JP7594909B2 - Nonaqueous electrolyte, nonaqueous electrolyte storage element, method for producing nonaqueous electrolyte storage element, and method for using nonaqueous electrolyte storage element - Google Patents

Description

The present invention relates to a non-aqueous electrolyte, a non-aqueous electrolyte storage element, a method for manufacturing a non-aqueous electrolyte storage element, and a method for using a non-aqueous electrolyte storage element.

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 two electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte storage elements.

Various studies have been conducted on the non-aqueous electrolytes used in such non-aqueous electrolyte storage elements.

For example, Japanese Patent Laid-Open Publication No. 2009-289414 (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 CF ₃ CH ₂ COOCH ₃ , 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).

JP 2017-168375 A (Patent Document 2) describes a "non-aqueous electrolyte for a non-aqueous electrolyte secondary battery comprising a non-aqueous solvent and an electrolyte salt, the non-aqueous 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 using a nonaqueous electrolyte solution in which "0.5 mass % of triethanolamine borate (BTR) was added to a nonaqueous solvent in which FEC and FMP were mixed 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 nonaqueous 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 was added, has a significantly lower DC resistance after high-temperature storage (hereinafter referred to as "post-storage resistance")" (paragraph 0089).

International Publication No. WO 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 cosolvent 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 wt% DFEA/30 wt% EC solvent ratio/1M LiPF ₆ 2 wt% FEC, 2 wt% LiBOB, 96 wt% cosolvent and LiPF ₆ " (paragraph 0201, Example 51). Here, DFEA stands for "2,2-difluoroethyl acetate" and its composition formula is described as "CH ₃ --COO--CH ₂ CF ₂ H" (paragraphs 0085 and 0024).

WO 2016/044088 (Patent Document 4) describes that "2,2-difluoroethyl acetate and...fluoroethylene carbonate were combined.... Sufficient _LiPF6 was added to make a 1 molar electrolyte composition" and electrolyte composition B was "combined with lithium bis(oxalato)borate to form electrolyte composition C" (paragraphs 0056 and 0057).

JP 2009-289414 A JP 2017-168375 A International Publication No. 2014/165748 International Publication No. 2016/044088

For example, an electricity storage element using a nonaqueous electrolyte described in Patent Document 1 exhibits a high discharge capacity retention rate even when stored in a high-temperature environment (e.g., 45°C) and at a high voltage (e.g., 4.4 V (vs. Li/Li ⁺ ) as a positive electrode potential at the end of charging). However, the inventors have newly found that such an electricity storage element has a problem in that the charge/discharge cycle performance is reduced in a low-temperature environment such as 0°C.

The present invention has been made based on the above circumstances, and its object is to provide a nonaqueous electrolyte having 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.

One aspect of the present invention, which has been made to solve the above problems, is a non-aqueous electrolyte that contains 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.

Another aspect of the present invention is a non-aqueous electrolyte storage element comprising the non-aqueous electrolyte.

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.

Another embodiment of the present invention is a method for using the nonaqueous electrolyte storage element having a positive electrode potential of 4.4 V (vs. Li/Li ⁺ ) or higher at an end-of-charge voltage during normal use.

The present invention provides a nonaqueous electrolyte having excellent charge/discharge cycle performance in a low-temperature environment, a nonaqueous electrolyte storage element including such a nonaqueous electrolyte, a method for manufacturing a nonaqueous electrolyte storage element, and a method for using a nonaqueous electrolyte storage element.

FIG. 1 is a perspective view showing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention. 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.

The configuration and effects of the present invention will be explained together with technical ideas. 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 characteristics. 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.

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 is a non-aqueous electrolyte containing a fluorinated cyclic carbonate and a fluorinated carboxylate ester having a group including a trifluoromethyl group.

The non-aqueous electrolyte contains a non-aqueous solvent containing a fluorinated cyclic carbonate and a fluorinated carboxylate ester 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 ester 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 ester 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 the slight charge bias on the negative electrode and decomposed to form a good coating. A coating containing fluorine atoms is formed on top of 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.

Furthermore, the non-aqueous electrolyte exhibits high capacity retention even when stored in a high-temperature environment. In other words, the non-aqueous electrolyte can exhibit excellent charge-discharge cycle performance in a low-temperature environment while maintaining high-temperature storage performance.

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.

The method for manufacturing the nonaqueous electrolyte storage element includes a step of placing the nonaqueous electrolyte described above into a container for the nonaqueous electrolyte storage element.

This manufacturing method makes it possible to produce a non-aqueous electrolyte storage element that has excellent charge/discharge cycle performance in a low-temperature environment.

