Nonaqueous electrolyte for storage element, nonaqueous electrolyte storage element, and method for manufacturing nonaqueous electrolyte storage element

Abstract

A non-aqueous electrolyte for a power storage element according to one embodiment of the present invention contains an imide salt (a), a salt (b) in which an anion charge center element is boron and which has an oxalate group, and a difluorophosphate (c). The content of the imide salt (a) is 3 or more in terms of molar ratio relative to the total content of the salt (b) in which an anion charge center element is boron and which has an oxalate group and the difluorophosphate (c). A non-aqueous electrolyte for a power storage element according to another embodiment of the present invention contains an imide salt (a), a salt (b) in which an anion charge center element is boron and which has an oxalate group, and a difluorophosphate (c). The content of the imide salt (a) is 25 mol% or more relative to the total content of all ion compounds.

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte for an electricity storage element, a non-aqueous electrolyte electricity storage element, and a method for producing a non-aqueous electrolyte electricity storage element. [Background technology]

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc. The non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. Furthermore, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte energy storage elements other than non-aqueous electrolyte secondary batteries.

[0003] Generally, nonaqueous electrolytes contain a nonaqueous solvent and an electrolyte salt that dissolves in the nonaqueous solvent, with other components added as needed. Lithium hexafluorophosphate (LiPF6) is widely used as the electrolyte salt. However, LiPF6 easily reacts with trace amounts of water contained in nonaqueous electrolyte storage elements, and the resulting hydrogen fluoride (HF) generated is known to degrade the charge-discharge performance of nonaqueous electrolyte storage elements. Meanwhile, imide salts such as LiN(SO2F)2 (LiFSI), LiN(C2F5SO2)2 (LiBETI), and LiN(CF3SO2)2 (LiTFSI) are also known as electrolyte salts used in nonaqueous electrolytes. Imide salts are less reactive with water than LiPF6 and rarely generate HF. Therefore, if an imide salt such as LiFSI is used as at least a portion of the electrolyte salt in a nonaqueous electrolyte, the nonaqueous electrolyte storage element using this salt is expected to exhibit good charge-discharge performance (see Patent Documents 1 and 2). Furthermore, salts in which the charge center element of the anion is boron, such as lithium bis(oxalato)borate (LiBOB) (see Patent Document 3), and difluorophosphates such as lithium difluorophosphate (LiPOF) (see Patent Document 4) are known as a type of charge-discharge cycle performance improving additive used in nonaqueous electrolytes that use LiPF as the electrolyte salt. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3878206 [Patent Document 2] Patent No. 5245373 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-050079 [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-031079 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in non-aqueous electrolyte storage elements, imide salts such as LiFSI, LiBETI, and LiTFSI are used as electrolyte salts at 1 mol dm -3 When used at a typical concentration around 4.0V (vs. Li / Li + It is known that when a nonaqueous electrolyte storage element is operated to achieve an operating potential of 1000 kJ / s or higher, oxidation corrosion of aluminum, which is used as a positive electrode substrate, etc., of the nonaqueous electrolyte storage element, is likely to occur. For this reason, even if an imide salt is used at a typical concentration instead of LiPF6 as the electrolyte salt of the nonaqueous electrolyte, sufficient improvement in charge-discharge cycle performance cannot be obtained. Furthermore, according to the findings of the inventors, when nonaqueous electrolytes using an imide salt at a typical concentration as the electrolyte salt or nonaqueous electrolytes using an imide salt as at least a part of the electrolyte salt and either a salt in which the charge center element of the anion is boron, such as LiBOB, or a difluorophosphate salt, such as LiPO2F2, as a charge-discharge cycle performance improving additive were prepared, and the charge-discharge cycle performance of lithium-ion secondary batteries, which are nonaqueous electrolyte storage elements, was examined, it was found that sufficient improvement in charge-discharge cycle performance could not be obtained.

[0006] The present invention has been made in light of the above circumstances, and its object is to provide a non-aqueous electrolyte storage element containing an imide salt as at least a part of an electrolyte salt, the positive electrode of which has a voltage of 4.0 V (vs. Li / Li + The present invention provides a non-aqueous electrolyte for an electricity storage element that can improve the charge-discharge cycle performance when operated so as to achieve an operating potential of at least 1000 kJ / s, a non-aqueous electrolyte electricity storage element including the same, and a method for manufacturing such a non-aqueous electrolyte electricity storage element. [Means for solving the problem]

[0007] One aspect of the present invention, which has been made to solve the above-mentioned problems, is a non-aqueous electrolyte for an energy storage element, comprising: an imide salt (a); a salt (b) having an anion whose charge center element is boron and which has an oxalate group; and a difluorophosphate (c), wherein the content of the imide salt (a) is 3 or more in terms of molar ratio to the total content of the salt (b) having an anion whose charge center element is boron and which has an oxalate group and the difluorophosphate (c).

[0008] Another embodiment of the present invention is a nonaqueous electrolyte for an energy storage element, comprising: an imide salt (a); a salt (b) having an anion whose charge center element is boron and which has an oxalate group; and a difluorophosphate (c), wherein the content of the imide salt (a) is 25 mol % or more with respect to the total content of all the ionic compounds.

[0009] Another aspect of the present invention is a nonaqueous electrolyte energy storage element including any one of the nonaqueous electrolytes for energy storage elements described above.

[0010] Another aspect of the present invention is a method for producing a nonaqueous electrolyte energy storage element using any one of the nonaqueous electrolytes for an energy storage element described above. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a nonaqueous electrolyte for an electricity storage element that can improve the charge-discharge cycle performance of a nonaqueous electrolyte electricity storage element, a nonaqueous electrolyte electricity storage element including the same, and a method for manufacturing such a nonaqueous electrolyte electricity storage element. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is an external perspective view showing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an electricity storage device constructed by assembling a plurality of nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] A non-aqueous electrolyte for a storage device according to one embodiment of the present invention contains an imide salt (a), a salt (b) having an oxalate group and in which the charge center element of the anion is boron, and a difluorophosphate salt (c).

[0014] According to the non-aqueous electrolyte for a storage element, the imide salt in the non-aqueous electrolyte is 1 mol dm -3 Even when used at typical concentrations around this range, oxidation corrosion of aluminum, such as the positive electrode substrate, can be suppressed, improving the charge-discharge cycle performance of nonaqueous electrolyte energy storage elements. While the reason for this effect in nonaqueous electrolyte energy storage elements is unclear, the following reasons are speculated. One cause of the deterioration of charge-discharge performance in nonaqueous electrolyte energy storage elements using LiPF6 as the electrolyte salt is corrosion of the positive electrode surface due to trace amounts of HF present near the positive electrode. This corrosion causes positive electrode active material components to leach from the positive electrode surface and diffuse to the negative electrode, resulting in precipitation of the positive electrode active material components on the negative electrode. This is thought to gradually increase the irreversible capacity and resistance of the negative electrode and reduce charge-discharge performance. The trace amounts of HF are speculated to be generated by decomposition of LiPF6 near the positive electrode. In contrast, imide salts exhibit strong interactions between cations and anions, resulting in the oxidation resistance of the anions. Therefore, it is speculated that nonaqueous electrolytes for energy storage elements containing imide salts rarely generate HF due to decomposition near the positive electrode, resulting in almost no corrosion of the positive electrode surface. As a result, it is presumed that an increase in the irreversible capacity and resistance of the negative electrode is suppressed, and the charge-discharge cycle performance of the nonaqueous electrolyte energy storage element can be improved. Furthermore, according to the nonaqueous electrolyte energy storage element, the charge center element of the anion is boron, and by containing both a salt having an oxalate group and a difluorophosphate, in addition to the above-mentioned suppression of corrosion of the positive electrode surface, the imide salt is added at a concentration of 1 mol dm -3 It is presumed that even when used at a general concentration around this range, oxidation corrosion of aluminum as a positive electrode substrate, etc. can be suppressed, etc. As a result, the effect of these salts as additives for improving charge-discharge cycle performance is fully exerted, and the charge-discharge cycle performance of nonaqueous electrolyte energy storage elements can be improved.

