Nonaqueous electrolyte and nonaqueous electrolyte storage element

Abstract

To provide a non-aqueous electrolyte with excellent charging / discharging cycle property under a low-temperature environment, a non-aqueous electrolyte power storage element provided with the non-aqueous electrolyte, and a method for producing the same.SOLUTION: A non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte salt and anion with a dicarboxylate group bonded to a boron atom. The non-aqueous solvent includes a fluorination cyclic carbonate and a fluorination carboxylic ester with a group containing a trifluoromethyl group.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a nonaqueous electrolyte, a nonaqueous electrolyte electricity storage element, a method for producing a nonaqueous electrolyte electricity storage element, and a method for using the nonaqueous electrolyte electricity storage element. [Background technology]

[0002] 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., due to their high energy density. 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. In addition, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte storage elements other than non-aqueous electrolyte secondary batteries.

[0003] Various studies have been conducted on the nonaqueous electrolyte used in such nonaqueous electrolyte storage elements.

[0004] For example, JP 2009-289414 A (Patent Document 1) describes a "non-aqueous electrolyte secondary battery" that "uses a mixed solvent in which 4-fluoroethylene carbonate (4-FEC), propylene carbonate (PC), and CF3CH2COOCH3, a fluorinated chain carboxylate, are mixed in a volume ratio of 20:5:75" (paragraph 0063, Example 7). Patent Document 1 also describes that "in the non-aqueous electrolyte secondary battery of Example 7, the capacity residual rate after storage and the capacity recovery rate were further improved" (paragraph 0077).

[0005] JP 2017-168375 A (Patent Document 2) describes "a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery comprising a nonaqueous solvent and an electrolyte salt, the nonaqueous solvent comprising 4-fluoroethylene carbonate (FEC) and methyl 3,3,3-trifluoropropionate (FMP), and further comprising a boric acid ester" (Claim 1). Furthermore, Patent Document 2 describes a battery that uses a non-aqueous electrolyte solution in which "0.5 mass % of triethanolamine borate (BTR) has been added to a non-aqueous solvent containing a mixture of FEC and FMP in a ratio of FEC:FMP = 10:90" (paragraphs 0078 and 0079), and states that "Comparing Example Battery 1-1 and Comparative Example Battery 1-1, which have in common the fact that they use a non-aqueous electrolyte solution containing FEC and FMP and differ in the presence or absence of BTR, it can be seen that Example Battery 1-1, to which BTR has been added, has a significantly lower DC resistance after high-temperature storage (hereinafter referred to as "post-storage resistance")" (paragraph 0089).

[0006] International Publication No. 2014 / 165748 (Patent Document 3) describes an electrolyte composition including "fluorinated acyclic carboxylic acid ester", "fluorinated acyclic carbonate", and "at least one fluorinated solvent selected from the group consisting of fluorinated acyclic ethers", "at least one co-solvent selected from the group consisting of ethylene carbonate, fluoroethylene carbonate, and propylene carbonate", "film-forming compound", and "electrolyte salt" (Claim 1). Patent Document 3 also describes a battery using "70% by weight DFEA / 30% by weight EC solvent ratio / 1M LiPF6, 2% by weight FEC, 2% by weight LiBOB, 96% by weight co-solvent, and LiPF6" (paragraph 0201, Example 51). Here, DFEA means "2,2-difluoroethyl acetate", and its composition formula is described as "CH3-COO-CH2CF2H" (paragraphs 0085 and 0024).

[0007] WO 2016 / 044088 (Patent Document 4) describes that "2,2-difluoroethyl acetate and fluoroethylene carbonate were combined... and sufficient LiPF6 was added to produce a 1 molar electrolyte composition," and electrolyte composition B was then "combined with lithium bis(oxalato)borate to form electrolyte composition C" (paragraphs 0056 and 0057). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2009-289414 A [Patent Document 2] JP 2017-168375 A [Patent Document 3] International Publication No. 2014 / 165748 [Patent Document 4] International Publication No. 2016 / 044088 Summary of the Invention [Problem to be solved by the invention]

[0009] For example, the electricity storage element using the non-aqueous electrolyte described in Patent Document 1 is capable of withstanding high voltages (for example, a positive electrode potential of 4.4 V (vs. Li / Li) at the end of charging) in a high temperature environment (for example, 45° C.). + )) shows a high discharge capacity retention rate. However, the inventors have newly found that such an energy storage element has a problem that the charge-discharge cycle performance is deteriorated in a low-temperature environment such as 0°C.

[0010] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a nonaqueous electrolyte that has excellent charge-discharge cycle performance in a low-temperature environment, as well as a nonaqueous electrolyte storage element including such a nonaqueous electrolyte, and a method for manufacturing the same. [Means for solving the problem]

[0011] One aspect of the present invention made to solve the above problems is a non-aqueous electrolyte containing a non-aqueous solvent, an electrolyte salt, and an anion in which a dicarboxylate group is bonded to a boron atom, wherein the non-aqueous solvent contains a fluorinated cyclic carbonate and a fluorinated carboxylate ester having a group including a trifluoromethyl group.

[0012] Another embodiment of the present invention is a nonaqueous electrolyte electricity storage element including the nonaqueous electrolyte.

[0013] Another aspect of the present invention is a method for producing a nonaqueous electrolyte storage element, comprising the step of placing the nonaqueous electrolyte in a container for the nonaqueous electrolyte storage element.

