Non-aqueous electrolyte solution for secondary battery, and secondary battery provided with said solution

Abstract

The purpose of the present invention is to provide: a non-aqueous electrolyte for a secondary battery that is exceptional in retention characteristics for electric capacity when stored in a high-temperature environment, the non-aqueous electrolyte satisfactorily suppressing any increase in internal resistance, and exhibiting excellent retention characteristics for electric capacity even if charging and discharging are repeated at a high charging voltage after storage in a high-temperature environment; and a secondary battery comprising the non-aqueous electrolyte. A non-aqueous electrolyte for a secondary battery according to the present invention contains an electrolyte and a non-aqueous solvent, and furthermore contains at least one fluorophosphate represented by chemical formula (1) as an additive. (In the formula, M+ represents an alkali metal ion. Each of R1 to R5 independently represents a hydrogen atom or a C1-10 hydrocarbon group. Alternatively, each of R1 to R5 independently is any one of a C1-10 hydrocarbon group and a C1-10 alkoxy group, and optionally selected combinations are bonded to each other to form an annular structure. n represents an integer of 0-10.)

Description

Nonaqueous electrolyte for secondary battery and secondary battery including same

[0001] The present invention relates to a nonaqueous electrolyte for secondary batteries, which is excellent in terms of the retention of electrical capacity and the suppression of an increase in internal resistance when stored in a high-temperature environment, for example, for secondary batteries used at high voltages, and which also has excellent retention of electrical capacity when repeatedly charged and discharged, and a secondary battery including the same.

[0002] Small, high-capacity lithium-ion secondary batteries have become widely used as power sources for portable information terminals such as laptops, digital cameras, mobile phones, smartphones, and tablets. In recent years, lithium-ion secondary batteries have been installed in electric vehicles (xEVs) as a key device toward achieving carbon neutrality. Their applications are also expanding to include household power sources and stationary storage batteries for power storage systems such as mega-solar power generation, leading to the development of lithium-ion secondary batteries with higher energy density.

[0003] In conventional lithium-ion secondary batteries, materials capable of reversibly inserting Li ions into the positive and negative electrode active materials are used. For example, the positive electrode active material is LiNiO ２ , LiCoO ２ , LiMn ２ O ４ , or LiFePO ４ The negative electrode active material is lithium metal, its alloy, a carbon material, a graphite material, or the like. The electrolyte used in lithium ion secondary batteries is a mixed solvent of ethylene carbonate, diethyl carbonate, propylene carbonate, etc., and LiPF ６ , LiBF ４ In this case, a material in which an electrolyte such as the above is dissolved is used.

[0004] It is generally understood that a stable film (SEI: Solid Electrolyte Interphase) with lithium ion conductivity but no electronic conductivity is formed at the interface between the electrode active material layer and the electrolyte. The process of lithium ion insertion and desorption into the electrode active material is highly reversible. However, long-term storage in a high-temperature environment or repeated charge / discharge cycles under high-voltage conditions can cause cracking, dissolution, or decomposition in the SEI. The resulting accumulation of gas and decomposition products inhibits lithium ion migration, increasing the internal resistance of the lithium-ion secondary battery and degrading battery performance. Therefore, it is important to determine how to form a stable SEI that can withstand the harsh operating conditions and usage environments of lithium-ion secondary batteries. From this perspective, efforts have traditionally been made to form a stable SEI by adding additives to the electrolyte.

[0005] For example, Patent Document 1 discloses an electrolyte solution for a secondary battery that contains a nonaqueous solvent containing a monofluorophosphate ester salt and a lithium salt as a solute. According to Patent Document 1, by using an electrolyte solution for a secondary battery having such a configuration, a high-quality film can be formed on the electrode interface, and this film formation can suppress decomposition of the electrolyte solution for a secondary battery, thereby improving the capacity residual rate and capacity recovery rate.

[0006] In addition, in lithium ion secondary batteries, improvements have been made to the positive electrode active material in order to improve the durability of charge / discharge cycles and increase the energy density in order to cope with harsh usage environments and operating conditions. For example, LiNiO ２ By using LiNiO, the charging voltage of the lithium ion secondary battery can be increased, and the energy density can be improved. ２ When LiNiO is used as the positive electrode active material, the theoretical value of the battery capacity is high, but the thermal stability of the battery characteristics is low. ２Instead of Li, ternary transition metal oxides containing cobalt and manganese are also used as positive electrode active materials. Furthermore, due to concerns about the amount of cobalt in the earth's reserves as a natural resource, positive electrode active materials in which the composition ratio of cobalt is reduced and the composition ratio of nickel is increased are being developed. For example, in Patent Document 2, Li ｘ (Ni ｙ M １－ｙ ) O ｚ (wherein M is one or more elements selected from the group consisting of Mn, Co, Mg, Al, Ti, Cr, Fe, Cu, and Zr, x is in the range of 0.9 to 1.2, y is in the range of 0.3 to 0.95, and z is in the range of 1.8 to 2.4), and the nickel composition ratio has been increased to 75%. This transition metal oxide is used as a positive electrode active material.

[0007] International Publication No. WO 2016 / 024496 International Publication No. WO 2010 / 113583

[0008] However, in the lithium ion secondary battery using the secondary battery electrolyte of Patent Document 1, satisfactory battery characteristics cannot be obtained when the battery is operated at a high charging voltage, and further improvement is required. In particular, in a high-voltage lithium ion secondary battery using a ternary transition metal oxide with a high nickel composition ratio as the positive electrode active material as disclosed in Patent Document 2, there are problems in that the electric capacity retention performance when stored in a high-temperature environment is insufficient and the internal resistance also increases.

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a nonaqueous electrolyte for a secondary battery, which has excellent electrical capacity retention characteristics when stored in a high-temperature environment in a secondary battery used at high voltage, effectively suppresses an increase in internal resistance, and exhibits good electrical capacity retention characteristics even when repeatedly charged and discharged at a high charging voltage after storage in a high-temperature environment, and a secondary battery including the same.

[0010] In order to solve the above-mentioned problems, the nonaqueous electrolyte solution for a secondary battery of the present invention is a nonaqueous electrolyte solution for a secondary battery comprising an electrolyte and a nonaqueous solvent, and further comprising at least one fluorophosphate represented by the following chemical formula (1) as an additive:

[0011] (In the formula, M ＋ represents an alkali metal ion. １ ~R ５ each independently represents a hydrogen atom; a hydrocarbon group having 1 to 10 carbon atoms; an alkoxy group having 1 to 10 carbon atoms; a hydrocarbon group having 1 to 10 carbon atoms and having at least one of a halogen atom or an unsaturated bond; or an alkoxy group having 1 to 10 carbon atoms and having at least one of a halogen atom or an unsaturated bond. １ ~R ５ are each independently any of the following: a hydrocarbon group having 1 to 10 carbon atoms; an alkoxy group having 1 to 10 carbon atoms; a hydrocarbon group having 1 to 20 carbon atoms and having at least one of a halogen atom, a hetero atom, or an unsaturated bond; or an alkoxy group having 1 to 20 carbon atoms and having at least one of a halogen atom, a hetero atom, or an unsaturated bond, any combination of which is bonded to each other to form a cyclic structure; and n represents an integer of 0 to 10.

[0012] In the above-mentioned configuration, the amount of the fluorophosphate added is preferably in the range of 0.01% by mass or more and 2% by mass or less with respect to the total mass of the non-aqueous electrolyte solution for secondary batteries.

[0013] In the above-mentioned configuration, the fluorophosphate is preferably lithium phenyl fluorophosphate, sodium phenyl fluorophosphate, lithium benzyl fluorophosphate, sodium benzyl fluorophosphate, 4-tert-amylphenyl lithium fluorophosphate, or sodium 4-tert-amylphenyl fluorophosphate.

[0014] The secondary battery according to the present invention is characterized by comprising at least the nonaqueous electrolyte solution for secondary batteries described above, a positive electrode, and a negative electrode.

[0015] In the above configuration, the charging voltage is preferably 4.3 V or higher.

[0016] In the above configuration, the secondary battery is preferably stored in an environment of 40° C. or higher after being charged with a charging voltage of 4.3 V or higher.

[0017] In the above-mentioned configuration, the positive electrode active material constituting the positive electrode active material layer in the positive electrode preferably contains a binary transition metal oxide or a ternary transition metal oxide.

[0018] According to the present invention, by adding at least one fluorophosphate salt represented by the chemical formula (1) as an additive to a nonaqueous electrolyte solution for a secondary battery, an increase in the internal resistance of the secondary battery can be effectively suppressed even when the secondary battery used at high voltage is stored in a high-temperature environment. Furthermore, the electrical capacity of the secondary battery can be well maintained, and the storage performance of the secondary battery can be improved. Furthermore, even when the secondary battery is charged and discharged at a high charging voltage after storage in a high-temperature environment, a high discharge capacity can be maintained, and the cycle characteristics can be improved.

[0019] 1 is a cross-sectional view showing an outline of a lithium ion secondary battery including a nonaqueous electrolyte solution for a secondary battery according to an embodiment of the present invention.

[0020] (Non-aqueous electrolyte for secondary battery) The non-aqueous electrolyte for secondary battery according to the present embodiment (hereinafter referred to as "nonaqueous electrolyte") contains at least an electrolyte, a non-aqueous solvent, and at least one fluorophosphate as an additive. The non-aqueous electrolyte of the present embodiment is suitably used as an electrolyte for secondary batteries such as lithium-ion secondary batteries used at high voltages.

