Solid electrolyte, ion conductor, sheet, and power storage device

Abstract

The present application provides a solid electrolyte (19), an ion conductor (10), a sheet (12), and a power storage device (11) capable of reducing an interfacial resistance. The solid electrolyte has a garnet crystal structure containing Li, La, Zr, and O, and the intensity of a secondary ion of Li2F + is 0.1% or greater relative to the intensity of Li + . The ion conductor contains the solid electrolyte and an electrolyte (23) obtained by dissolving a lithium salt in a nonaqueous solvent. The sheet contains the ion conductor and a binder that binds the solid electrolyte. The power storage device contains the solid electrolyte.

Description

Technical Field

[0001] This invention relates to solid electrolytes, ion conductors, plates, and energy storage devices. Background Technology

[0002] Patent document 1 discloses the following prior art: oxides having a garnet-type crystal structure containing Li, La and Zr have ionic conductivity and high electrochemical stability, and therefore the oxides are used as solid electrolytes in energy storage devices.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent No. 6797619 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] However, the solid electrolyte has a high interfacial resistance, which leads to the following problems: the migration of lithium ions between solid electrolytes and between solid batteries and electrolytes is hindered, making it impossible to reduce the internal resistance of the energy storage device.

[0008] This invention was made to solve this problem, and its purpose is to provide solid electrolytes, ion conductors, plates and energy storage devices that can reduce interfacial resistance.

[0009] Methods for solving problems

[0010] The first approach to achieve this objective is a solid electrolyte having a garnet-type crystal structure containing Li, La, Zr, and O, wherein, among the secondary ions detected by time-of-flight secondary ion mass spectrometry, Li₂F₂... + The strength relative to Li + The strength is above 0.1%.

[0011] The second method is as follows: In the first method, by using O 2- When sputtering as an ion source is used for depth-direction analysis, the intensity of secondary ions detected in Li₂F reaches at least a depth of 28 nm. + The strength relative to Li + The strength is above 0.1%.

[0012] The third approach is as follows: In the first or second approach, the solid electrolyte also contains Mg and Sr.

[0013] The fourth method is an ion conductor that includes a solid electrolyte of any one of the first to third methods and an electrolyte obtained by dissolving a lithium salt in a non-aqueous solvent.

[0014] The fifth type is a sheet containing a solid electrolyte of any one of the first to third types, or an ion conductor of the fourth type.

[0015] The sixth type is an electrode that includes a solid electrolyte of any one of the first to third types, or an ion conductor of the fourth type.

[0016] The seventh method is an electrode that is in contact with a protective layer of a solid electrolyte comprising any one of the first to third methods, or with a protective layer of an ion conductor comprising the fourth method.

[0017] The eighth type is a membrane that contains a solid electrolyte of any one of the first to third types, or an ion conductor of the fourth type.

[0018] The ninth method is a membrane that is in contact with a protective layer of a solid electrolyte comprising any one of the first to third methods, or with a protective layer of an ion conductor comprising the fourth method.

[0019] The tenth type is an energy storage device that includes electrodes of the sixth or seventh type, or a diaphragm of the eighth or ninth type.

[0020] Invention Effects

[0021] According to the solid electrolyte of the present invention, in the intensity of secondary ions detected by time-of-flight secondary ion mass spectrometry, Li₂F₂... + The strength relative to Li + The concentration of LiF is above 0.1%, therefore LiF exists on the surface. LiF has lithium-ion conductivity, thus it can reduce the interfacial resistance of solid electrolytes. Attached Figure Description

[0022] Figure 1 This is a cross-sectional view of the energy storage device including an ion conductor according to the first embodiment.

[0023] Figure 2 This is a schematic diagram illustrating the crystal structure of garnet.

[0024] Figure 3 This is a cross-sectional view of the diaphragm.

[0025] Figure 4 This is a cross-sectional view of the energy storage device according to the second embodiment.

[0026] Figure 5 This is a cross-sectional view of the energy storage device according to the third embodiment.

[0027] Figure 6(a) is a cross-sectional view of the insulator of the fourth embodiment, (b) is a cross-sectional view of the electrode of the fifth embodiment, and (c) is a cross-sectional view of the electrode of the sixth embodiment. Detailed Implementation

[0028] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Figure 1 This is a schematic cross-sectional view of the energy storage device 11 including the ion conductor 10 according to the first embodiment. The energy storage device 11 of this embodiment is a secondary battery that uses lithium ions as charge carriers. The energy storage device 11 sequentially includes a positive electrode layer 12, a separator 15, and a negative electrode layer 16. The positive electrode layer 12, the separator 15, and the negative electrode layer 16 are housed in a casing (not shown).

[0029] In the positive electrode layer 12, a current collector layer 13 and an active material layer 14 are stacked. The current collector layer 13 is a conductive component. The material of the current collector layer 13 can be, for example, a metal selected from Ni, Ti, Fe and Al, an alloy containing two or more of these elements, stainless steel, or a carbon material.

[0030] The active material layer 14 includes an ion conductor 10 and an active material 20. The ion conductor 10 includes a solid electrolyte 19. To reduce the resistance of the active material layer 14, a conductive additive may be included in the active material layer 14. Examples of conductive additives include carbon black, acetylene black, Ketjen black, carbon fiber, Ni, Pt, and Ag.