[0014] In another embodiment of the present invention, the positive electrode potential at the end of charge voltage during normal use is 4.4 V (vs. Li / Li + ) is a method for using the nonaqueous electrolyte electricity storage element. Effect of the Invention

[0015] Effects of the Invention According to the present invention, it is possible to provide a nonaqueous electrolyte having excellent charge-discharge cycle performance in a low-temperature environment, a nonaqueous electrolyte electricity storage element including such a nonaqueous electrolyte, a method for producing a nonaqueous electrolyte electricity storage element, and a method for using the nonaqueous electrolyte electricity storage element. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a perspective view showing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing an electricity storage device formed by assembling a plurality of nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The configuration and effects of the present invention will be explained together with the technical concept. However, the mechanism of action includes assumptions, and the correctness or incorrectness of such assumptions does not limit the present invention. The present invention can be embodied in various other forms without departing from its main features. Therefore, the embodiments and examples described below are merely illustrative in all respects and should not be interpreted in a restrictive manner. Furthermore, all modifications and changes within the scope of the claims are within the scope of the present invention.

[0018] The non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt, and an anion in which a dicarboxylate group is bonded to a boron atom, and the non-aqueous solvent contains a fluorinated cyclic carbonate and a fluorinated carboxylate ester having a group including a trifluoromethyl group.

[0019] The non-aqueous electrolyte contains a non-aqueous solvent containing a fluorinated cyclic carbonate and a fluorinated carboxylate having a group containing a trifluoromethyl group, and an anion having a dicarboxylate group bonded to a boron atom, and can exhibit excellent charge-discharge cycle performance in a low-temperature environment. The reason for this is unclear, but the following reason is speculated. The fluorinated cyclic carbonate, the fluorinated carboxylate having a group containing a trifluoromethyl group, and the anion having a dicarboxylate group bonded to a boron atom are all decomposed on the negative electrode to form a coating. Here, the fluorinated cyclic carbonate and the anion having a dicarboxylate group bonded to a boron atom are decomposed first at a noble potential, and then the fluorinated carboxylate having a group containing a trifluoromethyl group is decomposed to form a coating. At this time, the anion having a dicarboxylate group bonded to a boron atom is first adsorbed to a slight charge bias on the negative electrode and decomposed to form a good coating. A coating containing fluorine atoms is formed on it. That is, it is believed that the formation of a homogeneous coating in which boron atoms and fluorine atoms are in close proximity results in excellent charge-discharge cycle performance in a low-temperature environment.

[0020] Furthermore, the nonaqueous electrolyte exhibits a high capacity retention rate even when stored in a high-temperature environment, i.e., the nonaqueous electrolyte can exhibit excellent charge-discharge cycle performance in a low-temperature environment while maintaining high-temperature storage performance.

[0021] The nonaqueous electrolyte storage element is a nonaqueous electrolyte storage element (hereinafter, simply referred to as "storage element") that includes the nonaqueous electrolyte described above. The storage element exhibits excellent charge-discharge cycle performance in a low-temperature environment.

[0022] The method for producing a nonaqueous electrolyte storage element includes a step of placing the nonaqueous electrolyte in a container for a nonaqueous electrolyte storage element.

[0023] According to this manufacturing method, a nonaqueous electrolyte electricity storage element having excellent charge-discharge cycle performance in a low-temperature environment can be manufactured.

[0024] The method of using the nonaqueous electrolyte storage element is as follows: The positive electrode potential at the end of charge during normal use is 4.4V (vs. Li / Li + ) or more. The nonaqueous electrolyte storage element has a high capacity retention rate after storage in a high-temperature environment, and therefore this effect can be particularly fully exhibited when the storage element is used under charging conditions in which the positive electrode potential at the end-of-charge voltage during normal use is relatively high. Here, normal use refers to the case in which the nonaqueous electrolyte storage element is used under charging conditions recommended or specified for the nonaqueous electrolyte storage element, and in the case where a charger for the nonaqueous electrolyte storage element is prepared, the charger is used to use the nonaqueous electrolyte storage element. For example, in a storage element using graphite as the negative electrode active material, when the end-of-charge voltage is 5.0 V, the positive electrode potential is approximately 5.1 V (vs. Li / Li), although this depends on the design. + ).

[0025] The nonaqueous electrolyte, the nonaqueous electrolyte storage element, the method for producing the nonaqueous electrolyte storage element, and the method for using the nonaqueous electrolyte element according to the embodiments of the present invention will be described in detail below.

[0026] <Non-aqueous electrolyte> The non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt, and an anion in which a dicarboxylate group is bonded to a boron atom (hereinafter, simply referred to as an anion). Each component contained in the non-aqueous electrolyte will be described below.

[0027] [Non-aqueous solvent] The non-aqueous solvent contains a fluorinated cyclic carbonate and a fluorinated carboxylate having a group containing a trifluoromethyl group (hereinafter, also simply referred to as a "fluorinated carboxylate").

[0028] (Fluorinated cyclic carbonates) The term "fluorinated cyclic carbonate" refers to a compound in which some or all of the hydrogen atoms of a cyclic carbonate have been substituted with fluorine atoms.

[0029] Examples of the fluorinated cyclic carbonate include fluoroethylene carbonate (hereinafter also referred to as "FEC"), difluoroethylene carbonate, trifluoroethylene carbonate, tetrafluoroethylene carbonate, (fluoromethyl)ethylene carbonate, (difluoromethyl)ethylene carbonate, (trifluoromethyl)ethylene carbonate, bis(fluoromethyl)ethylene carbonate, bis(difluoromethyl)ethylene carbonate, bis(trifluoromethyl)ethylene carbonate, (fluoroethyl)ethylene carbonate, (difluoroethyl)ethylene carbonate, (trifluoroethyl)ethylene carbonate, 4-fluoro-4-methylethylene carbonate, 4,4-difluoro-5-methylethylene carbonate, 4,5-difluoro-4,5-dimethylethylene carbonate, etc. As the fluorinated cyclic carbonate, FEC is preferred. When the fluorinated cyclic carbonate is FEC, a stable coating can be formed on the negative electrode, particularly under high voltage, and the charge-discharge cycle performance can be improved. The fluorinated cyclic carbonate can be used alone or in combination of two or more.

