number of double bonds directly bonded to L. The onium cation represented by) is preferred.

  The “organic group” represented by the above R means a group having at least one hydrogen atom, fluorine atom, or carbon atom. The “group having at least one carbon atom” may be any group as long as it has at least one carbon atom, and may have another atom such as a halogen atom or a hetero atom, or a substituent. Good. Examples of the substituent include an amino group, an imino group, an amide group, a group having an ether bond, a group having a thioether bond, an ester group, a hydroxyl group, an alkoxy group, a carboxyl group, a carbamoyl group, a cyano group, a disulfide group, and a nitro group. Group, nitroso group, sulfonyl group and the like.

  Among the onium cations, chain quaternary ammonium such as tetraethylammonium, tetrabutylammonium and triethylmethylammonium; chain tertiary ammonium such as triethylammonium, tributylammonium, dibutylmethylammonium and dimethylethylammonium; 1-ethyl- Imidazolium such as 3-methylimidazolium and 1,2,3-trimethylimidazolium; and pyrrolidinium such as N, N-dimethylpyrrolidinium and N-ethyl-N-methylpyrrolidinium are more preferred because they are easily available. . More preferred are quaternary ammonium and imidazolium. From the viewpoint of reduction resistance, quaternary ammoniums such as tetraethylammonium, tetrabutylammonium, and triethylmethylammonium, which are classified as the chain onium cations, are more preferable.

  Examples of the sulfonylimide compound (compound having the sulfonylimide anion (1)) that dissociates in the nonaqueous electrolyte to generate the sulfonylimide anion (1) include the above-described sulfonylimide anion (1) and lithium, and if necessary, other compounds. Any combination with a cation may be used. For example, lithium bis (fluorosulfonyl) imide (LiFSI), lithium (fluorosulfonyl) (methylsulfonyl) imide, lithium (fluorosulfonyl) (ethylsulfonyl) imide, lithium (fluorosulfonyl) (Trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide, triethylmethylammonium bis (fluorosulfonyl) imide, tetraethylammonium bis (fluorosulfonyl) Imide, 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide, tetraethylammonium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, 1-ethyl-3-methylimidazolium (fluorosulfonyl) (trifluoromethylsulfonyl) ) Imide and the like.

  As the sulfonylimide compound, one type may be used alone, or two or more types may be used in combination. As the sulfonylimide compound, a commercially available product may be used, or a compound synthesized by a conventionally known method may be used.

1-2. Other anions The non-aqueous electrolyte of the present invention further comprises at least one anion represented by the following general formulas (7) and (8) and at least one selected from the group consisting of hexafluoroarsenate ions (AsF ₆ ⁻ ). Species anions may be included.
[PF _l (C _m F _{2m + 1} ) _6-l ] ^- (0 ≦ l ≦ 6, 1 ≦ m ≦ 4) (7)
_{[BF n (C o F 2o} + 1) 4-n] - (0 ≦ n ≦ 4,1 ≦ o ≦ 4) (8)

Examples of the anion represented by the general formula (7) (hereinafter sometimes referred to as anion (7)) include PF ₆ ⁻ , PF ₃ (CF ₃ ) ₃ ⁻ , PF ₃ (C ₂ F ₅ ) ₃ ⁻ , Preferred are PF ₃ (C ₃ F ₇ ) ₃ ⁻ and PF ₃ (C ₄ F ₉ ) ₃ ⁻ . More preferably ^{_{PF 6 -, PF 3 (C}} 2 F 5) 3 - and, even more preferably PF ₆ ^- is.

Examples of the anion represented by the general formula (8) (hereinafter sometimes referred to as anion (8)) include BF ₄ ⁻ , BF (CF ₃ ) ₃ ⁻ , BF (C ₂ F ₅ ) ₃ ⁻ , and BF ( C ₃ F ₇ ) _3- ^{and the} like are preferred, BF ₄ ^- and BF (CF ₃ ) ₃ ^- are more preferred, and BF ₄ ^- is more preferred.

