Sulfide-based positive electrode active material, positive electrode for energy storage element, energy storage element, and method for producing sulfide-based positive electrode active material

Abstract

To provide a sulfide-based positive electrode active material capable of improving discharge capacity of a power storage element.SOLUTION: A sulfide-based positive electrode active material has an inverse fluorite-type crystal structure and contains lithium, sulfur and nitrogen, and a content ratio of the sulfur to the nitrogen is between 4.0 and 32.0 in a mole ratio.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a sulfide-based positive electrode active material, a positive electrode for an electric storage device, and an electric storage device. [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. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as energy storage elements.

[0003] In recent years, with the aim of improving the safety of non-aqueous electrolyte secondary batteries, sulfide-based solid electrolytes with high ionic conductivity have been proposed as non-aqueous electrolytes in place of liquid electrolytes containing organic solvents or the like (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-109955 Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, in energy storage elements used in hybrid electric vehicles (hereinafter also referred to as "HEVs") and hybrid industrial machinery (heavy machinery, construction machinery, etc.), a positive electrode active material that can improve discharge capacity is desired to enhance charge / discharge performance, and the application of sulfide-based positive electrode active materials is being considered.

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a sulfide-based positive electrode active material that can improve the discharge capacity of an electricity storage element. [Means for solving the problem]

[0007] A sulfide-based positive electrode active material according to one aspect of the present invention has an antifluorite crystal structure and contains lithium, sulfur, and nitrogen, with the molar ratio of sulfur to nitrogen being 4.0 or more and 32.0 or less. [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to provide a sulfide-based positive electrode active material that can improve the discharge capacity of an electricity storage device. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic cross-sectional view of an all-solid-state battery, which is one embodiment of the energy storage element of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an energy storage device configured by assembling a plurality of energy storage elements according to one embodiment of the present invention. [Figure 3] FIG. 3 is an X-ray diffraction diagram of the sulfide-based positive electrode active materials of Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, an overview of the sulfide-based positive electrode active material, the positive electrode for an energy storage device, and the energy storage device disclosed in this specification will be described.

[0011] A sulfide-based positive electrode active material according to one aspect of the present invention has an antifluorite crystal structure and contains lithium, sulfur, and nitrogen, with the molar ratio of sulfur to nitrogen being 4.0 or more and 32.0 or less.

[0012] The sulfide-based positive electrode active material has an inverse fluorite crystal structure and contains lithium, sulfur, and nitrogen. The molar ratio of sulfur to nitrogen is 4.0 or more and 32.0 or less, thereby improving the discharge capacity of an energy storage device. While the reason for this is unclear, the following is presumed. The sulfide-based positive electrode active material has an inverse fluorite crystal structure and contains lithium, sulfur, and nitrogen. By including an appropriate amount of nitrogen, ionic conductivity is improved. Therefore, it is believed that the sulfide-based positive electrode active material can improve the discharge capacity of an energy storage device. On the other hand, it is presumed that an excessive nitrogen content increases the interaction between sulfur and nitrogen, resulting in a decrease in discharge capacity.

[0013] The sulfide-based positive electrode active material is preferably composited with a carbon material. By composited with the sulfide-based positive electrode active material and the carbon material, the discharge capacity of the energy storage device can be further improved. Here, "composite" refers to a composite formed by chemical or physical bonding between the sulfide-based positive electrode active material and the carbon material, or a composite formed by mechanically composited the sulfide-based positive electrode active material and the carbon material. The composite is a composite in which the sulfide-based positive electrode active material and the carbon material are present within a single particle. Examples of the composite include a composite in which the sulfide-based positive electrode active material and the carbon material form an aggregated state, and a composite in which at least a portion of the surface of the sulfide-based positive electrode active material is coated with a carbon material.

[0014] A positive electrode according to one aspect of the present invention contains the sulfide-based positive electrode active material, and because the positive electrode contains the sulfide-based positive electrode active material, the discharge capacity of the energy storage device can be improved.

[0015] The positive electrode preferably further contains a carbon material that is not composited with the sulfide-based positive electrode active material. By containing a carbon material that is not composited with the sulfide-based positive electrode active material, the discharge capacity of the energy storage device can be further improved.

[0016] An electric storage device according to one aspect of the present invention includes the positive electrode, and has a large discharge capacity.