[0015] Furthermore, according to the nonaqueous electrolyte for a storage battery element, by setting the content of the imide salt (a) to a molar ratio of 3 or more to the total content of the salt (b) having an oxalato group and in which the charge center element of the anion is boron, and the difluorophosphate (c), it is possible to sufficiently obtain an effect of improving the charge-discharge cycle performance of a nonaqueous electrolyte element using the nonaqueous electrolyte for a storage battery element.

[0016] Alternatively, according to the nonaqueous electrolyte for an energy storage element, by setting the content of the imide salt (a) to 25 mol % or more relative to the total content of all the ionic compounds in the nonaqueous electrolyte, it is possible to sufficiently obtain the effect of improving the charge-discharge cycle performance of a nonaqueous electrolyte element using the nonaqueous electrolyte for an energy storage element.

[0017] The non-aqueous electrolyte for an energy storage element preferably further contains a chain non-aqueous solvent containing fluorine in the molecule.

[0018] In the nonaqueous electrolyte for an energy storage element, by further containing a chain-like nonaqueous solvent containing fluorine in the molecule, which has high oxidation resistance and a relatively low viscosity, it is possible not only to suppress side reactions (such as oxidative decomposition of the nonaqueous solvent) that may occur during charge and discharge of the nonaqueous electrolyte energy storage element, but also to suppress an increase in the viscosity of the nonaqueous electrolyte and a decrease in ionic conductivity, etc. Therefore, by containing a chain-like nonaqueous solvent containing fluorine in the molecule, it is possible to further improve the charge and discharge cycle performance of a nonaqueous electrolyte energy storage element using the nonaqueous electrolyte for an energy storage element, particularly when operated at a high voltage.

[0019] A nonaqueous electrolyte electricity storage element according to one embodiment of the present invention is a nonaqueous electrolyte electricity storage element including any of the nonaqueous electrolytes for electricity storage elements described above.

[0020] According to the nonaqueous electrolyte storage element, by including any of the nonaqueous electrolytes for storage elements described above, the charge-discharge cycle performance can be further improved even when the nonaqueous electrolyte contains an imide salt as at least a part of the electrolyte salt.

[0021] A method for producing a nonaqueous electrolyte storage element according to one embodiment of the present invention is a method for producing a nonaqueous electrolyte storage element using any of the nonaqueous electrolytes for storage elements described above.

[0022] According to this production method, by using any of the nonaqueous electrolytes for energy storage elements described above, it is possible to produce a nonaqueous electrolyte energy storage element with improved charge-discharge cycle performance, even when the electrolyte salt contains an imide salt as at least a part of the electrolyte salt.

[0023] A nonaqueous electrolyte for an electricity storage element, a nonaqueous electrolyte electricity storage element, and a method for producing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention will be described in detail below.

[0024] <Non-aqueous electrolyte for energy storage elements> A nonaqueous electrolyte for a storage element according to one embodiment of the present invention (hereinafter also simply referred to as "nonaqueous electrolyte") contains an imide salt (a), a salt (b) having an oxalato group and in which the charge center element of the anion is boron, and a difluorophosphate salt (c). The nonaqueous electrolyte is not limited to a liquid. In other words, the nonaqueous electrolyte includes not only liquids in which these salts are dissolved in a nonaqueous solvent, but also solids, gels, and the like.

[0025] (imide salt) Examples of imide salts include lithium imide salts, sodium imide salts, potassium imide salts, etc. Among these, lithium imide salts are preferred.

[0026] Examples of the lithium imide salt include LiN(SO2F)2 (lithium bis(fluorosulfonyl)imide: LiFSI), LiN(CF3SO2)2 (lithium bis(trifluoromethanesulfonyl)imide: LiTFSI), LiN(C2F5SO2)2 (lithium bis(pentafluoroethanesulfonyl)imide: LiBETI), LiN(C4F9SO2)2 (lithium bis(nonafluorobutanesulfonyl)imide), FSO2-N-SO2-C4F9Li, and CF3-SO2- Examples include lithium sulfonylimide salts such as N-SO2-C4F9Li, CF3-SO2-N-SO2-N-SO2CF3Li2, CF3-SO2-N-SO2-CF2-SO2-N-SO2-CF3Li2, CF3-SO2-N-SO2-CF2-SO3Li2, and CF3-SO2-N-SO2-CF2-SO2-C(-SO2CF3)2Li2; and lithium phosphonylimide salts such as LiN(POF2)2 (lithium bis(difluorophosphonyl)imide: LiDFPI). One or more types of lithium imide salts can be used.

[0027] The lithium imide salt preferably has a fluorine atom, specifically, for example, a fluorosulfonyl group, a difluorophosphonyl group, a fluoroalkyl group, or the like. Among the lithium imide salts, lithium sulfonylimide salts are preferred, LiFSI, LiTFSI, and LiBETI are more preferred, and LiFSI is even more preferred. These imide salts are suitable for use in batteries having a positive electrode operating potential of 4.0 V (vs. Li / Li + ) or more. Therefore, when using such imide salts, the working potential of the positive electrode is 4.0 V (vs. Li / Li + This effectively suppresses oxidation corrosion of aluminum during charge and discharge, which may occur at or above the oxidative stress of the aluminum, thereby improving the charge and discharge cycle performance of the non-aqueous electrolyte energy storage element.

[0028] (Salts in which the charge center element of the anion is boron and which have an oxalato group) Examples of salts having an anion whose charge center element is boron and whose oxalate group include lithium salts, sodium salts, potassium salts, etc. Among these, lithium salts having an anion whose charge center element is boron and whose oxalate group are preferred.

[0029] Examples of the lithium salt having an oxalato group and an anion whose charge center element is boron include lithium bis(oxalato)borate (LiBOB) and lithium difluorooxalatoborate (LiDFOB). The lithium salt having an anion whose charge center element is boron and an oxalato group is preferably a salt further containing a fluorine atom, and specifically, LiDFOB is preferred. One or more types of lithium salts having an anion whose charge center element is boron and an oxalato group can be used.

[0030] (difluorophosphate) Examples of difluorophosphates include lithium difluorophosphate, sodium difluorophosphate, potassium difluorophosphate, etc. Among these, lithium difluorophosphate is preferred.

[0031] Examples of the lithium difluorophosphate include lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalate phosphate (LiDFOP), etc. Among these, LiPO2F2 is preferred. One or more types of lithium difluorophosphate can be used.

[0032] (Other ionic compounds) The nonaqueous electrolyte may or may not further contain other ionic compounds in addition to the imide salt (a), the salt (b) having an oxalate group and an anion whose charge center element is boron, and the difluorophosphate (c). Examples of other ionic compounds include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, etc., other than the imide salt (a), the salt (b) having an oxalate group and the difluorophosphate (c). Among these, lithium salts are preferred.

[0033] Examples of the lithium salt include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6, and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiSO3C4F9, LiC(SO2CF3)3, LiC(SO2C2F5)3, and LiC(SO2C4F9)3. Among these, LiPF6 and LiBF4 are preferred from the viewpoint of ionic conductivity, and LiPF6 is more preferred. Furthermore, LiBF4 may be more preferred from the viewpoint of stability against water. One or more of the other ionic compounds may be used.

[0034] When other ionic compounds are contained, the upper limit of the content of the other ionic compounds in the nonaqueous electrolyte is 1 mol dm -3 is sometimes preferred, and 0.1 mol dm -3 is sometimes more preferable, and 0.01 mol dm -3 By setting the content of the other ionic compounds to the above upper limit or less, it is possible to obtain good charge-discharge cycle performance of the nonaqueous electrolyte electricity storage element.