[0030] The lower limit of the content of the fluorinated cyclic carbonate in the non-aqueous solvent is preferably 1% by volume, more preferably 3% by volume, and even more preferably 5% by volume. By making the content of the fluorinated cyclic carbonate equal to or higher than the lower limit, the capacity retention rate of the storage element after storage in a high-temperature environment can be further increased. On the other hand, the upper limit of the content is, for example, preferably 50% by volume, more preferably 30% by volume, even more preferably 20% by volume, and particularly preferably 15% by volume. By making the content of the fluorinated cyclic carbonate equal to or lower than the upper limit, the charge-discharge cycle performance of the non-aqueous electrolyte storage element in a low-temperature environment can be made more sufficient.

[0031] (Fluorinated carboxylates) The fluorinated carboxylate is a fluorinated carboxylate having a group containing a trifluoromethyl group (-CF3).

[0032] Examples of the group containing a trifluoromethyl group include a trifluoromethyl group itself, and a monovalent hydrocarbon group having 1 to 3 carbon atoms in which some or all of the hydrogen atoms have been substituted with trifluoromethyl groups.

[0033] In this specification, the term "hydrocarbon group" includes linear hydrocarbon groups and branched hydrocarbon groups.

[0034] Examples of groups in which some or all of the hydrogen atoms of a monovalent hydrocarbon group having 1 to 3 carbon atoms have been substituted with trifluoromethyl groups include a 2,2,2-trifluoroethyl group, a 3,3,3-trifluoropropyl group, a 2,2,3,3,3-pentafluoropropyl group, a 4,4,4-trifluorobutyl group, and a 2,2,3,3,4,4,4-heptafluorobutyl group.

[0035] The group containing a trifluoromethyl group is preferably a 2,2,2-trifluoroethyl group.

[0036] The fluorinated carboxylate may, for example, be a compound represented by the following formula (2).

[0037] [ka]

[0038] In the above formula (2), R 4 and R 5 are each independently a monovalent hydrocarbon group having 1 to 4 carbon atoms or a monovalent fluorinated hydrocarbon group having 1 to 4 carbon atoms. 4 and R 5 At least one of the groups is a group containing a trifluoromethyl group.

[0039] As used herein, the term "fluorinated hydrocarbon group" refers to a hydrocarbon group in which some or all of the hydrogen atoms have been substituted with fluorine atoms. The term "fluorinated hydrocarbon group" also encompasses "groups containing a trifluoromethyl group."

[0040] Examples of the monovalent hydrocarbon group having 1 to 4 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, an s-butyl group, a t-butyl group, etc. Examples of the monovalent fluorinated hydrocarbon group having 1 to 4 carbon atoms include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a 2-fluoroethyl group, a 2,2-difluoroethyl group, a 3-fluoropropyl group, a 3,3-difluoropropyl group, a 3,3,3-trifluoropropyl group, a 2,2,3,3-tetrafluoropropyl group, a 2,2,3,3,3-pentafluoropropyl group, a 4,4,4-trifluorobutyl group, a 2,2,3,3,4,4-hexafluorobutyl group, a 2,2,3,3,4,4,4-heptafluorobutyl group, etc.

[0041] As the fluorinated carboxylate, from the viewpoint of charge / discharge cycle performance in a low-temperature environment, 2,2,2-trifluoroethyl acetate (hereinafter also referred to as "FEA") represented by the following formula (2-1), methyl 3,3,3-trifluoropropionate (hereinafter also referred to as "FMP") represented by the following formula (2-2), or a combination thereof is preferred.

[0042] [ka]

[0043] The lower limit of the content of the fluorinated carboxylate in the non-aqueous solvent is preferably 20% by volume, more preferably 30% by volume, and even more preferably 40% by volume. By setting the content to the lower limit or more, the charge / discharge cycle performance in a low-temperature environment can be improved. The upper limit of the content is preferably 95% by volume, and more preferably 90% by volume. By setting the content to the upper limit or less, an increase in resistance can be suppressed.

[0044] (Other non-aqueous solvents) The non-aqueous electrolyte may contain a non-aqueous solvent other than the fluorinated cyclic carbonate and the fluorinated carboxylate. As the other non-aqueous solvent, a known non-aqueous solvent that is generally used as a non-aqueous solvent for a general electric storage element may be used. As the other non-aqueous solvent, a cyclic carbonate other than the fluorinated cyclic carbonate (hereinafter, also simply referred to as "cyclic carbonate"), a chain carbonate, a fluorinated chain carbonate, an ester, an ether, an amide, a sulfone, a lactone, a nitrile, etc. may be mentioned. Among these, a cyclic carbonate or a chain carbonate is preferred, and it is more preferred to use a cyclic carbonate and a chain carbonate in combination.

[0045] Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. The cyclic carbonate may be one in which some or all of the hydrogen atoms are substituted with an atom or a substituent other than a fluorine atom, but is preferably one in which no substitution is performed. The cyclic carbonate is preferably EC, PC, or BC, more preferably PC or BC, and even more preferably PC.

[0046] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, etc. The chain carbonate may be one in which some or all of the hydrogen atoms are substituted with other atoms or substituents, but is preferably one in which no substitution is performed. The chain carbonate is preferably DEC, DMC, or EMC, and more preferably EMC.

[0047] When the non-aqueous solvent contains the cyclic carbonate or the chain carbonate, the upper limit of the total content of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 10% by volume, and more preferably 5% by volume, from the viewpoints of ionic conductivity and charge / discharge cycle performance in a low-temperature environment.

[0048] By configuring the non-aqueous solvent as described above, the dielectric constant, viscosity, and the like become appropriate, and the capacity retention rate, and the like of the electricity storage element can be further improved.

[0049] [Electrolyte salt] The non-aqueous electrolyte generally contains an electrolyte salt dissolved in a non-aqueous solvent. Examples of the electrolyte salt include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, etc. Among these, lithium salts are preferred.