In the nonaqueous electrolyte, the content of the sulfonylimide anion (1) is the same as that of the sulfonylimide anion (1), the anion represented by the general formulas (7) and (8), and the hexafluoroarsenate ion (AsF). ₆ ^- in total 100 mol% of), is preferably at least 20 mol%. Since the present invention suppresses the decomposition of the compound having a sulfonylimide anion, when the content of the sulfonylimide anion is less than 20 mol% of the total anion amount, the compound represented by the general formula (2) is added. This is because it is difficult to achieve the effect. The content of the sulfonylimide anion is more preferably 30 mol% or more, more preferably 50 mol% or more of the total anion amount.

The non-aqueous electrolyte of the present invention may further contain other anions. Other anions include trifluoromethanesulfonic acid ion (CF ₃ SO ₃ ⁻ ), perchlorate ion (ClO ₄ ⁻ ), tetracyanoborate ion ([B (CN) ₄ ] ⁻ ), tetrachloroaluminum ion ( AlCl ₄ ⁻ ), tricyanomethide ion (C [(CN) ₃ ] ⁻ ), dicyanamide ion (N [(CN) ₂ ] ⁻ ), tris (trifluoromethanesulfonyl) methide ion (C [(CF ₃ SO ₂ ) ₃ ] ⁻ ), hexafluoroantimonate ion (SbF ₆ ⁻ ), and dicyanotriazolate ion (DCTA).

  These anions can be used by being included in the non-aqueous electrolyte as an electrolyte salt in combination with the cation exemplified as the counter cation of the sulfonylimide anion.

The electrolyte salt of the non-aqueous electrolyte solution of the present invention (a compound having an imide anion, a compound having the anion (7), a compound having an anionic (8), AsF ₆ ^- compounds with and other electrolyte salt) total amount of Is not particularly limited, but is preferably 0.6 mol / L or more, more preferably 0.8 mol / L or more, still more preferably 1.0 mol / L or more, and 5.0 mol / L or less. It is preferably 3.0 mol / L or less, more preferably 2.0 mol / L or less. If the concentration of the electrolyte salt 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, the battery performance may be poor.

1-3. Lithium cation The non-aqueous electrolyte of the present invention contains a lithium cation. The lithium cation may be derived from a sulfonylimide compound, or may be derived from the above-mentioned electrolyte salt. The concentration of lithium cation contained in the non-aqueous electrolyte of the present invention is preferably 0.6 mol / L or more, more preferably 0.8 mol / L or more, and further preferably 1.0 mol / L or more. . If the lithium cation concentration is too high, the viscosity of the electrolytic solution may increase and the ionic conductivity may decrease. Therefore, it is preferable to use the lithium cation at a saturation concentration or lower. It is more preferably at most 5.0 mol / L, even more preferably at most 2.0 mol / L.

  Here, the “concentration of lithium cation in the non-aqueous electrolyte” is a value based on the total amount of lithium cations contained in the non-aqueous electrolyte of the present invention. For example, an electrolyte salt having two or more lithium cations may be used. When used, the total amount of lithium cations generated from the used electrolyte salt, and when the sulfonylimide compound contains a lithium cation, the lithium cation contained in the sulfonylimide compound and the electrolyte salt used in combination The value obtained based on the total amount of lithium cation and lithium cation is defined as the concentration of lithium cation in the non-aqueous electrolyte. When a lithium cation is not contained in the sulfonylimide compound, the total of the concentrations of the electrolyte salt (containing lithium cation) used in combination may be considered as the lithium cation concentration.

1-4. Compound represented by general formula (2) The non-aqueous electrolyte solution of the present invention includes a compound represented by the following general formula (2).

(In the general formula (2), M is B or P, A ^{a +} is a metal ion, hydrogen ion, or onium ion, a is 1 to 3, b is 1 to 3, p is b / a, and m is 1 to 3. , N represents 0 to 4, q represents 0 or 1, R ¹ represents a C ₁ -C ₁₀ alkylene group or a C ₁ -C ₁₀ halogenated alkylene group, R ² represents fluorine or a C ₁ -C ₁₀ The fluorinated alkyl group, X ¹ and X ² each independently represent O or S.)