[0035] The content of the imide salt (a) according to one embodiment of the present invention is a molar ratio of 3 or more relative to the total content of the salt (b) in which the charge center element of the anion is boron and which has an oxalate group, and the difluorophosphate (c). In other words, when the molar amount of the imide salt (a) contained in a given mass or volume of non-aqueous electrolyte is A (mol), the molar amount of the salt (b) in which the charge center element of the anion is boron and which has an oxalate group, and the molar amount of the difluorophosphate (c) contained in the non-aqueous electrolyte is B (mol), and C (mol), respectively, A / (B+C) is 3 or more. The lower limit of the molar ratio of the content of the imide salt (a) is preferably 3.5, more preferably 5. By setting the molar ratio of the content of the imide salt at or above the lower limit, the charge-discharge cycle performance of the non-aqueous electrolyte element can be improved. On the other hand, the upper limit of the molar ratio of this content is preferably 50, more preferably 25, even more preferably 20, and even more preferably 10. By setting the molar ratio of the content of the imide salt (a) to the above upper limit or less, it is expected that the nonaqueous electrolyte storage element will have good discharge performance in low temperature environments and high rate discharge performance.

[0036] According to another embodiment of the present invention, the content of the imide salt (a) is 25 mol% or more relative to the total content of all ionic compounds in the nonaqueous electrolyte. The lower limit of the content of the imide salt (a) is more preferably 27 mol%, and even more preferably 30 mol%. By setting the content of the imide salt (a) to the above-mentioned lower limit or more, the charge-discharge cycle performance of the nonaqueous electrolyte element can be improved. On the other hand, the upper limit of this content is preferably 99 mol%, more preferably 95 mol%, even more preferably 93 mol%, and even more preferably 92 mol%. By setting the content of the imide salt (a) to the above-mentioned upper limit or less, it is expected that the nonaqueous electrolyte storage element will achieve good discharge performance and high-rate discharge performance in low-temperature environments.

[0037] The content of the imide salt (a) in the non-aqueous electrolyte may be, for example, 0.1 mol dm -3 is preferred, and 0.3 mol dm -3is more preferred, and 0.5 mol dm -3 is more preferred, and 0.7 mol dm -3 By setting the content of the imide salt (a) to the above lower limit or more, the charge-discharge cycle performance of the nonaqueous electrolyte storage element can be improved. On the other hand, the upper limit of this content is 2 mol dm -3 is preferred, and 1.4 mol dm -3 is more preferred, and 1.2 mol dm -3 is more preferred, and 1.1 mol dm -3 By setting the content of the imide salt (a) to the above upper limit or less, it is expected that the nonaqueous electrolyte storage element will have good discharge performance in a low-temperature environment and high-rate discharge performance. According to the nonaqueous electrolyte storage element, even if the imide salt is used at a general concentration that is not such a high concentration, the positive electrode will not exceed 4.0 V (vs. Li / Li + ) or more, oxidation corrosion of aluminum is suppressed. Therefore, in this nonaqueous electrolyte storage element, it is not necessary to use a high concentration of imide salt. Furthermore, when a high concentration of imide salt is used, the viscosity of the nonaqueous electrolyte usually increases. In contrast, in this nonaqueous electrolyte storage element, by setting the concentration of the imide salt in the nonaqueous electrolyte to a normal concentration that is not high, such as in the above-mentioned range, it is possible to suppress an increase in the viscosity of the nonaqueous electrolyte, and it is expected that the nonaqueous electrolyte storage element will achieve good discharge performance and high-rate discharge performance in low-temperature environments.

[0038] The contents of the salt (b) having an oxalato group and in which the charge center element of the anion is boron and the difluorophosphate (c) in the nonaqueous electrolyte are not particularly limited as long as they are within a range that realizes the molar ratio with the content of the imide salt (a) or the content of the imide salt relative to the total content of all ionic compounds in the nonaqueous electrolyte. The lower limit of the content of these salts is, for example, 0.001 mol dm -3 is preferred, and 0.005 mol dm -3 is more preferred, and 0.01 mol dm -3 is more preferred, and 0.02 mol dm -3By setting the content of these salts to the above lower limit or more, the charge / discharge cycle performance of the nonaqueous electrolyte storage element can be improved. On the other hand, the upper limit of the content of these salts is 0.2 mol dm -3 is preferred, and 0.15 mol dm -3 is more preferred, and 0.1 mol dm -3 By setting the content of these salts to the above upper limit or less, it is expected that the nonaqueous electrolyte storage element will have good discharge performance in a low-temperature environment and high-rate discharge performance.

[0039] (non-aqueous solvent) The nonaqueous electrolyte may be a known nonaqueous solvent commonly used in nonaqueous electrolytes for general energy storage devices. Examples of the nonaqueous solvent include carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, sulfones, lactones, and nitriles. One or more of the nonaqueous solvents may be used.

[0040] (A chain-like non-aqueous solvent containing fluorine in the molecule) The nonaqueous electrolyte preferably further includes a chain-like nonaqueous solvent containing fluorine in the molecule. Examples of the chain-like nonaqueous solvent containing fluorine in the molecule include chain-like fluorinated carbonates, chain-like fluorinated carboxylic acid esters, chain-like fluorinated ethers, chain-like fluorinated phosphate esters, and chain-like fluorinated sulfonic acid esters. One or more of the chain-like nonaqueous solvents containing fluorine in the molecule can be used. The chain-like nonaqueous solvent containing fluorine in the molecule preferably contains fluorine as a fluorinated hydrocarbon group. The number of carbon atoms in the fluorinated hydrocarbon group is not particularly limited, as long as it is 1 or more, and may be 2 or more. The upper limit is not particularly limited, but is preferably 8, more preferably 5, even more preferably 4, and even more preferably 3. The number of fluorine atoms contained in the molecule is not particularly limited, as long as it is 1 or more, and is preferably 3 or more. The upper limit is not particularly limited, and may be, for example, a hydrocarbon group in which all hydrogen atoms are substituted with fluorine atoms, such as a perfluoroalkyl group, but is preferably 17 or less, more preferably 9 or less, even more preferably 7, and even more preferably 5. The carbon to which the fluorine atom is bonded is not particularly limited, and may be a carbon located at the terminal of the hydrocarbon group, or may be a carbon other than the terminal.

[0041] (chain fluorinated carbonate) Examples of the chain fluorinated carbonate include fluoromethyl methyl carbonate, bis(fluoromethyl)carbonate, difluoromethyl methyl carbonate, 2,2-difluoroethyl methyl carbonate, ethyl-(2,2-difluoroethyl)carbonate, bis(2,2-difluoroethyl)carbonate, 2,2,2-trifluoroethyl methyl carbonate (TFEMC), ethyl-(2,2,2-trifluoroethyl)carbonate, bis(2,2,2-trifluoroethyl)carbonate (FDEC), bis(2,2,3,3-tetrafluoropropyl)carbonate, and bis(2,2,3,3,3-pentafluoropropyl)carbonate. One or more of the above fluorinated chain carbonates can be used. The fluorinated chain carbonate is required to be electrochemically stable, have low viscosity, and not have too low a solubility of the electrolyte salt. Therefore, it is preferable that the proportion of fluorine atoms contained in the molecule is not too large, and it is preferable that the molecular weight is not too large. From this viewpoint, 2,2,2-trifluoroethyl methyl carbonate (TFEMC), ethyl-(2,2,2-trifluoroethyl) carbonate, and bis(2,2,2-trifluoroethyl) carbonate (FDEC) are preferred, and 2,2,2-trifluoroethyl methyl carbonate (TFEMC) is more preferred.