[0021] In lithium-ion secondary batteries, it is believed that during initial charging, an irreversible decomposition reaction of the nonaqueous electrolyte occurs at the interface between the electrode and the nonaqueous electrolyte, resulting in the formation of a film on the electrode surface. The properties of the formed film, such as voltage resistance, thermal stability, ionic conductivity, morphology, and compactness, are thought to vary significantly depending on the electrode active material, the type and composition of the nonaqueous solvent, electrolyte, and additives in the nonaqueous electrolyte. In this embodiment, a fluorophosphate phenyl salt is added as an additive to the nonaqueous electrolyte to form a film on the surface of the electrode (electrode active material layer). The properties of this film, such as voltage resistance and thermal stability, are believed to provide excellent capacity retention and suppression of internal resistance increases when stored in a high-temperature environment when the lithium-ion secondary battery is used at a high charging voltage. Furthermore, it is believed that the capacity is well maintained even when repeatedly charged and discharged after storage in a high-temperature environment, resulting in excellent cycle characteristics.

[0022] In this specification, the phrase "containing a fluorophosphate as an additive" in a non-aqueous electrolyte means that the fluorophosphate is contained so as to function as an additive, separate from the electrolyte and the non-aqueous solvent. Here, the fluorophosphate functioning as an additive means, as described above, that a coating having properties such as voltage resistance and thermal stability is formed on the surface of the electrode (electrode active material layer).

[0023] <Fluorophosphate> The fluorophosphate of the present embodiment is represented by the following chemical formula (1).

[0024]

[0025] In the chemical formula (1), the M ＋ represents an alkali metal ion. The alkali metal ion is not particularly limited, and examples thereof include lithium ion, sodium ion, potassium ion, rubidium ion, and cesium ion. These ions may be used alone or in combination of two or more.

[0026] Said M ＋Among the above-listed examples, lithium ions are preferred from the viewpoint of battery characteristics. Furthermore, lithium ions and sodium ions are preferred from the viewpoint of availability and ease of synthesis of fluorophosphates.

[0027] In the chemical formula (1), R １ ~R ５ each independently represents a hydrogen atom; a hydrocarbon group having 1 to 10 carbon atoms; an alkoxy group having 1 to 10 carbon atoms; a hydrocarbon group having 1 to 10 carbon atoms and having at least one of a halogen atom or an unsaturated bond (hereinafter referred to as a "hydrocarbon group having a halogen atom or the like"); or an alkoxy group having 1 to 10 carbon atoms and having at least one of a halogen atom or an unsaturated bond (hereinafter referred to as an "alkoxy group having a halogen atom or the like"). Alternatively, R １ ~R ５ are each independently any of the following: a hydrocarbon group having 1 to 10 carbon atoms; an alkoxy group having 1 to 10 carbon atoms; a hydrocarbon group having 1 to 20 carbon atoms and having at least one of a halogen atom, a heteroatom, or an unsaturated bond; or an alkoxy group having 1 to 20 carbon atoms and having at least one of a halogen atom, a heteroatom, or an unsaturated bond, and any selected combinations thereof are bonded to each other to form a cyclic structure.

[0028] In this specification, when a range of carbon atoms is expressed, the range means that all integer carbon atoms within the range are included. Therefore, for example, a hydrocarbon group having "1 to 3 carbon atoms" means all hydrocarbon groups having 1, 2, and 3 carbon atoms.

[0029] The R １ ~R ５The hydrocarbon group having 1 to 10 carbon atoms in the formula (I) is not particularly limited, and examples thereof include linear or branched chain alkyl groups and cyclic alkyl groups. Examples of chain alkyl groups include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, and decyl groups. The cyclic alkyl groups are not particularly limited, and examples thereof include cyclopentyl and cyclohexyl groups. The number of carbon atoms in the hydrocarbon group is preferably 1 to 8, and more preferably 1 to 6.

[0030] The R １ ~R ５ The alkoxy group having 1 to 10 carbon atoms in the formula (I) is not particularly limited, and examples thereof include chain alkoxy groups and cyclic alkoxy groups. The chain alkoxy group is not particularly limited, and examples thereof include methoxy group, ethoxy group, propoxy group, butoxy group, pentoxy group, and hexoxy group. The cyclic alkoxy group is not particularly limited, and examples thereof include cyclopentoxy group and cyclohexoxy group. The number of carbon atoms in the alkoxy group is preferably 1 to 8, and more preferably 1 to 6.

[0031] The R １ ~R ５ In the above, a hydrocarbon group having 1 to 10 carbon atoms and containing a halogen atom refers to a functional group in which some or all of the hydrogen atoms in the hydrocarbon group have been substituted with halogen atoms. Examples of halogen atoms include fluorine, chlorine, bromine, and iodine atoms. Of the hydrocarbon group having 1 to 10 carbon atoms and containing a halogen atom, a hydrocarbon group having 1 to 8 carbon atoms and containing a halogen atom is preferred, and a hydrocarbon group having 1 to 6 carbon atoms and containing a halogen atom is more preferred.

[0032] The R １ ~R ５ In the above, the hydrocarbon group having 1 to 10 carbon atoms and an unsaturated bond refers to a hydrocarbon group having a carbon-carbon double bond or a triple bond. The number of unsaturated bonds is preferably in the range of 1 to 6, more preferably in the range of 1 to 4, and particularly preferably in the range of 1 to 3.

[0033] The hydrocarbon group having a halogen atom or the like is not particularly limited, and examples thereof include a 2-iodoethyl group, a 2-bromoethyl group, a 2-chloroethyl group, a 2-fluoroethyl group, a 1,2-diiodoethyl group, a 1,2-dibromoethyl group, a 1,2-dichloroethyl group, a 1,2-difluoroethyl group, a 2,2-diiodoethyl group, a 2,2-dibromoethyl group, a 2,2-dichloroethyl group, a 2,2-difluoroethyl group, a 2,2,2-tribromoethyl group, a 2,2,2-trichloroethyl group, and a 2-iodoethyl group. chain-like halogen-containing alkyl groups such as a 2-iodocyclohexyl group, a 2-bromocyclohexyl group, a 2-chlorocyclohexyl group, and a 2-fluorocyclohexyl group; chain-like alkenyl groups such as a 2-propenyl group, an isopropenyl group, a 2-butenyl group, and a 3-butenyl group; chain-like alkenyl groups such as a 2-cyclopentenyl group, a 2-cyclohexenyl group, and a 3-cyclohexenyl group; Cyclic alkenyl groups; chain alkynyl groups such as 2-propynyl group, 1-butynyl group, 2-butynyl group, 3-butynyl group, 1-pentynyl group, 2-pentynyl group, 3-pentynyl group, and 4-pentynyl group; aryl groups such as phenyl group; alkoxyphenyl groups such as 3-methoxyphenyl group, 4-methoxyphenyl group, 3,5-dimethoxyphenyl group, and 4-phenoxyphenyl group; 2-iodophenyl group, 2-bromophenyl group, 2-chlorophenyl group, 2-fluorophenyl group, halogen-containing phenyl groups such as an iodophenyl group, a 3-bromophenyl group, a 3-chlorophenyl group, a 3-fluorophenyl group, a 4-iodophenyl group, a 4-bromophenyl group, a 4-chlorophenyl group, a 4-fluorophenyl group, a 3,5-diiodophenyl group, a 3,5-dibromophenyl group, a 3,5-dichlorophenyl group, and a 3,5-difluorophenyl group; and naphthyl groups such as a 1-naphthyl group, a 2-naphthyl group, and a 3-amino-2-naphthyl group.

[0034] The R １ ~R ５In the above, an alkoxy group having 1 to 10 carbon atoms and containing a halogen atom refers to a functional group in which some or all of the hydrogen atoms in the alkoxy group have been substituted with halogen atoms. Examples of halogen atoms include fluorine, chlorine, bromine, and iodine atoms. Of the alkoxy group having 1 to 10 carbon atoms and containing a halogen atom, an alkoxy group having 1 to 8 carbon atoms and containing a halogen atom is preferred, and an alkoxy group having 1 to 6 carbon atoms and containing a halogen atom is more preferred.

[0035] The R １ ~R ５ In the above, the alkoxy group having 1 to 10 carbon atoms and an unsaturated bond refers to, for example, an alkoxy group having a carbon-carbon double bond or a triple bond. The number of unsaturated bonds is preferably in the range of 1 to 6, more preferably in the range of 1 to 4, and particularly preferably in the range of 1 to 3.

[0036] The alkoxy group having a halogen atom or the like is not particularly limited, and examples thereof include a 2-iodoethoxy group, a 2-bromoethoxy group, a 2-chloroethoxy group, a 2-fluoroethoxy group, a 1,2-diiodoethoxy group, a 1,2-dibromoethoxy group, a 1,2-dichloroethoxy group, a 1,2-difluoroethoxy group, a 2,2-diiodoethoxy group, a 2,2-dibromoethoxy group, a 2,2-dichloroethoxy group, a 2,2-difluoroethoxy group, a 2,2,2-tribromoethoxy group, a 2,2,2 -A chain halogen-containing alkoxy group such as a trichloroethoxy group, a 2,2,2-trifluoroethoxy group, and a 1,1,1,3,3,3-hexafluoro-2-propoxy group; a cyclic halogen-containing alkoxy group such as a 2-iodocyclohexoxy group, a 2-bromocyclohexoxy group, a 2-chlorocyclohexoxy group, and a 2-fluorocyclohexoxy group; a chain alkenylalkoxy group such as a 2-propenoxy group, an isopropenoxy group, a 2-butenoxy group, and a 3-butenoxy group; a 2-cyclopentylalkoxy group such as a 2-isopropyl alkoxy group, ... cyclic alkenylalkoxy groups such as phenoxy group, 2-cyclohexenoxy group, and 3-cyclohexenoxy group; chain alkynylalkoxy groups such as 2-propynoxy group, 1-butynoxy group, 2-butynoxy group, 3-butynoxy group, 1-pentynoxy group, 2-pentynoxy group, 3-pentynoxy group, and 4-pentynoxy group; aryloxy groups such as phenoxy group, 3-methylphenoxy group, 4-methylphenoxy group, and 3,5-dimethylphenoxy group; and 2-iodophenoxy group, Examples of halogen-containing phenoxy groups include a 2-bromophenoxy group, a 2-chlorophenoxy group, a 2-fluorophenoxy group, a 3-iodophenoxy group, a 3-bromophenoxy group, a 3-chlorophenoxy group, a 3-fluorophenoxy group, a 4-iodophenoxy group, a 4-bromophenoxy group, a 4-chlorophenoxy group, a 4-fluorophenoxy group, a 3,5-diiodophenoxy group, a 3,5-dibrophenoxy group, a 3,5-dichlorophenoxy group, and a 3,5-difluorophenoxy group.