  Lithium-ion secondary batteries using an electrolyte solution containing a sulfonylimide compound represented by LiFSI have been found to exhibit reduced rate characteristics after storage when subjected to a high-voltage storage test at a high voltage exceeding 4.2 V and a high temperature. Was. The present inventors have studied and found that, because LiFSI improves the activity of the positive electrode, the electrolyte solution decomposes near the positive electrode, decomposition products are deposited on the negative electrode, and the opposing separator may be clogged. It was found that this was the cause of the decrease in rate characteristics. Furthermore, as a result of continued study, it was found that the addition of the compound represented by the general formula (2) to the electrolytic solution suppressed the deposition of decomposition products on the negative electrode. This is because the compound represented by the general formula (2) undergoes reductive decomposition at the negative electrode at the time of the first charge and forms a film on the negative electrode, so that the decomposition product of the electrolytic solution material decomposed near the positive electrode is deposited on the negative electrode. It is presumed that the deposition is suppressed. Further, the sulfonylimide compound also has an effect of suppressing the transition metal from being eluted as ions from the positive electrode. Together with these effects, the rate characteristics of the lithium ion secondary battery using the non-aqueous electrolyte of the present invention have become excellent even after storage at high voltage and high temperature.

As the metal ion in the general formula (2), the same as the above-mentioned inorganic cation can be exemplified, and Li ⁺ , Na ⁺ , Mg ²⁺ and Ca ²⁺ are more preferable, and Li ⁺ is particularly preferable. The same onium ion as the above-mentioned onium cation can be exemplified. Therefore, a, b, and p are preferably 1.

The C ₁ -C ₁₀ alkylene group of R ^{1 in} the general formula (2) includes a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, a hexylene group, a heptylene group, an octylene group, a nonylene group, and a decalene group. And these may be branched. As the C ₁ -C ₁₀ halogenated alkylene group, a group in which part or all of the hydrogen atoms of the C ₁ -C ₁₀ alkylene group is replaced with F, Cl, Br or I (in particular, F is preferable, and for example, Fluoromethylene group and fluoroethylene group). As R ¹ , an alkylene group having 1 to 4 carbon atoms or a fluorinated alkylene group having 1 to 4 carbon atoms in which a part of hydrogen of the alkylene group is substituted by fluorine is preferable. More preferably, it is an alkylene group having 1 to 2 carbon atoms or a fluorinated alkylene group. q is 0 or 1, and when q is 0, it represents a direct bond of a carbonyl group, and the compound of the general formula (2) is oxalate borate or oxalate phosphonium. q is preferably 0.

R ² is a fluorine atom or a C ₁ -C ₁₀ fluorinated alkyl group, and examples of the C ₁ -C ₁₀ fluorinated alkyl group include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoro group; Examples include an ethyl group, a trifluoroethyl group, a tetrafluoroethyl group, a perfluoroethyl group, a fluoropropyl group, a perfluoropropyl group, a perfluorobutyl group, and a perfluorooctyl group. R ² is preferably a fluorinated alkyl group having 1 to 2 carbon atoms or a fluorine atom, and more preferably a fluorine atom.

X ¹ and X ² each independently represent O or S, but both are preferably O from the viewpoint of availability.

In the general formula (2), when M is B, m is more preferably 1 or 2, and when m is 1, n is 2 and R ² is F at this time. preferable. When m is 2, n is 0. When M is P, m is 1-3, when m is 1, n is 4, when m is 2, n is 2, and when m is 3, n is 0.

  Specific examples of the compound represented by the general formula (2) include difluorooxalate borate, bisoxalate borate, tetrafluorophosphonium, difluorobisoxalate phosphonium, and trisoxalate phosphonium. One or more kinds can be used.

  The compound represented by the general formula (2) is preferably used in a range of 0.01 to 5% by mass in 100% by mass of the nonaqueous electrolyte. If the amount is less than 0.01% by mass, the effect of suppressing the decomposition of the electrolytic solution during high-temperature storage under high voltage may be insufficient. It is not preferable because battery performance itself may decrease due to the increase. More preferably, it is 0.1 to 2% by mass.

1-5. Medium The non-aqueous electrolyte of the present invention contains a medium. As the medium, a conventionally known carbonate-based solvent and a non-aqueous solvent other than the carbonate-based solvent, and a conventionally known solvent used in batteries such as a medium such as a polymer and a polymer gel may be used. The medium preferably contains a carbonate-based solvent from the viewpoint of battery performance.