[0042] (chain fluorinated carboxylic acid ester) Examples of the chain fluorinated carboxylic acid ester include methyl 2,2-difluoroacetate, methyl 2,2,2-trifluoroacetate, ethyl 2,2,2-trifluoroacetate, methyl 3,3,3-trifluoropropionate (FMP), ethyl 3,3,3-trifluoropropionate, methyl 4,4,4-trifluorobutyrate, ethyl 4,4,4-trifluorobutyrate, trifluoromethyl acetate, 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate (TFEA), trifluoromethyl propionate, 2,2,2-trifluoroethyl propionate, trifluoromethyl butyrate, 2,2-difluoroethyl butyrate, 2,2,2-trifluoroethyl butyrate (TFB), 2,2,2-trifluoroethyl 2,2,2-trifluoroacetate, 2,2,2-trifluoroethyl 3,3,3-trifluoropropionate, and 2,2,2-trifluoroethyl 4,4,4-trifluorobutyrate. The chain fluorinated carboxylic acid ester may be used alone or in combination with two or more thereof. The chain fluorinated carboxylic acid ester is required to be electrochemically stable, have low viscosity, and not have too low a solubility of the electrolyte salt. Therefore, it is preferable that the proportion of fluorine atoms contained in the molecule is not too high, and it is also preferable that the molecular weight is not too high. From this viewpoint, 2,2,2-trifluoroethyl acetate, methyl 3,3,3-trifluoropropionate (FMP), 2,2,2-trifluoroethyl acetate (TFEA), 2,2,2-trifluoroethyl propionate, and 2,2,2-trifluoroethyl butyrate (TFB) are preferred, methyl 3,3,3-trifluoropropionate (FMP), 2,2,2-trifluoroethyl acetate (TFEA), and 2,2,2-trifluoroethyl butyrate (TFB) are more preferred, and 2,2,2-trifluoroethyl butyrate (TFB) is even more preferred.

[0043] (chain fluorinated ether) Examples of chain fluorinated ethers include 1,1,2,2-tetrafluoro-2,2,2-trifluoroethyl ether (TFEE), 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether, and 1,1,2,2-tetrafluoropropyl-1,1,2,3,3,3-hexafluoropropyl ether. One or more of the chain fluorinated ethers can be used. The chain fluorinated ethers are required to be electrochemically stable, have low viscosity, and not have too low a solubility of the electrolyte salt. Therefore, it is preferable that the proportion of fluorine atoms contained in the molecule is not too high, and that the molecular weight is not too high. From this viewpoint, 1,1,2,2-tetrafluoro-2,2,2-trifluoroethyl ether (TFEE) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) are preferred, and 1,1,2,2-tetrafluoro-2,2,2-trifluoroethyl ether (TFEE) is more preferred.

[0044] The content of the chain-like non-aqueous solvent containing fluorine in the molecule is not particularly limited, but the lower limit of the content of the chain-like non-aqueous solvent containing fluorine in the molecule relative to the total non-aqueous solvent is preferably 10% by volume, more preferably 20% by volume, and even more preferably 30% by volume. On the other hand, the upper limit of the content of the chain-like non-aqueous solvent containing fluorine in the molecule is preferably 95% by volume, more preferably 90% by volume, and even more preferably 80% by volume. By setting the total content of the chain-like non-aqueous solvent containing fluorine in the molecule to the above lower limit or more, the operating potential of the positive electrode can be increased, particularly to 4.0 V (vs. Li / Li + On the other hand, by setting the content of the chain nonaqueous solvent containing fluorine in the molecule to the above upper limit or less, the viscosity of the nonaqueous electrolyte, the lithium ion conductivity, etc. can be optimized.

[0045] (Other non-aqueous solvents) The nonaqueous electrolyte may be a known nonaqueous solvent commonly used as a nonaqueous solvent for general nonaqueous electrolytes for energy storage devices, other than the chain nonaqueous solvent containing fluorine in the molecule. Examples of the nonaqueous solvent include carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, sulfones, lactones, and nitriles. One or more of the nonaqueous solvents may be used.

[0046] (carbonate) Among the non-aqueous solvents, it is preferable to use a carbonate. The carbonate may be a cyclic carbonate or a chain carbonate. The carbonate may be a carbonate having no substituent or a carbonate having a substituent. Examples of the substituent include a halogen atom such as a chlorine atom or a bromine atom, and a hydroxy group. One or more of the carbonates may be used.

[0047] Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, styrene carbonate, catechol carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, fluoroethylene carbonate (FEC), 4,4-difluoroethylene carbonate (DFEC), 4,5-difluoroethylene carbonate, trifluoropropylene carbonate (4-(trifluoromethyl)-ethylene carbonate), 4-fluoro-4-methylethylene carbonate, 4-fluoro-5-methylethylene carbonate, 4-(fluoromethyl)-ethylene carbonate, etc. The cyclic carbonate is required to have a low viscosity and not too low a solubility of the electrolyte salt. Therefore, carbonates having a molecular weight that is not too large are preferred, and carbonates having no substituents are also preferred. From this viewpoint, ethylene carbonate (EC) and propylene carbonate (PC) are preferred, and ethylene carbonate (EC), which is less susceptible to oxidation and reduction, is more preferred.

[0048] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, di-n-butyl carbonate, diisobutyl carbonate, di-t-butyl carbonate, methyl-n-propyl carbonate, n-butyl methyl carbonate, isobutyl methyl carbonate, t-butyl methyl carbonate, ethyl-n-propyl carbonate, n-butyl ethyl carbonate, isobutyl ethyl carbonate, and t-butyl ethyl carbonate. The chain carbonate must have low viscosity and not too low a solubility of the electrolyte salt. Therefore, a carbonate with a molecular weight that is not too large is preferred, and a carbonate without a substituent is also preferred. From this viewpoint, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, and methyl-n-propyl carbonate are preferred, and diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are more preferred.

[0049] The cyclic carbonate and the chain carbonate, including the chain fluorinated carbonate, can be used in combination, and one or more of each can be used. When the cyclic carbonate and the chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate, including the chain fluorinated carbonate (cyclic carbonate: chain carbonate) is not particularly limited, but can be, for example, 5:95 or more and 95:5 or less. The content of the carbonate is not particularly limited, but the lower limit of the content of the carbonate, including the chain fluorinated carbonate, relative to the total non-aqueous solvent is preferably 10% by volume, more preferably 20% by volume, and even more preferably 30% by volume. On the other hand, the upper limit of the content of the carbonate, including the chain fluorinated carbonate, may be 100% by volume, 95% by volume, 90% by volume, or 80% by volume. In addition, the lower limit of the content of the cyclic carbonate in the total nonaqueous solvent is preferably 5% by volume, more preferably 10% by volume, and even more preferably 20% by volume. Meanwhile, the upper limit is preferably 80% by volume, more preferably 60% by volume, and even more preferably 50% by volume. By setting the content of the cyclic carbonate in the total nonaqueous solvent to the above lower limit or more, the charge / discharge performance of the nonaqueous electrolyte storage element can be more effectively improved. Meanwhile, by setting the content of the cyclic carbonate to the above upper limit or less, the viscosity of the nonaqueous electrolyte, lithium ion conductivity, and the like can be optimized. Furthermore, the lower limit of the content of the chain carbonate, including the chain fluorinated carbonate, in the total nonaqueous solvent is preferably 10% by volume, more preferably 20% by volume, and even more preferably 30% by volume. Meanwhile, the upper limit is preferably 95% by volume, more preferably 90% by volume, and even more preferably 80% by volume. By setting the content of the chain carbonate to be equal to or greater than the above lower limit, the viscosity of the non-aqueous electrolyte, lithium ion conductivity, etc. can be optimized. On the other hand, by setting the content of the fluorinated cyclic carbonate to be equal to or less than the above upper limit, the charge / discharge performance of the non-aqueous electrolyte storage element can be more effectively improved.