[0037] The R １ ~R ５As described above, each of the groups may independently be any of a hydrocarbon group having 1 to 10 carbon atoms; an alkoxy group having 1 to 10 carbon atoms; a hydrocarbon group having 1 to 20 carbon atoms and having at least one of a halogen atom, a hetero atom, or an unsaturated bond; or an alkoxy group having 1 to 20 carbon atoms and having at least one of a halogen atom, a hetero atom, or an unsaturated bond, and any selected combination thereof may be bonded to each other to form a cyclic structure.

[0038] The R １ ~R ５ When the ring structure is formed, the hydrocarbon group having 1 to 10 carbon atoms and the alkoxy group having 1 to 10 carbon atoms are the same as those described above, and therefore, detailed description thereof will be omitted.

[0039] Also, the R １ ~R ５ When the hydrocarbon group having 1 to 20 carbon atoms and containing a halogen atom forms the aforementioned cyclic structure, the hydrocarbon group refers to a functional group in which some or all of the hydrogen atoms in the hydrocarbon group have been substituted with halogen atoms. Examples of halogen atoms include fluorine, chlorine, bromine, and iodine atoms. Of the hydrocarbon group having 1 to 20 carbon atoms and containing a halogen atom, a hydrocarbon group having 3 to 10 carbon atoms and containing a halogen atom is preferred, and a hydrocarbon group having 3 to 5 carbon atoms and containing a halogen atom is more preferred.

[0040] Also, the R １ ~R ５ When the above-mentioned hydrocarbon group having 1 to 20 carbon atoms and containing a heteroatom forms a cyclic structure, the hydrocarbon group refers to a functional group in which some or all of the hydrogen atoms and carbon atoms in the hydrocarbon group are substituted with heteroatoms. The heteroatom represents an atom such as oxygen, nitrogen, or sulfur. Among the hydrocarbon groups having 1 to 20 carbon atoms and containing a heteroatom, hydrocarbon groups having 2 to 10 carbon atoms are preferred, and hydrocarbon groups having 2 to 5 carbon atoms are more preferred.

[0041] Also, the R １ ~R ５When the hydrocarbon group forms the aforementioned cyclic structure, the hydrocarbon group having 1 to 20 carbon atoms and an unsaturated bond means, for example, a hydrocarbon group having a carbon-carbon double bond or a triple bond. The number of unsaturated bonds is preferably in the range of 1 to 6, more preferably in the range of 1 to 4, and particularly preferably in the range of 1 to 3.

[0042] Also, the R １ ~R ５ When the alkoxy group having 1 to 20 carbon atoms and containing a halogen atom forms the aforementioned cyclic structure, the alkoxy group refers to a functional group in which some or all of the hydrogen atoms in the alkoxy group have been substituted with halogen atoms. Examples of halogen atoms include fluorine, chlorine, bromine, and iodine atoms. Among the alkoxy groups having 1 to 20 carbon atoms and containing a halogen atom, alkoxy groups having 3 to 10 carbon atoms and containing a halogen atom are preferred, and alkoxy groups having 3 to 5 carbon atoms and containing a halogen atom are more preferred.

[0043] Also, the R １ ~R ５ In the case where the alkoxy group having 1 to 20 carbon atoms and containing a heteroatom forms the aforementioned cyclic structure, the alkoxy group refers to a functional group in which some or all of the hydrogen atoms and carbon atoms in the alkoxy group are substituted with heteroatoms. The heteroatom represents an atom such as oxygen, nitrogen, or sulfur. Among the alkoxy groups having 1 to 20 carbon atoms and containing a heteroatom, an alkoxy group having 2 to 10 carbon atoms and containing a heteroatom is preferred, and an alkoxy group having 2 to 5 carbon atoms and containing a heteroatom is more preferred.

[0044] Also, the R １ ~R ５ In the case where the alkoxy group forms the cyclic structure, the alkoxy group having 1 to 20 carbon atoms and an unsaturated bond means, for example, an alkoxy group having a carbon-carbon double bond or a triple bond. The number of unsaturated bonds is preferably in the range of 1 to 6, more preferably in the range of 1 to 4, and particularly preferably in the range of 1 to 3.

[0045] The R １ ~R ５Specific examples of the cyclic structure formed by any combination thereof include, for example, straight-chain alkylene groups such as methylene, ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, and nonylene; iodomethylene, diiodomethylene, bromomethylene, dibromomethylene, fluoromethylene, difluoromethylene, iodoethylene, 1,1-diiodoethylene, 1,2-diiodoethylene, triiodoethylene, and tetraiodoethylene; halogen-containing linear alkylene groups such as a chloroethylene group, a chloroethylene group, a 1,1-dichloroethylene group, a 1,2-dichloroethylene group, a trichloroethylene group, a tetrachloroethylene group, a fluoroethylene group, a 1,1-difluoroethylene group, a 1,2-difluoroethylene group, a trifluoroethylene group, and a tetrafluoroethylene group; and cyclic hydrocarbon groups such as a cyclohexylene group, a phenylene group, a benzylene group, a naphthylene group, an anthracylene group, a naphthasylene group, and a pentasylene group.Further, straight-chain alkylenedioxy groups such as methylenedioxy group, ethylenedioxy group, propylenedioxy group, butylenedioxy group, pentylenedioxy group, hexylenedioxy group, heptylenedioxy group, octylenedioxy group, and nonylenedioxy group; cyclic alkylenedioxy groups such as cyclohexylenedioxy group; iodomethylenedioxy group, diiodomethylenedioxy group, bromomethylenedioxy group, dibromomethylenedioxy group, fluoromethylenedioxy group, difluoromethylenedioxy group, iodoethylenedioxy group, 1,1-diiodoethylenedioxy group, 1,2-diiodoethylenedioxy group, triiodoethylenedioxy group, tetraiodoethylenedioxy group, chloro ... Examples of the halogen-containing linear alkylenedioxy group include ethylenedioxy group, 1,1-dichloroethylenedioxy group, 1,2-dichloroethylenedioxy group, trichloroethylenedioxy group, tetrachloroethylenedioxy group, fluoroethylenedioxy group, 1,1-difluoroethylenedioxy group, 1,2-difluoroethylenedioxy group, trifluoroethylenedioxy group, and tetrafluoroethylenedioxy group; arylenedioxy group, such as phenylenedioxy group, benzylenedioxy group, naphthylenedioxy group, anthracylenedioxy group, naphthasylenedioxy group, and pentasylenedioxy group; and those in which part or all of these functional groups have been replaced with halogen atoms, etc. Examples of halogen atoms include fluorine, chlorine, bromine, and iodine atoms.

[0046] In the chemical formula (1), n ​​represents an integer of 0 to 10. When n is 0, the fluorophosphate represented by the chemical formula (1) has a structure in which the phosphoric acid skeleton and the benzene skeleton are directly bonded, as shown below.

[0047] (In the formula, M ＋ , and R １ ~R ５ is the same as in the previous case.)

[0048] When n is an integer of 1 to 10, -(CH ２ ) ｎ- is a methylene group (n=1), an ethylene group (n=2), a propylene group (n=3), a butylene group (n=4), a pentylene group (n=5), a hexylene group (n=6), a heptylene group (n=7), an octylene group (n=8), a nonylene group (n=9), or a decanylene group (n=10). Among these, from the viewpoint of availability, a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, and a hexylene group, in which n is in the range of 1 to 6, are preferred. In addition, -(CH ２ ) ｎ The - may be either a straight chain or a branched chain.

[0049] Of the fluorophosphates represented by chemical formula (1), lithium phenyl fluorophosphate, sodium phenyl fluorophosphate, lithium benzyl fluorophosphate, sodium benzyl fluorophosphate, 4-tert-amylphenyl lithium fluorophosphate, and 4-tert-amylphenyl sodium fluorophosphate are preferred from the viewpoint of resource amount and availability of raw materials.

[0050] The amount of fluorophosphate added is preferably within the range of 0.01% by mass to 2% by mass, more preferably within the range of 0.05% by mass to 1% by mass, and even more preferably within the range of 0.1% by mass to 0.5% by mass, relative to the total mass of the non-aqueous electrolyte. By setting the amount to 0.01% by mass or more, a high discharge capacity can be maintained even after storage in a high-temperature environment, and cycle characteristics can be further improved even at high charge voltages. On the other hand, by setting the amount to 2% by mass or less, the solubility of the fluorophosphate in the non-aqueous solvent can be maintained at a good level. Here, "solubility" means that 0.005 g or more of the fluorophosphate dissolves in 100 g of non-aqueous solvent at 25°C.

[0051] <Electrolyte> As the electrolyte, a conventionally known electrolyte used in various secondary batteries can be used. From the viewpoint of solubility in a non-aqueous solvent and the characteristics of the secondary battery, the electrolyte preferably has an alkali metal ion as a cation. Examples of alkali metal ions include lithium ions, sodium ions, and potassium ions. For example, when the secondary battery is a lithium ion secondary battery, a lithium salt can be used as the electrolyte.