[0050] Examples of the lithium salt include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiPF2(C2O4)2, LiClO4, and LiN(SO2F)2, and lithium salts having a fluorinated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0051] The lower limit of the content of the electrolyte salt in the nonaqueous electrolyte is preferably 0.1 mol / L, more preferably 0.3 mol / L, even more preferably 0.5 mol / L, and particularly preferably 0.8 mol / L. The upper limit of the content is not particularly limited, but is preferably 2.5 mol / L, more preferably 2 mol / L, and even more preferably 1.5 mol / L.

[0052] [Anion] The anion is an anion in which a dicarboxylate group is bonded to a boron atom.

[0053] As used herein, the term "dicarboxylate group" refers to a group in which one hydrogen atom has been removed from each of the two carboxy groups of a dicarboxylic acid.

[0054] Examples of dicarboxylic acids that provide dicarboxylate groups include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, etc. Among these, oxalic acid or malonic acid is preferred, and oxalic acid is more preferred.

[0055] The dicarboxylate group includes a group in which one hydrogen atom has been removed from each of the two carboxy groups of the above-mentioned dicarboxylic acid, such as an oxalate group, a malonate group, a succinate group, a glutarate group, an adipinate group, etc. Among these, an oxalate group or a malonate group is preferred, and an oxalate group is more preferred.

[0056] The anion preferably further contains a fluorine atom. By the anion further containing a fluorine atom, a good coating film can be formed. More specifically, the anion preferably contains a fluorine atom bonded to a boron atom.

[0057] The anion is preferably an anion represented by the following formula (1).

[0058] [ka]

[0059] In the above formula (1), R 1 is a single bond or a divalent hydrocarbon group having 1 to 4 carbon atoms. n is 1 or 2. When n is 2, two R 1 are the same or different. R 2 and R 3 are each independently a fluorine atom or a monovalent fluorinated hydrocarbon group having 1 to 3 carbon atoms.

[0060] R 1 Examples of the divalent hydrocarbon group having 1 to 4 carbon atoms represented by the formula (I) include groups in which two hydrogen atoms have been removed from a chain hydrocarbon such as methane, ethane, n-propane, or n-butane.

[0061] R 2 and R 3 Examples of the monovalent fluorinated hydrocarbon group having 1 to 3 carbon atoms represented by the formula (I) include groups in which some or all of the hydrogen atoms of a monovalent hydrocarbon group such as a methyl group, an ethyl group, or an n-propyl group have been substituted with fluorine atoms.

[0062] R 1 is preferably a single bond or a group in which two hydrogen atoms have been removed from a methyl group, and more preferably a single bond.

[0063] R 2 and R 3 As the group, a fluorine atom is preferable.

[0064] As n, 1 is preferable.

[0065] Examples of the anion represented by the above formula (1) include a difluorooxalate borate anion represented by the following formula (1-1), a bisoxalate borate anion represented by the following formula (1-2), a difluoromalonate borate anion represented by the following formula (1-3), a bismalonate borate anion represented by the following formula (1-4), and a malonate oxalate borate anion represented by the following formula (1-5).

[0066] [ka]

[0067] The anion is preferably a difluorooxalate borate anion represented by the above formula (1-1) or a bis(oxalate) borate anion represented by the above formula (1-2), and more preferably a difluorooxalate borate anion represented by the above formula (1-1). By using these anions as the anion, a good coating film can be formed.

[0068] The anion is usually contained in the non-aqueous electrolyte in the form of a salt with a cation. Examples of the cation include an alkali metal cation, an alkaline earth metal cation, and an onium cation. Among these, an alkali metal cation is preferred, and a lithium ion is more preferred.

[0069] Examples of compounds that provide the above anion include lithium difluorooxalatoborate (hereinafter also referred to as "LiDFOB") represented by the following formula (1-1-1) and lithium bis(oxalatoborate) (hereinafter also referred to as "LiBOB") represented by the following formula (1-2-1).

[0070] [ka]

[0071] The compound that provides the anion is preferably lithium difluorooxalate represented by the above formula (1-1-1).

[0072] The lower limit of the content of the anion in the non-aqueous electrolyte is preferably 0.01% by mass, more preferably 0.05% by mass, even more preferably 0.1% by mass, even more preferably 0.3% by mass, and even more preferably 0.5% by mass, based on the total mass of the non-aqueous electrolyte. By setting the content of the anion to the lower limit or more, the capacity retention rate after the charge / discharge cycle of the storage element in a low-temperature environment can be increased. The upper limit of the content is preferably 5% by mass, more preferably 3% by mass, even more preferably 2% by mass, even more preferably 1% by mass, and particularly preferably 0.8% by mass. By setting the content of the anion to the upper limit or less, an increase in resistance can be suppressed.

[0073] [Other additives] The non-aqueous electrolyte may contain additives other than the non-aqueous solvent, the electrolyte salt, and the anion, as necessary. Examples of the additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; and halogen compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole. Examples of the additives include methyl anisole compounds, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride, ethylene sulfite, propylene sulfite, dimethyl sulfite, dimethyl sulfate, ethylene sulfate, thioanisole, diphenyl disulfide, dipyridinium disulfide, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. When the non-aqueous electrolyte contains other additives, the upper limit of the content of the other additives is preferably 5% by mass, more preferably 1% by mass, and even more preferably 0.1% by mass, based on the total mass of the non-aqueous electrolyte.

[0074] <Non-aqueous electrolyte electricity storage element> The nonaqueous electrolyte storage element according to one embodiment of the present invention includes a nonaqueous electrolyte. The nonaqueous electrolyte storage element usually includes a positive electrode and a negative electrode. Hereinafter, a secondary battery will be described as an example of the nonaqueous electrolyte storage element. The positive electrode and the negative electrode are usually stacked or wound with a separator between them to form an electrode body. The electrode body is housed in a container for the nonaqueous electrolyte storage element, and the container for the nonaqueous electrolyte storage element is filled with a nonaqueous electrolyte. The nonaqueous electrolyte is interposed between the positive electrode and the negative electrode. The container for the nonaqueous electrolyte storage element may be a known metal container, a resin container, or the like that is usually used as a container for a secondary battery.