[0050] (additives) The non-aqueous electrolyte may contain additives as other components. Examples of additives include cyclic carbonates having carbon-carbon unsaturated bonds, such as vinylene carbonate (VC), methyl vinylene carbonate, ethyl vinylene carbonate, vinyl ethylene carbonate (VEC), styrene carbonate, and catechol carbonate; cyclic halogenated carbonates, such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); aromatic compounds, such as biphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexyl benzene, t-butyl benzene, t-amyl benzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds, such as 2-fluorobiphenyl, o-cyclohexyl fluorobenzene, and p-cyclohexyl fluorobenzene; halogenated anisole compounds, such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; succinic anhydride, glutaric anhydride, maleic anhydride, and anhydrous carboxylic acid. Acid anhydrides such as citraconic acid, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4' -bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, and the like. These additives may be used alone or in combination of two or more.The upper limit of the content of the additive in the non-aqueous electrolyte may be preferably 5% by mass, more preferably 2% by mass, even more preferably 1% by mass, and even more preferably 0.1% by mass.

[0051] <Non-aqueous electrolyte energy storage element> A nonaqueous electrolyte storage element according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. Hereinafter, a nonaqueous electrolyte secondary battery will be described as an example of a nonaqueous electrolyte storage element. The positive electrode and negative electrode are usually stacked or wound alternately with a separator interposed therebetween to form an electrode assembly. This electrode assembly is housed in a container, and the container is filled with the nonaqueous electrolyte. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. The container may be a known metal container, a resin container, or the like, that is typically used as a container for nonaqueous electrolyte secondary batteries.

[0052] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode mixture layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0053] The positive electrode substrate is electrically conductive. Examples of materials for the substrate include metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof. Among these, aluminum and aluminum alloys are preferred due to their balance of high conductivity and cost. Furthermore, when the positive electrode substrate is aluminum or an aluminum alloy, the effects of the present invention, such as suppressing the deterioration of the charge-discharge performance of the nonaqueous electrolyte storage element, which is presumed to be primarily caused by oxidation corrosion of aluminum, and improving the charge-discharge cycle performance, can be more fully achieved. Examples of the positive electrode substrate include foil and vapor-deposited film, with foil being preferred from a cost perspective. In other words, aluminum foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085P and A3003P, as specified in JIS-H-4000 (2014).

[0054] The intermediate layer is a coating layer on the surface of the positive electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode mixture layer. The configuration of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles. The term "conductive" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 7 "Non-conductive" means that the volume resistivity is 10 Ω·cm or less. 7 This means that the resistance is greater than Ω·cm.

[0055] The positive electrode mixture layer is a layer formed from a positive electrode mixture. This positive electrode mixture contains a positive electrode active material and, if necessary, further contains optional components such as a conductive agent, a binder, a thickener, a filler, and a dispersant.

[0056] Examples of the positive electrode active material include lithium transition metal composite oxides and polyanion compounds. Examples of the lithium transition metal composite oxides include Li x MeO y (Me represents at least one transition metal) x CoO2,Li x NiO2,Li x MnO3,Li x Ni α Co (1-α) O2,Li x Ni α Co β Al (1-α-β) O 2、 Li x Ni α Mn β Co (1-α-β) O2,Li 1+x (Ni α Mn β Co (1-α-β) ) 1-x Li with spinel-type crystal structure, such as O2 x Mn2O4,Li x Ni α Mn (2-α) O4, etc.) Also, examples of polyanion compounds include Li w Mex (XO y ) z (Me represents at least one transition metal, and X represents, for example, P, Si, B, or V) (LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc.). The elements or polyanions in these compounds may be partially substituted with other elements or anion species. In the positive electrode mixture layer, one of these compounds may be used alone, or two or more may be used in combination.

[0057] The positive electrode active material has a positive electrode potential of 4.0 V (vs. Li / Li) at the end-of-charge voltage during normal use of the nonaqueous electrolyte storage element. + ) is preferably contained. Such a positive electrode active material is preferably a lithium transition metal composite oxide. In the nonaqueous electrolyte storage element, the positive electrode has a potential of 4.0 V (vs. Li / Li + ) or more, it has good charge-discharge cycle performance. + ) or more, a nonaqueous electrolyte storage element can be obtained that has an increased energy density and improved charge / discharge cycle performance. Here, "normal use" refers to a case where the nonaqueous electrolyte storage element is used under charge / discharge conditions recommended or specified for the nonaqueous electrolyte storage element. With regard to charging conditions, if a charger for the nonaqueous electrolyte storage element is provided, "normal use" refers to a case where the nonaqueous electrolyte storage element is used with the charger.

[0058] The lower limit of the content of the lithium transition metal composite oxide in the total positive electrode active material is preferably 50% by mass, more preferably 90% by mass, and even more preferably 99% by mass. The positive electrode active material may be substantially composed of only the lithium transition metal composite oxide. By increasing the content of the lithium transition metal composite oxide in this way, it is possible to further increase the energy density while maintaining good high-rate discharge performance in low-temperature environments.

[0059] The above positive electrode active material preferably contains a positive electrode active material capable of having a positive electrode operating potential of 4.35 V (vs. Li / Li + ) or more at the charging cut-off voltage during normal use of the non-aqueous electrolyte storage element. The positive electrode active material capable of having a positive electrode operating potential of 4.35 V (vs. Li / Li + ) or more may be a positive electrode active material capable of reversible insertion and extraction of lithium ions at a potential of 4.35 V (vs. Li / Li + ) or more. Examples of such a positive electrode active material include Li 1+x (Ni α [[ID=##**##]]Mn β Co (1-α-β) ) 1-x O2 (0 < x < 1, 0 ≤ α < 0.5, 0.5 < β ≤ 1, 0 ≤ 1 - α - β < 0.5), and Li x Ni α Mn (2-α) O4 such as LiNi 0.5 Mn 1.5 O4, which is an example of a spinel-type crystal structure, and examples of polyanion compounds such as LiNiPO4, LiCoPO4, Li2CoPO4F, Li2MnSiO4, etc. can be mentioned.

[0060] The content of the positive electrode active material in the above positive electrode binder layer can be, for example, 80% by mass or more and 98% by mass or less, and preferably 90% by mass or more.

[0061] The above conductive agent is not particularly limited as long as it is a conductive material that does not adversely affect the performance of the storage element. Examples of such a conductive agent include carbon blacks such as natural or artificial graphite, furnace black, acetylene black, ketjen black, metals, conductive ceramics, etc., and acetylene black is preferred. Examples of the shape of the conductive agent include powdery, fibrous, etc.

[0062] Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-HFP), polyethylene, polypropylene, polyacrylonitrile, polyamide, polyimide, polyamide-imide, polyacrylic acid, polymethyl methacrylate (PMMA), ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, and styrene-butadiene rubber (SBR).

[0063] Examples of the thickener include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. In addition, when the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like.

[0064] The filler is not particularly limited as long as it does not adversely affect the performance of the energy storage device. Examples of the main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, and glass.

[0065] The positive electrode active material layer may further contain other optional components such as additives as necessary. Examples of additives include phosphorus oxoacids. However, in the nonaqueous electrolyte storage element, some or all of the phosphorus oxoacids may be modified. A positive electrode mixture paste obtained by mixing at least a positive electrode active material and a phosphorus oxoacid is applied to the surface of a positive electrode substrate and dried, forming a coating containing phosphorus atoms on the positive electrode. This results in a positive electrode in which the peak attributed to P2p is at 135 eV or less in the X-ray photoelectron spectroscopy spectrum of the positive electrode mixture.

[0066] The phosphorus oxoacid refers to a compound having a structure in which a hydroxy group (-OH) and an oxy group (=O) are bonded to a phosphorus atom. Examples of the phosphorus oxoacid include phosphoric acid (H3PO4), phosphonic acid (H3PO3), phosphinic acid (H3PO2), pyrophosphoric acid (H4P2O7), and polyphosphoric acid. Among these, phosphoric acid and phosphonic acid are preferred, and phosphonic acid is more preferred.