The method of using the nonaqueous electrolyte storage element is such that the positive electrode potential at the end of charge voltage during normal use is 4.4 V (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 can particularly fully exhibit this effect 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 where the nonaqueous electrolyte storage element is used by adopting charging conditions recommended or specified for the nonaqueous electrolyte storage element, and when 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, the positive electrode potential is about 5.1 V (vs. Li/Li ⁺ ) when the end of charge voltage is 5.0 V, depending on the design.

The following provides a detailed description of the nonaqueous electrolyte, nonaqueous electrolyte storage element, manufacturing method for the nonaqueous electrolyte storage element, and method of using the nonaqueous electrolyte element according to embodiments of the present invention.

<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.

[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").

(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.

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. FEC is preferred as the fluorinated cyclic carbonate. 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.

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 setting the content of the fluorinated cyclic carbonate to the lower limit or more, 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 preferably 50% by volume, more preferably 30% by volume, even more preferably 20% by volume, and particularly preferably 15% by volume. By setting the content of the fluorinated cyclic carbonate to the upper limit or less, the charge/discharge cycle performance of the non-aqueous electrolyte storage element in a low-temperature environment can be made more sufficient.

(Fluorinated carboxylate ester)
The fluorinated carboxylate is a fluorinated carboxylate having a group containing a trifluoromethyl group (-CF ₃ ).

Examples of groups containing the above-mentioned trifluoromethyl group include the trifluoromethyl group itself and 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.

In this specification, "hydrocarbon group" includes linear hydrocarbon groups and branched hydrocarbon groups.

Examples of groups in which some or all of the hydrogen atoms in 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.

As the group containing the above trifluoromethyl group, a 2,2,2-trifluoroethyl group is preferred.

Examples of the fluorinated carboxylic acid ester include compounds represented by the following formula (2):

In the above formula (2), ^R4 and ^R5 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, provided that at least one of ^R4 and ^R5 is a group containing a trifluoromethyl group.

In this specification, the term "fluorinated hydrocarbon group" refers to a group in which some or all of the hydrogen atoms in a hydrocarbon group have been replaced with fluorine atoms. In addition, "fluorinated hydrocarbon group" includes "groups containing a trifluoromethyl group."

Examples of monovalent hydrocarbon groups having 1 to 4 carbon atoms include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, and t-butyl groups. Examples of monovalent fluorinated hydrocarbon groups having 1 to 4 carbon atoms include fluoromethyl, difluoromethyl, trifluoromethyl, 2-fluoroethyl, 2,2-difluoroethyl, 3-fluoropropyl, 3,3-difluoropropyl, 3,3,3-trifluoropropyl, 2,2,3,3-tetrafluoropropyl, 2,2,3,3,3-pentafluoropropyl, 4,4,4-trifluorobutyl, 2,2,3,3,4,4-hexafluorobutyl, and 2,2,3,3,4,4,4-heptafluorobutyl groups.

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

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.

(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 non-aqueous electrolyte for a 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.

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, 1,2-diphenylvinylene carbonate, and the like. The cyclic carbonate may be one in which some or all of the hydrogen atoms are substituted with atoms or substituents other than fluorine atoms, but is preferably one in which no substitutions are made. As the cyclic carbonate, EC, PC, or BC are preferred, PC or BC are more preferred, and PC is even more preferred.

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 replaced with other atoms or substituents, but it is preferable that the chain carbonate is not replaced. As the chain carbonate, DEC, DMC or EMC is preferable, and EMC is more preferable.

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.

By configuring the non-aqueous solvent as described above, the dielectric constant, viscosity, etc. become appropriate, thereby further improving the capacity retention rate, etc. of the storage element.

[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.

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.

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.

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

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.

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.

Dicarboxylate groups include groups in which one hydrogen atom has been removed from each of the two carboxy groups of the dicarboxylic acid described above, 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.

It is preferable that the anion further contains a fluorine atom. By the anion further containing a fluorine atom, a good coating film can be formed. More specifically, it is preferable that the anion has a fluorine atom bonded to a boron atom.

The anion is preferably an anion represented by the following formula (1):

In the above formula (1), ^R1 is a single bond or a divalent hydrocarbon group having 1 to 4 carbon atoms. n is 1 or 2. When n is 2, the two ^R1s are the same or different. ^R2 and ^R3 are each independently a fluorine atom or a monovalent fluorinated hydrocarbon group having 1 to 3 carbon atoms.

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

Examples of the monovalent fluorinated hydrocarbon group having 1 to 3 carbon atoms represented by ^R2 and ^R3 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.