[0075] [Non-aqueous electrolyte] The nonaqueous electrolyte used in the nonaqueous electrolyte storage element is the nonaqueous electrolyte according to the embodiment of the present invention described above.

[0076] [Positive electrode] The positive electrode preferably has a positive electrode substrate and a positive electrode mixture layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0077] The positive electrode substrate has electrical conductivity. As the material of the substrate, metals such as aluminum, titanium, tantalum, stainless steel, or alloys thereof are used. Among these, aluminum or aluminum alloys are preferred in terms of the balance between potential resistance, high electrical conductivity, and cost. In addition, the positive electrode substrate may be formed in the form of foil, vapor deposition film, or the like, and foil is preferred in terms of cost. That is, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. In addition, examples of aluminum or aluminum alloys include A1085P, A3003P, and the like specified in JIS-H-4000 (2014).

[0078] 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 composite 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. Note that "having electrical conductivity" 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 7 This means that the resistance is greater than Ω·cm.

[0079] The positive electrode mixture layer is a layer formed from a so-called positive electrode mixture containing a positive electrode active material. The positive electrode mixture layer contains optional components such as a conductive agent, a binder, a thickener, and a filler as necessary.

[0080] As the positive electrode active material, a metal oxide is usually used. Specific examples of the positive electrode active material include Li x MOy (M 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 Mn β Co (1-α-β) Li with spinel crystal structure such as O2 x Mn2O4, Li x Ni α Mn (2-α) O4, etc.), Li w Me x (A.O. y ) z (Me represents at least one type of transition metal, and A represents, for example, P, Si, B, V, etc.) polyanion compounds (LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc.) in the above formula. 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 of them may be used in combination.

[0081] The conductive agent is not particularly limited as long as it is a conductive material that does not adversely affect the performance of the electric storage element. Examples of such conductive agents include natural or artificial graphite, carbon black such as furnace black, acetylene black, and ketjen black, metals, conductive ceramics, etc., and acetylene black is preferred. The conductive agent may be in the form of powder, fiber, etc.

[0082] Examples of the binder include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyimide, etc.; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, etc.; polysaccharide polymers, etc.

[0083] 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.

[0084] The filler is not particularly limited as long as it does not adversely affect the battery performance. Examples of the main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, and glass.

[0085] [Negative electrode] The negative electrode preferably has a negative electrode substrate and a negative electrode mixture layer disposed on the negative electrode substrate directly or via an intermediate layer. The intermediate layer may have the same structure as the intermediate layer of the positive electrode.

[0086] The negative electrode substrate can be configured in the same manner as the positive electrode substrate, but the material is a metal such as copper, nickel, stainless steel, nickel-plated steel, or an alloy thereof, and copper or a copper alloy is preferred. That is, the negative electrode substrate is preferably a copper foil. Examples of the copper foil include rolled copper foil and electrolytic copper foil.

[0087] The negative electrode mixture layer is formed from a so-called negative electrode mixture containing a negative electrode active material. The negative electrode mixture layer also contains optional components such as a conductive agent, a binder, a thickener, and a filler as necessary. The optional components such as the conductive agent, the binder, the thickener, and the filler can be the same as those in the positive electrode mixture layer.

[0088] The negative electrode active material is usually a material capable of absorbing and releasing lithium ions. Specific examples of the negative electrode active material include metals or semimetals such as Si and Sn, metal oxides or semimetal oxides such as Si oxide and Sn oxide, polyphosphate compounds, and carbon materials such as graphite and non-graphitic carbon (easily graphitized carbon or non-graphitizable carbon).

[0089] Furthermore, the negative electrode mixture layer may contain typical nonmetallic 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 metallic elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W.

[0090] [Separator] The separator may be made of a material such as woven fabric, nonwoven fabric, or 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 made mainly of a polyolefin such as polyethylene or polypropylene from the viewpoint of strength, or of polyimide or aramid from the viewpoint of oxidation and decomposition resistance. These resins may be combined.

[0091] An inorganic layer may be disposed between the separator and the electrode (usually, the positive electrode). This inorganic layer is a porous layer also called a heat-resistant layer. A separator having an inorganic layer formed on one side of a porous resin film may also be used. The inorganic layer is usually composed of inorganic particles and a binder, and may contain other components. As the inorganic particles, Al2O3, SiO2, aluminosilicate, etc. are preferable.

[0092] The secondary battery (electric storage element) has a high capacity retention rate after storage in a high-temperature environment, and can be used at a high operating voltage. For example, the positive electrode potential at the end-of-charge voltage during normal use of the secondary battery is, for example, 4.0 V (vs. Li / Li + ) or more, but 4.4V (vs. Li / Li+ On the other hand, the upper limit of the positive electrode potential at the end of charge voltage during normal use is 5.1 V (vs. Li / Li + ).

[0093] <Method of Manufacturing Nonaqueous Electrolyte 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 includes a step of putting a nonaqueous electrolyte into a container for the nonaqueous electrolyte storage element (hereinafter also referred to as a "nonaqueous electrolyte injection step").

[0094] (Nonaqueous electrolyte injection process) The nonaqueous electrolyte injection step can be performed by a known method, except that the nonaqueous electrolyte according to the embodiment of the present invention described above is used as the nonaqueous electrolyte. That is, the nonaqueous electrolyte is prepared and the prepared nonaqueous electrolyte is injected into the container for the nonaqueous electrolyte storage element.

