JP6637237B2 - 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 describes that when phosphorous acid is added to an electrolytic solution, a protective film of a lithium salt is formed on an electrode surface, and swelling of the battery during high-temperature storage is suppressed.

  On the other hand, while the applicant of the present application has continued to study using a sulfonylimide anion compound as an electrolyte of an electrolytic solution for a lithium ion secondary battery, the use of a sulfonate and / or a sulfone compound as an additive has led to Have found that the corrosion of steel is suppressed, and have already filed an application (Patent Document 2).

JP 2008-41635 A JP-A-2004-13704

  By the way, various studies have been made on the improvement of the battery characteristics as described above, but with the expansion of applications, further higher energy density is required for batteries. In order to increase the energy density of a battery, it is necessary to drive the battery under a high voltage. The inventors of the present application are studying a battery electrolyte solution that can withstand a higher voltage and does not deteriorate even when stored at a high temperature. However, when a battery using a non-aqueous electrolyte containing a sulfonylimide anion compound is driven under a high voltage, the electrolyte is decomposed at the positive electrode, decomposition products are deposited on the negative electrode, and the opposing separator is exposed. There is a problem that clogging occurs and the capacity and rate characteristics of the battery are reduced. When phosphorous acid described in Patent Document 1 is added to the electrolytic solution, a protective film is formed on the surface of the positive electrode, so that decomposition of the electrolytic solution and deposition of decomposition products on the negative electrode can be suppressed, but the resistance of the positive electrode increases. In addition, the recovery capacity and rate characteristics after high-temperature storage are reduced. Patent Document 2 does not discuss the problem of use under a high voltage exceeding 4.2 V.

  The present invention has been made in view of the above circumstances, and its purpose is to reduce the capacity and rate characteristics of a battery even when stored at high temperatures or when used under high voltage. An object of the present invention is to provide a non-aqueous electrolyte which can be suppressed 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 phosphoric acid compound represented by the following general formula (2):

(In the above general formula (2), R ^a and R ^b each independently represent hydrogen, a C _{1 to} C ₄ alkyl group or a phenyl group which may have a substituent, and p and q represent 0 Or 1.)
It is characterized by including.

  The non-aqueous electrolyte of the present invention preferably further contains a compound having a sulfone structure. In this case, the embodiment in which the compound having a sulfone structure is a cyclic sulfonic acid ester and the embodiment in which the phosphoric acid compound is phosphorous acid are both preferred embodiments of the present invention.

The non-aqueous electrolyte of the present invention further comprises 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 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)

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

  The non-aqueous electrolyte of the present invention suppresses the decomposition of the electrolyte material at the positive electrode, and can also suppress an increase in the resistance of the positive electrode. It is effective in suppressing deterioration of capacity and rate characteristics after storage at high voltage and high temperature. 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 preferable 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 a compound having a sulfonylimide anion by a radical, 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) This is because the effect of adding is difficult to develop. 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.

Sum of electrolyte salts (compound having sulfonylimide anion, compound having anion (7), compound having anion (8), compound having AsF ₆ ^-1 and other electrolyte salts) in the non-aqueous electrolyte of the present invention. The amount 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. It is preferably at most 3.0 mol / L, more preferably at most 2.0 mol / L. 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. The lithium cation concentration of the non-aqueous electrolyte of the present invention is 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. Phosphoric acid compound The nonaqueous electrolyte solution of the present invention contains a phosphoric acid compound represented by the following general formula (2) as an essential component.

(In the above general formula (2), R ^a and R ^b each independently represent hydrogen, a C _{1 to} C ₄ alkyl group or a phenyl group which may have a substituent, and p and q represent 0 Or 1.)

  Since the phosphoric acid-based compound forms a protective film on the surface of the positive electrode, even when the sulfonylimide compound enhances the activity of the positive electrode, the electrolytic solution decomposes near the positive electrode, and decomposition products are deposited on the negative electrode to face each other. Clogging of the separator can be suppressed. When the phosphoric acid-based compound is not used in combination with a compound having a sulfone structure described below, if it is added alone in a large amount, the battery performance, particularly the rate characteristics after high-temperature storage at high voltage may be reduced.