[0031] Active material 20 can be exemplified as metal oxides containing transition metals, sulfide-based active materials, and organic active materials. Metal oxides containing transition metals can be exemplified as metal oxides containing one or more elements selected from Mn, Co, Ni, Fe, Cr, and V, as well as Li. Examples of metal oxides containing transition metals include LiCoO2 and LiNi. 0.8 Co 0.15 Al 0.05 O2, LiMn2O4, LiNiVO4, LiNi 0.5 Mn 1.5 O4, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 and LiFePO4.

[0032] To inhibit the reaction between the active material 20 and the solid electrolyte 19, a coating layer can be formed on the surface of the active material 20. Examples of coating layers include Al2O3, ZrO2, LiNbO3, and Li4Ti5O3. 12 LiTaO3, LiNbO3, LiAlO2, Li2ZrO3, Li2WO4, Li2TiO3, Li2B4O7, Li3PO4 and Li2MoO4.

[0033] Examples of sulfur-based active substances include S, TiS2, NiS, FeS2, Li2S, MoS3, and sulfur-carbon composites. Examples of organic active substances include free radical compounds such as 2,2,6,6-tetramethylpiperidinoxy-4-yl methacrylate and polytetramethylpiperidinoxy vinyl ether, quinone compounds, axialene compounds, tetracyanobenzoquinone dimethane, and phenazine oxides.

[0034] The separator 15 isolates the positive electrode layer 12 from the negative electrode layer 16, making them electrically insulated from each other. The separator 15 is composed of an ion conductor 10. The ion conductor 10 contains a solid electrolyte 19 and an electrolyte solution (described later). The ion conductor 10 may also contain a binder.

[0035] In the negative electrode layer 16, a current collector layer 17 and an active material layer 18 are stacked. The current collector layer 17 is a conductive component. The material of the current collector layer 17 can be a metal selected from Ni, Ti, Fe, Cu and Si, an alloy containing two or more of these elements, stainless steel, or a carbon material.

[0036] The active material layer 18 comprises an ion conductor 10 and an active material 21. To reduce the resistance of the active material layer 18, a conductive additive may be included. Examples of conductive additives include carbon black, acetylene black, Ketjen black, carbon fiber, Ni, Pt, and Ag. Examples of active material 21 include Li, Li-Al alloys, and Li₄Ti₅O₂. 12 Graphite, In, Si, Si-Li alloy and SiO. The active material layers 14 and 18 may also contain a binder, similar to the diaphragm 15.

[0037] The energy storage device 11 is manufactured, for example, as follows: A slurry is prepared by mixing an electrolyte obtained by dissolving a lithium salt in a non-aqueous solvent with a solid electrolyte 19 to obtain a solution in which a binder is dissolved in a solvent. The slurry is formed into a sheet and then dried to obtain a green sheet (electrolyte sheet) for use in the separator 15.

[0038] Active material 20 is mixed into a mixture obtained by dissolving lithium salt in a non-aqueous solvent and a solid electrolyte 19, and then further mixed to obtain a solution obtained by dissolving a binder in a solvent to prepare a slurry. After coating the slurry onto the current collector layer 13, it is dried to obtain a green sheet (positive electrode sheet) for the positive electrode layer 12.

[0039] Active material 21 is mixed into a mixture obtained by dissolving lithium salt in a non-aqueous solvent and a solid electrolyte 19, and then further mixed to obtain a solution obtained by dissolving a binder in a solvent to prepare a slurry. After coating the slurry onto the current collector layer 17, it is dried to obtain a green sheet (negative electrode sheet) for the negative electrode layer 16.

[0040] After cutting the electrolyte sheet, positive electrode sheet, and negative electrode sheet into specified shapes, they are stacked in the order of positive electrode sheet, electrolyte sheet, and negative electrode sheet, and then pressed together to form an integrated unit. Terminals (not shown) are connected to current collector layers 13 and 17 respectively and sealed into a housing (not shown), resulting in an energy storage device 11 containing a positive electrode layer 12, a separator 15, and a negative electrode layer 16. Thus, the sheet containing the solid electrolyte 19 can be transformed into an electrolyte sheet, a positive electrode sheet, and a negative electrode sheet via an ion conductor 10.

[0041] Solid electrolyte 19 is a composite oxide with a garnet-type crystal structure containing Li, La, and Zr. This garnet-type crystal structure is based on the general formula C3A2B3O. 12 express.

[0042] Figure 2 This diagram schematically illustrates a garnet-type crystal structure. In this structure, the C site Sc coordinates dodecahedrally with oxygen atom Oa, the A site Sa coordinates octahedrally with oxygen atom Oa, and the B site Sb coordinates tetrahedrally with oxygen atom Oa. In a typical garnet-type crystal structure, solid electrolyte 19 may contain Li at sites octahedrally coordinated with oxygen atom Oa, forming vacancies V. Vacancy V is, for example, a site sandwiched between B sites Sb1 and Sb2. The Li present in vacancy V coordinates octahedrally with oxygen atom Oa forming an octahedron containing the tetrahedron face Fb1 forming B site Sb1 and the tetrahedron face Fb2 forming B site Sb2. For example, Li7La3Zr2O has a garnet-type crystal structure. 12 In this context, La can occupy site C (Sc), Zr can occupy site A (Sa), and Li can occupy site B (Sb) and the void (V).

[0043] Garnet-type crystal structure has the same X-ray diffraction pattern as CSD (Cambridge Structural Database) X-ray diffraction file No. 422259 (Li7La3Zr2O). 12 Similar XRD patterns. Solid electrolyte 19 differs from No. 422259 sometimes in the types of constituent elements, Li concentration, etc., thus sometimes resulting in different diffraction angles and intensity ratios. This representative crystal structure is cubic (space group Ia-3d (- indicates an overline signifying rotation-inversion operation), JCPDS: 84-1753).

