Nonaqueous electrolyte and nonaqueous electrolyte energy storage device

Abstract

To provide a nonaqueous electrolyte for a nonaqueous electrolyte power storage element capable of reducing resistance under a low temperature environment of the nonaqueous electrolyte power storage element. And to provide a nonaqueous electrolyte power storage element having low resistance in a low-temperature environment.SOLUTION: A nonaqueous electrolyte for a nonaqueous electrolyte energy storage device according to one aspect of the present invention includes a lithium salt containing a phosphorus element and two fluorine elements bonded to the phosphorus element, and an alkylene sulfate, wherein the lithium salt contains lithium (difluorophosphonyl) fluorosulfonylimide, lithium difluorobis (oxalate) Phosphates or combinations thereof.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte storage element. [Background technology]

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

[0003] In order to improve the performance of non-aqueous electrolyte storage elements, the addition of various additives to non-aqueous electrolytes has been investigated (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-086790 Summary of the Invention [Problem to be solved by the invention]

[0005] Some additives can be decomposed on the surface of the positive electrode to form a coating that can improve the output characteristics of a nonaqueous electrolyte storage element. Examples of such additives include lithium salts such as lithium (difluorophosphonyl)fluorosulfonylimide and lithium difluorobis(oxalate)phosphate. The lithium salts can particularly reduce the resistance of a nonaqueous electrolyte storage element in a low-temperature environment. However, because the lithium salts can also be decomposed on the surface of the negative electrode, they may not be able to fully demonstrate the effect of reducing the resistance of a nonaqueous electrolyte storage element in a low-temperature environment.

[0006] The present invention has been made in light of the above circumstances, and has an object to provide a nonaqueous electrolyte for a nonaqueous electrolyte storage element that can reduce the resistance of the nonaqueous electrolyte storage element in a low-temperature environment, and to provide a nonaqueous electrolyte storage element that has low resistance in a low-temperature environment. [Means for solving the problem]

[0007] A non-aqueous electrolyte for a non-aqueous electrolyte storage element according to one aspect of the present invention comprises a lithium salt containing a phosphorus element and two fluorine elements bonded to the phosphorus element, and an alkylene sulfate, wherein the lithium salt is lithium (difluorophosphonyl)fluorosulfonylimide, lithium difluorobis(oxalate)phosphate, or a combination thereof.

[0008] A nonaqueous electrolyte storage element according to another aspect of the present invention includes a positive electrode, a negative electrode, and the nonaqueous electrolyte. [Effects of the Invention]

[0009] The nonaqueous electrolyte for a nonaqueous electrolyte storage element according to one aspect of the present invention can reduce the resistance of the nonaqueous electrolyte storage element in a low-temperature environment. The nonaqueous electrolyte electricity storage element according to another aspect of the present invention has low resistance in a low-temperature environment. [Brief explanation of the drawings]

[0010] [Figure 1]FIG. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element. [Figure 2] FIG. 2 is a schematic diagram showing one embodiment of an electricity storage device configured by assembling a plurality of nonaqueous electrolyte electricity storage elements. DETAILED DESCRIPTION OF THE INVENTION

[0011] First, an overview of the nonaqueous electrolyte and nonaqueous electrolyte storage element disclosed in this specification will be described.

[0012] (1) A nonaqueous electrolyte for a nonaqueous electrolyte storage element according to one aspect of the present invention includes a lithium salt containing a phosphorus element and two fluorine elements bonded to the phosphorus element, and an alkylene sulfate, wherein the lithium salt is lithium (difluorophosphonyl)fluorosulfonylimide, lithium difluorobis(oxalate)phosphate, or a combination thereof.