R ^a and R ^b in the general formula (2) represent hydrogen, a C _{1 to} C ₄ alkyl group such as a methyl group, an ethyl group, a butyl group, and a propyl group, or a phenyl group which may have a substituent. It is. R ^a and R ^b may be the same or different. Examples of the substituent include a carboxy group, a cyano group, an N-substituted amino group, a nitro group, an alkyl group having 1 to 20 carbon atoms, a fluoroalkyl group, an alkoxy group, an alkoxycarbonyl group, an acyl group, a cyanoalkyl group, a cyanoalkoxy group, Examples thereof include a cyanoalkylsulfonyl group and an alkylsulfonyl group. If p and q are both 1, the P (phosphorus atom), so that a hydroxyl group, an alkoxy group of C ₁ -C _4, or a phenoxy group is attached.

Specific examples of the phosphoric acid compound include phosphorous acid (H ₃ PO ₃ ) in which R ^a and R ^b are both hydrogen and p is 0 and q is 1 in the general formula (2). In the general formula (2), R ^a is hydrogen, R ^b is a phenyl group, and p and q are both 0, and in the above general formula (2), both R ^a and R ^b are phenyl groups. And diphenyl phosphate in which p and q are both 1, and the like, and among them, phosphorous acid is preferable.

  The phosphoric acid compound is preferably used in a range of 0.01% by mass or more and 1% by mass or less 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 at a high voltage may be insufficient. If the amount exceeds 1% by mass, the battery performance itself deteriorates. There is a fear that this is not preferable. More preferably, it is 0.03% by mass or more and 0.5% by mass or less, and still more preferably 0.05% by mass or more and 0.2% by mass or less.

1-5. Compound having a sulfone structure The non-aqueous electrolyte of the present invention preferably contains a compound having a sulfone structure as an additive. The compound having a sulfone structure is a sulfonate and / or a sulfone compound. The protective coating on the positive electrode surface formed by adding a phosphoric acid compound to the electrolytic solution suppresses decomposition of the electrolytic solution material near the positive electrode and prevents clogging of the separator, but increases the resistance of the positive electrode, Battery performance will be reduced. The sulfonic acid ester and / or sulfone compound has an effect of suppressing such an increase in the resistance of the positive electrode. The mechanism is presumed to be because the sulfonic acid ester and / or the sulfone compound forms a film before the phosphoric acid-based compound, and suppresses the growth of a high-resistance protective film derived from the phosphoric acid compound. As a result, it is considered that the lithium secondary battery provided with the nonaqueous electrolytic solution of the present invention hardly causes deterioration in characteristics after high-temperature storage at high voltage, in combination with the effect of the above-mentioned phosphoric acid compound. The combined use of a phosphoric acid compound and a compound having a sulfone structure also suppresses the generation of decomposition products of the electrolytic solution, and is particularly effective for improving the rate characteristics after storage at high temperatures. It is a preferred embodiment of the liquid.

The sulfonic acid ester is a compound represented by R ¹ —S (O) ₂ —O—R ² , wherein R ¹ and R ² represent a hydrocarbon group having 1 to 10 carbon atoms. The hydrocarbon group may be chain-like, branched-chain, cyclic, or may have two or more of these structures, and may have some or all of the hydrogen atoms bonded to carbon constituting the hydrocarbon group. May be substituted with halogen. More preferably, it is a hydrocarbon group having 1 to 6 carbon atoms. Specific examples include an alkyl group, an aryl group, a fluoroalkyl group, a fluoroaryl group, and the like. R ¹ and R ² may be the same or different, and R ¹ and R ² may combine to form a ring. Specific sulfonic acid esters include 1,3-propane sultone, 1,3-butane sultone, 1,4-butane sultone, 2,4-butane sultone, 1,5-pentane sultone and 2,4-pentane sultone Sulfonic acid esters such as 1,4-hexanesultone and 4,6-heptanesultone; and linear sulfonic acid esters such as methyl methanesulfonate, methyl benzenesulfonate and methyl trifluoromethanesulfonate. Among the sulfonic acid esters, cyclic sulfonic acid esters are preferable, 1,3-propane sultone, 1,4-butane sultone, and 2,4-butane sultone are more preferable, and 1,3-propane sultone, 1,4 and 1,4 are more preferable. -Butane sultone, particularly preferably 1,4-butane sultone.