[0044] return Figure 1 This will be explained. Solid electrolytes 19 can typically be exemplified by Li7La3Zr2O. 12A portion of the constituent elements of solid electrolyte 19 may be replaced by other elements, or trace amounts of other elements may be added without replacing the constituent elements. Other elements may be exemplified by at least one element selected from the group consisting of Mg, Al, Si, Ca, Ti, V, Ga, Sr, Y, Nb, Sn, Sb, Ba, Hf, Ta, W, Bi, Rb and the lanthanides (excluding La).

[0045] Solid electrolytes 19, for example, can be Li6La3Zr 1.5 W 0.5 O 12 Li 6.15 La3Zr 1.75 Ta 0.25 Al 0.2 O 12 Li 6.15 La3Zr 1.75 Ta 0.25 Ga 0.2 O 12 Li 6.25 La3Zr2Ga 0.25 O 12 Li 6.4 La3Zr 1.4 Ta 0.6 O 12 Li 6.5 La3Zr 1.75 Te 0.25 O 12 Li 6.75 La3Zr 1.75 Nb 0.25 O 12 Li 6.9 La3Zr 1.675 Ta 0.289 Bi 0.036 O 12 Li 6.46 Ga 0.23 La3Zr 1.85 Y 0.15 O 12 Li 6.8 La 2.95 Ca 0.05 Zr 1.75 Nb 0.25 O 12 Li 7.05 La 3.00 Zr 1.95 Gd 0.05 O 12 Li 6.20 Ba 0.30 La 2.95 Rb 0.05Zr2O 12 .

[0046] Solid electrolyte 19 is particularly preferred to be a solid electrolyte containing at least one of Mg and element A (A being at least one element selected from the group consisting of Ca, Sr, and Ba) and the molar ratio of each element satisfies all of the following (1) to (3); or, a solid electrolyte containing both Mg and element A and the molar ratio of each element satisfies all of the following (4) to (6). To improve the ionic conductivity of solid electrolyte 19, element A is preferably Sr.

[0047] (1) 1.33≤Li / (La+A)≤3

[0048] (2) 0 ≤ Mg / (La+A) ≤ 0.5

[0049] (3) 0 ≤ A / (La+A) ≤ 0.67

[0050] (4) 2.0 ≤ Li / (La+A) ≤ 2.6

[0051] (5) 0.01 ≤ Mg / (La+A) ≤ 0.14

[0052] (6) 0.04 ≤ A / (La+A) ≤ 0.17

[0053] The median particle size of the equivalent circular diameter of the solid electrolyte 19 appearing in the cross-section of the diaphragm 15 is preferably 0.5-10 μm, more preferably 0.5-6 μm. This is to ensure that the surface area of ​​the solid electrolyte 19 is of an appropriate size, thereby ensuring the migration of Li ions between the electrolyte present on the surface of the solid electrolyte 19 and the solid electrolyte 19.

[0054] To determine the median particle size of the solid electrolyte 19, scanning electron microscopy (SEM) images of the solid electrolyte 19 appearing in cross-sections of the diaphragm 15 (ground surface, surface obtained by focused ion beam (FIB) irradiation, and surface obtained by ion milling) were first analyzed. The equivalent circle diameter was calculated based on the area of ​​each particle in the solid electrolyte 19, and the volumetric particle size distribution was determined. The median particle size is the equivalent circle diameter at which the cumulative frequency in the particle size distribution reaches 50%. To ensure accuracy, the image used to determine the particle size distribution was set to 400 μm in the diaphragm 15. 2 The above area.

[0055] In addition to a solid electrolyte 19 having a garnet-type crystal structure containing Li, La, Zr, and O, the ion conductor 10 may also contain one or more other solid electrolytes. Other solid electrolytes may include crystalline, amorphous oxide-based solid electrolytes such as perovskite-type, NASICON-type, and LISICON-type, as well as hydride-based solid electrolytes.

[0056] Perovskite-type solid electrolytes can be listed as containing at least oxides of Li, Ti, and La, such as La. 2 / 3- X Li 3X TiO3. NASICON-type solid electrolytes can be exemplified by oxides containing at least Li, M (where M is one or more elements selected from Ti, Zr, and Ge), and P, such as Li(Al,Ti)2(PO4)3 and Li(Al,Ge)2(PO4)3. LISICON-type solid electrolytes can be exemplified by Li... 14 Zn(GeO4)4. Examples of hydride-based solid electrolytes include hydrides of alkali metals or alkaline earth metals, containing at least one element from Group 13 of the periodic table (e.g., B, Al, Ga, In, Ta). Examples include LiBH4 and LiAlH4.

[0057] Figure 3 It is diaphragm 15 (refer to) Figure 1 A cross-sectional view of the lithium salt. The ion conductor 10 contains an electrolyte 23 (non-aqueous electrolyte) obtained by dissolving the lithium salt in a non-aqueous solvent. The lithium salt is a compound used for the acceptance and donation of cations between the positive electrode layer 12 and the negative electrode layer 16. The anion of the lithium salt can be exemplified by a halide ion (I...). - Cl - ,Br - etc.), SCN - BF4 - BF3 (CF3) - BF3 (C2F5) - PF6 - ClO4 - SbF6 - N(SO2F)2 - N(SO2CF3)2 - N(SO2C2F5)2 - B(C6H5)4 - B(O2C2H4)2 - C(SO2F)3 - C(SO2CF3)3 - CF3COO - CF3SO2O - C6F5SO2O - B(O2C2O2)2 - RCOO - (R is an alkyl, phenyl, or naphthyl group with 1-4 carbon atoms, etc.)