[0013] The nonaqueous electrolyte described in (1) above can reduce the resistance of a nonaqueous electrolyte storage element in a low-temperature environment. While the reason for this is unclear, the following is presumed. The nonaqueous electrolyte contains lithium (difluorophosphonyl)fluorosulfonylimide, lithium difluorobis(oxalate)phosphate, or a combination thereof as a lithium salt. When such a conventional nonaqueous electrolyte is used in a nonaqueous electrolyte storage element, the lithium salt can be oxidatively decomposed on the positive electrode surface to form a coating. This coating formed on the positive electrode surface can reduce the resistance of the nonaqueous electrolyte storage element, particularly in a low-temperature environment. On the other hand, the lithium salt can also be reductively decomposed on the negative electrode surface to form a coating. This coating formed on the negative electrode surface can increase the resistance of the nonaqueous electrolyte storage element in a low-temperature environment. For this reason, the lithium salt may not fully exhibit its resistance-reducing effect in the low-temperature environment. In contrast, the nonaqueous electrolyte contains an alkylene sulfate. The alkylene sulfate is reductively decomposed on the negative electrode surface in preference to the lithium salt, thereby forming a coating on the negative electrode surface. This suppresses the reductive decomposition of the lithium salt on the negative electrode surface, and the lithium salt can reliably form a good coating on the positive electrode surface. This coating forms a good ion conduction path on the positive electrode surface while suppressing side reactions and the like on the positive electrode surface. Therefore, the nonaqueous electrolyte can reduce the resistance of a nonaqueous electrolyte storage element in a low-temperature environment.

[0014] (2) In the nonaqueous electrolyte described in (1) above, the content of the lithium salt may be 0.1% by mass or more and 1.0% by mass or less, and the content of the alkylene sulfate may be 0.1% by mass or more and 2.0% by mass or less.

[0015] The nonaqueous electrolyte described in (2) above, when the content of the lithium salt and the alkylene sulfate is within the above range, can more reliably reduce the resistance of the nonaqueous electrolyte storage element in a low-temperature environment.

[0016] (3) In the non-aqueous electrolyte described in (1) or (2) above, the alkylene sulfate may be ethylene sulfate.

[0017] The nonaqueous electrolyte described in (3) above can more reliably reduce the resistance of the nonaqueous electrolyte storage element in a low-temperature environment.

[0018] (4) A nonaqueous electrolyte storage element according to another aspect of the present invention includes a positive electrode, a negative electrode, and the nonaqueous electrolyte according to any one of (1) to (3) above.

[0019] The nonaqueous electrolyte storage element described in (4) above has low resistance in a low-temperature environment because it contains the nonaqueous electrolyte.

[0020] (5) In the nonaqueous electrolyte storage element according to (4), the potential of the positive electrode at the end-of-charge voltage during normal use is 3.9 V vs. Li / Li + It may be more than that.

[0021] In the nonaqueous electrolyte storage element described in (5) above, the potential of the positive electrode at the end-of-charge voltage during normal use is equal to or higher than the lower limit, so that the lithium salt is easily oxidatively decomposed on the surface of the positive electrode, and therefore, a nonaqueous electrolyte storage element with low resistance in a low-temperature environment can be more easily obtained.

[0022] The term "normal use" refers to a case where a nonaqueous electrolyte storage element is used under recommended or specified charging conditions for the nonaqueous electrolyte storage element. For example, if a charger for the nonaqueous electrolyte storage element is provided, the term refers to a case where the nonaqueous electrolyte storage element is used with the charger.

[0023] Hereinafter, a nonaqueous electrolyte, a nonaqueous electrolyte energy storage element, an energy storage device, a method for manufacturing a nonaqueous electrolyte energy storage element according to one embodiment of the present invention, and other embodiments will be described in detail. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0024] [Non-aqueous electrolyte] A nonaqueous electrolyte for a nonaqueous electrolyte storage element according to one embodiment of the present invention contains an additive. The additive contains at least the first and second additives described below. The nonaqueous electrolyte may be a nonaqueous electrolytic solution containing a nonaqueous solvent, an electrolyte salt dissolved or mixed in the nonaqueous solvent, and the additives.

[0025] The first additive is a lithium salt containing a phosphorus element and two fluorine elements bonded to the phosphorus element. The lithium salt is lithium (difluorophosphonyl)fluorosulfonylimide (LiFSPI), lithium difluorobis(oxalate)phosphate (LiFOP), or a combination thereof. As the first additive, one of the lithium salts may be used alone, or both of the lithium salts may be used in combination.

[0026] As the first additive, lithium (difluorophosphonyl)fluorosulfonylimide is preferred from the viewpoint of reducing the resistance of a nonaqueous electrolyte storage element using the nonaqueous electrolyte in a low-temperature environment.