The sulfone compound is a compound having a sulfone bond (R ¹ -SO ₂ -R ² ) in the molecule (R ¹ and R ² are the same as the sulfonic acid ester). Specific examples of the sulfone compound include a cyclic sulfone compound such as sulfolane and 3-methylsulfolane, and a chain sulfone compound such as ethylmethyl sulfone, diphenyl sulfone and bis (4-fluorophenyl) sulfone. Among the compounds having a sulfonyl group, sulfolane, 3-methylsulfolane, and ethylmethylsulfone are preferable, and sulfolane is more preferable.

  The sulfonic acid ester and / or the sulfone compound may be used alone or in a combination of two or more.

The content of the compound having a sulfone structure is preferably 0.01 to 3% by mass in 100% by mass of the nonaqueous electrolyte. When the amount is less than 0.01% by mass, the effect derived from the compound having a sulfone structure may not be sufficiently obtained. On the other hand, when the amount exceeds 3% by mass, the battery performance itself may be reduced. . The content of the compound having a sulfone structure is more preferably 0.1 to 2% by mass, and still more preferably 0.5 to 1.5% by mass.
In addition, 100% by mass of the nonaqueous electrolyte refers to all components constituting the electrolyte such as a sulfonylimide compound, another electrolyte salt, a compound having a sulfone structure, a phosphoric acid compound, a solvent, and additives used as necessary. Is the total amount of

1-6. Solvent The non-aqueous electrolyte of the present invention contains a solvent. The solvent is not particularly limited as long as it can dissolve various components contained in the nonaqueous electrolytic solution of the present invention.Non-aqueous solvents and media such as polymers and polymer gels, and conventionally known solvents used for batteries include Either can be used.

  As the non-aqueous solvent, a solvent having a large dielectric constant, a high solubility of the sulfonylimide compound, a boiling point of 60 ° C. or higher, and a wide electrochemical stability range is preferable. More preferably, the organic solvent has 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; chain-like carbonic esters such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, diphenyl carbonate and methylphenyl carbonate; ethylene carbonate, propylene carbonate, 2,3- Cyclic carbonates such as dimethyl ethylene carbonate, butylene carbonate, vinylene carbonate and 2-vinyl ethylene carbonate; fluoroethylene carbonate, trans-difluoroethylene carbonate And fluorinated cyclic carbonates such as cis-difluoroethylene carbonate; aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate, ethyl propionate, ethyl acetate, propyl acetate, butyl acetate and amyl acetate; Aromatic carboxylic esters such as methyl benzoate and ethyl benzoate; lactones such as γ-butyrolactone, γ-valerolactone, and δ-valerolactone; trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, phosphoric acid Phosphate esters such as triethyl; trifluoroethyldimethylphosphate, bis (trifluoroethyl) methylphosphate, tris (trifluoroethyl) phosphate, pentafluoropropyldimethylphosphate, trifluoroethylmethylethylphosphate, phosphorus Pentafluoropropionate Halogenated phosphoric acid esters such as methylethyl and trifluoroethylmethylpropyl phosphate; phosphazene compounds such as monophosphazene, cyclic phosphazene and chain phosphazene; acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile; Nitriles such as 2-methylglutaronitrile, valeronitrile, butyronitrile, isobutyronitrile; N-methylformamide, N-ethylformamide, N, N-dimethylformamide, N, N-dimethylacetamide, N-methylpyrrolidinone, Amides such as N-vinylpyrrolidone; sulfur compounds such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, 2,4-dimethyl sulfolane: ethylene glycol Alcohols such as propylene glycol, ethylene glycol monomethyl ether and ethylene glycol monoethyl ether; sulfoxides such as dimethyl sulfoxide, methyl ethyl sulfoxide and diethyl sulfoxide; aromatic nitriles such as benzonitrile and tolunitrile; nitromethane; -Dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2 (1H) -pyrimidinone, 3-methyl-2-oxazolidinone, and the like.