[0058] The preferred anion for lithium salts is N(SO₂F)₂, which has a sulfonyl group -S(=O)₂-. - N(SO2CF3)2 - N(SO2C2F5)2 - Isosulfonamides, PF6 - Isohalinated phosphate ions are preferred because, for sulfonamide anions, even with increased salt concentration, the effects of increased electrolyte viscosity and decreased ionic conductivity are minimal, and the degree of dissociation of halophosphate ions is high. Furthermore, by forming a highly stable and low-resistivity film (SEI), both can reduce the reductive decomposition of non-aqueous solvents and expand the reduction-side potential window, thus making them the preferred choice.

[0059] Non-aqueous solvents are broadly classified into molecular solvents, which are mostly composed of molecules, and ionic liquids, which are composed of cations and anions. To broaden the potential window of non-aqueous electrolytes, molecular solvents are preferably aprotic solvents. Examples of aprotic solvents include cyclic esters, chain esters, aliphatic carboxylic acid esters, phosphate esters, nitriles, amides, sulfur compounds, ketones, ethers, nitro compounds, fluorinated solvents, and sulfone solvents. Mixtures of these solvents are also possible.

[0060] Cyclic esters can be exemplified by carbonates such as propylene carbonate, ethylene carbonate, butyl carbonate, vinylene carbonate, vinyl ethylene carbonate, and fluoroethylene carbonate; lactones such as β-propiolactone, γ-butyrolactone, δ-valerolactone, α-pyranone, and coumarins. Chain esters can be exemplified by carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Aliphatic carboxylic acid esters can be exemplified by methyl formate, methyl acetate, and ethyl propionate. Phosphate esters can be exemplified by trimethyl phosphate. Nitriles can be exemplified by acetonitrile, propionitrile, butyronitrile, and benzonitrile.

[0061] Amides can be exemplified as formamide, N-methylformamide, dimethylformamide, N-methylacetamide, dimethylacetamide, N-methylpropionamide, hexamethylphosphoramide, and N-methylpyrrolidone. Sulfur compounds can be exemplified as dimethyl sulfoxide, sulfolane, dimethylthioformamide, and N-methylthiopyrrolidone. Ketones can be exemplified as acetone, 4-methyl-2-pentanone, and acetylacetone. Ethers can be exemplified as tetrahydrofuran and ethylene glycol dimethyl ether. Nitro compounds can be exemplified as nitromethane and nitrobenzene. Fluorinated solvents are compounds and their derivatives in which hydrogen atoms of hydrocarbons are replaced with fluorine atoms.

[0062] Examples of sulfone solvents include trimethylene sulfone, sulfolane, difluorosulfolane, dimethylsulfolane, monofluorosulfolane, 3-methylsulfolane, ethylmethylsulfone, and ethylisopropylsulfone. Sulfone solvents have high thermal stability and are therefore preferred.

[0063] The higher the relative permittivity of the solvent, and the easier it is for ions to solvate, the easier the reaction of the electrolyte dissolving and dissociating into free ions in the molecular solvent. Therefore, solvents with a larger relative permittivity εr (εr > 20) are preferred. Examples of molecular solvents with a relative permittivity greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, and nitro compounds. Of course, it is also possible to mix solvents with a relative permittivity greater than 20 with solvents with a relative permittivity less than 20 to adjust the viscosity, etc.

[0064] Ionic liquids are compounds composed of cations and anions, and are liquids at room temperature and pressure. Using an ionic liquid as the solvent for a non-aqueous electrolyte can improve its flame retardancy. Ammonium and imidazole are preferred ionic liquids. pyrrolidine and piperidine One or more species in the group are classified as cationic species.

[0065] There are no particular limitations on the anionic composition of ionic liquids. An example of anionic composition is BF4. - N(SO2F)2 - Inorganic anions, B(C6H5)4 - CH3SO3 - CF3SO3 - N(SO2CF3)2 - N(SO2C4F9)2 - Organic anions, etc.

[0066] Ionic liquids can also be solvated ionic liquids. Examples of solvated ionic liquids include liquids obtained by dissolving lithium salts in sulfone solvents such as sulfolane or sulfolane derivatives, or in ethylene glycol dimethyl ether solvents such as tetraethylene glycol dimethyl ether.

[0067] The ion conductor 10 may contain a binder for bonding the solid electrolyte 19. Examples of binders include rubbery polymers such as fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and styrene-butadiene rubber. Examples of fluorinated resins include vinylidene fluoride polymers, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

[0068] Polymers of vinylidene fluoride can be exemplified as homopolymers of vinylidene fluoride and copolymers of vinylidene fluoride with comonomers. Comonomers can be listed as halogenated monomers (excluding vinylidene fluoride) and non-halogenated comonomers. Examples of halogenated monomers include: chlorine-containing monomers such as vinyl chloride; fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, trifluorochloroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ethers. Examples of non-halogenated comonomers include: olefins such as ethylene and propylene; acrylic monomers such as acrylic acid, methacrylic acid, their esters or salts; and vinyl monomers such as acrylonitrile, vinyl acetate, and styrene. One or more of these comonomers polymerize with vinylidene fluoride to form copolymers.