[0027] The second additive is an alkylene sulfate. The alkylene sulfate is a compound containing a structure in which an alkylene group is bonded to an oxysulfonyloxy group (—OS(═O)2-O—). The alkylene sulfate is a compound represented by the following formula: [ka]

[0028] In the above formula, R 1is a linear or branched alkylene group having 2 to 5 carbon atoms. That is, the compound represented by the above formula is a cyclic sulfate, and may be, for example, a cyclic sulfate containing a 5-membered ring structure or a cyclic sulfate containing a 6-membered ring structure. Examples of the compound represented by the above formula include 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, etc. 1 Compounds containing hetero elements are not included.

[0029] Examples of the second additive include ethylene sulfate (DTD), 2,3-propylene sulfate, 1,3-propylene sulfate, and 4,5-pentene sulfate. Among these, ethylene sulfate is preferred. The second additive may be used alone or in combination of two or more.

[0030] The lower limit of the content of the first additive is preferably 0.1% by mass, more preferably 0.2% by mass, and even more preferably 0.3% by mass, from the viewpoint of reducing the resistance of the nonaqueous electrolyte storage element in a low-temperature environment. On the other hand, the upper limit of the content of the first additive is preferably 2.0% by mass, more preferably 1.5% by mass, and even more preferably 1.2% by mass, from the viewpoint of suppressing reductive decomposition of the first additive on the negative electrode surface of the nonaqueous electrolyte storage element. Furthermore, the content of the first additive is preferably 0.1% by mass or more to 2.0% by mass or less, more preferably 0.2% by mass or more to 1.5% by mass or less, and even more preferably 0.3% by mass or more to 1.2% by mass or less. Here, when multiple first additives are mixed, the "content of the first additive" refers to the total content of the multiple first additives.

[0031] The lower limit of the content of the second additive is preferably 0.1% by mass, more preferably 0.3% by mass, and even more preferably 0.5% by mass, from the viewpoint of suppressing the reductive decomposition of the first additive on the negative electrode surface of the nonaqueous electrolyte storage element. On the other hand, the upper limit of the content of the second additive is preferably 2.0% by mass, more preferably 1.8% by mass, and even more preferably 1.6% by mass, from the viewpoint of reducing the resistance of the nonaqueous electrolyte storage element in a low-temperature environment. Furthermore, the content of the second additive is preferably 0.1% by mass or more and 2.0% by mass or less, more preferably 0.3% by mass or more and 1.8% by mass or less, and even more preferably 0.5% by mass or more and 1.6% by mass or less. Here, when two or more second additives are mixed, the "content of the second additive" refers to the total content of the two or more second additives.

[0032] The non-aqueous electrolyte may further contain other additives in addition to the first additive and the second additive. Other additives include, for example, halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB) and lithium difluorooxalateborate (LiFOB); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, and succinic anhydride. , glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathione) oran), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives may be used alone or in combination of two or more.

[0033] The total content of other additives contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass, based on the total mass of the non-aqueous electrolyte. By setting the total content of other additives within the above ranges, it is possible to improve capacity retention or cycle performance after high-temperature storage, and further improve safety.

[0034] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0035] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0036] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0037] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0038] The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of cations constituting the electrolyte salt include lithium ions, sodium ions, potassium ions, magnesium ions, onium ions, etc. Among these, lithium ions are preferred.

[0039] Examples of electrolyte salts containing lithium ions include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB) and lithium difluorooxalateborate (LiFOB), and halogenated hydrocarbon group-containing lithium salts 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.

[0040] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3 More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0041] The non-aqueous electrolyte may be a combination of the non-aqueous electrolytic solution and a solid electrolyte.

[0042] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc., and is solid at room temperature (for example, 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

[0043] Examples of sulfide solid electrolytes for lithium ion secondary batteries include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 Ge-P2S 12 etc.