[0095] The manufacturing method may include the following steps in addition to the non-aqueous electrolyte injection step. 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 body in which the positive electrode and the negative electrode are alternately stacked by stacking or rolling them with a separator interposed therebetween, and a step of housing the positive electrode and the negative electrode (electrode body) in a container for a non-aqueous electrolyte storage element. Usually, after housing the electrode body in a container for a non-aqueous electrolyte storage element, the non-aqueous electrolyte is injected into the container for a non-aqueous electrolyte storage element, but this order may be reversed. After these steps, a secondary battery (non-aqueous electrolyte storage element) can be obtained by sealing the injection port.

[0096] <Method of using non-aqueous electrolyte storage element> The method of using the nonaqueous electrolyte storage element is as follows: The positive electrode potential at the end of charge during normal use is 4.4V (vs. Li / Li +) or more. The nonaqueous electrolyte storage element has a high capacity retention rate after storage in a high-temperature environment, and therefore this effect can be particularly fully exhibited when the storage element is used under charging conditions in which the positive electrode potential at the end-of-charge voltage during normal use is relatively high. For example, in a storage element using graphite as the negative electrode active material, when the end-of-charge voltage is 5.0 V, the positive electrode potential is approximately 5.1 V (vs. Li / Li), although this depends on the design. + ).

[0097] <Other embodiments> The present invention is not limited to the above embodiment, and can be embodied in various modified and improved forms in addition to the above embodiment. For example, the positive electrode or the negative electrode does not need to have an intermediate layer. In addition, the positive electrode and the negative electrode of the nonaqueous electrolyte storage element do not need to have a clear layer structure. For example, the positive electrode may have a structure in which a positive electrode composite is supported on a mesh-shaped positive electrode base material.

[0098] In the above embodiment, the nonaqueous electrolyte storage element is mainly a nonaqueous electrolyte secondary battery, but the nonaqueous electrolyte storage element may be another type of nonaqueous electrolyte storage element, such as a capacitor (electric double layer capacitor, lithium ion capacitor, etc.).

[0099] FIG. 1 is a schematic diagram of a rectangular nonaqueous electrolyte storage element 1 (nonaqueous electrolyte secondary battery) which is one embodiment of the nonaqueous electrolyte storage element according to the present invention. The figure is a see-through view of the inside of the container. In the nonaqueous electrolyte storage element 1 shown in FIG. 1, an electrode body 2 is housed in a container 3 for the nonaqueous electrolyte storage element. The electrode body 2 is formed by winding a positive electrode having a positive electrode mixture layer and a negative electrode having a negative electrode mixture layer 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'. In addition, the container 3 for the nonaqueous electrolyte storage element is filled with a nonaqueous electrolyte according to one embodiment of the present invention.

[0100] The configuration of the nonaqueous electrolyte storage element according to the present invention is not particularly limited, and examples thereof include a cylindrical battery, a prismatic battery (rectangular battery), and a flat battery. The present invention can also be realized as an electricity storage device including a plurality of the nonaqueous electrolyte storage elements. One embodiment of the electricity storage device is shown in FIG. 2. In FIG. 2, an electricity storage device 30 includes a plurality of electricity storage units 20. Each electricity storage unit 20 includes a plurality of nonaqueous electrolyte storage elements 1. The electricity 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). EXAMPLES

[0101] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0102] The abbreviations of the non-aqueous solvents and additives used in the examples, comparative examples and reference examples are shown below.

[0103] [Non-aqueous solvent] EMC: Ethyl methyl carbonate FEC: Fluoroethylene carbonate FMP: methyl 3,3,3-trifluoropropionate FEA: 2,2,2-trifluoroethyl acetate DFEA: 2,2-difluoroethyl acetate

[0104] [Additives] LiDFOB: Lithium difluorooxalatoborate LiBOB: Lithium bis(oxalato)borate VEC: Vinyl ethylene carbonate

[0105] [Example 1] (Preparation of non-aqueous electrolyte) FEC and FMP were mixed in a volume ratio of 10:90 to prepare a non-aqueous solvent. Lithium hexafluorophosphate (LiPF6) was dissolved as an electrolyte salt in a content of 1.2 mol / L in this non-aqueous solvent, and LiDFOB was dissolved as an additive in a content of 0.2 mass% relative to the total mass of the non-aqueous solvent and electrolyte to prepare a non-aqueous electrolyte.

[0106] (Preparation of positive electrode) As the positive electrode active material, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 was used. A positive electrode paste was prepared containing the positive electrode active material, polyvinylidene fluoride (PVdF), and acetylene black (AB) in a mass ratio of 94:3:3 (solid weight equivalent) and N-methylpyrrolidone as a dispersion medium. This positive electrode paste was used to prepare a positive electrode paste having a positive electrode active material content of 18.6 mg / cm2 per unit electrode area. 2 The mixture was applied to both sides of a strip of aluminum foil as a positive electrode substrate so that the mixture was contained therein, and then dried to remove N-methylpyrrolidone, which was a dispersion medium. The mixture was pressed with a roller press to form a positive electrode composite layer, and then dried under reduced pressure at 100°C for 14 hours to remove moisture from the positive electrode composite layer. In this way, a positive electrode was obtained.

[0107] (Preparation of negative electrode) Graphite was used as the negative electrode active material. A negative electrode paste was prepared by mixing the negative electrode active material (graphite), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a mass ratio of 97:2:1 (solid content equivalent) and using water as a dispersion medium. The negative electrode paste was mixed with the negative electrode active material at a concentration of 9.9 mg / cm2 per unit electrode area. 2 The mixture was applied to both sides of a strip-shaped copper foil current collector as a negative electrode substrate so that the mixture was contained therein, and then dried to remove water as a dispersion medium. The mixture was pressed with a roller press to form a negative electrode composite layer, and then dried under reduced pressure at 100° C. for 12 hours to remove moisture in the negative electrode composite layer. In this way, a negative electrode was obtained.