[0069] Because the solid electrolyte 19 is highly reactive, even when treated in an inert gas atmosphere, it will react with trace amounts of moisture and carbon dioxide present in the inert gas to form a resistive layer such as lithium hydroxide or lithium carbonate on its surface. Therefore, it is effective to perform surface treatment on the solid electrolyte 19 before the resistive layer is formed, thus providing components that effectively reduce resistance on the surface of the solid electrolyte 19. Examples of surface treatment processes include vapor deposition and plating. In this embodiment, the solid electrolyte 19 is wet-milled (wet-pulverized) in an inert gas atmosphere in the presence of a fluorine-containing solution. Examples of fluorine-containing solutions include an electrolyte obtained by dissolving a fluorine-containing electrolyte in a non-aqueous solvent, and a fluorine-containing non-aqueous solvent.

[0070] The surface of the solid electrolyte 19, after the resistive layer has been removed by grinding, immediately comes into contact with the solution, causing the components of the solution to react with the solid electrolyte 19 to form a surface layer 22. The solution used in wet grinding adheres to the surface layer 22 on the solid electrolyte 19 immediately after grinding. Upon removal of the adhered solution, the surface layer 22 of the solid electrolyte 19 is revealed.

[0071] Time-of-flight secondary ion mass spectrometry (TOF-SIMS) enables the analysis of chemical species present on the outermost surface (surface layer 22) of the solid electrolyte 19. TOF-SIMS is a method for mass analysis of secondary ions released by irradiating the surface of the solid electrolyte 19 with primary ions. When used in combination with sputtering, TOF-SIMS can also provide information related to the compositional distribution in the depth direction of the solid electrolyte 19.

[0072] The intensity of positive secondary ions in the outermost surface (surface layer 22) of solid electrolyte 19, as measured by TOF-SIMS, was found to be Li₂F₂. + The strength relative to Li + Its strength is above 0.1%. Li₂F +The strength originates from LiF. Since LiF is present in the surface layer 22 of the solid electrolyte 19, it can reduce the formation of lithium hydroxide and lithium carbonate (resistive layer) caused by the reaction of atmospheric moisture and carbon dioxide with the solid electrolyte 19. LiF has lithium-ion conductivity, thus reducing the interfacial resistance of the solid electrolyte 19.

[0073] By using O 2- When sputtering as an ion source is used for depth-direction analysis using TOF-SIMS, the intensity of positive secondary ions detected in Li₂F reaches at least a depth of 28 nm. + The strength relative to Li + The strength is above 0.1%. Since the thickness of the surface layer 22 containing LiF can be ensured to reach 28 nm, the stability of the surface layer 22 can be improved.

[0074] The concentration of lithium salt in the non-aqueous electrolyte is preferably 4.0 mol / kg or less. This is because when the salt concentration of the non-aqueous electrolyte exceeds 4.0 mol / kg, the lithium-ion conductivity tends to decrease significantly due to the increase in the viscosity of the non-aqueous electrolyte. The salt concentration of the non-aqueous electrolyte is determined, for example, as follows. Here, the ion conductor 10 constituting the membrane 15 will be described, but the ion conductor 10 constituting the active material layers 14 and 18 can also be determined in the same way.

[0075] First, the diaphragm 15 is pulverized and then immersed in a solvent to dissolve the non-aqueous electrolyte contained in the diaphragm 15. The diaphragm is then separated into solid and liquid components using a centrifuge. The content of Li in the separated liquid component is determined by high-frequency inductively coupled plasma (ICP) analysis.

[0076] Additionally, the types of non-aqueous solvents contained in the diaphragm 15 are determined, for example, by gas chromatography-mass spectrometry (GC-MS). Thermogravimetric differential calorimetry (TG-DTA) is used to analyze the identified non-aqueous solvents (hereinafter referred to as "standard substances") and the diaphragm 15. The analytical results of the standard substances are compared with the analytical results of the diaphragm 15 to determine the content of non-aqueous solvents contained in the diaphragm 15. Based on the Li content in the liquid component and the content of non-aqueous solvents in the diaphragm 15, the molality (mol / kg) of lithium salts in the non-aqueous electrolyte is calculated.

[0077] In the ion conductor 10, the volume ratio of the solid electrolyte 19 to the total volume of the non-aqueous electrolyte is preferably 52% or more and less than 100%, more preferably 61% or more and less than 100%. By combining the solid electrolyte 19 with the non-aqueous electrolyte, the interfacial resistance of the solid electrolyte 19 can be reduced, thus increasing the operational stability of the energy storage device 11 equipped with the ion conductor 10.

[0078] The content (volume %) of solid electrolyte 19 and non-aqueous electrolyte is determined by freezing the diaphragm 15 or embedding it in a tetrafunctional epoxy resin and curing it, and then analyzing it using a SEM equipped with an energy-dispersive X-ray spectrometer (EDS) at a randomly selected 5000x magnification field of view from the cross-section of the diaphragm 15. During the analysis, the distribution of La, Zr, and S is determined, or the contrast of the reflected electron image is analyzed to determine the area of ​​the solid electrolyte 19 and the area of ​​the non-aqueous electrolyte. The proportion of the area in the cross-section of the diaphragm 15 is considered as the proportion of the volume of the ion conductor 10 of the diaphragm 15, thus obtaining the content (volume %) of solid electrolyte 19 and non-aqueous electrolyte.