[0017] The configuration of a sulfide-based positive electrode active material, the configuration of a positive electrode for an energy storage device, the configuration of an energy storage device, and other embodiments according to one embodiment of the present invention will be described in detail. Note that the names of the components (components) used in each embodiment may differ from the names of the components (components) used in the background art.

[0018] <Sulfide-based positive electrode active material> The sulfide-based positive electrode active material has an inverse fluorite crystal structure and contains lithium, sulfur, and nitrogen.

[0019] The lower limit of the molar ratio of sulfur to nitrogen in the sulfide-based positive electrode active material is 4.0, preferably 4.5. Meanwhile, the upper limit of the molar ratio of sulfur to nitrogen is 32.0, preferably 19.0. When the molar ratio of sulfur to nitrogen in the sulfide-based positive electrode active material is within the above range, the discharge capacity of the energy storage element can be improved.

[0020] The sulfide-based positive electrode active material preferably has a composition expressed by the molar ratio (100-x)Li2S·xLi3N (3≦x≦19). When the sulfide-based positive electrode active material has a composition expressed by the above general formula, the discharge capacity of the energy storage device can be further improved. The sulfide-based positive electrode active material has a structure doped with Li3N, which has a crystal structure similar to Li2S, widening the bottleneck for lithium ion migration and increasing the lithium ion concentration within the crystal structure. This improves lithium conductivity. Furthermore, when the molar ratio of Li2S to Li3N is within the above range, ionic conductivity can be enhanced while suppressing the interaction between Li2S and nitrogen. As a result, the sulfide-based positive electrode active material can further improve the discharge capacity of the energy storage device.

[0021] The sulfide-based positive electrode active material is preferably composited with a carbon material. By composited with the sulfide-based positive electrode active material and a carbon material, the discharge capacity of the energy storage device can be further improved. Examples of the carbon material include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofiber, pitch-based carbon fiber, 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. As the carbon material, one of these materials may be used alone, or two or more may be used in combination.

[0022] The sulfide-based positive electrode active material can improve the discharge capacity of the energy storage device.

[0023] <Positive electrodes for energy storage elements> A positive electrode according to one embodiment of the present invention is a positive electrode for an energy storage device having the sulfide-based positive electrode active material described above. 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.

[0024] The positive electrode substrate is conductive. Conductivity is determined by a volume resistivity of 107 Ω·cm measured in accordance with JIS-H-0505 (1975). Positive electrode substrates are typically made of metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof. Among these, aluminum or aluminum alloys are preferred due to their high potential resistance, conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred due to 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-H4160 (2006).

[0025] 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 secondary battery.

[0026] 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.

[0027] The positive electrode active material layer contains a positive electrode active material.

[0028] The positive electrode active material layer contains the sulfide-based positive electrode active material described above as the positive electrode active material. The positive electrode active material may contain other positive electrode active materials in addition to the sulfide-based positive electrode active material. However, the molar ratio of the other positive electrode active materials to the sulfide-based positive electrode active material is preferably 20 or less, more preferably 10 or less. The positive electrode active material may be substantially composed of the sulfide-based positive electrode active material. By having the positive electrode active material be substantially composed of the sulfide-based positive electrode active material, the discharge capacity of the energy storage device can be improved.

[0029] The other positive electrode active materials can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as a 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), 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), 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), 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). Lithium transition metal composite oxides with spinel crystal structure 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. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0030] 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).

[0031] 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.

[0032] The total content of the positive electrode active material in the positive electrode active material layer is preferably 40% by mass to 95% by mass, more preferably 45% by mass to 93% by mass, and even more preferably 50% by mass to 90% by mass. By setting the total 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.

[0033] The positive electrode preferably further contains a carbon material that is not composited with the sulfide-based positive electrode active material. By containing a carbon material that is not composited with the sulfide-based positive electrode active material, the discharge capacity of the energy storage device can be further improved. As this carbon material, the same materials as the carbon materials exemplified above can be used. From the viewpoints of electronic conductivity and coatability, carbon black is preferred, and acetylene black is particularly preferred.

[0034] The positive electrode active material layer may contain other optional components such as a conductive agent, a binder, a thickener, and a filler, as required.