  Of these, chain carbonates, cyclic carbonates, aliphatic carboxylic esters, lactones and ethers are preferred, and dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate (ethylene carbonate), propylene carbonate ( Propylene carbonate), γ-butyrolactone, γ-valerolactone and the like are more preferred. One of the above non-aqueous solvents may be used alone, or two or more may be used in combination.

  When a polymer or polymer gel is used as a solvent, the following method may be employed. That is, various components (sulfonylimide compounds, other electrolyte salts, compounds having a sulfone structure, phosphoric acid compounds, and additives used as necessary) are added to the above-mentioned non-aqueous solvent in a polymer formed by a conventionally known method. ), Etc., by dropping a solution in which various components and the like and a non-aqueous solvent are impregnated and supported; melting and mixing the polymer and various components at a temperature equal to or higher than the melting point of the polymer; A method of impregnating a non-aqueous solvent with a polymer; a method of mixing an electrolyte solution in which various components and the like are dissolved in a non-aqueous solvent in advance with a polymer, forming a film by a casting method or a coating method, and volatilizing the organic solvent ( As described above, a gel electrolyte); a method in which a polymer, various components, and the like are melted at a temperature equal to or higher than the melting point of the polymer, 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.

1-7. 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. Acid anhydrides, carboxylic anhydrides such as cyclopentanetetracarboxylic dianhydride; sulfur-containing compounds such as ethylene sulfide, busulfan, tetramethylthiuram monosulfide, trimethylene glycol sulfate; 1-methyl-2-pyrrolidinone; Nitrogen-containing compounds such as 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide; monofluorophosphates, difluorophosphates and the like Phosphate; heptane, octane, cyclohepta Saturated hydrocarbon compounds 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, 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.

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 higher than 4.2 V. 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. 1 was dissolved in a concentration of 0.1% by mass of phosphorous acid as a compound represented by the general formula (2). 2 was prepared.

  The above non-aqueous electrolyte solution No. 1, 1,4-butane sultone as a compound having a sulfone structure was dissolved at a concentration of 1% by mass. 3 was prepared.

  The above non-aqueous electrolyte solution No. 1 was dissolved in 0.5% by mass of phosphorous acid and 1% by mass of 1,4-butanesultone. 4 was prepared.

The above non-aqueous electrolyte solution No. LiPF ₆ concentration was changed to 0.6 mol / L, and lithium bis (fluorosulfonyl) imide was used as a compound having a sulfonylimide anion (1) and generating a sulfonylimide anion (1) in a non-aqueous electrolyte. (Hereinafter sometimes referred to as LiFSI) was dissolved at a concentration of 0.6 mol / L. 5 was prepared.

  The above non-aqueous electrolyte solution No. 5 was dissolved in 0.1% by mass of phosphorous acid. 6 was prepared.

  The above non-aqueous electrolyte solution No. 5, 1,4-butane sultone was dissolved so as to be 1% by mass. 7 was prepared.