[0044] [Non-aqueous electrolyte energy storage element] A nonaqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, the nonaqueous electrolyte described above, and a container that accommodates the electrode assembly and nonaqueous electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a stack of positive electrodes and negative electrodes with separators interposed therebetween is wound. The nonaqueous electrolyte is present in a state contained in the positive electrode, negative electrode, and separator. As an example of a nonaqueous electrolyte storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0045] <Positive electrode> The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0046] The lower limit of the positive electrode potential at the end of charge voltage during normal use is 3.8 V vs. Li / Li +is preferable, 4.0V vs. Li / Li + is more preferable, 4.2V vs. Li / Li + When the potential is equal to or higher than the lower limit, the first additive contained in the non-aqueous electrolyte is easily oxidized and decomposed on the surface of the positive electrode. From the viewpoint of further increasing the capacity retention rate after charge-discharge cycles, the upper limit of the potential of the positive electrode at the end-of-charge voltage during normal use is 4.8 V vs. Li / Li. + is preferable, 4.6V vs. Li / Li + is more preferable, 4.4V vs. Li / Li + The potential of the positive electrode at the end-of-charge voltage during normal use is more preferably 3.8 V vs. Li / Li. + More than 4.8V vs.Li / Li + Less than 4.0V vs. Li / Li is preferred + More than 4.6V vs.Li / Li + The following is more preferable: 4.2V vs. Li / Li + More than 4.4V vs.Li / Li + The following is more preferable: The above potential can be adjusted by appropriately selecting the type of positive electrode active material described later.

[0047] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 -2 The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0048] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the nonaqueous electrolyte storage element.

[0049] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0050] The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0051] The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 crystal structure, lithium transition metal composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having an α-NaFeO2 crystal structure include Li[Li x Ni (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li x Ni γ Mn β Co (1-x-γ-β)]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β). Lithium transition metal composite oxides with spinel-type crystal structures include Li x Mn2O4, Li x Ni γ Mn (2-γ) Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. As the positive electrode active material, a lithium transition metal composite oxide is preferred, and a lithium transition metal composite oxide having an α-NaFeO2 crystal structure is more preferred, from the viewpoint of easily oxidizing and decomposing the first additive contained in the non-aqueous electrolyte on the positive electrode surface. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0052] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material becomes easier to manufacture and handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. The "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

[0053] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0054] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within this range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0055] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0056] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the nonaqueous electrolyte storage element can be increased.

[0057] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0058] The content of the binder in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the positive electrode active material can be stably maintained.

[0059] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be deactivated in advance by methylation or the like. When a thickener is used, the content of the thickener in the positive electrode active material layer can be 0.1% by mass or more and 8% by mass or less, and typically 5% by mass or less is preferred, and 2% by mass or less is more preferred. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a thickener.

[0060] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene; inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; carbonates such as calcium carbonate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate; nitrides such as aluminum nitride and silicon nitride; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and artificial products thereof. When a filler is used, the content of the filler in the positive electrode active material layer can be 0.1% by mass or more and 8% by mass or less, typically 5% by mass or less is preferred, and 2% by mass or less is more preferred. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0061] The positive electrode active material 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, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0062] <Negative electrode> The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0063] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0064] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while also increasing the energy density per volume of the nonaqueous electrolyte storage element.

[0065] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0066] The negative electrode active material 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, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0067] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、Examples of the material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0068] "Graphite" refers to graphite that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0069] "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0070] Here, the "discharged state" of a carbon material refers to a state in which the carbon material, which is a negative electrode active material, is discharged so that lithium ions that can be absorbed and released during charging and discharging are sufficiently released from the carbon material. For example, this refers to a state in which the open circuit voltage of a half cell using a negative electrode containing a carbon material as a negative electrode active material as a working electrode and metallic Li as a counter electrode is 0.7 V or higher.

[0071] "Non-graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.36 nm or more and 0.42 nm or less.

[0072] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0073] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the negative electrode active material can be easily produced or handled. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the negative electrode active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and classification method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material layer may be in the form of a foil.

[0074] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved. When the negative electrode active material is metallic Li, the content of the negative electrode active material in the negative electrode active material layer may be 99% by mass or more, or may be 100% by mass.

[0075] <separator> The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0076] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of safety of the nonaqueous electrolyte electricity storage element.

[0077] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0078] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0079] <Non-aqueous electrolyte> The nonaqueous electrolyte of the nonaqueous electrolyte storage element can be the same as the nonaqueous electrolyte in the above-described embodiment.

[0080] The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries. FIG. 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0081] [Electricity storage device] The nonaqueous electrolyte energy storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements in automotive power sources such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), power sources for electronic devices such as personal computers and communication terminals, or power storage power sources, etc. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit. 2 shows an example of an electricity storage device 30 in which electricity storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte electricity storage elements 1, are further assembled. The electricity storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte electricity storage elements 1, a bus bar (not shown) that electrically connects two or more electricity storage units 20, etc. The electricity storage unit 20 or the electricity storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte electricity storage elements.