[0035] The other conductive agent is not particularly limited as long as it is a material having conductivity other than the above-mentioned carbon material, and examples of such conductive agents include metals and conductive ceramics.

[0036] The total 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 total content of the conductive agent within this range, the energy density of the energy storage element can be increased.

[0037] 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.

[0038] The binder content 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 active material can be stably held.

[0039] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0040] The filler is not particularly limited, and examples of the filler 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, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0041] 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.

[0042] The positive electrode contains the sulfide-based positive electrode active material, and therefore the discharge capacity of the energy storage device can be improved.

[0043] <Energy storage element> An energy storage element according to one embodiment of the present invention includes an electrode assembly having the positive electrode, negative electrode, and separator, a non-aqueous electrolyte, and a container for accommodating the electrode assembly and non-aqueous 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 is wound with separators interposed therebetween. As an example of an energy storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0044] [Positive electrode] The positive electrode of the energy storage element is the positive electrode according to one embodiment of the present invention, and the energy storage element has a large discharge capacity.

[0045] [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.

[0046] 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.

[0047] 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 increasing the energy density per volume of the secondary battery.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] "Graphite" refers to a graphite material 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.

[0052] "Non-graphitic carbon" refers to 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.

[0053] Here, the "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbonaceous material as a negative electrode active material as a working electrode and metallic Li as a counter electrode, this refers to a state in which the open circuit voltage is 0.7 V or higher.

[0054] "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.

[0055] "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.

[0056] 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 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 the powder 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 may be in the form of a foil.

[0057] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0058] [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 materials for the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these materials, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is 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.

[0059] 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 under 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, barium titanate, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; 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 and barium fluoride; covalent 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 the safety of the energy storage device.

[0060] 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.

[0061] 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.

[0062] [Non-aqueous electrolyte] The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte may be a nonaqueous electrolytic solution. The nonaqueous electrolytic solution contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0068] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated 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.

[0069] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3More 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. 3 More 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.

[0070] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiFOB), and lithium bis(oxalato)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, and cyclohexyl. Aromatic compounds such as benzene, 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, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, Glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2- dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 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, and the like.These additives may be used alone or in combination of two or more.

[0071] The content of the additive 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. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0072] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0073] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc. and is solid at room temperature (e.g., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

[0074] Examples of sulfide solid electrolytes include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 Ge-P2S 12 etc.

[0075] The shape of the energy storage element of this embodiment is not particularly limited, and examples thereof include a cylindrical battery, a prismatic battery, a flat battery, a coin battery, and a button battery.

[0076] FIG. 1 shows an energy storage element 1 as an example of a prismatic battery. Note that this 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.

[0077] [Configuration of the power storage device] The energy storage element of this embodiment can be mounted as an energy storage device configured by assembling a plurality of energy storage elements 1 in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one energy storage element included in the energy storage device. 2 shows an example of an energy storage device 30 in which energy storage units 20, each of which is an assembly of two or more electrically connected energy storage elements 1, are further assembled. The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, etc. The energy storage units 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more energy storage elements.

[0078] [Method for producing sulfide-based positive electrode active material] The sulfide-based positive electrode active material can be produced, for example, by the following method, although not particularly limited thereto: That is, the method for producing the sulfide-based positive electrode active material preferably comprises treating a material containing lithium, sulfur, and nitrogen by a mechanochemical method.

[0079] The above material is usually a mixture of two or more compounds containing at least one element selected from the group consisting of lithium, sulfur, and nitrogen. The material (mixture) may contain the elements lithium, sulfur, and nitrogen. A single compound may contain two or more elements selected from the group consisting of lithium, sulfur, and nitrogen. For example, an example of a compound containing lithium and sulfur is Li2S, which will be described later, and an example of a compound containing lithium and nitrogen is Li3N.

[0080] The sulfide-based positive electrode active material obtained by this production method has an inverse fluorite crystal structure.

[0081] The mechanochemical method (also known as mechanochemical processing) refers to a synthesis method that utilizes mechanochemical reactions. Mechanochemical reactions are chemical reactions such as crystallization, solid-solution reactions, and phase transition reactions that utilize high energy generated locally due to mechanical energy such as friction and compression during the crushing process of solid materials. It is believed that in this manufacturing method, the mechanochemical process results in a reaction that forms a sulfide-based positive electrode active material with an inverse fluorite crystal structure and containing lithium, sulfur, and nitrogen. Examples of equipment used in the mechanochemical method include grinding and dispersing machines such as ball mills, bead mills, vibration mills, turbo mills, mechanofusion machines, and disk mills. Among these, ball mills are preferred. Ball mills made of tungsten carbide (WC) or zirconium oxide (ZrO2) are suitable.