[0067] The lower limit of the content (addition amount) of phosphorus oxoacid in the positive electrode mixture paste is preferably 0.05 parts by mass, more preferably 0.1 parts by mass, and even more preferably 0.3 parts by mass, per 100 parts by mass of the positive electrode active material. Meanwhile, the upper limit of this content is preferably 5 parts by mass, more preferably 3 parts by mass, and even more preferably 2 parts by mass. By setting the content of phosphorus oxoacid within the above range, it is possible to efficiently obtain an effect of improving the charge-discharge cycle performance, particularly of the nonaqueous electrolyte storage element.

[0068] It is presumed that the phosphorus atoms are present in the coating formed on the surface of the positive electrode mixture. That is, it is presumed that the phosphorus atoms derived from phosphorus oxoacid are present in the coating formed on the surface of the positive electrode mixture, and therefore the peak attributed to P2p is present at a position of 135 eV or less in the X-ray photoelectron spectroscopy spectrum of the positive electrode mixture. In the X-ray photoelectron spectroscopy spectrum of the positive electrode mixture, the upper limit of the position of the peak attributed to P2p is preferably 134 eV. On the other hand, the lower limit of the position of this peak may be 130 eV or 132 eV.

[0069] The sample (positive electrode mixture) used for measuring the X-ray photoelectron spectroscopy spectrum is prepared as follows: The nonaqueous electrolyte storage element is discharged at a current of 0.1 C to the discharge end voltage during normal use to achieve a fully discharged state. The fully discharged storage element is disassembled to remove the positive electrode, which is then thoroughly washed with dimethyl carbonate and dried under reduced pressure at room temperature. The dried positive electrode is cut to a predetermined size (e.g., 2 × 2 cm), and this is used as the sample for measuring the X-ray photoelectron spectroscopy spectrum. The operations from disassembling the nonaqueous electrolyte storage element to preparing the sample for measuring the X-ray photoelectron spectroscopy spectrum are performed in an argon atmosphere with a dew point of −60°C or lower. The prepared sample is sealed in a transfer vessel, maintained in an argon atmosphere with a dew point of −60°C or lower, and introduced into the sample chamber of the X-ray photoelectron spectroscopy measurement device. The equipment and measurement conditions used for measuring the X-ray photoelectron spectroscopy spectrum are as follows. Equipment: KRATOS ANALYTICAL's "AXIS NOVA" X-ray source: monochromated AlKα Accelerating voltage: 15 kV Analysis area: 700μm x 300μm Measurement range: P2p = 145 to 128 eV, C1s = 300 to 272 eV Measurement interval: 0.1 eV Measurement time: P2p = 72.3 seconds / time, C1s = 70.0 seconds / time Accumulation count: P2p = 15 times, C1s = 8 times

[0070] The peak positions in the above spectrum are determined as follows: First, the position of the C1s peak attributed to sp2 carbon is set to 284.8 eV, and the binding energies of all the obtained spectra are corrected. Next, the corrected spectra are subjected to a leveling process by removing the background using the linear method. In the leveled spectrum, the binding energy at which the peak intensity attributed to P2p shows the highest value is determined to be the position of the peak attributed to P2p.

[0071] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode mixture layer disposed on the negative electrode substrate directly or via an intermediate layer, which may have the same structure as the intermediate layer of the positive electrode.

[0072] The negative electrode substrate is electrically conductive. The negative electrode substrate can have the same structure as the positive electrode substrate, but the material used is a metal such as copper, nickel, stainless steel, nickel-plated steel, or aluminum, or an alloy thereof, with copper or a copper alloy being preferred. That is, copper foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0073] The negative electrode mixture layer is formed from a negative electrode mixture containing a negative electrode active material. The negative electrode mixture forming the negative electrode mixture layer further contains optional components such as a conductive agent, a binder, a thickener, a filler, a dispersant, etc. as needed. The optional components such as the conductive agent, the binder, the thickener, the filler, the dispersant, etc. may be the same as those in the positive electrode mixture.

[0074] As the negative electrode active material, a material capable of absorbing and releasing lithium ions is usually used. Specific examples of the negative electrode active material include metallic Li; metals or semimetals such as Si, Sn, and Sb; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0075] Furthermore, the negative electrode mixture (negative electrode mixture layer) may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, and Ge; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W.

[0076] The negative electrode active material preferably contains a material that has a low operating potential during normal use of the nonaqueous electrolyte storage element, and is preferably a carbon material such as graphite or non-graphitic carbon.

[0077] Note that "graphite" refers to graphite that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0078] "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. The crystallite size Lc of non-graphitic carbon is usually 0.80 to 2.0 nm. Examples of non-graphitic carbon include non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch-derived materials, and alcohol-derived materials.

[0079] Here, the "discharged state" refers to a state in which an open circuit voltage is 0.7 V or higher in a single-electrode battery using a negative electrode containing a carbon material as the negative electrode active material as a working electrode and metallic Li as a counter electrode. Since the potential of the metallic Li counter electrode in the open circuit state is approximately equal to the redox potential of Li, the open circuit voltage in the single-electrode battery is approximately equal to the potential of the negative electrode containing the carbon material relative to the redox potential of Li. In other words, an open circuit voltage of 0.7 V or higher in the single-electrode battery means that lithium ions capable of being absorbed and released during charging and discharging have been sufficiently released from the carbon material, which is the negative electrode active material.

[0080] "Non-graphitizable carbon" means the above d 002 The term "non-graphitizable carbon" refers to a carbon material with a particle size of 0.36 nm or more and 0.42 nm or less. Non-graphitizable carbon is generally a type of non-graphitizable carbon that is difficult to form into a graphite structure with three-dimensional stacking regularity.

[0081] "Graphitizable carbon" means the above d 002 Graphitizable carbon is a carbon material with a particle size of 0.34 nm or more and less than 0.36 nm. Graphitizable carbon is generally a non-graphitic carbon that has the property of easily forming a graphite structure with three-dimensional stacking regularity.

[0082] The content of the negative electrode active material in the negative electrode mixture layer can be, for example, from 80% by mass to 99% by mass, and preferably from 90% by mass to 98% by mass.

[0083] (separator) The separator may be made of, for example, a woven fabric, a nonwoven fabric, or a porous resin film. Among these, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of nonaqueous electrolyte retention. The separator may be primarily made of a polyolefin such as polyethylene or polypropylene from the viewpoint of strength, or a polyimide or aramid from the viewpoint of resistance to oxidation and decomposition. These resins may also be combined.

[0084] An inorganic insulating layer may be disposed between the separator and the electrode (usually a positive electrode). This inorganic insulating layer is a porous layer also known as a heat-resistant layer. A separator having an inorganic insulating layer formed on one or both surfaces of a porous resin film can also be used. The inorganic insulating layer is usually composed of inorganic insulating particles and a binder, and may contain other components. Preferred inorganic insulating particles include Al2O3, SiO2, and aluminosilicate.

[0085] (Operating voltage of non-aqueous electrolyte energy storage element) In the nonaqueous electrolyte storage element, the lower limit of the maximum potential of the positive electrode is, for example, 4.0 V (vs. Li / Li + ) and 4.2V (vs. Li / Li + ), but 4.4V (vs. Li / Li + ) and 4.5V (vs. Li / Li + ) and 4.6V (vs. Li / Li+ ) and 4.7V (vs. Li / Li + The upper limit of the maximum potential of the positive electrode may be, for example, 5.4 V (vs. Li / Li + ) and 5.0V (vs. Li / Li + The nonaqueous electrolyte energy storage element may have a maximum positive electrode potential of 4.0 V (vs. Li / Li), which is a potential at which oxidation corrosion of aluminum is likely to occur when an imide salt is used. + ) or higher, the charge-discharge cycle performance of the non-aqueous electrolyte energy storage element is improved. Therefore, when the positive electrode is charged to 4.0 V (vs. Li / Li + ) or higher. Such a high maximum potential of the positive electrode allows the nonaqueous electrolyte electricity storage element to have a high energy density.