  Examples of the carbonate solvents include chain carbonates such as dimethyl carbonate, ethyl methyl carbonate (ethyl methyl carbonate), diethyl carbonate (diethyl carbonate), diphenyl carbonate, and methylphenyl carbonate; ethylene carbonate (ethylene carbonate), propylene carbonate (propylene carbonate). Carbonates), saturated cyclic carbonates such as 2,3-dimethylethylene carbonate (2,3-butanediyl carbonate), 1,2-butylene carbonate and erythritan carbonate; vinylene carbonate, methylvinylene carbonate (MVC; 4-methyl- 1,3-dioxol-2-one), ethylvinylene carbonate (EVC; 4-ethyl-1,3-dioxol-2-one), ethylene 2-vinyl carbonate (4-vinyl-1,3-dioxolan-2-) ON) and phenylethylene Cyclic carbonates having an unsaturated bond such as carbonate (4-phenyl-1,3-dioxolan-2-one); fluorine-containing cyclic carbonates such as fluoroethylene carbonate, 4,5-difluoroethylene carbonate and trifluoropropylene carbonate Esters are exemplified, and among these, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate and propylene carbonate are preferred. These can be used alone or in combination of two or more.

  As the other solvent, a solvent having a large dielectric constant, a high solubility of the electrolyte salt, a boiling point of 60 ° C. or more, 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 an organic solvent 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; aromatic carboxylic esters such as methyl benzoate and ethyl benzoate; lactones such as γ-butyrolactone, γ-valerolactone, δ-valerolactone; Phosphate esters such as trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, and triethyl phosphate; acetonitrile, propionitrile, methoxypropionitrile, glutaroni Nitriles such as ril, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, isobutyronitrile; dimethylsulfone, ethylmethylsulfone, diethylsulfone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane Sulfur compounds; aromatic nitriles such as benzonitrile and tolunitrile; nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2 (1H)- Examples include pyrimidinone and 3-methyl-2-oxazolidinone. Among these, γ-butyrolactone, γ-valerolactone, and the like are preferable as those used in combination with the carbonate-based solvent. When these are used in combination, one kind may be used alone, or two or more kinds may be used in combination.

  When the above-mentioned polymer or polymer gel is used, the following method may be adopted. That is, a solution prepared by dissolving an electrolyte salt in the above-mentioned carbonate-based solvent or another non-aqueous solvent (hereinafter, simply referred to as a non-aqueous solvent) is dropped onto a polymer formed by a conventionally known method, A method of impregnating and supporting an electrolyte salt and a non-aqueous solvent; a method of melting and mixing the polymer and the electrolyte salt at a temperature equal to or higher than the melting point of the polymer, forming a film, and impregnating the non-aqueous solvent with the method (above, gel A method of mixing a polymer with a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent in advance and forming a film by a casting method or a coating method and volatilizing the organic solvent; a temperature higher than the melting point of the polymer. A method in which a polymer and an electrolyte salt are melted, mixed, and molded (intrinsic polymer electrolyte).

  Examples of the polymer used in the above method include polyether polymers such as polyethylene oxide (PEO), which is a homopolymer or copolymer of epoxy compounds (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.), polypropylene oxide, and the like. Methacrylic polymers such as polymethyl methacrylate (PMMA), nitrile polymers such as polyacrylonitrile (PAN), fluoropolymers such as polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene, and copolymers thereof. Is mentioned.

  From the viewpoint of preventing decomposition of the solvent and suppressing deterioration of cycle characteristics (life) when the nonaqueous electrolyte of the present invention is used for a lithium ion secondary battery, it is recommended to use a cyclic carbonate as a solvent. Is done. Examples of the cyclic carbonate include the above-described saturated cyclic carbonate, a cyclic carbonate having an unsaturated bond, a fluorine-containing cyclic carbonate, and the like. Among these, saturated cyclic carbonate is preferred from the viewpoint of cost, and ethylene carbonate and propylene carbonate are particularly preferred. One kind of the cyclic carbonate may be used alone, or two or more kinds may be used in combination.