[0082] When treating with a ball mill, the ball rotation speed during treatment can be, for example, 100 rpm to 1,000 rpm. The treatment time can be, for example, 0.1 hours to 10 hours. This treatment can be carried out in an inert gas atmosphere such as argon or an active gas atmosphere, but is preferably carried out in an inert gas atmosphere.

[0083] In preparing the sulfide-based positive electrode active material positive electrode, it is preferable to subject a mixture containing the sulfide-based positive electrode active material and a carbon material to mechanical milling. By subjecting the mixture containing the sulfide-based positive electrode active material and the carbon material to mechanical milling, the sulfide-based positive electrode active material is composited with the carbon material, thereby enabling the reliable production of a positive electrode active material that can further improve the discharge capacity of a storage device. It is also more preferable to subject a mixture containing the sulfide-based positive electrode active material, a carbon material, and a solid electrolyte to mechanical milling. By also subjecting the solid electrolyte to composite formation, ionic conductivity can also be improved.

[0084] Here, mechanical milling refers to a process of pulverizing, mixing, or compounding materials by applying mechanical energy such as impact, shear stress, or friction. Examples of equipment used for mechanical milling include grinding and dispersing machines such as ball mills, bead mills, vibration mills, turbo mills, mechanofusion mills, and disk mills. Among these, ball mills are preferred. Suitable ball mills include those made of tungsten carbide (WC) and zirconium oxide (ZrO2). Note that the mechanical milling process does not necessarily involve a mechanochemical reaction. It is believed that such mechanical milling processes compound the sulfide-based positive electrode active material and the carbon material, improving electronic conductivity.

[0085] When treating with a ball mill, the ball rotation speed during treatment can be, for example, 100 rpm to 1,000 rpm. The treatment time can be, for example, 0.1 hours to 10 hours. This treatment can be carried out in an inert gas atmosphere such as argon or an active gas atmosphere, but is preferably carried out in an inert gas atmosphere.

[0086] [Method of manufacturing positive electrodes for energy storage devices] The positive electrode for an electric storage device can be produced by using the sulfide-based positive electrode active material. That is, the positive electrode for an electric storage device can be produced, for example, by the following method, although not particularly limited thereto.

[0087] The positive electrode can be fabricated by, for example, applying a positive electrode mixture paste to a positive electrode substrate directly or via an intermediate layer, and then drying. The positive electrode mixture paste contains each component constituting the positive electrode mixture, such as a sulfide-based positive electrode active material, and optional components such as a carbon material, other conductive agents, and binders. The positive electrode mixture paste may further contain a dispersion medium.

[0088] [Method of manufacturing an energy storage element] The energy storage element can be manufactured by using the sulfide-based positive electrode active material. That is, a method for manufacturing the energy storage element includes fabricating a positive electrode using the sulfide-based positive electrode active material. The method for manufacturing the energy storage element can be appropriately selected from known methods. The method for manufacturing the energy storage element includes, for example, preparing an electrode assembly, preparing a non-aqueous electrolyte, and housing the electrode assembly and the non-aqueous electrolyte in a container. Preparing the electrode assembly includes preparing the positive electrode and negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and negative electrode with a separator interposed therebetween.

[0089] 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.

[0090] <Other embodiments> The energy storage device 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 can be added to the configuration of another embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0091] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) has been described, but the energy storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0092] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode. [Example]

[0093] 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.

[0094] [Examples 1 to 3, Comparative Examples 2 and 3] Li2S and Li3N were mixed in an agate mortar in the molar ratio shown in Table 1, and the mixture was then placed in an 80 mL sealed zirconia pot containing 160 g of 4 mm diameter zirconia balls. These steps were carried out in an argon atmosphere with a dew point of -50°C or lower. The mixture was treated for 17 hours at an orbital speed of 400 rpm using a planetary ball mill (FRITSCH, model number Premium line P-7). Through this mechanochemical treatment, sulfide-based positive electrode active materials of Examples 1 to 3 and Comparative Examples 2 and 3 were obtained.