[0086] The maximum potential of the positive electrode is preferably the positive electrode potential at the end-of-charge voltage of the nonaqueous electrolyte storage element during normal use. Generally, when the positive electrode frequently reaches a high potential due to repeated charge and discharge, the discharge capacity is likely to decrease. Therefore, when the maximum potential of the positive electrode frequently reaches, for example, 4.4 V (vs. Li / Li) due to repeated charge and discharge, + ) or more, the effect of improving the charge-discharge cycle performance of the nonaqueous electrolyte storage element is more significantly exhibited.

[0087] <Method of manufacturing nonaqueous electrolyte energy storage element> The nonaqueous electrolyte storage element is preferably produced by the following method. That is, the method for producing a nonaqueous electrolyte storage element according to one embodiment of the present invention is a method for producing a nonaqueous electrolyte storage element, which comprises injecting into a container a nonaqueous electrolyte for a storage element (nonaqueous electrolyte injection step) that includes an imide salt (a), a salt (b) in which the charge center element of the anion is boron and that has an oxalate group, and a difluorophosphate salt (c). The nonaqueous electrolyte injection step can be carried out by a known method. That is, a nonaqueous electrolyte having a desired composition may be prepared, and the prepared nonaqueous electrolyte may be injected into a container.

[0088] In addition to the non-aqueous electrolyte injection step, the manufacturing method may include the following steps. That is, the manufacturing method may include, for example, a step of preparing a positive electrode, a step of preparing a negative electrode, a step of forming an electrode assembly in which the positive electrode and the negative electrode are alternately stacked by stacking or winding them with a separator interposed therebetween, and a step of housing the positive electrode and the negative electrode (electrode assembly) in a container. Typically, after housing the electrode assembly in the container, the non-aqueous electrolyte is injected into the container, but this order may be reversed. After these steps, the injection port is sealed to obtain a non-aqueous electrolyte storage element.

[0089] <Other embodiments> The present invention is not limited to the above-described embodiment, and various modifications and improvements can be made to the present invention. For example, the positive electrode or negative electrode may not have an intermediate layer. Furthermore, the positive electrode of the nonaqueous electrolyte storage element may not have a clear layer formed from the positive electrode mixture. For example, the positive electrode may have a structure in which the positive electrode mixture is supported on a mesh-like positive electrode substrate.

[0090] Although the above-described embodiments have been described mainly with reference to the nonaqueous electrolyte secondary battery as the nonaqueous electrolyte storage element, other nonaqueous electrolyte storage elements may also be used, such as capacitors (electric double layer capacitors, lithium ion capacitors).

[0091] FIG. 1 is a schematic diagram of a rectangular nonaqueous electrolyte storage element 1, which is one embodiment of the nonaqueous electrolyte storage element according to the present invention. The diagram is a see-through view of the inside of a container. The nonaqueous electrolyte storage element 1 shown in FIG. 1 includes an electrode assembly 2 housed in a container 3. The electrode assembly 2 is formed by winding a positive electrode including a positive electrode mixture and a negative electrode including a negative electrode mixture with a separator interposed therebetween. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 4′, and the negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 5′. A nonaqueous electrolyte is poured into the container 3.

[0092] The shape of the non-aqueous electrolyte storage element according to the present invention is not particularly limited, and examples include a cylindrical battery, a prismatic battery (rectangular battery), a flat battery, a coin-type battery, a button-type battery, and the like. The present invention can also be realized as a power storage device including a plurality of the non-aqueous electrolyte storage elements. An embodiment of the power storage device is shown in FIG. 2. In FIG. 2, the power storage device 30 includes a plurality of power storage units 20. Each power storage unit 20 includes a plurality of non-aqueous electrolyte storage elements 1. The power storage device 30 can be mounted as a power source for automobiles such as electric vehicles (EVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs).

Example

[0093] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to the following examples.

[0094] (Production of positive electrode) As the positive electrode active material, a lithium transition metal composite oxide having an α-NaFeO2 structure and represented by Li 1+α Me 1-α O2 (Me is a transition metal containing Mn, 0.1 < α < 0.2, and the molar ratio of Mn to Me, Mn / Me, is 0.5 < Mn / Me) was used. N-methylpyrrolidone (NMP) was used as the dispersion medium, and the positive electrode active material, acetylene black (AB) as the conductive agent, polyvinylidene fluoride (PVDF) as the binder, and phosphonic acid (H3PO3) as the additive were mixed at a mass ratio of 93.5:4.5:1.5:0.5 in terms of solid content to obtain a positive electrode mixture paste. This positive electrode mixture paste was applied to one side of an aluminum foil which is the positive electrode substrate and dried to form a positive electrode mixture layer on the positive electrode substrate. Also, the positive electrode mixture paste was similarly applied to the opposite side of the aluminum foil and dried. The positive electrode mixture layer formed on the opposite side was peeled off after drying. Thus, a positive electrode was obtained.

[0095] (Production of negative electrode) Graphite was used as the negative electrode active material. Using water as a dispersant, the negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in a mass ratio of 97:2:1 (solid content equivalent) to obtain a negative electrode mixture paste. This negative electrode mixture paste was applied to one side of copper foil, which served as the negative electrode substrate, and dried to form a negative electrode mixture layer on the negative electrode substrate. This is how the negative electrode was obtained.

[0096] [Example 1] (Preparation of non-aqueous electrolyte) A mixed solvent of ethylene carbonate (EC) and 2,2,2-trifluoroethyl butyrate (TFB) in a volume ratio of 10:90 was added with 1.0 mol dm of lithium bis(fluorosulfonyl)imide (LiFSI) as imide salt (a). -3 , the charge center element of the anion is boron, and as a salt (b) having an oxalato group, lithium difluorooxalatoborate (LiDFOB) is used at 0.05 mol dm -3 , and 0.05 mol dm lithium difluorophosphate (LiDFP) as difluorophosphate (c). -3 The nonaqueous electrolyte of Example 1 was prepared by dissolving each of the above components at a concentration of 1.0.

[0097] (Fabrication of non-aqueous electrolyte energy storage element) The positive electrode and the negative electrode were laminated with a microporous polyolefin separator interposed therebetween to produce an electrode assembly, which was then housed in a container made of a metal-resin composite film, the nonaqueous electrolyte was poured into the container, and the container was then sealed by thermal welding to obtain the nonaqueous electrolyte storage element (laminate-type nonaqueous electrolyte secondary battery) of Example 1.

[0098] [Examples 2 to 5, Comparative Examples 1 to 5] The nonaqueous electrolyte storage elements of Examples 2 to 5 and Comparative Examples 1 to 5 were obtained in the same manner as in Example 1, except that the type and concentration of the electrolyte salt, and the type and mixing volume ratio of the solvent were as shown in Tables 1 and 2.

[0099] The electrolyte salts and solvents in the table represent the following compounds. LiFSI: Lithium bis(fluorosulfonyl)imide LiDFOB: Lithium difluorooxalatoborate LiPO2F2: Lithium difluorophosphate EC: Ethylene carbonate TFB: 2,2,2-trifluoroethyl butyrate TFEMC: 2,2,2-trifluoroethyl methyl carbonate

[0100] [Battery evaluation] (Initial charge / discharge) The obtained nonaqueous electrolyte storage elements of Examples 1 to 5 and Comparative Examples 1 to 5 were initially charged and discharged under the following conditions. Charging was performed at a constant current of 0.1 C at 25°C up to 4.5 V, followed by charging at a constant voltage of 4.5 V. Charging was terminated when the charging current reached 0.05 C. After a 10-minute pause, discharging was performed at a constant current of 0.1 C at 25°C down to 2.0 V. A 10-minute pause was then performed after discharging. This cycle of charging, discharging, and pausing constituted one cycle, and this cycle was repeated twice.