  The above non-aqueous electrolyte solution No. 5 was dissolved in 0.5% by mass of phosphorous acid and 1% by mass of 1,4-butanesultone. 8 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 Measurement of 0.2 C recovery capacity at 4.4 V and rate characteristics after standing (1 C / 0.2 C) A lithium ion secondary battery with a rated charging voltage of 4.4 V was manufactured using a coin cell, and a charge / discharge test was performed. Using a device (ACD-01, manufactured by Asuka Electronics Co., Ltd.), a constant current constant voltage charge of 0.02 C cut at 1 C to 4.4 V was performed in an environment of a temperature of 25 ° C., and then 2 C at 0.2 C. The capacity when discharging to 0.75 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 up 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 recovery capacity after standing / initial 0.2C recovery 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

Comparing Comparative Examples 1-1, 2-1 and 3-1 with Comparative Example 2-1 in which the charging voltage was 4.2 V and phosphoric acid was added in an amount of 0.1% by mass, the rate was slightly lowered after standing. . The battery performance of Comparative Example 3-1 to which butane sultone was added was almost the same as that of Comparative Example 1-1 containing only LiPF ₆ . When the charging voltage was increased to 4.4 V, the capacity and rate characteristics of Comparative Examples 1-2, 2-2, 3-2, and 4 were all lower than at 4.2 V.

On the other hand, in Comparative Example 5 to which LiFSI was added, the rate after standing was significantly reduced. It is considered that LiFSI enhances the activity of the positive electrode, and decomposition of the non-aqueous electrolyte occurs near the positive electrode. In Comparative Example 7 in which phosphorous acid was added to the LiFSI-added system, an improvement in the rate after standing was observed. In Comparative Example 6 to which butane sultone was added, no improvement was observed in the rate after standing. In Example 2 in which both phosphorous acid and butane sultone were added in the LiFSI-added system, the rate after standing was improved to twice the level of Comparative Example 5. Thereby, the combined effect of phosphorous acid and butane sultone was confirmed.

Claims (8)

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
PF ₆ ^- and,
A lithium cation,
A phosphoric acid compound represented by the following general formula (2):
(In the above general formula (2), R ^a and R ^b each independently represent hydrogen, a C _{1 to} C ₄ alkyl group or a phenyl group which may have a substituent, and p and q represent 0 Or 1.)
Furthermore, it includes a compound having a sulfone structure,
The compound having the sulfone structure includes 1,3-propanesultone, 1,3-butanesultone, 1,4-butanesultone, 2,4-butanesultone, 1,5-pentanesultone, 2,4-pentanesultone, Consists of 1,4-hexanesultone, 4,6-heptanesultone, methyl methanesulfonate, methyl benzenesulfonate, methyl trifluoromethanesulfonate, ethylmethylsulfone, diphenylsulfone, and bis (4-fluorophenyl) sulfone and at least one der Ru nonaqueous electrolyte selected from the group,
A non-aqueous electrolyte used for a lithium ion secondary battery having a rated charging voltage exceeding 4.2 V. The non-aqueous electrolyte according to claim 1, wherein the rated charging voltage is higher than 4.2V and not higher than 4.6V. The anion represented by the general formula (1), non-aqueous electrolyte according to claim 1 or 2 is bis (fluorosulfonyl) imide anion. Compounds having the sulfone structure include 1,3-propane sultone, 1,3-butane sultone, 1,4-butane sultone, 2,4-butane sultone, 1,5-pentanes sultone, 2,4-pentanes sultone, The non-aqueous electrolyte according to any one of claims 1 to 3, which is at least one selected from the group consisting of 1,4-hexanesultone and 4,6-heptanesultone. The non-aqueous electrolyte according to any one of claims 1 to 4 , wherein the phosphoric acid compound is phosphorous acid. Furthermore, the following general formula (8) represented by anion and hexafluoroarsenate ion ^- nonaqueous according to any one of claims 1 to 5 including at least one anion selected from the group consisting of (AsF ₆₎ Electrolyte.
_{[BF n (C o F 2o} + 1) 4-n] - (0 ≦ n ≦ 4,1 ≦ o ≦ 4) (8) The content of the phosphoric acid compound is 0.01% by mass or more and 1% by mass or less,
The content of the compound having a sulfonic structure 0.01 mass% or more, the non-aqueous electrolyte according to any one of claims 1 to 6 3% by mass or less.   A lithium ion secondary battery comprising the nonaqueous electrolyte according to claim 1.