[0079] The Li-ion conductivity of the ion conductor 10 is determined by the type of solid electrolyte 19, the non-aqueous electrolyte, and the salt concentration. The preferred lithium-ion conductivity of the ion conductor 10 at 25°C is 1.0 × 10⁻⁶. -5 S / cm or higher. This is to ensure the output density of the energy storage device 11 containing the ion conductor 10.

[0080] Reference Figure 4 The second embodiment will be described. In the first embodiment, an energy storage device 11 using a solid electrolyte 19 in the electrolyte was described. In the second embodiment, an ion conductor 10 is used in a liquid lithium-ion battery using a non-aqueous electrolyte in the electrolyte. The same symbols are used for parts that are the same as those described in the first embodiment, and the following descriptions are omitted. Figure 4 This is a cross-sectional view of the energy storage device 24 according to the second embodiment.

[0081] The energy storage device 24 sequentially comprises a positive electrode layer 12, a separator 25, and a negative electrode layer 16. These are housed within a casing (not shown). The separator 25 is composed of a porous body that is durable for the active materials 20 and 21 contained in the positive electrode layer 12 and the negative electrode layer 16, as well as the electrolyte, and allows lithium ions to pass through but does not have electronic conductivity. Examples of separator 25 include nonwoven fabrics or porous membranes made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc. The non-aqueous electrolyte is the same as that described in the first embodiment, and therefore its description is omitted.

[0082] The energy storage device 24 of the second embodiment contains an ion conductor 10 in the positive electrode layer 12 and the negative electrode layer 16, thus increasing its operational stability in the same way as the energy storage device 11 of the first embodiment.

[0083] Reference Figure 5The third embodiment will be described. In the first and second embodiments, the case where the ion conductor 10 is included in the positive electrode layer 12, the separator 15, and the negative electrode layer 16 was described. In the third embodiment, the case where the ion conductor 10 is included in the protective layers 29 and 32 will be described. The same symbols are used for parts that are the same as those described in the first and second embodiments, and the following descriptions are omitted. Figure 5 This is a cross-sectional view of the energy storage device 26 according to the third embodiment.

[0084] The energy storage device 26 comprises, in sequence, a positive electrode layer 27, a separator 25, and a negative electrode layer 30. These are housed within a casing (not shown). The energy storage device 26 is a liquid lithium-ion battery using a non-aqueous electrolyte.

[0085] In the positive electrode layer 27, a current collector layer 13 and an active material layer 28 are stacked. The active material layer 28 contains active material 20. In order to reduce the resistance of the active material layer 28, conductive additives such as carbon black, acetylene black, Ketjen black, carbon fiber, Ni, Pt and Ag can be included in the active material layer 28.

[0086] A protective layer 29 is disposed between the separator 25 and the negative electrode layer 30. The protective layer 29 includes an ion conductor 10.

[0087] In the negative electrode layer 30, an active material layer 31, a protective layer 32, and a current collector layer 17 are sequentially stacked. The active material layer 31 is composed of, for example, Li, a Li-Al alloy, a Li-Sn alloy, a Li-Si alloy, a Li-Mg alloy, a Li-Si alloy, or a Si-Li alloy. The protective layer 32 contains an ion conductor 10. The protective layers 29 and 32 are configured by laminating sheet-shaped bodies containing ion conductor 10, coating the ion conductor 10 into the separator 25 and the current collector layer 17, etc.

[0088] The solid electrolyte 19, containing a garnet-type crystal structure comprising Li, La, Zr, and O within the ion conductor 10, exhibits resistance to reduction of the metallic lithium in the active material layer 31. Therefore, the operational stability of the energy storage device 26 is increased due to the protective layer 29. Furthermore, the protective layer 29 suppresses short circuits caused by the dendrite growth of metallic lithium. The protective layer 32, situated between the active material layer 31 and the current collector layer 17, suppresses the deterioration of the current collector layer 17.

[0089] Reference Figure 6 The fourth to sixth embodiments will be described. It should be noted that the same symbols are used for parts that are the same as those described in the first to third embodiments, and the following descriptions are omitted. Figure 6 (a) is a cross-sectional view of the insulator 33 according to the fourth embodiment.

[0090] The insulator 33 includes a diaphragm 25 and a protective layer 29 in contact with the diaphragm 25. The diaphragm 25 includes a first interface 34 and a second interface 35 opposite to the first interface 34, and the protective layer 29 is disposed on the first interface 34 and the second interface 35. By means of the protective layer 29 disposed on the diaphragm 25, short circuits caused by dendrite growth of lithium metal contained in the energy storage device can be reduced. Even if a short circuit occurs in the energy storage device and the diaphragm 25 is to undergo thermal deformation, the protective layer 29 can maintain the shape of the diaphragm 25 and suppress the occurrence of thermal runaway of the energy storage device.

[0091] Figure 6 (b) is a cross-sectional view of the electrode 36 according to the fifth embodiment. The electrode 36 includes a positive electrode layer 12 and a protective layer 29 in contact with the active material layer 14 of the positive electrode layer 12. The protective layer 29 is disposed on the electrode 36 at the interface 37 of the active material layer 14 opposite to the surface on which the current collector layer 13 is disposed. The protective layer 29 disposed at the interface 37 of the active material layer 14 can reduce dendrite growth from the negative electrode layer 16 of the energy storage device.