[0108] (Preparation of non-aqueous electrolyte storage element) A polyolefin microporous film coated with an inorganic layer was used as the separator. The positive electrode and the negative electrode were laminated with the separator interposed therebetween to prepare an electrode body. The electrode body was housed in an aluminum rectangular container for a nonaqueous electrolyte storage element, and a positive electrode terminal and a negative electrode terminal were attached. The nonaqueous electrolyte was poured into the container for a nonaqueous electrolyte storage element, and the container was sealed to obtain a nonaqueous electrolyte storage element (secondary battery) of Example 1.

[0109] (Examples 2 to 6 and Comparative Examples 1 to 3) Nonaqueous electrolyte storage elements of Examples 2 to 6 and Comparative Examples 1 to 3 were obtained in the same manner as in Example 1, except that the type and content of the electrolyte salt, the type and volume ratio of the nonaqueous solvent, or the type and amount of additive were as shown in Tables 1 and 2. In the tables, "-" indicates that the corresponding nonaqueous solvent or additive was not used.

[0110] Example 7 A nonaqueous electrolyte storage element of Example 7 was obtained in the same manner as in Example 2, except that the electrolyte salts contained 0.4 mol / L of lithium hexafluorophosphate (LiPF6) and 0.8 mol / L of lithium bis(fluorosulfonyl)amide (LiFSA). In the table, "-" indicates that the corresponding nonaqueous solvent or additive was not used.

[0111] (Reference Examples 1 to 3) Non-aqueous electrolyte storage elements of Reference Examples 1 to 3 were obtained in the same manner as Comparative Example 1, Example 5, and Example 2, respectively, except that DFEA was used instead of FMP as the non-aqueous solvent. In the table, "-" indicates that the corresponding non-aqueous solvent or additive was not used. Here, "DFEA" is represented by the following formula (3). "DFEA" is a compound having a similar structure to "FEA", but in which the terminal of the group bonded to the oxyoxy atom of the carbonyloxy group is a difluoromethyl group (-CF2H) instead of a trifluoromethyl group (-CF3).

[0112] [ka]

[0113] Among the nonaqueous electrolyte storage elements prepared by the above-mentioned procedure, a plurality of nonaqueous electrolyte storage elements according to Comparative Examples 1 and 2 and Example 2 were prepared, and those subjected to the "initial charge / discharge" described below were subjected to a "45°C storage test" or a "0°C charge / discharge cycle test." The nonaqueous electrolyte storage elements according to Comparative Example 3, Examples 1, 3 to 7, and Reference Examples 1 to 3 were subjected to the "initial charge / discharge" and then subjected to a "0°C charge / discharge cycle test."

[0114] [evaluation] <Initial charge / discharge> The obtained nonaqueous electrolyte storage element was initially charged and discharged at 25° C. in accordance with the following steps (1) to (4). (1) Charging process The battery is charged at a constant current of 0.2 C up to 4.3 V. After reaching 4.3 V, the voltage is maintained, and charging ends 8 hours after the start of charging (so-called constant current / constant voltage charging), followed by a 10-minute break. (2)Discharge process Discharge the battery at a constant current of 0.2 C up to 2.75 V. When the battery reaches 2.75 V, discharge is completed and a 10-minute break is performed. (3) Charging process The battery is charged at a constant current of 1 C up to 4.3 V. After reaching 4.3 V, the voltage is maintained at that level, and charging ends when 3 hours have elapsed since the start of charging, followed by a 10-minute break. (4)Discharge process Discharge the battery at a constant current of 1 C up to 2.75 V. When the battery reaches 2.75 V, the discharge is completed and a 10-minute break is performed. The discharge capacity in the above step (4) was recorded as the "second discharge capacity in the initial charge / discharge test."

[0115] <45℃ storage test> The nonaqueous electrolyte electricity storage element after the above initial charge / discharge was subjected to a storage test at 45° C. in the following manner. (1') Charging process The battery is charged at a constant current of 1 C in a thermostatic chamber at 25° C. After the voltage reaches 4.3 V, the battery is maintained at that voltage, and charging ends when 3 hours have elapsed since the start of charging, after which a 10-minute break is taken. (2') 45℃ storage process The nonaqueous electrolyte electricity storage element was placed in an open circuit state and left to stand (preserved) in a thermostatic chamber at 45° C. After 30 days had passed since the start of the preservation, the preservation process was terminated, and the electricity storage element was taken out and left to stand at room temperature for 5 hours or more. (3') Discharge process In a thermostatic chamber at 25° C., constant current discharge is performed at a current value of 1 C down to 2.75 V. When 2.75 V is reached, the discharge ends, and a 10-minute break is performed. After steps (1'), (2'), and (3'), steps (3) and (4) of the initial charge / discharge test were performed in a thermostatic chamber at 25°C, and the discharge capacity at that time was recorded as "discharge capacity after storage." The percentage of the discharge capacity after storage to the second discharge capacity in the initial charge / discharge test was calculated as "capacity retention rate after 45°C storage test / %."