[0101] (X-ray photoelectron spectroscopy measurement) Each nonaqueous electrolyte storage element in a fully discharged state after the initial charge / discharge was disassembled in an argon atmosphere with a dew point of −60°C or lower, and the positive electrode was removed. The positive electrode was washed with dimethyl carbonate and then dried under reduced pressure at room temperature. The resulting positive electrode was sealed in a transfer vessel in an argon atmosphere and introduced into the sample chamber of an X-ray photoelectron spectroscopy spectrometer. X-ray photoelectron spectroscopy measurement of the positive electrode mixture surface of the positive electrode was performed under the conditions described above. From the obtained spectra, a peak attributed to P2p was confirmed between 133 and 134 eV in the positive electrode mixtures of the nonaqueous electrolyte storage elements of Examples 1 to 5 and Comparative Examples 1 to 5, which all used phosphonic acid in the positive electrode mixture paste. Note that a peak attributed to P2p derived from LiPF6 was also confirmed at 136 eV in the positive electrode mixtures of Examples 2 to 5 and Comparative Examples 4 and 5, which contained LiPF6 as the electrolyte salt.

[0102] (Charge-discharge cycle test) Each nonaqueous electrolyte storage element in a fully discharged state after the initial charge and discharge was stored in a thermostatic chamber at 45°C for 2 hours, and then charged at a constant current of 0.5C up to 4.5V, and then charged at a constant voltage of 4.5V. The charge was terminated when the charge current reached 0.05C. The positive electrode potential (= maximum potential of the positive electrode) at the end of charge was 4.6V (vs. Li / Li + After charging, the battery was allowed to rest for 10 minutes, and then discharged at a constant current of 0.5 C to 2.0 V. After discharging, a 10-minute rest was allowed. This cycle of charging, discharging, and resting constituted one cycle, and this cycle was repeated 100 times. The charging, discharging, and resting were all performed in a constant temperature bath at 45°C.

[0103] (Capacity maintenance rate) For each nonaqueous electrolyte storage element, the discharge capacity at the 100th cycle relative to the discharge capacity at the first cycle in the charge-discharge cycle test is shown in Tables 1 and 2 as "capacity maintenance rate / %."

[0104] [Table 1]

[0105] [Table 2]

[0106] As can be seen from a comparison between Example 1 and Comparative Example 1 in Table 1 above, a nonaqueous electrolyte storage element in which the nonaqueous electrolyte contains an imide salt (a), a salt (b) having an oxalate group and a difluorophosphate (c), and the charge center element of the anion is boron, has a high capacity retention rate and good charge-discharge cycle performance compared to a case in which a nonaqueous electrolyte contains an imide salt (a), the charge center element of the anion is boron, and does not contain a salt (b) having an oxalate group and a difluorophosphate (c).

[0107] Furthermore, as can be seen from a comparison of Comparative Examples 1 to 3, when the nonaqueous electrolyte contains imide salt (a), the charge center element of the anion is boron, and the salt (b) having an oxalate group or the difluorophosphate (c) is contained alone, the capacity retention rate of the nonaqueous electrolyte storage element using these salts is actually lower than when neither of these salts is contained, and sufficiently good charge-discharge cycle performance is not obtained. In other words, when the nonaqueous electrolyte contains imide salt (a), it can be said that a significant improvement in charge-discharge cycle performance is only achieved by containing both the salt (b) having an oxalate group and the difluorophosphate (c) whose charge center element of the anion is boron.

[0108] On the other hand, as can be seen from a comparison between Comparative Examples 4 and 5 in Table 2 above, when the nonaqueous electrolyte does not contain imide salt (a), even if it contains both salt (b) having an oxalate group and difluorophosphate (c) and the charge center element of the anion is boron, the charge-discharge cycle performance is improved, but the effect is slight. Thus, the effect of a nonaqueous electrolyte storage element using a nonaqueous electrolyte containing imide salt (a), salt (b) having an oxalate group and difluorophosphate (c) and the charge center element of the anion is boron, and the effect of good charge-discharge cycle performance can be said to be a unique effect that could not be predicted from conventional knowledge, which only occurs when these elements are combined.

[0109] Furthermore, as can be seen from a comparison between Examples 2 to 5 and Comparative Example 4, when the non-aqueous electrolyte containing the imide salt (a), the salt (b) whose anion has boron as the charge center element and which has an oxalate group, and the difluorophosphate salt (c) further contains LiPF as an electrolyte salt, the charge-discharge cycle performance is improved compared to when only LiPF is contained.

[0110] In Example 2, the content of the imide salt (a) was 3 in terms of molar ratio to the total content of the salt (b) having an oxalato group and the difluorophosphate salt (c), in which the charge center element of the anion was boron. In other words, 1 dm 3The molar amount A of the imide salt (a) contained in the nonaqueous electrolyte is 0.3 mol, the molar amount B of the salt (b) whose anion has boron as the charge center element and has an oxalate group is 0.05 mol, and the molar amount C of the difluorophosphate salt (c) is 0.05 mol, so A / (B+C)=0.3 / (0.05+0.05)=3. Furthermore, the content of the imide salt (a) is 27 mol% of the total content of all ionic compounds. That is, the content of all ionic compounds is 0.3+0.05+0.05 +0.7 = 1.1 mol dm -3 Therefore, (0.3 / 1.1) × 100 = 27. Thus, it can be seen that when the content of the imide salt (a) is 3 or more in molar ratio to the total content of the salt (b) having an oxalate group and the difluorophosphate (c) in which the charge center element of the anion is boron, or is 25 mol % or more to the total content of all the ionic compounds, the effect of the present invention, that is, improved charge-discharge cycle performance of the nonaqueous electrolyte energy storage element, can be obtained.

[0111] As shown in Examples 1 and 2, the effects of the present invention are exhibited regardless of the nonaqueous solvent composition, but can be said to be effectively exhibited when the nonaqueous electrolyte for a storage battery element contains a chain nonaqueous solvent containing fluorine in the molecule, such as a chain carboxylic acid ester or a chain fluorinated carbonate. [Industrial Applicability]

[0112] The present invention is applicable to nonaqueous electrolyte energy storage elements used in electronic devices such as personal computers and communication terminals, and as power sources for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). [Explanation of symbols]

[0113] 1. Non-aqueous electrolyte energy storage element 2 Electrode body 3 containers 4 Positive terminal 4' Positive lead 5 Negative terminal 5' negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. A non-aqueous electrolyte for an electric storage element that satisfies the following (1) (however, excluding "a non-aqueous electrolyte that contains a chain carbonate solvent, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, and a difluorophosphate, and has an electron density value of 2.28 to 2.64Φ*Φ"); (1) A compound including an imide salt (a), a salt (b) having an oxalato group and an anion whose charge center element is boron, and a difluorophosphate salt (c), The content of the imide salt (a) is 25 mol% or more based on the total content of all ionic compounds, And contains a chain fluorinated carboxylic acid ester or a chain fluorinated carbonate, Non-aqueous electrolyte for energy storage elements.

2. The imide salt (a) is LiN(SO ２ F) ２ , LiN(CF ３ SO ２ ) ２ , or LiN(C ２ F ５ SO ２ ) ２ The nonaqueous electrolyte for an electric storage element according to claim 1 ,

3. 3. The non-aqueous electrolyte for a storage element according to claim 1, wherein the charge center element of the anion is boron, and the salt (b) having an oxalato group is lithium bis(oxalato)borate or lithium difluorooxalatoborate.

4. 4. The non-aqueous electrolyte for an energy storage element according to claim 1, wherein the difluorophosphate (c) is lithium difluorophosphate or lithium difluorobis(oxalato)phosphate.

5. A non-aqueous electrolyte energy storage element comprising the non-aqueous electrolyte for an energy storage element according to any one of claims 1 to 4.

6. A method for producing a nonaqueous electrolyte storage element, using the nonaqueous electrolyte for a storage element according to any one of claims 1 to 4.

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