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.

^R2 and ^R3 are preferably a fluorine atom.

n is preferably 1.

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).

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

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.

Examples of compounds that provide the above anion include lithium difluorooxalate borate (hereinafter also referred to as "LiDFOB") represented by the following formula (1-1-1) and lithium bis(oxalate borate) (hereinafter also referred to as "LiBOB") represented by the following formula (1-2-1).

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

The lower limit of the content ratio of the anion in the nonaqueous 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 nonaqueous electrolyte. By setting the content ratio 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 ratio 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.

[Other additives]
The nonaqueous electrolyte may contain additives other than the nonaqueous 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; halogen compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole. Examples of the anisole compounds include 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.

<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.

[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.

[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.

The positive electrode substrate is conductive. Metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof, are used as the material of the substrate. Among these, aluminum or aluminum alloys are preferred in terms of the balance between potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil and vapor deposition film, with foil being preferred in terms of cost. In other words, aluminum foil or aluminum alloy 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).

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 "conductive" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 ⁷ Ω·cm or less, and "non-conductive" means that the volume resistivity is more than 10 ⁷ Ω·cm.

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.

As the positive electrode active material, a metal oxide is usually used. Specific examples of the positive electrode active material include composite oxides represented by Li _x MO _y (M represents at least one transition metal) (such as Li _x CoO ₂ , Li _x NiO ₂ , Li _x MnO ₃ , Li _x Ni _{α Co (1-α)} O ₂ , and Li _x Ni _α Mn _β Co _(1-α-β) O ₂ having a layered α-NaFeO 2 type crystal structure, and Li _x Mn ₂ O ₄ and Li _x Ni _α Mn _(2-α) O ₄ having a spinel type crystal structure), polyanion compounds represented by Li _w Me _x (AO _y ) _z (Me represents at least one transition metal, and A represents, for example, P, Si, B, V, etc.) (LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , Li ₂ CoPO ₄ F, etc. 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 mixed and used.

The 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 conductive agents include natural or artificial graphite, carbon black such as furnace black, acetylene black, and ketjen black, metals, conductive ceramics, etc., with acetylene black being preferred. The conductive agent may be in the form of a powder, fiber, etc.

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.

Examples of the thickener include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. In addition, if 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.

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

[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.

The negative electrode substrate can be constructed in the same manner as the positive electrode substrate, but the material used is a metal such as copper, nickel, stainless steel, or nickel-plated steel, or an alloy thereof, with copper or a copper alloy being preferred. In other words, copper foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

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 a conductive agent, a binder, a thickener, and a filler can be the same as those used in the positive electrode mixture layer.

As the negative electrode active material, a material capable of absorbing and releasing lithium ions is usually used. Specific negative electrode active materials include, for example, metals or semi-metals such as Si and Sn; metal oxides or semi-metal oxides such as Si oxide and Sn oxide; polyphosphate compounds; and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon).

Furthermore, the negative electrode composite 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.

[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.

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, Al ₂ O ₃ , SiO ₂ , aluminosilicate, etc. are preferred.

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 may be, for example, 4.0 V (vs. Li/Li ⁺ ) or more, but is preferably 4.4 V (vs. Li/Li ⁺ ) or more. 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 ⁺ ).

<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").

(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, The prepared non-aqueous electrolyte may be poured into a container for a non-aqueous electrolyte storage element.

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 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.

<Method of using non-aqueous electrolyte electricity storage element>
The method of using the nonaqueous electrolyte storage element is such that the positive electrode potential at the end-of-charge voltage during normal use is 4.4 V (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 can particularly fully exhibit this effect 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, the positive electrode potential is about 5.1 V (vs. Li/Li ⁺ ) when the end-of-charge voltage is 5.0 V, although this depends on the design.

<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.

In the above embodiment, the nonaqueous electrolyte storage element is mainly a nonaqueous electrolyte secondary battery, but other nonaqueous electrolyte storage elements may be used. Examples of other nonaqueous electrolyte storage elements include capacitors (electric double layer capacitors, lithium ion capacitors), etc.

1 shows 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 perspective 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 a 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.

The configuration of the nonaqueous electrolyte storage element according to the present invention is not particularly limited, and examples include a cylindrical battery, a square battery (rectangular battery), and a flat battery. The present invention can also be realized as an electricity storage device having a plurality of the above-mentioned nonaqueous electrolyte storage elements. One embodiment of the electricity storage device is shown in FIG. 2. In FIG. 2, the electricity storage device 30 has a plurality of electricity storage units 20. Each of the electricity storage units 20 has a plurality of nonaqueous electrolyte storage elements 1. The above-mentioned electricity storage device 30 can be installed as a power source for automobiles such as electric vehicles (EVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs).