The cyclic carbonate is preferably used in a range where the molar ratio (cyclic carbonate / Li ⁺ ) to all lithium cations contained in the nonaqueous electrolyte is 1 or more and 5 or less. When the cyclic carbonate is used within the above range, the deterioration of the cycle characteristics (life) of the lithium ion secondary battery is further suppressed.

  It is known that one of the causes of the deterioration of the cycle characteristics is the decomposition of the solvent constituting the non-aqueous electrolyte. However, by setting the amount of the cyclic carbonate to be in the above range with respect to lithium ions, deterioration of cycle characteristics can be further suppressed. By determining the amount of the cyclic carbonate to be used in accordance with the amount of lithium ions, the amount of the cyclic carbonate (free cyclic carbonate) that is not solvated with the lithium ions present in the non-aqueous electrolyte can be reduced. That is, it is considered that the amount of free cyclic carbonate that can participate in the decomposition reaction is reduced, so that the decomposition reaction of the solvent is less likely to occur, and the deterioration of the cycle characteristics is suppressed.

When the molar ratio (cyclic carbonate / Li ⁺ ) is too large, free cyclic carbonate excessively present in the non-aqueous electrolyte is oxidized and / or reductively decomposed, resulting in deterioration of cycle characteristics. On the other hand, if the molar ratio is too small, the amount of cyclic carbonate is too small, and an effect derived from the cyclic carbonate (for example, an effect of forming a film on the negative electrode and suppressing the decomposition of the non-aqueous electrolyte, etc.) is obtained. There is a possibility that the non-aqueous electrolyte may be in a dry state due to the consumption of the solvent due to the repetition of charge / discharge (formation of a film, decomposition, etc.). Therefore, the cyclic carbonate is more preferably used in a molar ratio to lithium ions (cyclic carbonate / Li ⁺ ) of 1 or more and 4 or less, more preferably 3 or less, and even more preferably 2.0 or less. And particularly preferably 1.8 or less.

The molar ratio of the cyclic carbonate to the lithium cation (cyclic carbonate / Li ⁺ ) may be calculated based on the specific gravity and the molar mass of the cyclic carbonate. For example, in the case of ethylene carbonate, the specific gravity may be calculated as 1.321 and the molar mass as 88.06.

  In this case, the amount of the other solvent used is preferably 50% by volume or more, more preferably 55% by volume or more, even more preferably 60% by volume, based on 100% by volume of the total of the cyclic carbonate and the other solvent. % Or more, preferably 99% by volume or less, more preferably 95% by volume or less, and still more preferably 90% by volume or less.

1-6. Other components The non-aqueous electrolyte according to the present invention may contain an additive for improving various characteristics of the lithium ion secondary battery.
Additives include acetic anhydride, propionic anhydride, butyric anhydride, crotonic anhydride, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, and cyclohexanedicarboxylic acid. Carboxylic acid anhydrides such as acid anhydride and cyclopentanetetracarboxylic dianhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolane, dimethyl sulfone, Sulfur-containing compounds such as tetramethylthiuram monosulfide and trimethylene glycol sulfate; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazo Ridinone, N-methyls Nitrogen-containing compounds such as Shin'imido; monofluorophosphate, phosphates such as difluoro phosphate; heptane, octane, saturated hydrocarbon compounds cycloheptane and the like; and the like.

The additive is preferably used in a range of 0.1% by mass or more and 10% by mass or less in 100% by mass of the nonaqueous electrolyte of the present invention (more preferably, 0.2% by mass or more, 8% by mass or less, More preferably, it is 0.3% by mass or more and 5% by mass or less. When the amount of the additive is too small, the effect derived from the additive may not be easily obtained.On the other hand, even when other additives are used in a large amount, the effect corresponding to the added amount is hardly obtained, and There is a possibility that the viscosity of the non-aqueous electrolyte increases and the conductivity decreases.
Here, 100% by mass of the non-aqueous electrolyte means the sum of all components contained in the non-aqueous electrolyte, such as the above-mentioned sulfonylimide compound, electrolyte salt, solvent, and appropriately used additives.

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 inserting and extracting lithium ions, a negative electrode containing a negative electrode active material capable of inserting and extracting 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 accommodated 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 non-aqueous electrolyte 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 auxiliary agent, a binder, and the like on a positive electrode current collector, and is usually formed in a sheet shape.