[0052] The electrolyte preferably has a fluorine-containing anion. The fluorine-containing anion is not particularly limited, and examples thereof include BF ４ － , P.F. ６ － , B.F. ３ CF ３ － , B.F. ３ C ２ F ５ － , C.F. ３ SO ３ － , C ２ F ５ SO ３ － , C ３ F ７ SO ３ － , C ４ F ９ SO ３ － , N(SO ２ F) ２ － , N(CF ３ SO ２ ) ２ － , N(C ２ F ５ SO ２ ) ２ － , N(CF ３ SO ２ ) (CF ３ CO) － , N(CF ３ SO ２ ) (C ２ F ５ SO ２ ) －, and C(CF ３ SO ２ ) ３ － These electrolytes having fluorine-containing anions can be used alone or in combination of two or more. Among the fluorine-containing anions, BF is particularly preferred from the viewpoint of safety and stability of the non-aqueous electrolyte, as well as improvement of electrical conductivity and cycle characteristics. ４ － , P.F. ６ － and N(CF ３ SO ２ ) ２ － is preferred, and BF ４ － and PF ６ － is particularly preferred.

[0053] The concentration of the electrolyte relative to the non-aqueous solvent is not particularly limited, but is typically in the range of 0.1 M to 2 M, preferably in the range of 0.15 M to 1.8 M, more preferably in the range of 0.2 M to 1.5 M, and particularly preferably in the range of 0.3 M to 1.2 M. By setting the electrolyte concentration to 0.1 M or more, it is possible to prevent the electrical conductivity of the non-aqueous electrolyte from becoming insufficient. On the other hand, by setting the electrolyte concentration to 2 M or less, it is possible to suppress a decrease in electrical conductivity due to an increase in the viscosity of the non-aqueous electrolyte, and to prevent a decrease in secondary battery performance.

[0054] <Non-aqueous solvent> The non-aqueous solvent (organic solvent) used in the non-aqueous electrolyte solution is not particularly limited, and examples thereof include cyclic carbonate esters, chain carbonate esters, phosphate esters, cyclic ethers, chain ethers, lactone compounds, chain esters, nitrile compounds, amide compounds, and sulfone compounds. These non-aqueous solvents can be used alone or in combination of two or more. Among these non-aqueous solvents, cyclic carbonate esters and chain carbonate esters are preferred in this embodiment.

[0055] The cyclic carbonate is not particularly limited, and examples thereof include cyclic carbonates in which at least a portion of the hydrogen atoms are substituted with fluorine atoms, such as ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, trans-difluoroethylene carbonate, and cis-difluoroethylene carbonate. These cyclic carbonates can be used alone or in combination of two or more.

[0056] The chain carbonate ester is not particularly limited, and examples thereof include dimethyl carbonate; ethyl methyl carbonate; diethyl carbonate; fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, trifluoromethyl methyl carbonate, bis(fluoromethyl)carbonate, bis(difluoro)methyl carbonate, bis(trifluoromethyl)carbonate, 2-fluoroethyl methyl carbonate, ethyl fluoromethyl carbonate, 2,2-difluoroethyl methyl carbonate, 2-fluoroethyl fluoromethyl carbonate, ethyl difluoromethyl carbonate, 2,2,2-trifluoroethyl methyl carbonate, 2,2-difluoroethyl fluoromethyl carbonate, 2-fluoroethyl methyl carbonate, Examples of the carbonate ester include linear carbonates in which at least a portion of the hydrogen atoms has been substituted with a fluorine atom, such as difluoroethyl difluoromethyl carbonate, ethyl trifluoromethyl carbonate, ethyl (2-fluoroethyl) carbonate, ethyl (2,2-difluoroethyl) carbonate, bis(2-fluoroethyl) carbonate, ethyl (2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl-2'-fluoroethyl carbonate, bis(2,2-difluoroethyl) carbonate, 2,2,2-trifluoroethyl-2'-fluoroethyl carbonate, 2,2,2-trifluoroethyl-2',2'-difluoroethyl carbonate, and bis(2,2,2-trifluoroethyl) carbonate. These linear carbonate esters can be used alone or in combination of two or more.

[0057] The phosphate ester is not particularly limited, and examples thereof include phosphate esters in which at least a portion of the hydrogen atoms has been substituted with a fluorine atom, such as trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, trifluoroethyl dimethyl phosphate, bis(trifluoroethyl)methyl phosphate, and tris(trifluoroethyl)phosphate. These phosphate esters may be used alone or in combination of two or more.

[0058] The cyclic ether is not particularly limited, and examples thereof include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, etc. These cyclic ethers can be used alone or in combination of two or more.

[0059] The chain ether is not particularly limited, and examples thereof include dimethoxyethane, ethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc. These chain ethers can be used alone or in combination of two or more.

[0060] The lactone compound is not particularly limited, and examples thereof include γ-butyrolactone, γ-valerolactone, δ-valerolactone, etc. These lactone compounds may be used alone or in combination of two or more.

[0061] The chain ester is not particularly limited, and examples thereof include chain esters in which at least a portion of hydrogen atoms is substituted with a fluorine atom, such as methyl propionate, methyl acetate, ethyl acetate, methyl formate, methyl difluoroacetate, and ethyl trifluoroacetate. These chain esters can be used alone or in combination of two or more.

[0062] The nitrile compound is not particularly limited, and examples thereof include acetonitrile, adiponitrile, valeronitrile, etc. These nitrile compounds may be used alone or in combination of two or more.

[0063] The amide compound is not particularly limited, and examples thereof include dimethylformamide.

[0064] The sulfone compound is not particularly limited, and examples thereof include sulfolane, sulfolene, 3-methylsulfolane, etc. These sulfone compounds may be used alone or in combination of two or more.

[0065] <Other Additives> In the nonaqueous electrolyte solution of the present embodiment, other additives may be contained in addition to the nonaqueous solvent, electrolyte, and fluorophosphate for the purpose of improving the performance of the secondary battery.

[0066] The other additives are not particularly limited, and examples thereof include at least one compound selected from the group consisting of phosphorus compounds, boron compounds, acid anhydrides, cyclic carbonates having unsaturated bonds, sulfur compounds, and amines having an acetoacetyl group.

[0067] The phosphorus compound is not particularly limited in type and may be selected from a variety of compounds as long as it does not impair the properties of the nonaqueous electrolyte solution of the present embodiment and the secondary battery using the same. Examples of the phosphorus compound include phosphines, phosphonic acids, phosphates, phosphate ester salts, and phosphorus complex salts.

[0068] Furthermore, the phosphines are not particularly limited and include, for example, trimelylphosphine, triethylphosphine, triisopropylphosphine, and triphenylphosphine. The phosphonic acids are not particularly limited and include, for example, dimethyl phosphite, diethyl phosphite, dibutyl phosphite, diphenyl phosphite, and bis(2,2,2-trifluoroethyl)phosphite. The phosphates are not particularly limited and include, for example, lithium fluorophosphate, sodium fluorophosphate, potassium fluorophosphate, lithium difluorophosphate, sodium difluorophosphate, and potassium difluorophosphate. The phosphate ester salt is not particularly limited, and examples thereof include lithium dimethyl phosphate, sodium dimethyl phosphate, potassium dimethyl phosphate, lithium diethyl phosphate, sodium diethyl phosphate, potassium diethyl phosphate, lithium dipropyl phosphate, sodium dipropyl phosphate, potassium dipropyl phosphate, lithium diphenyl phosphate, sodium diphenyl phosphate, potassium diphenyl phosphate, lithium bis(2,2,2-trifluoroethyl)phosphate, sodium bis(2,2,2-trifluoroethyl)phosphate, potassium bis(2,2,2-trifluoroethyl)phosphate, lithium bis(1,1,1,3,3,3-hexafluoroisopropyl)phosphate, sodium bis(1,1,1,3,3,3-hexafluoroisopropyl)phosphate, potassium bis(1,1,1,3,3,3-hexafluoroisopropyl)phosphate, etc. The phosphorus complex salt is not particularly limited, and examples thereof include lithium difluorobisoxalate phosphate, sodium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, and sodium tetrafluorooxalate phosphate. As the phosphorus compound, lithium difluorophosphate, lithium difluorobisoxalate phosphate, and lithium tetrafluorooxalate phosphate are preferred from the viewpoints of availability and battery characteristics.

[0069] The boron compound is not particularly limited in type, and various compounds can be selected as long as they do not impair the properties of the nonaqueous electrolyte solution of the present embodiment and the secondary battery using the same. Examples of the boron compound include borate, boric acid ester, and boron complex salt.

[0070] The borate is not particularly limited, and examples thereof include lithium tetraborate, sodium tetraborate, potassium tetraborate, lithium tetrafluoroborate, sodium tetrafluoroborate, and potassium tetrafluoroborate. The borate ester is not particularly limited, and examples thereof include trimethyl borate, triethyl borate, triisopropyl borate, tributyl borate, tripentyl borate, trihexyl borate, triheptyl borate, triphenyl borate, tris(2,2,2-triiodoethyl) borate, tris(2,2,2-tribromoethyl) borate, tris(2,2,2-trichloroethyl) borate, tris(2,2,2-trifluoroethyl) borate, tris(4-iodophenyl) borate, tris(4-bromophenyl) borate, tris(4-chlorophenyl) borate, tris(4-fluorophenyl) borate, diethylmethyl borate, and ethyldimethyl borate. The boron complex salt is not particularly limited, and examples thereof include lithium difluorooxalatoborate, sodium difluorooxalatoborate, potassium difluorooxalatoborate, lithium bisoxalatoborate, sodium bisoxalatoborate, potassium bisoxalatoborate, lithium bissalicylateborate, sodium bissalicylateborate, potassium bissalicylateborate, lithium bis[1,2'-benziolate(2)-O,O']borate, sodium bis[1,2'-benziolate(2)-O,O']borate, and potassium bis[1,2'-benziolate(2)-O,O']borate. From the viewpoints of availability and battery characteristics, lithium difluorooxalatoborate, lithium bisoxalatoborate, and lithium bissalicylateborate are preferred as the boron compound.