[0095] [Comparative Example 1] A sulfide-based positive electrode active material containing only Li2S was used as Comparative Example 1.

[0096] (X-ray diffraction measurement of sulfide-based positive electrode active material) X-ray diffraction measurements were performed on the sulfide-based positive electrode active materials of Example 1 and Comparative Example 1 using the following method. The sulfide-based solid electrolyte powders of the Examples and Comparative Examples were packed into an airtight X-ray diffraction sample holder under an argon atmosphere with a dew point of −50°C or lower. Powder X-ray diffraction measurements were performed using an X-ray diffractometer (Rigaku Corporation, "miniFlex II"). The radiation source was CuKα radiation, the tube voltage was 30 kV, and the tube current was 15 mA. Diffracted X-rays were passed through a 30 μm-thick Kβ filter and detected with a high-speed one-dimensional detector (model number: D / teX Ultra2). The sampling width was 0.01°, the scan speed was 5° / min, the divergence slit width was 0.625°, the receiving slit width was 13 mm (open), and the scattering slit width was 8 mm. Figure 3 shows the X-ray diffraction patterns of the sulfide-based positive electrode active materials of Example 1 and Comparative Example 1. It was confirmed that all of them had an inverse fluorite crystal structure as the main phase.

[0097] (Characteristics of X-ray diffraction patterns) 3, which shows the X-ray diffraction patterns of the sulfide-based positive electrode active materials of Example 1 and Comparative Example 1, it can be confirmed that the diffraction peak of the Li2S doped with Li3N of Example 1 is shifted to a lower angle side compared to the Li2S single phase of Comparative Example 1. This is presumably due to the substitutional solid solution of Li3N in Li2S.

[0098] (Preparation of solid electrolyte) Li2S (0.478 g) and P2S5 (0.772 g) were mixed in an agate mortar. The mixture was placed in a sealed 80 mL zirconia pot containing 160 g of 4 mm diameter zirconia balls. These steps were carried out in an argon atmosphere with a dew point of -50°C or less. Mechanical milling was carried out for 45 hours using a planetary ball mill (FRITSCH, model number Premium line P-7) at an orbital speed of 510 rpm to obtain a sulfide solid electrolyte.

[0099] (Preparation of positive electrode mixture) In an argon atmosphere with a dew point of −50° C. or lower, the sulfide-based positive electrode active material obtained in each example and comparative example, acetylene black, and sulfide solid electrolyte were weighed out in a mass ratio of sulfide-based positive electrode active material:acetylene black:sulfide solid electrolyte = 50:10:40. These were mixed in an agate mortar and placed in a zirconia pot, which was then covered with a lid. This was dry-treated in a planetary ball mill at an orbital speed of 300 rpm for 15 minutes to prepare a positive electrode mixture that was a mixed powder of the sulfide-based positive electrode active material, acetylene black, and sulfide solid electrolyte.

[0100] (Fabrication of all-solid-state batteries) 80 mg of 75Li2S·25P2S5 (LPS), a sulfide-based solid electrolyte, was placed in a powder molding machine with an inner diameter of 10 mm and then pressure-molded using a hydraulic press to create a separator. After releasing the pressure, 5 mg of positive electrode mixture was placed on one side of the separator and pressure-molded at 360 MPa for 5 minutes. After releasing the pressure, a negative electrode (pre-bonded lithium foil and indium foil) was placed on the other side of the separator and pressure-molded at 45 MPa. This resulted in a 10 mm diameter laminate consisting of a positive electrode active material layer, separator layer, and negative electrode active material layer. A rectangular polytetrafluoroethylene plate approximately 30 mm square with a 10 mm diameter through-hole was prepared. The resulting laminate was placed in this through-hole and sandwiched between two sheets of stainless steel foil so that the center of the polytetrafluoroethylene plate was covered. This was then vacuum-sealed in an aluminum laminate film exterior. At this time, each end of the lead terminal made of nickel foil previously attached to each stainless steel foil was led out from the sealing portion of the exterior body. Both sides of this exterior body were sandwiched between two polytetrafluoroethylene sheets, each approximately 40 mm square, and then both sides of this were sandwiched between two stainless steel plates, each approximately 60 mm square, and the stainless steel plates were fastened together with screws so that a pressure of 50 MPa was applied to the laminate. These steps were carried out in an argon atmosphere with a dew point of −50°C or lower. In this way, all-solid-state batteries equipped with the solid electrolytes of Examples 1 to 3 and Comparative Examples 2 and 3 were obtained.