  The method for producing the positive electrode is not particularly limited. For example, (i) a positive electrode active material composition obtained by dissolving or dispersing a positive electrode mixture in a dispersion solvent is applied to a positive electrode current collector by a doctor blade method or the like; A method in which the current collector is immersed in the positive electrode active material composition and then dried; (ii) a sheet obtained by kneading and forming the positive electrode active material composition and drying is joined to the positive electrode current collector via a conductive adhesive. (Iii) coating or casting the positive electrode active material composition to which the liquid lubricant has been added on the positive electrode current collector to form a desired shape, and then removing the liquid lubricant. And then stretching in a uniaxial or multiaxial direction; and the like. Further, if necessary, the dried positive electrode mixture layer may be pressurized. Thereby, the adhesive strength with the positive electrode current collector is increased, and the electrode density is also increased.

  The material of the positive electrode current collector, the positive electrode active material, the conductive auxiliary agent, the binder, and the solvent (solvent for dispersing or dissolving the positive electrode mixture) used in the positive electrode active material composition are not particularly limited, and may be any of conventionally known materials. Materials can be used, and for example, each material described in JP-A-2014-13704 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, more preferably 85 parts by mass or more, and still more preferably 90 parts by mass or more, based on 100 parts by mass of the positive electrode mixture. Yes, more preferably 98 parts by mass or less, even more preferably 97 parts by mass or less.

  When the conductive auxiliary is used, the content of the conductive auxiliary in the positive electrode mixture is preferably in the range of 0.1% by mass to 10% by mass with respect to 100% by mass of the positive electrode mixture. Preferably, it is 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 becomes extremely poor, and the load characteristics and the discharge capacity may be deteriorated. On the other hand, if it is too large, the bulk density of the positive electrode mixture layer becomes high, and it is necessary to further increase the content of the binder, which is not preferable.

When a binder is used, the content of the binder in the positive electrode mixture is preferably 0.1% by mass to 10% by mass relative 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 assistant may be detached from the current collector. On the other hand, if the amount is too large, the internal resistance may increase, which may adversely affect battery characteristics.
The 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, energy, etc.), ionic conductivity, and the like.

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

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

2-3. Separator The separator is arranged 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. For example, each material described in JP-A-2014-13704 can be used.

2-4. Battery exterior material A battery element equipped with a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, etc. is housed in a battery exterior material to protect the battery element from external impact and environmental degradation when using a lithium ion secondary battery. You. In the present invention, the material of the battery exterior material is not particularly limited, and any conventionally known exterior materials 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 of the lithium ion secondary battery may be used, such as a cylindrical shape, a square shape, a laminate type, a coin shape, and a large size. Can be. When used as a high-voltage power supply (several tens of volts to several hundreds of volts) to be mounted on an electric vehicle, a hybrid electric vehicle, or the like, 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 4.2 V or more. The effect of the present invention is particularly remarkable when used at a voltage exceeding 4.2V. More preferably, it is 4.3 V or more, and further preferably, it is 4.35 V or more. The energy density can be increased as the rated charging voltage is higher, but if it is too high, it may be difficult to ensure safety. Therefore, the rated charging voltage is preferably 4.6 V or less. More preferably, it is 4.5 V or less.

  Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to the following Examples, and may be appropriately modified within a range that can conform to the purpose of the preceding and the following. It is, of course, possible to implement them, and all of them are included in the technical scope of the present invention.

1. Preparation of Non-Aqueous Electrolyte Lithium hexafluorophosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd.) in a non-aqueous solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a ratio of 3: 7 (volume ratio). , An electrolyte salt) and a non-aqueous electrolyte solution having a LiPF ₆ concentration of 1.2 mol / L. 1 was prepared.

  The above non-aqueous electrolyte solution No. In Example 1, lithium bisoxalate borate (LiBOB) was dissolved in a concentration of 1% by mass as a compound represented by the general formula (2). 2 was prepared.

Lithium bis (fluorosulfonyl) was used as a compound having a sulfonylimide anion (1) and producing a sulfonylimide anion (1) in a nonaqueous electrolyte, with the concentration of LiPF ₆ in the nonaqueous solvent being 1 mol / L. An imide (hereinafter sometimes referred to as LiFSI) was dissolved at a concentration of 0.2 mol / L, and LiBOB was dissolved at a concentration of 1% by mass. 3 was prepared.