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

The abbreviations for the non-aqueous solvents and additives used in the examples, comparative examples, and reference examples are shown below.

[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

[Additives]
LiDFOB: Lithium difluorooxalate borate LiBOB: Lithium bis(oxalate) borate VEC: Vinyl ethylene carbonate

[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 (LiPF ₆ ) as an electrolyte salt was dissolved in the non-aqueous solvent at a content of 1.2 mol/L, and LiDFOB as an additive was dissolved at a content of 0.2 mass% relative to the total mass of the non-aqueous solvent and the electrolyte to prepare a non-aqueous electrolyte.

(Preparation of Positive Electrode)
LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used as the positive electrode active material. A positive electrode paste containing the positive electrode active material: polyvinylidene fluoride (PVdF): acetylene black (AB) = 94: 3: 3 (solid equivalent) by mass ratio and N-methylpyrrolidone as a dispersion medium was prepared. This positive electrode paste was applied to both sides of a strip-shaped aluminum foil as a positive electrode substrate so that the positive electrode active material was contained at 18.6 mg / cm ² per unit electrode area, and the dispersion medium N-methylpyrrolidone was removed by drying. This was pressed by a roller press machine to form a positive electrode mixture layer, and then dried under reduced pressure at 100 ° C. for 14 hours to remove moisture in the positive electrode mixture layer. In this way, a positive electrode was obtained.

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

(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.

(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.

(Example 7)
A nonaqueous electrolyte storage element of Example 7 was obtained in the same manner as in Example 2, except that the electrolyte salt contents were 0.4 mol/L of lithium hexafluorophosphate (LiPF ₆ ) and 0.8 mol/L of lithium bis(fluorosulfonyl)amide (LiFSA). Note that "-" in the table indicates that the corresponding nonaqueous solvent or additive was not used.

(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 (-CF ₂ H) rather than a trifluoromethyl group (-CF ₃ ).

Among the nonaqueous electrolyte storage elements prepared by the above-mentioned procedure, multiple 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."

[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 step: Constant current charging is performed at a current value of 0.2 C up to 4.3 V. After reaching 4.3 V, the voltage is maintained, and charging is completed 8 hours after the start of charging (so-called constant current constant voltage charging), followed by a 10-minute break.
(2) Discharge step: Discharge the battery at a constant current of 0.2 C down to 2.75 V. When the battery voltage reaches 2.75 V, the battery is discharged to the end, and a 10-minute break is then performed.
(3) Charging step: Constant current charging is performed at a current value of 1 C up to 4.3 V. After reaching 4.3 V, the voltage is maintained, and charging is terminated when 3 hours have elapsed since the start of charging, followed by a 10-minute break.
(4) Discharge step: Discharge at a constant current of 1 C down to 2.75 V. When 2.75 V is reached, discharge is terminated, and a 10-minute pause is performed.
The discharge capacity in the above step (4) was recorded as the "second discharge capacity in the initial charge/discharge test."

<45°C storage test>
The nonaqueous electrolyte storage element after the above initial charge/discharge was subjected to a storage test at 45° C. in the following manner.
(1') Charging step: The battery is charged at a constant current of 1 C in a thermostatic chamber at 25° C. up to 4.3 V. After the voltage reaches 4.3 V, the voltage is maintained, and charging is terminated when 3 hours have elapsed since the start of charging, followed by a 10-minute break.
(2') 45° C. storage step The nonaqueous electrolyte storage element was placed in an open circuit state and left to stand (store) in a thermostatic chamber at 45° C. After 30 days had passed since the start of storage, the storage step was terminated, and the storage element was taken out and left to stand at room temperature for 5 hours or more.
(3') Discharging Step: 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 pause is performed.
After steps (1'), (2'), and (3'), steps (3) and (4) in the initial charge and discharge 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 and discharge test was calculated as "capacity retention rate after 45° C. storage test/%."