[0082] [Method of manufacturing nonaqueous electrolyte energy storage element] The method for manufacturing the nonaqueous electrolyte storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing a nonaqueous electrolyte, and housing the electrode assembly and the nonaqueous electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0083] The method for placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and then the inlet may be sealed.

[0084] [Other embodiments] The nonaqueous electrolyte storage element of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, or part of the configuration of one embodiment may be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0085] In the above embodiment, the non-aqueous electrolyte is described as containing the first additive as an additive, but the lithium salt described as the first additive may also be used as the electrolyte salt.

[0086] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery), but the type, shape, size, capacity, etc. of the nonaqueous electrolyte storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors. [Example]

[0087] 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.

[0088] [Example 1] (Preparation of positive electrode) As the positive electrode active material, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 was prepared. Next, a positive electrode mixture paste was prepared using N-methylpyrrolidone (NMP) as a dispersion medium, containing the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 90.5:7.0:2.5 in terms of solid content. The positive electrode mixture paste was applied to both sides of aluminum foil as a positive electrode substrate, dried, and pressed. This resulted in a positive electrode in which positive electrode active material layers were laminated on both sides of the positive electrode substrate.

[0089] (Preparation of negative electrode) A negative electrode mixture paste was prepared by mixing graphite (Gr) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, carboxymethyl cellulose (CMC) as the thickener, and water as the dispersion medium. The mass ratio of Gr, SBR, and CMC was 98:1:1 (solid content equivalent). The negative electrode mixture paste was applied to both sides of copper foil as the negative electrode substrate, dried, and pressed. This produced a negative electrode.

[0090] (Preparation of non-aqueous electrolyte) A non-aqueous solvent was prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35. LiPF6 was added as an electrolyte salt at a concentration of 1.2 mol / dm 3 To this mixed solution, lithium (difluorophosphonyl) fluorosulfonylimide (LiFSPI) and ethylene sulfate (DTD) were added as additives at a content of 0.5% by mass and 0.5% by mass, respectively, to prepare a non-aqueous electrolyte.

[0091] (Fabrication of non-aqueous electrolyte energy storage element) The positive electrode and the negative electrode were stacked with a polyolefin microporous membrane separator interposed therebetween to prepare an electrode assembly, which was then housed in a container made of a metal resin composite film, the nonaqueous electrolyte was poured into the container, and the container was then sealed by heat welding to obtain the nonaqueous electrolyte storage element of Example 1.

[0092] [Example 2, Comparative Examples 1 to 5] The nonaqueous electrolyte storage elements of Example 2 and Comparative Examples 1 to 5 were obtained by the same procedure as in Example 1, except that the types and contents of additives were as shown in Table 1. In Table 1, "LiFSPI" means lithium (difluorophosphonyl)fluorosulfonylimide, "LiFOP" means lithium difluorobis(oxalate)phosphate, "LiDFP" means lithium difluorophosphate, "DTD" means ethylene sulfate, and "VC" means vinylene carbonate.

[0093] [evaluation] (1-1)Initial charging / discharging The obtained nonaqueous electrolyte storage element was subjected to initial charge and discharge under the following conditions. In a thermostatic chamber at 25°C, constant current charging was performed with a charging current of 1.0 C and a cut-off voltage of 4.10 V, followed by constant voltage charging at 4.10 V. The charge was terminated until the total charge time reached 3 hours. A 10-minute rest period was then provided. Constant current discharging was performed with a discharging current of 1.0 C and a cut-off voltage of 2.50 V. The discharge capacity at this time was taken as the initial discharge capacity. Note that the positive electrode potential at the cut-off voltage of 4.10 V for each nonaqueous electrolyte storage element was at least 4.10 V vs. Li / Li. + That was all.