Except that the concentration of LiPF _{6 and} the concentration of LiFSI in the non-aqueous solvent were set to 0.6 mol / L and 0.6 mol / L, respectively, In the same manner as in Non-aqueous electrolyte solution No. 1, 4 was prepared. This non-aqueous electrolyte solution No. 4 was dissolved in LiBOB so as to be 1% by mass. 5 was prepared.

2. Manufacture of coin type lithium ion secondary battery Positive electrode active material (LiCoO ₂ ), conductive agent 1 (acetylene black, AB), conductive agent 2 (graphite), and binder (polyvinylidene fluoride, PVdF) were 93: 2. : A positive electrode having a mass ratio of 2: 3, artificial graphite as a negative electrode active material, a conductive additive (vapor-grown carbon fiber, “VGCF (registered trademark)”, manufactured by Showa Denko KK), and a binder (polyfluorinated A negative electrode having a mass ratio of vinylidene (PVdF) of 95.7: 0.5: 3.8 was used. A separator (made of polyethylene, 16 μm) was impregnated with a non-aqueous electrolyte to prepare a coin cell.

3. Battery evaluation 3-1. Measurement of 0.2 C recovery capacity and rate characteristics after standing (1 C / 0.2 C) at 4.4 V. Using a coin cell, a lithium ion secondary battery with a rated charging voltage of 4.4 V was manufactured. Using a charge / discharge tester (ACD-01, manufactured by Asuka Electronics Co., Ltd.), a constant current and constant voltage charge of 0.02C cut at 1C to 4.4V under an environment of 25 ° C. The capacity when the battery was discharged at 2.C to 2.75 V was measured. This capacity was used as the initial 0.2 C capacity. Then, after performing the same charge, it discharged to 2.75V at 1C, and measured the initial 1C capacity. Thereafter, the battery was charged at a constant current and a constant voltage (CCCV) for 3 hours at 1 C to 4.4 V, and left at 80 ° C. for 3 days.

After leaving the battery for 24 hours in an environment of a temperature of 25 ° C., the battery was discharged to 2.75 V at a temperature of 25 ° C. and a discharge rate of 1 C, and the discharge capacity was measured. Thereafter, constant-current constant-voltage charging was performed at 1 C at 0.02 C cut to 4.4 V, and constant-current discharging was performed at 0.2 C to 2.75 V. The discharge capacity at this time was 0.2 C recovery capacity after standing. From the values of the initial 0.2 C capacity and the 0.2 C recovery capacity after standing, a 0.2 C recovery capacity (%) was determined by the following equation. The results are shown in Table 1.
0.2C recovery capacity (%) = (0.2C capacity after standing / initial 0.2C capacity) × 100
Furthermore, constant-current constant-voltage charging was performed at 4.4 C with a cut of 0.02 C at 1 C, and then constant-current discharge was performed at 2.75 V with 1 C. The capacity at this time was 1 C capacity after standing. The ratio of the 1C capacity after standing to the 0.2C capacity after standing (the following formula) is also shown in Table 1 as the rate after standing.
Rate after standing (%) = (1C / 0.2C) × 100

  No. 1 and No. Comparing No. 2, although both the 0.2 C recovery capacity and the rate after standing were slightly improved, No. 3 is No. Compared with No. 2, the 0.2C recovery capacity and the rate after standing were more excellent, and the combined effect of LiFSI and LiBOB could be confirmed. In addition, No. 1 and No. 4 and No. When No. 5 is compared, In No. 4, although the rate after the high-temperature storage was particularly decreased due to the increase in LiFSI, No. 4 in which LiBOB was added was used. 5 was the most excellent value among the five non-aqueous electrolytes. This is because a film is formed on the negative electrode due to decomposition of LiBOB, decomposition products of the electrolytic solution near the positive electrode are prevented from depositing on the negative electrode, and the elution of transition metal from the positive electrode is suppressed by LiFSI. It is considered that it is.