<0°C 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 step: Constant current charging is performed at a current value of 1 C up to 4.3 V. After reaching 4.3 V, the voltage is maintained, and charging is terminated when 3 hours have elapsed since the start of charging, followed by a 10-minute break.
(2'') Discharge step: Discharge at a constant current of 1 C down to 2.75 V. When 2.75 V is reached, discharge is terminated 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 relative to the discharge capacity at the first cycle was calculated as "capacity maintenance rate after 0°C cycle test/%". In addition, the difference in the capacity maintenance rate after 0°C cycle test with respect to Comparative Example 1 or Reference Example 1, which did not use an additive, was calculated as "difference in capacity maintenance 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 obtained. 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 obtained as "difference in battery thickness change after 0°C cycle test from non-additive product/mm". For Reference Examples 2 and 3, the difference from Reference Example 1 was obtained, and for Comparative Example 3 and Examples 1 to 7, the difference from Comparative Example 1 was obtained. If this value is a negative value, it means that the use of the additive can suppress the increase in battery thickness after the 0°C cycle test.

In the above test, when the voltage of the nonaqueous electrolyte storage element was 4.3 V, the potential of the positive electrode was 4.4 V (vs. Li/Li ⁺ ).

For the nonaqueous electrolyte storage elements of 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.

Comparing Comparative Example 1 and Comparative Example 2, it can be seen that by using FEC and FMP as the nonaqueous solvent, the "capacity retention rate after 45°C storage test" is improved compared to when FEC and EMC are used, but on the other hand, the "capacity retention rate after 0°C cycle test" (hereinafter sometimes referred to as low-temperature characteristics) is decreased.
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.

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 an unadded product (% difference)" and "difference in battery thickness change after 0°C cycle test from an unadded product (mm difference)" obtained by the "0°C charge-discharge cycle test" are summarized in Table 2.

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

Comparing Example 5 with Comparative Examples 1 and 3, it can be seen that the low-temperature properties are improved even when LiBOB is used as an additive. However, when VEC is used, the properties are significantly reduced.

Comparing Example 7 with Comparative Example 1, it can be seen that the improvement in low-temperature characteristics is observed even when LiFSA is used as the electrolyte salt.

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

Comparing Reference Example 1 with Reference Examples 2 and 3, it can be seen that when DFEA is used as the non-aqueous solvent, the effect of improving low-temperature characteristics is small even when LiDFOB is added, and there is no effect of suppressing the increase in battery thickness after a charge-discharge cycle test at 0° C. That is, it can be seen 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, DFEA had the largest LUMO and FMP had the smallest LUMO among DFEA, FEA and FMP. It is said that the larger the LUMO value, the better the reduction resistance. That is, FEA and FMP are more easily reduced than DFEA, and when comparing "fluorinated carboxylate having a group containing a difluoromethyl group" and "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, in a non-aqueous electrolyte storage element using FEA or FMP as a non-aqueous solvent, which are more easily reduced than DFEA, it is believed that FEA or FMP is decomposed on the negative electrode together with LiDFOB containing the additive, and a homogeneous coating is formed in which boron atoms and fluorine atoms are in close proximity, thereby exhibiting excellent charge-discharge cycle performance in a low-temperature environment.

The present invention can be applied to nonaqueous electrolyte storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

Reference Signs List 1 Non-aqueous electrolyte storage element 2 Electrode body 3 Container for non-aqueous electrolyte storage element 4 Positive electrode terminal 4' Positive electrode lead 5 Negative electrode terminal 5' Negative electrode lead 20 Storage unit 30 Storage device

Claims (6)

A non-aqueous solvent;
An electrolyte salt;
and an anion having a dicarboxylate group bonded to a boron atom;
the non-aqueous solvent contains a fluorinated cyclic carbonate and a fluorinated carboxylate having a group containing a trifluoromethyl group,
the fluorinated cyclic carbonate is fluoroethylene carbonate,
the fluorinated carboxylate is methyl 3,3,3-trifluoropropionate or a combination of methyl 3,3,3-trifluoropropionate and 2,2,2-trifluoroethyl acetate;
The non-aqueous electrolyte , wherein the anion is a difluorooxalatoborate anion . The non-aqueous electrolyte according to claim 1 , wherein the content of the fluorinated cyclic carbonate in the non-aqueous solvent is equal to or greater than 1 vol % and less than 20 vol % . 3. The non-aqueous electrolyte according to claim 2, wherein a content of the anion with respect to a total mass of the non-aqueous solvent and the electrolyte salt is 0.01% by mass or more and 1.0% by mass or less. A nonaqueous electrolyte storage element comprising the nonaqueous electrolyte according to claim 1 . A method for producing a nonaqueous electrolyte storage element, comprising: placing the nonaqueous electrolyte according to claim 1 in a container for the nonaqueous electrolyte storage element. 5. The method for using the nonaqueous electrolyte storage element according to claim 4 , wherein the positive electrode potential at the end-of-charge voltage during normal use is 4.4 V (vs. Li/Li ⁺ ) or higher.

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