[0071] The acid anhydride is not particularly limited in type, and various types can be selected as long as it does not impair the characteristics of the nonaqueous electrolyte solution of the present embodiment and the secondary battery using the same. Specific examples of the acid anhydride include acetic anhydride, propionic anhydride, butyric anhydride, valeric anhydride, hexanoic anhydride, heptanoic anhydride, octanoic anhydride, nonanoic anhydride, decanoic anhydride, eicosanoic anhydride, docosanoic anhydride, benzoic anhydride, 4-methoxybenzoic anhydride, diphenylacetic anhydride, crotonic anhydride, cyclohexanecarboxylic anhydride, elaidic anhydride, isobutyric anhydride, isovaleric anhydride, lauric anhydride, linoleic anhydride, myristic anhydride, angelica anhydride, hexano ... hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic anhydride, hexanoic linear carboxylic acid anhydrides such as phthalic anhydride, chlorodifluoroacetic anhydride, trichloroacetic anhydride, difluoroacetic anhydride, trifluoroacetic anhydride, and 4-trifluoromethylbenzoic anhydride; phthalic anhydride, 3-acetamidophthalic anhydride, 4,4'-carbonyldiphthalic anhydride, 4,4'-biphthalic anhydride, 3-iodophthalic anhydride, 3-bromophthalic anhydride, 3-chlorophthalic anhydride, 3-fluorophthalic anhydride, 4-iodophthalic anhydride, 4-bromophthalic anhydride, and 4-chlorophthalic anhydride; Phthalic anhydride, 4-fluorophthalic anhydride, 4,5-diiodophthalic anhydride, 4,5-dibromophthalic anhydride, 4,5-dichlorophthalic anhydride, 4,5-difluorophthalic anhydride, 4,4'-sulfonyldiphthalic anhydride, 3-nitrophthalic anhydride, 4-nitrophthalic anhydride, exo-3,6-epoxyhexahydrophthalic anhydride, exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic anhydride, tetraiodophthalic anhydride, tetrachlorophthalic anhydride, tetrafluorophthalic anhydride Phthalic anhydride, 4-tert-butylphthalic anhydride, 4-ethynylphthalic anhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, succinic anhydride, (R)-(+)-2-acetoxysuccinic anhydride, (S)-(-)-2-acetoxysuccinic anhydride, 2-buten-1-ylsuccinic anhydride, butylsuccinic anhydride, decylsuccinic anhydride, 2,3-dimethylsuccinic anhydride, 2-dodecen-1-ylsuccinic anhydride, dodecylsuccinic anhydride, octadecene-ic anhydride, (2,7-octadien-1-yl)succinic anhydride, n-octylsuccinic anhydride, hexadecylsuccinic anhydride, maleic anhydride, 2,3-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride, 2-(2-carboxyethyl)-3-methyl-maleic anhydride, 2,3-dimethylmaleic anhydride, 2,3-diphenylmaleic anhydride, phenylmaleic anhydride, 4-pentene-1,2-dicarboxylic anhydride, 2,3-anthracenedicarboxylic anhydride, bicyclo[2,2,2]octo- 5-ene-2,3-dicarboxylic anhydride, 4-bromo-1,8-naphthalenedicarboxylic anhydride, (±)-trans-1,2-cyclohexanedicarboxylic anhydride, cis-4-cyclohexene-1,2-dicarboxylic anhydride, 2,5-dibromo-3,4-thiophenedicarboxylic anhydride, 5,6-dihydro-1,4-dithiine-2,3-dicarboxylic anhydride, 2,2'-biphenyldicarboxylic anhydride, 4-methylcyclohexane-1,2-dicarboxylic anhydride, 3-methyl-4-cyclohexene-1, 2-Dicarboxylic anhydride, 4-methyl-4-cyclohexene-1,2-dicarboxylic anhydride, 2,3-naphthalenedicarboxylic anhydride, 3,4-thiophenedicarboxylic anhydride, 1,8-naphthalenedicarboxylic anhydride, 5-norbornene-2,3-dicarboxylic anhydride, 1,2-cyclopropanedicarboxylic anhydride, glutaric anhydride, 3,3-pentamethyleneglutaric anhydride, 2,2-dimethylglutaric anhydride, 3,3-dimethylglutaric anhydride, 3-methylglutaric anhydride, 2-phthalimidoglycan Glutaric anhydride, 3,3-tetramethyleneglutaric anhydride, N-methylisatoic anhydride, 4-iodoisatoic anhydride, 4-bromoisatoic anhydride, 4-chloroisatoic anhydride, 4-fluoroisatoic anhydride, 5-iodoisatoic anhydride, 5-bromoisatoic anhydride, 5-chloroisatoic anhydride, 5-fluoroisatoic anhydride, itaconic anhydride, caronic anhydride, citraconic anhydride, diglycolic anhydride, 1,2-naphthalic anhydride, pyromellitic anhydride, HET anhydride, and 2,2,3,3,4,Cyclic carboxylic acid anhydrides such as 4-hexafluoropentanedioic anhydride; linear sulfonic acid anhydrides such as trifluoromethanesulfonic acid anhydride and p-toluenesulfonic acid anhydride; cyclic sulfonic acid anhydrides such as 2-sulfobenzoic acid anhydride, tetraiodo-O-sulfobenzoic acid anhydride, tetrabromo-O-sulfobenzoic acid anhydride, tetrachloro-O-sulfobenzoic acid anhydride, and tetrafluoro-O-sulfobenzoic acid anhydride; linear phosphinic acid anhydrides such as diphenylphosphinic acid; cyclic phosphonic acid anhydrides such as 1-propanephosphonic acid anhydride; and 3,4-diiodophenyl Examples of suitable acid anhydrides include boronic acid anhydride, 3,4-dibromophenylboronic acid anhydride, 3,4-dichlorophenylboronic acid anhydride, 3,4-difluorophenylboronic acid anhydride, 4-iodophenylboronic acid anhydride, 4-bromophenylboronic acid anhydride, 4-chlorophenylboronic acid anhydride, 4-fluorophenylboronic acid anhydride, m-terphenylboronic acid anhydride, 3,4,5-triiodophenylboronic acid anhydride, 3,4,5-tribromophenylboronic acid anhydride, 3,4,5-trichlorophenylboronic acid anhydride, and 3,4,5-trifluorophenylboronic acid anhydride. Among these acid anhydrides, maleic acid anhydride is preferred from the viewpoints of availability and battery characteristics.

[0072] The cyclic carbonate having an unsaturated bond is not particularly limited in type, and various types can be selected as long as it does not impair the properties of the nonaqueous electrolyte solution of the present embodiment and the secondary battery using the same. Examples of cyclic carbonates having an unsaturated bond include vinylene carbonate, iodovinylene carbonate, bromovinylene carbonate, chlorovinylene carbonate, fluorovinylene carbonate, 1,2-diiodovinylene carbonate, 1,2-dibromovinylene carbonate, 1,2-dichlorovinylene carbonate, 1,2-difluorovinylene carbonate, methylvinylene carbonate, iodomethylvinylene carbonate, bromomethylvinylene carbonate, chloromethylvinylene carbonate, fluoromethylvinylene carbonate, and dichlorovinylene carbonate. Examples of the cyclic carbonate having an unsaturated bond include methyl vinylene carbonate, dibromomethyl vinylene carbonate, dichloromethyl vinylene carbonate, difluoromethyl vinylene carbonate, triiodomethyl vinylene carbonate, tribromomethyl vinylene carbonate, trichloromethyl vinylene carbonate, trifluoromethyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, butyl vinylene carbonate, dimethyl vinylene carbonate, diethyl vinylene carbonate, dipropyl vinylene carbonate, and vinyl ethylene carbonate. From the viewpoints of availability and battery characteristics, vinylene carbonate and vinyl ethylene carbonate are preferred as the cyclic carbonate having an unsaturated bond.

[0073] The sulfur compound is not particularly limited in type, and various compounds can be selected as long as they do not impair the properties of the nonaqueous electrolyte solution of the present embodiment and the secondary battery using the same. Examples of the sulfur compound include sulfates, sulfonates, sulfonic acid esters, and sulfones.

[0074] The sulfate salt is not particularly limited, and examples thereof include lithium sulfate, sodium sulfate, potassium sulfate, lithium fluorosulfate, sodium fluorosulfate, potassium fluorosulfate, lithium methylsulfate, sodium methylsulfate, potassium methylsulfate, lithium ethylsulfate, sodium ethylsulfate, and potassium ethylsulfate. The sulfonate salt is not particularly limited, and examples thereof include lithium trifluoromethanesulfonate, sodium trifluoromethanesulfonate, and potassium trifluoromethanesulfonate. The sulfonic acid ester is not particularly limited, and examples thereof include 1,3-propane sultone, 2,4-butane sultone, 1,4-butane sultone, ethylene sulfite, methyl methanesulfonate, and ethyl methanesulfonate. The sulfones are not particularly limited, and examples thereof include dimethyl sulfone, diethyl sulfone, diphenyl sulfone, methyl phenyl sulfone, sulfolane, and sulfolene. From the viewpoints of ease of availability and battery characteristics, lithium fluorosulfate, lithium trifluoromethanesulfonate, 1,3-propane sultone, and ethylene sulfite are preferred as the sulfur compound.