[0094] (1-2) Initial resistance in low temperature environments After the above initial charge / discharge, the initial resistance of each nonaqueous electrolyte storage element was measured in a low-temperature environment in the following manner. In a thermostatic chamber at 25°C, constant-current charging was performed at a charging current of 1.0 C until the SOC (State of Charge) reached 50%. After storing the cells in a thermostatic chamber at -10°C for 3 hours, the cells were discharged at currents of 0.2 C, 0.5 C, and 1.0 C at -10°C for 30 seconds each. After each discharge, constant-current charging was performed at a charging current of 0.2 C until the SOC reached 50%. The relationship between the current and the voltage 10 seconds after the start of discharge at each discharge current was plotted, and the DC resistance was calculated from the slope of the straight line obtained from the three plots, which was used as the initial resistance (initial DCR) in a low-temperature environment. Table 1 shows the ratio of the initial DCR of each nonaqueous electrolyte energy storage element to the initial DCR of Comparative Example 4.

[0095] (1-3) Charge / discharge cycle test After measuring the initial resistance in the low-temperature environment, each nonaqueous electrolyte storage element was subjected to a charge-discharge cycle test at 60° C. in the following manner. Constant current charging was performed with a charging current of 4.0 C and a cut-off voltage of 4.02 V (corresponding to an SOC of 85%). Subsequently, constant current discharging was performed with a discharging current of 4.0 C and a cut-off voltage of 3.4 V (corresponding to an SOC of 15%). A 10-minute rest period was provided after each charge and discharge. This charge / discharge cycle was repeated 4,290 times. The positive electrode potential at the cut-off voltage of 4.02 V for each nonaqueous electrolyte storage element was at least 4.02 V vs. Li / Li. + That was all.

[0096] (1-4) Discharge capacity after charge-discharge cycle test and resistance in low temperature environment After the charge-discharge cycle test, the discharge capacity of each nonaqueous electrolyte storage element was measured using the same procedure as the initial charge-discharge test, and this was defined as the discharge capacity after the charge-discharge cycle test. Furthermore, the percentage of the discharge capacity after the charge-discharge cycle test relative to the initial discharge capacity was calculated as the capacity retention rate [%]. The capacity retention rates are shown in Table 1. The DC resistance of each element was measured in the same manner as for the initial resistance in a low-temperature environment, and this was taken as the post-cycling DCR. Table 1 shows the ratio of the post-cycling DCR of each nonaqueous electrolyte electricity storage element to the post-cycling DCR of Comparative Example 4.

[0097] [Table 1]

[0098] As shown in Table 1, Comparative Examples 2 and 3, which used LiFSPI or LiFOP and VC as additives, had large initial DCRs and post-cycling DCRs, and small capacity retention rates. In contrast, Examples 1 and 2, which used LiFSPI or LiFOP and DTD as additives, had small initial DCRs and post-cycling DCRs, and large capacity retention rates. This suggests that the combination of LiFSPI and LiFOP with DTD significantly reduces resistance in low-temperature environments.

[0099] On the other hand, in Comparative Example 5, which used LiDFP and DTD as additives, the initial DCR, post-cycling DCR, and capacity retention were all improved compared to Comparative Example 1, which used LiDFP and VC. However, in Comparative Example 5, neither the initial DCR nor the post-cycling DCR was improved compared to Comparative Example 4, which used LiDFP alone. These results suggest that the effect of reducing resistance in low-temperature environments exerted by the combination with DTD as in Examples 1 and 2 is an effect unique to LiFSPI and LiFOP. [Industrial Applicability]

[0100] 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]

[0101] 1. Non-aqueous electrolyte energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. a lithium salt containing a phosphorus element and two fluorine elements bonded to the phosphorus element, and an alkylene sulfate; The non-aqueous electrolyte for a non-aqueous electrolyte storage element is one in which the lithium salt is lithium (difluorophosphonyl)fluorosulfonylimide, lithium difluorobis(oxalate)phosphate, or a combination thereof.

2. 2. The non-aqueous electrolyte according to claim 1, wherein the content of the lithium salt is from 0.1% by mass to 1.0% by mass, and the content of the alkylene sulfate is from 0.1% by mass to 2.0% by mass.

3. 3. The non-aqueous electrolyte according to claim 1, wherein the alkylene sulfate is ethylene sulfate.

4. A nonaqueous electrolyte storage element comprising a positive electrode, a negative electrode, and the nonaqueous electrolyte according to claim 1 or 2.

5. The potential of the positive electrode at the end of charge voltage during normal use is 3.9 V vs. Li / Li ＋ 5. The nonaqueous electrolyte electricity storage element according to claim 4, wherein the nonaqueous electrolyte electricity storage element is a nonaqueous electrolyte electricity storage element.

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