[0101] [evaluation] (Charge / discharge test) The all-solid-state batteries obtained using the sulfide-based positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to a charge-discharge test in an argon atmosphere with a dew point of −50° C. or less and at 25° C. The discharge was performed at a discharge current density of 0.4 mA / cm. 2 The discharge was performed at a constant current (CC) with a discharge cut-off voltage of 0.7 V. Charging was performed at a current density of 0.4 mA / cm. 2 The battery was charged at a constant current (CC) with a maximum charging voltage of 3 V. The rest time between charging and discharging was 10 minutes. Under these conditions, charging and discharging were carried out once each. The discharge capacity, expressed per unit mass of the positive electrode active material, is shown in Table 1.

[0102] [Table 1]

[0103] As shown in Table 1, when the sulfide-based positive electrode active materials of Examples 1 to 3, which have an inverse fluorite crystal structure, contain lithium, sulfur, and nitrogen, and have a molar ratio of sulfur to nitrogen of 4.0 to 32.0, were used, the discharge capacity was a large value exceeding 150 mAh / g. On the other hand, when the sulfide-based positive electrode active materials of Comparative Examples 1 to 3, which have a sulfur to nitrogen content outside the above range, were used, the discharge capacity was lower than that of Examples 1 to 3.

[0104] The above results demonstrate that the sulfide-based positive electrode active material can improve the discharge capacity of an electricity storage device. [Industrial Applicability]

[0105] The sulfide-based positive electrode active material is suitable for use as a positive electrode active material in an electricity storage element used as a power source for electronic devices such as personal computers and communication terminals, automobiles, and the like. [Explanation of symbols]

[0106] 1. 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 Electricity storage device

Claims

1. It has an inverse fluorite crystal structure, consisting of lithium, sulfur and nitrogen, The sulfide-based positive electrode active material has a molar ratio of sulfur to nitrogen of 4.0 or more and 32.0 or less.

2. Having an inverse fluorite crystal structure, containing lithium, sulfur and nitrogen, a molar ratio of the sulfur content to the nitrogen content of 4.0 or more and 32.0 or less; A sulfide-based positive electrode active material having a structure in which Li 2 S is doped with Li 3 N.

3. Having an inverse fluorite crystal structure, containing lithium, sulfur and nitrogen, The sulfide-based positive electrode active material has a molar ratio of sulfur to nitrogen of 4.9 or more and 32.0 or less.

4. A positive electrode for a storage element, comprising the sulfide-based positive electrode active material according to any one of claims 1 to 3.

5. An energy storage element comprising the positive electrode according to claim 4 .

6. Preparing a mixture of two or more materials containing at least one element selected from the group consisting of lithium, sulfur, and nitrogen; The above materials are reacted to obtain a sulfide-based positive electrode active material that is composed of lithium, sulfur, and nitrogen and has an inverse fluorite crystal structure. Equipped with The method for producing a sulfide-based positive electrode active material, wherein the molar ratio of sulfur to nitrogen contained in the mixture is 4.0 or more and 32.0 or less.

7. A method for producing a mixture comprising: preparing a mixture comprising Li 2 S and Li 3 N; The above materials are reacted to obtain a sulfide-based positive electrode active material containing lithium, sulfur, and nitrogen and having an inverse fluorite crystal structure. Equipped with The method for producing a sulfide-based positive electrode active material, wherein the molar ratio of sulfur to nitrogen contained in the mixture is 4.0 or more and 32.0 or less.

8. Preparing a mixture of two or more materials containing at least one element selected from the group consisting of lithium, sulfur, and nitrogen; The above materials are reacted to obtain a sulfide-based positive electrode active material containing lithium, sulfur, and nitrogen and having an inverse fluorite crystal structure. Equipped with The method for producing a sulfide-based positive electrode active material, wherein the molar ratio of sulfur to nitrogen contained in the mixture is 4.9 or more and 32.0 or less.

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