[0075] The amines having an acetoacetyl group are not particularly limited in type and can be selected from a variety of amines as long as they do not impair the characteristics of the nonaqueous electrolyte solution of the present embodiment and the secondary battery using the same. Examples of amines having an acetoacetyl group include N,N-dimethylacetoacetamide, N,N-diethylacetoacetamide, N,N-dipropylacetoacetamide, N,N-dibutylacetoacetamide, N,N-ethylmethylacetoacetamide, N,N-methylpropylacetoacetamide, and N,N-butylmethylacetoacetamide.

[0076] The content of the other additives can be appropriately set as needed, as long as it does not impair the characteristics of the secondary battery. The content of the other additives is typically preferably within a range of 0.05% by mass to 20% by mass, more preferably within a range of 0.1% by mass to 10% by mass, and particularly preferably within a range of 0.5% by mass to 5% by mass, relative to the total mass of the non-aqueous electrolyte. By setting the content of the other additives to 0.05% by mass or more, the effect of the other additives, i.e., the formation of a more stable coating on the electrode surface, can be achieved. On the other hand, by setting the content of the other additives to 20% by mass or less, excessive reduction in the solubility of the other additives in the non-aqueous solvent can be suppressed. Here, "solubility" means that the other additive dissolves in an amount of 0.005 g or more in 100 g of non-aqueous solvent at 25°C.

[0077] The cyclic carbonate esters described for use as nonaqueous solvents, in which at least some of the hydrogen atoms have been substituted with fluorine atoms, and the chain carbonate esters, in which at least some of the hydrogen atoms have been substituted with fluorine atoms, can also be used as additives separate from the nonaqueous solvent. In this case, the content of the cyclic carbonate esters and chain carbonate esters is preferably within a range of 0.05% by mass to 20% by mass, more preferably within a range of 0.1% by mass to 10% by mass, and particularly preferably within a range of 0.5% by mass to 5% by mass, relative to the total mass of the nonaqueous electrolyte. By making the content of the cyclic carbonate esters and chain carbonate esters 0.05% by mass or more, they can function as additives, forming a more stable film on the electrode surface. On the other hand, by making the content of the cyclic carbonate esters and chain carbonate esters 20% by mass or less, excessive reduction in the solubility of the cyclic carbonate esters and chain carbonate esters in the nonaqueous solvent can be suppressed.

[0078] <Production of Non-Aqueous Electrolyte for Secondary Battery> Next, a method for producing the non-aqueous electrolyte of this embodiment will be described below. The non-aqueous electrolyte of this embodiment is produced, for example, by adding the electrolyte salt to the non-aqueous solvent (organic solvent) and then adding at least one of the fluorophosphates. Furthermore, other conventionally known additives may be added. In this case, it is preferable to use the non-aqueous solvent, the electrolyte salt, the fluorophosphate, and other additives by purifying them in advance to a degree that does not reduce production efficiency, thereby minimizing the amount of impurities. When multiple types of the fluorophosphates or other additives are used, the order of addition can be appropriately determined as needed.

[0079] (Secondary Battery) Next, the secondary battery of the present invention will be described below using a lithium ion secondary battery as an example. Fig. 1 is a cross-sectional view showing an outline of a lithium ion secondary battery equipped with a nonaqueous electrolyte according to this embodiment. Note that the secondary battery of the present invention can be applied to not only lithium ion secondary batteries but also sodium ion secondary batteries, potassium ion secondary batteries, magnesium ion secondary batteries, calcium ion secondary batteries, etc.

[0080] As shown in FIG. 1 , the lithium-ion secondary battery according to this embodiment has a structure in which a stack of a positive electrode 1, a separator 3, a negative electrode 2, and a spacer 7 is housed in an internal space formed by a positive electrode can 4 and a negative electrode can 5, in this order from the positive electrode can 4 side. A spring 8 is interposed between the negative electrode can 5 and the spacer 7, thereby appropriately pressing and fixing the positive electrode 1 and the negative electrode 2. A nonaqueous electrolyte containing the additive of this embodiment is impregnated between the positive electrode 1, the separator 3, and the negative electrode 2. The positive electrode can 4 and the negative electrode can 5 are joined by being sandwiched with a gasket 6 interposed between them, thereby sealing the stack.

[0081] The positive electrode active material in the positive electrode active material layer of the positive electrode 1 is not particularly limited, and examples thereof include a transition metal compound having a structure in which lithium ions can diffuse, or an oxide of the transition metal compound and lithium. More specifically, the positive electrode active material is, for example, LiCoO ２ LiNiO ２ LiMnO２ ; LiMn ２ O ４ ; Li ２ MnO ３ and LiMeO ２ (Me = Mn, Co, Ni) solid solution; LiFePO ４ ; LiMn ｘ Fe ｙ P.O. ４ (0<x<1, 0<y<1, x+y=1); LiCoPO ４ LiMnPO ４ ; LiNiPO ４ ; Li ２ CoP ２ O ７ ; Li ２ FePO ４ F; binary transition metal oxide; ternary transition metal oxide; LiFeF ３ ; TiO ２ , V ２ O ５ , and MoO ３ oxides such as TiS ２ , and sulfides such as FeS. Furthermore, examples of the positive electrode active material include conductive polymers such as polyacetylene, polyparaphenylene, polyaniline, and polypyrrole; activated carbon; radical-generating polymers; and carbon materials. Among the positive electrode active materials listed above, binary transition metal oxides and ternary transition metal oxides are preferred in this embodiment.

[0082] The binary transition metal oxide is not particularly limited, and examples thereof include LiNi ｘ１ Co ｙ１ O ｚ１ (0<x1<2, 0<y1<2, 2≦z1≦4, 1≦x1+y1≦2), LiNi ｘ２ Mn ｙ２ O ｚ２ (0<x2<2, 0<y2<2, 2≦z2≦4, 1≦x2+y2≦2), LiCo ｘ３ Mn ｙ３ O ｚ３ (0<x3<2, 0<y3<2, 2≦z3≦4, 1≦x3+y3≦2), etc. These binary transition metal oxides may have some of their oxygen atoms replaced with other non-metal atoms such as phosphorus atoms, boron atoms, or fluorine atoms.

[0083] The LiNi ｘ１ Co ｙ１ O ｚ１ , and LiNi ｘ２ Mn ｙ２ O ｚ２ In the binary transition metal oxide comprising the above formula, x1 and x2 represent the composition ratio of nickel in the binary transition metal oxide. x1 and x2 are each greater than 0 and less than 2, preferably greater than 0.1 and less than 1.9, and more preferably greater than 0.5 and less than 1.5. x1 and x2 can be controlled by adjusting the amount of nickel supplied during the production of the positive electrode active material. Furthermore, y1 represents the composition ratio of cobalt in the binary transition metal oxide. y1 is greater than 0 and less than 2, preferably greater than 0.1 and less than 1.9, and more preferably greater than 0.5 and less than 1.5. y1 can be controlled by adjusting the amount of cobalt supplied during the production of the positive electrode active material. y2 represents the composition ratio of manganese in the binary transition metal oxide. y2 is greater than 0 and less than 2, preferably greater than 0.1 and less than 1.9, and more preferably greater than 0.5 and less than 1.5. y2 can be controlled by adjusting the amount of manganese supplied when producing the positive electrode active material. ｘ２ Mn ｙ２ O ｚ２ As a binary transition metal oxide consisting of LiNi, from the viewpoint of availability and battery characteristics, ０．５ Mn １．５ O ４ is preferred.

[0084] The LiCo ｘ３ Mn ｙ３ O ｚ３In the binary transition metal oxide comprising the above, x3 represents the composition ratio of cobalt in the binary transition metal oxide. x3 is greater than 0 and less than 2, preferably 0.1 to 1.9, and more preferably 0.5 to 1.5. x3 can be controlled by adjusting the amount of cobalt supplied during the production of the positive electrode active material. y3 represents the composition ratio of manganese in the binary transition metal oxide. y3 is greater than 0 and less than 2, preferably 0.1 to 1.9, and more preferably 0.5 to 1.5. y3 can be controlled by adjusting the amount of manganese supplied during the production of the positive electrode active material.

[0085] The ternary transition metal oxides include LiNi ｘ４ Co ｙ４ Mn ｚ４ O ２ (0<x4<1, 0<y4<1, 0<z4<1, x4+y4+z4=1), and LiNi ｘ５ Co ｙ５ Al ｚ５ O ２ (0<x5<1, 0<y5<1, 0<z5<1, x5+y5+z5=1). These ternary transition metal oxides may have some of the oxygen atoms substituted with other non-metal atoms such as phosphorus atoms, boron atoms, or fluorine atoms.

[0086] The LiNi ｘ４ Co ｙ４ Mn ｚ４ O ２In the ternary transition metal oxide comprising the above, x4 represents the composition ratio of nickel in the ternary transition metal oxide. x4 is greater than 0 and less than 1, preferably 0.6 or greater but less than 1, and more preferably 0.6 or greater but less than 0.9. x4 can be controlled by adjusting the amount of nickel supplied during the production of the positive electrode active material. y4 represents the composition ratio of cobalt in the ternary transition metal oxide. y4 is greater than 0 and less than 1, preferably 0.05 or greater but less than 0.2, and more preferably 0.05 or greater but less than 0.1. y4 can be controlled by adjusting the amount of cobalt supplied during the production of the positive electrode active material. z4 represents the composition ratio of manganese in the ternary transition metal oxide. z4 is greater than 0 and less than 1, preferably 0.05 or greater but less than 0.2, and more preferably 0.05 or greater but less than 0.1. The value of z4 can be controlled by adjusting the amount of manganese supplied during the production of the positive electrode active material.