3-2. ICP emission spectroscopy of negative electrode (Co detection amount)
The negative electrode after the high-temperature storage test was analyzed using an ICP emission spectrometer (manufactured by Shimadzu Corporation). After washing the negative electrode after the high-temperature storage test with 50 ml of ethyl methyl carbonate, the negative electrode active material was peeled off, dissolved in 1 g of nitric acid, and diluted 15 times with pure water to prepare a sample for measurement. Spectroscopic analysis was performed. Table 2 shows an indexed value where the amount of detected Co of the negative electrode of the battery in the case of the rated charging voltage of 4.2 V using the non-aqueous electrolyte 1 is defined as 100 (%).

  Non-aqueous electrolyte containing LiFSI It was confirmed that all of the samples Nos. 3 to 5 had a smaller amount of Co detection than the case where no LiFSI was contained.

Measurement of 0.2 C recovery capacity and rate characteristic after standing (1 C / 0.2 C) at 3-3.4.2 V The addition effect of the compound represented by the general formula (2) in the lithium ion secondary battery of the present invention is as follows. In particular, it becomes remarkable when used at a voltage exceeding 4.2V. An experiment was conducted to confirm the effect at 4.2 V. Except changing 4.4V to 4.2V, it carried out similarly to 3-1 and measured the 0.2C recovery capacity and the rate characteristic after standing. The amount of Co detection was also measured in the same manner as in 3-2. Table 3 shows the results.

  As is clear from Table 3, in the charged state of 4.2 V, even if LiBOB was added, the recovery capacity and the rate characteristics after standing were reduced, and the effect of adding LiBOB was not recognized. At a high voltage of 4.4 V, a film derived from LiBOB is formed on the negative electrode, and the decomposition of the electrolytic solution near the positive electrode suppresses deposition on the negative electrode, whereas at 4.2 V, It is considered that such a film derived from LiBOB becomes a resistive film, which deteriorates the battery performance after high-temperature storage.

  As described above, it was found that the addition of LiBOB was extremely effective at a high voltage exceeding 4.2 V, but had no effect at a voltage of 4.2 V or less.

Claims (4)

A non-aqueous electrolyte used for a lithium ion secondary battery having a rated charging voltage exceeding 4.2 V,
The non-aqueous electrolyte,
General formula (1) [N (XSO ₂ ) (FSO ₂ )] ^- (In general formula (1), X represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms. And an anion represented by
At least one anion selected from the group consisting of an anion represented by the following general formulas (7) and (8) and a hexafluoroarsenate ion (AsF ₆ ⁻ );
[PF _l (C _m F _{2m + 1} ) _6-l ] ^- (0 ≦ l ≦ 6, 1 ≦ m ≦ 4) (7)
_{[BF n (C o F 2o} + 1) 4-n] - (0 ≦ n ≦ 4,1 ≦ o ≦ 4) (8)
A lithium cation,
A compound represented by the following general formula (2):
(In the general formula (2), M is B or P, A ^{a +} is a metal ion, hydrogen ion, or onium ion, a is 1 to 3, b is 1 to 3, p is b / a, and m is 1 to 3. , N represents 0 to 4, q represents 0 or 1, R ¹ represents a C ₁ -C ₁₀ alkylene group or a C ₁ -C ₁₀ halogenated alkylene group, and R ² represents fluorine or a C ₁ -C ₁₀ The fluorinated alkyl group, X ¹ and X ² each independently represent O or S.)
And the media only contains,
The content of the anion represented by the general formula (1) is the same as the content of the anion represented by the general formula (1), the anion represented by the general formulas (7) and (8), and hexafluoroarsenate. 30 mol% or more of the total 100 mol% with the ion (AsF ₆ ⁻ );
Wherein said medium is, ethylene carbonate, propylene carbonate, 2,3-dimethyl ethylene carbonate, and contains Mukoto at least one saturated cyclic carbonate-based solvent is selected from carbonic acid 1,2-butylene and the group consisting of erythritan carbonate Non-aqueous electrolyte.   The non-aqueous electrolyte according to claim 1, comprising 0.01 to 5% by mass of the compound represented by the general formula (2).   3. The non-aqueous electrolyte according to claim 1, wherein the compound represented by the general formula (2) is a bisoxalyl borate salt. Lithium ion secondary battery comprising the nonaqueous electrolytic solution according to any one of claims 1-3.