[0116] <0℃ charge / discharge cycle test> The nonaqueous electrolyte storage element after the above initial charge and discharge was left to stand in a thermostatic chamber at 0° C. for 5 hours, and then a 0° C. charge and discharge cycle test was carried out in the following manner. (1'') Charging process The battery is charged at a constant current of 1 C up to 4.3 V. After reaching 4.3 V, the voltage is maintained at that level, and charging ends when 3 hours have elapsed since the start of charging, followed by a 10-minute break. (2'')Discharge process Discharge the battery at a constant current of 1 C up to 2.75 V. When the battery reaches 2.75 V, the discharge is completed and a 10-minute break is performed. The above steps (1'') and (2'') were counted as one charge and discharge, and 350 charge and discharge cycles were performed. The percentage of the discharge capacity at the 350th cycle to the discharge capacity at the first cycle was calculated as "capacity retention rate after 0°C cycle test / %". The difference in the capacity retention rate after 0°C cycle test with respect to Comparative Example 1 or Reference Example 1, in which no additive was used, was calculated as "difference in capacity retention rate after 0°C cycle test from non-additive product / %". For Reference Examples 2 and 3, the difference with respect to Reference Example 1 was calculated, and for Comparative Example 3 and Examples 1 to 7, the difference with respect to Comparative Example 1 was calculated. This value is an index showing the magnitude of the effect of improving the charge and discharge cycle performance in a low temperature environment by using the additive. The thickness of the nonaqueous electrolyte storage element was measured before and after the test, and the thickness change before and after the 0°C charge-discharge cycle test was determined. The difference in thickness change before and after the 0°C charge-discharge cycle test for Comparative Example 1 or Reference Example 1, which did not use an additive, was determined as "difference in battery thickness change after 0°C cycle test from a non-additive product / mm." For Reference Examples 2 to 3, the difference from Reference Example 1 was determined, and for Comparative Example 3 and Examples 1 to 7, the difference from Comparative Example 1 was determined. If this value is a negative value, it means that the use of the additive has suppressed the increase in battery thickness after the 0°C cycle test.

[0117] In the above test, when the voltage of the non-aqueous electrolyte storage element was 4.3 V, the potential of the positive electrode was 4.4 V (vs. Li / Li + ) was.

[0118] For the nonaqueous electrolyte storage elements according to Example 2, Comparative Example 1, and Comparative Example 2, the "capacity retention rate / % after 45°C storage test" obtained in the above-mentioned "45°C storage test" and the "capacity retention rate / % after 0°C cycle test" obtained in the "0°C charge-discharge cycle test" are summarized in Table 1.

[0119] [Table 1]

[0120] Comparing Comparative Example 1 and Comparative Example 2, it can be seen that the use of FEC and FMP as the nonaqueous solvent improves the "capacity retention rate after a 45°C storage test" compared to the use of FEC and EMC, but on the other hand, the "capacity retention rate after a 0°C cycle test" (hereinafter sometimes referred to as low-temperature characteristics) decreases. Comparing Example 2 with Comparative Example 1, it can be seen that by adding LiDFOB as an additive, the "capacity retention rate after a 45°C storage test" is improved, and further the "capacity retention rate after a 0°C cycle test" is greatly improved. These results demonstrate that the nonaqueous electrolyte of the present invention maintains high-temperature storage performance while exhibiting excellent charge-discharge cycle performance in a low-temperature environment.

[0121] For the nonaqueous electrolyte storage elements of Examples 1 to 7, Comparative Examples 1 and 3, and Reference Examples 1 to 3, the "difference in capacity retention rate after 0°C cycle test from a non-added product / %" and "difference in battery thickness change after 0°C cycle test from a non-added product / mm" obtained by the "0°C charge-discharge cycle test" are summarized in Table 2.

[0122] [Table 2]

[0123] Comparing Examples 1 to 4 with Comparative Example 1, it was found that the use of LiDFOB as an additive improved the low-temperature characteristics and suppressed the increase in battery thickness after a 0°C charge-discharge cycle test, regardless of the amount of LiDFOB added.

[0124] Comparing Example 5 with Comparative Examples 1 and 3, it can be seen that the low-temperature properties were improved even when LiBOB was used as an additive, but were significantly reduced when VEC was used.

[0125] Comparing Example 7 with Comparative Example 1, it is apparent that the effect of improving low-temperature characteristics can be seen even when LiFSA is used as the electrolyte salt.

[0126] Comparing Example 6 with Comparative Example 1, it can be seen that the effect of improving low-temperature characteristics was observed even when FEA was used as the non-aqueous solvent. In addition, the increase in the battery thickness after the 0°C charge-discharge cycle test was also suppressed.

[0127] Comparing Reference Example 1 with Reference Examples 2 and 3, it is found that when DFEA is used as the non-aqueous solvent, the addition of LiDFOB has little effect on improving low-temperature characteristics, and also has no effect on suppressing the increase in battery thickness after a charge-discharge cycle test at 0° C. In other words, it is found that the effect of improving low-temperature characteristics is more remarkable when FEA or FMP is used as the non-aqueous solvent. The following reasons are considered for this mechanism. As a result of LUMO (Lowest Unoccupied Molecular Orbital) calculation by PM3 (Parameterized Model number 3) method, LUMO of DFEA was the largest among DFEA, FEA and FMP, and FMP was the smallest. It is said that the larger the LUMO value, the better the reduction resistance. In other words, FEA and FMP are more easily reduced than DFEA, and when comparing "fluorinated carboxylate having a group containing a difluoromethyl group" with "fluorinated carboxylate having a group containing a trifluoromethyl group", it can be said that "fluorinated carboxylate having a group containing a trifluoromethyl group" tends to be more easily reduced. Therefore, a nonaqueous electrolyte storage element using FEA or FMP, which is more easily reduced than DFEA, as a nonaqueous solvent is decomposed on the negative electrode together with LiDFOB containing FEA or FMP as an additive, and a homogeneous coating in which boron atoms and fluorine atoms are close to each other is formed, which is considered to exhibit excellent charge-discharge cycle performance in a low-temperature environment. [Industrial Applicability]

[0128] The present invention can be applied to nonaqueous electrolyte electricity storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles. [Explanation of symbols]

[0129] 1. Non-aqueous electrolyte storage element 2 Electrode body 3. Container for non-aqueous electrolyte storage element 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 solvent; An electrolyte salt; An anion with a dicarboxylate group attached to a boron atom Contains The non-aqueous solvent contains a fluorinated cyclic carbonate and a group containing a trifluoromethyl group. A non-aqueous electrolyte containing a fluorinated carboxylate that has a function of preventing oxidation of the electrolyte (excluding those containing lithium bis(fluorosulfonyl)imide and those containing lithium fluorosulfate).

2. A non-aqueous electrolyte storage element comprising the non-aqueous electrolyte described in claim 1.

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