[0087] The LiNi ｘ５ Co ｙ５ Al ｚ５ O ２In the ternary transition metal oxide comprising the above, x5 represents the composition ratio of nickel in the ternary transition metal oxide. x5 is greater than 0 and less than 1, preferably 0.6 or greater but less than 1, and more preferably 0.6 or greater but less than 0.9. x5 can be controlled by adjusting the amount of nickel supplied during the production of the positive electrode active material. y5 represents the composition ratio of cobalt in the ternary transition metal oxide. y5 is greater than 0 and less than 1, preferably 0.05 or greater but less than 0.2, and more preferably 0.05 or greater but less than 0.1. y5 can be controlled by adjusting the amount of cobalt supplied during the production of the positive electrode active material. z5 represents the composition ratio of aluminum in the ternary transition metal oxide. z5 is greater than 0 and less than 1, preferably 0.05 or greater but less than 0.2, and more preferably 0.05 or greater but less than 0.1. z5 can be controlled by adjusting the amount of aluminum supplied during the production of the positive electrode active material.

[0088] Among the positive electrode active materials exemplified above, LiCoO ２ LiNiO ２ ; Li ２ MnO ３ and LiMeO ２ (Me = Mn, Co, Ni) solid solution; LiNi ｘ４ Co ｙ４ Mn ｚ４ O ２ (0<x4<1, 0<y4<1, 0<z4<1, x4+y4+z4=1), or LiNi ｘ５ Co ｙ５ Al ｚ５ O ２ (0<x5<1, 0<y5<1, 0<z5<1, x5+y5+z5=1) is preferred, and LiNi is more preferred. ｘ４ Co ｙ４ Mn ｚ４ O ２ (0.6≦x4<1, 0<y4≦0.4, 0<z4≦0.4, x4+y4+z4=1), or LiNi ｘ５ Co ｙ５ Alｚ５ O ２ A ternary transition metal oxide consisting of (0.6≦x5<1, 0<y5≦0.4, 0<z5≦0.4, x5+y5+z5=1) is used.

[0089] The positive electrode 1 can be obtained by pressure molding the positive electrode active material listed above together with a known conductive additive and binder, or by mixing the positive electrode active material together with a known conductive additive and binder in an organic solvent such as pyrrolidone to form a paste, applying the paste to a current collector such as aluminum foil, and then drying the paste.

[0090] The material for the negative electrode active material layer in the negative electrode 2 is not particularly limited as long as it is a material capable of absorbing and releasing lithium, and examples thereof include metal composite oxides, lithium metal, lithium alloys, silicon, silicon-based alloys, tin-based alloys, metal oxides, and carbon materials.

[0091] The metal composite oxide is not particularly limited, and examples thereof include Li ４ Ti ５ O １２ , Li ｘ Fe ２ O ３ (0≦x≦1), Li ｘ WO ２ (0≦x≦1), Sn ｘ Me １ １－ｘ Me ２ ｙ O ｚ (Me １ = Mn, Fe, Pb, Ge, Me ２ =Al, B, P, Si, elements of groups 1 to 3 of the periodic table, halogens, where 0≦x≦1, 1≦y≦3, 1≦z≦8), etc.

[0092] The metal oxide is not particularly limited, and examples thereof include SnO, SnO ２ , SiO ｘ (0<x<2), PbO, PbO ２ , Pb ２ O ３ , Pb ３ O ４ , Sb ２ O ３ , Sb ２ O ４, Sb ２ O ５ , GeO, GeO ２ , Bi ２ O ３ , Bi ２ O ４ , Bi ２ O ５ etc.

[0093] The carbon material is not particularly limited, and examples thereof include natural graphite, artificial graphite, boronized graphite, fluorinated graphite, mesocarbon microbeads, pitch-based graphitized carbon fibers, carbon nanotubes, hard carbon, and fullerene.

[0094] The electrode material in foil or powder form can be used for the negative electrode 2. In the case of powder form, it can be obtained by pressure molding together with a known conductive additive and binder, or by mixing together with a known conductive additive and binder in an organic solvent such as pyrrolidone to form a paste, which is then applied to a current collector such as copper foil and then dried.

[0095] In the lithium-ion secondary battery according to this embodiment, a separator 3 is typically interposed between the positive electrode 1 and the negative electrode 2 to prevent short-circuiting between them. The material and shape of the separator 3 are not particularly limited, but a material that is easily permeable to the nonaqueous electrolyte, insulating, and chemically stable is preferred. Examples include microporous films, sheets, and nonwoven fabrics made of various polymeric materials, those with glass coatings on their surfaces, and glass fiber nonwoven fabrics. Specific examples of polymeric materials include polyolefin-based polymers such as nylon (registered trademark), nitrocellulose, polyacrylonitrile, polyvinylidene fluoride, polyethylene, and polypropylene. From the viewpoints of electrochemical and chemical stability, polyolefin-based polymers are preferred.

[0096] The lithium ion secondary battery of this embodiment can be charged at a high charging voltage, preferably 4.3 V or higher, more preferably in the range of 4.3 V to 5 V, even more preferably in the range of 4.35 V to 4.8 V, and particularly preferably in the range of 4.4 V to 4.7 V.

[0097] Furthermore, the lithium-ion secondary battery of the present embodiment has excellent storage performance, suppressing an increase in internal resistance even when stored in a high-temperature environment after charging. Furthermore, even after storage in a high-temperature environment, the battery maintains a good discharge capacity and exhibits excellent cycle characteristics, even when repeatedly charged and discharged at a high charging voltage. Here, "storage in a high-temperature environment" means storage at a temperature of 40°C or higher, preferably 40°C or higher and 80°C or lower, more preferably 40°C or higher and 70°C or lower, and even more preferably 40°C or higher and 60°C or lower. Furthermore, "repeated charging and discharging at a high charging voltage" means storage at a charging voltage of preferably 4.3V or higher, more preferably 4.3V or higher and 5V or lower, even more preferably 4.35V or higher and 4.8V or lower, and particularly preferably 4.4V or higher and 4.7V or lower.

[0098] The shape of the lithium ion secondary battery of this embodiment is not particularly limited, and examples thereof include a cylindrical type, a square type, a laminate type, and the like in addition to the coin type cell shown in FIG.

[0099] The lithium ion secondary battery having the nonaqueous electrolyte solution of this embodiment is suitable as an on-board power source for electric vehicles (xEVs) and hybrid electric vehicles, which require high operating voltage and high-temperature storage performance.

[0100] The secondary battery according to this embodiment can exhibit excellent cycle characteristics even in a high-temperature environment, and the nonaqueous electrolyte solution according to this embodiment can be suitably used for, for example, a lithium-ion secondary battery. However, the lithium-ion secondary battery shown in FIG. 1 is an example of one embodiment of the secondary battery of the present invention, and the secondary battery of the present invention is not limited thereto.

[0101] Preferred manufacturing examples and working examples of the present invention are described in detail below. However, the described manufacturing examples, materials, blending amounts, etc., do not limit the scope of the present invention unless otherwise specified.

[0102] <Preparation of Phenyllithium Fluorophosphate> Phenyllithium fluorophosphate as a fluorophosphate salt was prepared by the following method.

[0103] Specifically, in a 250 mL recovery flask equipped with a stir bar, 8.7 g of phenyl phosphate and 4 g of 50% by weight hydrofluoric acid were dissolved in 40 mL of pyridine solvent (non-aqueous solvent), and 28.6 g of trichloroacetonitrile was added in small portions while stirring at 60°C. The mixture was then stirred for 15 hours and then concentrated under reduced pressure. Subsequently, 50 mL of a 1.0 mol / L aqueous lithium hydroxide solution was added to the mixture, which was then neutralized with an ion exchange resin and filtered. The excess water and solvent were then removed by evaporation under reduced pressure. The resulting product was further purified to yield 2.2 g of a white solid.

[0104] The obtained white solid was subjected to anion analysis by ion chromatography (trade name: IC-850, manufactured by Metrohm Japan Co., Ltd.), and a peak corresponding to phenyl fluorophosphate ions was observed at a retention time of 8.8 min, with a relative area of ​​93%. Furthermore, when cation analysis was performed by ion chromatography (trade name: Dionex ICS-1500, manufactured by Thermo Fisher Scientific Co., Ltd.), a peak corresponding to lithium ions was detected. This confirmed that the obtained white solid was phenyl lithium fluorophosphate.

[0105] Example 1 Preparation of Non-Aqueous Electrolyte Solution In an argon atmosphere dry box with a dew point of −70° C. or less, a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=1:1, manufactured by Kishida Chemical Co., Ltd., lithium battery grade) was dissolved in LiPF ６ LiPF ６ The mixture of LiPF ６ The concentration was adjusted to 1.0 mol / liter.

[0106] Next, LiPF ６ Phenyllithium fluorophosphate was added to a mixed solvent containing the above. The amount of phenyllithium fluorophosphate added was 0.3 mass % relative to the total mass of the nonaqueous electrolyte solution. In this way, a nonaqueous electrolyte solution according to this example was prepared.

[0107] Example 2 In this example, benzyllithium fluorophosphate was used instead of phenyllithium fluorophosphate. The amount of benzyllithium fluorophosphate added was 0.3 mass% relative to the total mass of the nonaqueous electrolyte solution. Other than these, the nonaqueous electrolyte solution of this example was prepared in the same manner as in Example 1.

[0108] Example 3 In this example, 4-tert-amylphenyllithium fluorophosphate was used instead of phenyllithium fluorophosphate. The amount of 4-tert-amylphenyllithium fluorophosphate added was 0.3 mass% relative to the total mass of the nonaqueous electrolyte. Other than these, the nonaqueous electrolyte of this example was prepared in the same manner as in Example 1.