[0092] Figure 6 (c) is a cross-sectional view of the electrode 38 according to the sixth embodiment. The electrode 38 includes a negative electrode layer 16 and a protective layer 29 in contact with the active material layer 18 of the negative electrode layer 16. The protective layer 29 is disposed on the interface 39 of the active material layer 18 opposite to the surface on which the current collector layer 17 is disposed. The protective layer 29 disposed on the interface 39 of the active material layer 18 can reduce dendrite growth from the negative electrode layer 16 of the energy storage device.

[0093] The insulator 33 can be disposed in the energy storage device in place of the diaphragm 25 of the energy storage device 24 in the second embodiment and the energy storage device 26 in the third embodiment. In the insulator 33, one of the two protective layers 29 disposed at the interfaces 34 and 35 of the diaphragm 25 can be omitted.

[0094] Electrode 36 may be disposed in the energy storage device in place of the positive electrode layers 12 and 27 of the energy storage device 24 in the second embodiment and the energy storage device 26 in the third embodiment. Electrode 38 may be disposed in the energy storage device in place of the negative electrode layers 16 and 30 of the energy storage device 24 in the second embodiment and the energy storage device 26 in the third embodiment.

[0095] Example

[0096] The invention has been described in more detail through embodiments, but the invention is not limited to these embodiments.

[0097] (Preparation of solid electrolytes)

[0098] To become Li 6.95 Mg 0.15 La 2.75 Sr 0.25Zr 2.0 O 12 The following method was used: weighing Li₂CO₃, MgO, La(OH)₃, SrCO₃, and ZrO₂. Considering the volatilization of Li during calcination, Li₂CO₃ was added in excess (approximately 15 mol%). The weighed raw materials, along with ethanol and zirconium oxide pellets, were added to a nylon can and mixed using a ball mill for 15 hours. The slurry removed from the can was dried and placed on an MgO plate, then pre-calcined at 1200°C for 10 hours. A binder was added to the pre-calcined material, and the mixture was then mixed using a ball mill in an inactive solvent for 60 hours. The slurry removed from the can was dried to obtain the raw material powder for the solid electrolyte.

[0099] The raw material powder was placed into a mold with an inner diameter of 32.5 mm and pressed into a circular plate with a diameter of approximately 32.5 mm and a thickness of approximately 2.5 mm. A cold isostatic press was used to apply a pressure of 1.5 t / cm² to the circular plate. 2 The hydrostatic pressure is applied to obtain a shaped body. The shaped body is coated with raw material powder and fired at 1100°C for 4 hours in an atmospheric atmosphere to obtain a sintered body of solid electrolyte.

[0100] (Example)

[0101] A non-aqueous electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide (LiFSI) in tetraethylene glycol dimethyl ether to achieve a lithium salt concentration of 0.1 mol / L. The non-aqueous electrolyte was then added dropwise to water-resistant abrasive paper with a silicon carbide abrasive material (particle size 320 as specified in JIS R6010:2000) fixed on it, and the circular surfaces of the sintered body were wet-polished one by one under an argon atmosphere. This yielded the solid electrolyte of the example.

[0102] (Comparative Example)

[0103] Using water-resistant abrasive paper with a fixed silicon carbide abrasive material (320 grit size as specified in JIS R6010:2000), the circular surfaces of the sintered body were dry-polished one by one under an argon atmosphere. Otherwise, the operation was the same as in Example 1 to obtain the solid electrolyte of the comparative example.

[0104] (Measurement of interfacial resistance)

[0105] Using an insulator-made cylinder, a solid electrolyte, polished on both sides, is clamped and fixed from both sides through a gasket. A non-aqueous electrolyte of the same type as used in the polishing process is added to both sides of the cylinder. The working electrode and reference electrode, made of metallic lithium, are immersed in the non-aqueous electrolyte, and AC impedance is measured using a four-terminal method. For the solid electrolyte of the comparative example, which underwent dry polishing, the non-aqueous electrolyte of the example is injected for AC impedance measurement. The AC impedance measurement conditions are set as follows: temperature 25°C, voltage 10mV, and frequency 1MHz-10mHz. The interface resistance is determined by separating the impedance at the solid electrolyte / non-aqueous electrolyte interface using a Nyquist plot.

[0106] (Measurement using TOF-SIMS)

[0107] The solid electrolyte, whose interfacial resistance was measured, was washed by shaking in dimethyl carbonate and then stored in a transfer container under an argon atmosphere. After the solid electrolyte was introduced into the chamber of the TOF-SIMS, the outermost surface and depth direction of the circular surface of the solid electrolyte were analyzed.

[0108] Regarding the analysis conditions for the outermost surface of solid electrolytes using TOF-SIMS, both methods use a primary ion source: Bi32O3. 2+ Cluster ions, accelerating voltage: 30kV, grating size: 500×500μm 2 The secondary ion Li was determined using TOF-SIMS. + Li2OH + Li3CO3 + Li2F + The strength of Li₂OH is calculated. + Li3CO3 + Li2F + The strength relative to Li + The intensity ratios are shown in Table 1.

[0109] Regarding the sputtering conditions for depth-direction analysis of solid electrolytes using TOF-SIMS, both are set as sputtering ion sources: O 2- Accelerating voltage: 1kV; Grating size: 500×500μm 2 Using a combination of sputtering, the outermost surface was analyzed under the above conditions, and secondary ion Li was measured at each sputtering time. + Li2F + The intensity.