[0109] Example 4 In this example, 4-tert-amylphenyllithium fluorophosphate was used instead of phenyllithium fluorophosphate. The amount of 4-tert-amylphenyllithium fluorophosphate added was 0.01% by mass relative to the total mass of the nonaqueous electrolyte. Other than these, the nonaqueous electrolyte of this example was prepared in the same manner as in Example 1.

[0110] Example 5 In this example, the volume ratio of EC:DMC was changed from 1:1 to 1:4, and 4-tert-amylphenyllithium fluorophosphate was used instead of phenyllithium fluorophosphate. The amount of 4-tert-amylphenyllithium fluorophosphate added was 2% by mass relative to the total mass of the nonaqueous electrolyte. Other than these, the nonaqueous electrolyte of this example was prepared in the same manner as in Example 1.

[0111] Comparative Example 1 In this comparative example, ethyllithium fluorophosphate was used instead of phenyllithium fluorophosphate. The amount of ethyllithium fluorophosphate added was 0.5% by mass relative to the total mass of the nonaqueous electrolyte solution. Other than these, the nonaqueous electrolyte solution according to this comparative example was prepared in the same manner as in Example 1.

[0112] Comparative Example 2 In this comparative example, methyllithium fluorophosphate was used instead of phenyllithium fluorophosphate. The amount of methyllithium fluorophosphate added was 0.5% by mass relative to the total mass of the nonaqueous electrolyte solution. Other than these, the nonaqueous electrolyte solution of this comparative example was prepared in the same manner as in Example 1.

[0113] Comparative Example 3 In this comparative example, a non-aqueous electrolyte solution according to this comparative example was prepared in the same manner as in Example 1, except that phenyllithium fluorophosphate was not added.

[0114] Comparative Example 4 In this comparative example, the volume ratio of EC:DMC was changed from 1:1 to 1:4, and the nonaqueous electrolyte solution according to this comparative example was prepared in the same manner as in Example 1, except that phenyllithium fluorophosphate was not added.

[0115] (Evaluation of High-Temperature Storage Performance and Cycle Characteristics) <Preparation of Coin Cells> Coin-type lithium ion secondary batteries (coin cells) as shown in FIG. 1 were prepared using the nonaqueous electrolyte solutions prepared in Examples 1 to 5 and Comparative Examples 1 to 4, respectively, and their electrochemical characteristics were evaluated.

[0116] That is, the positive electrode was made of LiNi cut to a diameter of 11.3 mm. ０．６ Co ０．２ Mn ０．２ O ２ A cathode (manufactured by Piotrec Corporation) was used, and glass filter paper (product name: GC-50, manufactured by Advantec Toyo Kaisha) was used as the separator. Furthermore, natural graphite (manufactured by Piotrec Corporation) cut to a diameter of 13 mm was used for the negative electrode. The positive electrode, separator, and negative electrode were stacked in this order to form a laminate, which was then impregnated with the nonaqueous electrolyte prepared in Examples 1 to 5 and Comparative Examples 1 to 4, and the laminate was then sealed to produce each coin cell. The coin cells were all assembled in an argon glove box with a dew point of -70°C or lower.

[0117] <Coin Cell Break-in> Each of the produced coin cells was charged in a thermostatic chamber at 25°C with a charging current of 0.3 mA / cm ２ The battery was charged to a final voltage of 4.3 V at a charging current of 0.075 mA / cm ２The potential was maintained at 4.3 V until the discharge current was 0.3 mA / cm or less. ２ The battery was discharged at 1000 kJ / min to a final voltage of 3.0 V. Under these charge / discharge conditions, three cycles of charge / discharge were carried out by a constant current / voltage method, and the discharge capacity at the third cycle was taken as the initial discharge capacity.

[0118] <High-Temperature Storage Test> Next, after the break-in, each coin cell was placed in a thermostatic chamber at 25°C and charged to 0.3 mA / cm ２ The battery was charged to a final voltage of 4.3 V at a charging current of 0.075 mA / cm ２ The charging was terminated by maintaining a constant potential of 4.3 V until the voltage dropped below 4.3 V. Thereafter, the temperature in the thermostatic chamber was changed to 60° C., and each of the charged coin cells was stored at high temperature for two weeks.

[0119] After two weeks, the temperature in the thermostatic chamber was returned to 25°C, and the discharge current for each coin cell after high-temperature storage was 0.3 mA / cm ２ The capacity at this time was taken as the discharge capacity after high-temperature storage. Table 1 shows the ratio of the discharge capacity after storage to the initial discharge capacity, which is taken as 100.

[0120] <Measurement of Internal Resistance of Coin Cells> The internal resistance of each coin cell was determined by an AC impedance method. After high-temperature storage, each coin cell was placed in a thermostatic chamber at 25°C and charged at a current of 0.3 mA / cm. ２ The coin cells were charged to 4.3 V at 100 V, and a sine wave with an amplitude of ±10 mV and a frequency of 1 MHz to 50 mHz was superimposed on the resultant Nyquist plot to determine the internal resistance of each coin cell. Table 1 shows the ratio of the internal resistance of each coin cell to the internal resistance of the coin cell using the non-electrolyte solution of Comparative Example 3, which is set to 100.

[0121]

[0122] <Cycle characteristic test 1> After high-temperature storage, each coin cell was placed in a thermostatic chamber at 25°C and charged at a current of 1.5 mA / cm ２ The battery was charged to a final voltage of 4.2 V at a charging current of 0.075 mA / cm ２ The potential was maintained at 4.2 V until the discharge current was 1.5 mA / cm or less. ２The battery was discharged at 100 V to a final voltage of 3.0 V. Under these charge / discharge conditions, 100 charge / discharge cycles were performed using a constant current / voltage method. Table 2 shows the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle, which is set to 100.

[0123]

[0124] <Cycle characteristic test 2> After high-temperature storage, each coin cell was placed in a thermostatic chamber at 25°C and charged at a current of 1.5 mA / cm ２ The battery was charged to a final voltage of 4.4 V at a charging current of 0.075 mA / cm ２ The potential was maintained at 4.4 V until the discharge current was 1.5 mA / cm or less. ２ The battery was discharged at 100 volts until the final voltage reached 3.0 V. Under these charge / discharge conditions, 100 charge / discharge cycles were performed using a constant current / constant voltage method. Table 3 shows the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle, which is set to 100.

[0125]

[0126] (Results) As can be seen from Table 1, the coin cells using the nonaqueous electrolytes of Examples 1 to 5 had higher discharge capacities and lower internal resistances within the coin cells after being charged at a charging voltage of 4.3 V and stored at 60°C for two weeks, compared to the coin cells using the nonaqueous electrolytes of Comparative Examples 1 to 4.

[0127] Furthermore, as shown in Table 2, in cycle characteristic test 1 conducted at a charging voltage of 4.2 V, no significant difference in discharge capacity was observed between the coin cells using the nonaqueous electrolytes of Examples 1 to 5 and the coin cells using the nonaqueous electrolytes of Comparative Examples 1 to 4. However, in cycle characteristic test 2 conducted at a charging voltage of 4.4 V, the coin cells using the nonaqueous electrolytes of Examples 1 to 5 had a higher discharge capacity after 100 charge / discharge cycles than the coin cells using the nonaqueous electrolytes of Comparative Examples 1 to 4.

[0128] These results confirm that the lithium ion secondary batteries using the nonaqueous electrolyte solutions of Examples 1 to 5 have excellent storage performance in high-temperature environments, suppress an increase in the internal resistance of the battery, and have excellent cycle characteristics even at high charging voltages.

[0129] 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode can 5 Negative electrode can 6 Gasket 7 Spacer 8 Spring

Claims

1. A non-aqueous electrolyte solution for a secondary battery comprising an electrolyte and a non-aqueous solvent, the non-aqueous electrolyte solution further comprising at least one fluorophosphate salt represented by the following chemical formula (1) as an additive: (In the formula, M ＋ represents an alkali metal ion. １ ~R ５ each independently represents a hydrogen atom; a hydrocarbon group having 1 to 10 carbon atoms; an alkoxy group having 1 to 10 carbon atoms; a hydrocarbon group having 1 to 10 carbon atoms and having at least one of a halogen atom or an unsaturated bond; or an alkoxy group having 1 to 10 carbon atoms and having at least one of a halogen atom or an unsaturated bond. １ ~R ５ are each independently any of a hydrocarbon group having 1 to 10 carbon atoms; an alkoxy group having 1 to 10 carbon atoms; a hydrocarbon group having 1 to 20 carbon atoms and having at least one of a halogen atom, a hetero atom, or an unsaturated bond; or an alkoxy group having 1 to 20 carbon atoms and having at least one of a halogen atom, a hetero atom, or an unsaturated bond, any combination of which is bonded to each other to form a cyclic structure. n represents an integer of 0 to 10.

2. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the amount of the fluorophosphate added is in the range of 0.01% by mass or more and 2% by mass or less based on the total mass of the non-aqueous electrolyte for a secondary battery.

3. The nonaqueous electrolyte for a secondary battery according to claim 1, wherein the fluorophosphate is lithium phenyl fluorophosphate, sodium phenyl fluorophosphate, lithium benzyl fluorophosphate, sodium benzyl fluorophosphate, lithium 4-tert-amylphenyl fluorophosphate, or sodium 4-tert-amylphenyl fluorophosphate.

4. A secondary battery comprising at least the nonaqueous electrolyte for secondary batteries according to any one of claims 1 to 3, a positive electrode, and a negative electrode.

5. The secondary battery according to claim 4, having a charging voltage of 4.3 V or higher.

6. The secondary battery according to claim 5, which is stored in an environment of 40° C. or higher after being charged with a charging voltage of 4.3 V or higher.

7. The secondary battery according to claim 4, wherein the positive electrode active material constituting the positive electrode active material layer in the positive electrode contains a binary transition metal oxide or a ternary transition metal oxide.

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