[0110] After performing depth orientation analysis, a standard sample with a 100 nm thick SiO2 film formed on a Si substrate was sputtered under the same conditions as the TOF-SIMS depth orientation analysis. The time until the Si substrate was exposed was measured, and the sputtering velocity was calculated. The sputtering time was multiplied by the sputtering velocity to calculate the depth at each sputtering time. For each depth, Li2F was calculated. + The strength relative to Li + The ratio of intensity to depth is shown in Table 2.

[0111] [Table 1]

[0112] [Table 2]

[0113] As shown in Table 1, the interface resistance in this example is 20 Ωcm. 2 In contrast, the comparative example is 79 Ωcm. 2 It can be seen that, compared with the solid electrolyte of the comparative example which was dry-milled under an argon atmosphere, the solid electrolyte of the example which was wet-milled using a non-aqueous electrolyte under an argon atmosphere has a lower interfacial resistance.

[0114] According to Table 1, Li₂OH derived from lithium hydroxide + The ratio of strength, Li3CO3 derived from lithium carbonate + The intensity ratios were not significantly different between the examples and comparative examples. However, regarding Li2F derived from LiF... + The proportion of LiF in the solid electrolyte of the examples was 2.9%, while that in the comparative examples was less than 0.1%, showing a large difference. It can be inferred that the LiF in the solid electrolyte of the examples was formed during the grinding of the solid electrolyte in the presence of a non-aqueous electrolyte. Since LiF has lithium-ion conductivity, it can be inferred that the decrease in the interfacial resistance of the solid electrolyte of the examples is due to the influence of the LiF present on the outermost surface of the solid electrolyte.

[0115] As shown in Table 2, in the embodiments, as the analysis depth increases, Li2F + The strength relative to Li + The proportion of intensity gradually decreases, but remains above 0.1% up to a depth of 28 nm. It can be inferred that the surface layer containing LiF has a thickness of approximately 28 nm, thus suggesting that it can improve the stability of the lithium-ion conductive surface layer.

[0116] The examples illustrate the formation of a low-resistivity surface layer on the surface of a sintered solid electrolyte by wet milling in an argon atmosphere in the presence of a non-aqueous electrolyte. However, this is not a limitation. Clearly, wet milling of solid electrolyte particles in an inactive atmosphere in the presence of a non-aqueous electrolyte can form a low-resistivity surface layer on the surface of the solid electrolyte (particles).

[0117] Oxide-based solid electrolytes with a garnet-type crystal structure containing Li, La, and Zr possess excellent characteristics, including bulk conductivity comparable to that of non-aqueous electrolytes and electrochemical stability against lithium metal. However, oxide-based solid electrolytes exhibit high interfacial resistance, making it difficult to achieve practical battery characteristics in storage devices obtained by press-forming solid electrolyte particles. In contrast, the embodiments have shown that solid electrolytes with reduced interfacial resistance have been obtained. Therefore, even without forming and sintering the solid electrolyte particles to create a high-density sintered body, storage devices with practical battery characteristics can be obtained by press-forming solid electrolyte particles.

[0118] The present invention has been described above based on the embodiments, but the present invention is not limited to any of the above embodiments, and it can be readily inferred that various modifications and variations can be made without departing from the spirit of the present invention.

[0119] In this embodiment, the energy storage device 11 is described as an energy storage device having a positive electrode layer 12 on one side of the current collector layer 13 with an active material layer 14 and a negative electrode layer 16 on one side of the current collector layer 17 with an active material layer 18, but it is not necessarily limited to this. For example, the elements in this embodiment can certainly be applied to an energy storage device having electrode layers (so-called bipolar electrodes) on both sides of the current collector layer 13 with active material layers 14 and 18 respectively. If the bipolar electrodes and the separator 15 are alternately stacked and housed in a housing (not shown), a so-called bipolar structure energy storage device can be obtained.

[0120] In this embodiment, the case where the active material layers 14 and 18 and the separator 15 all contain the ion conductor 10 has been described, but it is not necessarily limited to this. In the energy storage device, it is sufficient that at least one of the active material layers 14 and 18 and the separator 15 contains the ion conductor 10.

[0121] In this embodiment, energy storage devices 11, 24, and 26, which are composed of lithium-ion batteries, have been described, but are not necessarily limited to this. Obviously, other energy storage devices may also include a solid electrolyte 19. Other energy storage devices may include electrochemical capacitors. Electrochemical capacitors may include redox capacitors utilizing redox reactions, and asymmetric units, i.e., hybrid capacitors, formed by combining a double-layer capacitor with a solid electrolyte 19.

[0122] Although the description is omitted in the embodiment, it is of course possible to provide a protective layer 29 between the active material layer 18 and the separators 15, 25, or between the current collector layer 17 and the active material layer 18. Providing a protective layer 29 between the active material layer 18 and the separators 15, 25 can reduce short circuits caused by dendrites. Providing a protective layer 29 between the current collector layer 17 and the active material layer 18 can reduce the deterioration of the current collector layer 17.

[0123] Symbol Explanation

[0124] 10 Ion Conductors

[0125] 11, 24, 26 Storage devices

[0126] 12 Positive electrode layer (plate, electrode)

[0127] 15. Diaphragm (piece)

[0128] 16. Negative electrode layer (plate, electrode)

[0129] 19 Solid electrolytes

[0130] 23. Non-aqueous electrolyte (electrolyte)

[0131] 25 Diaphragm

[0132] 29, 32 Protective layers

Claims

1. A solid electrolyte having a garnet-type crystal structure comprising Li, La, Zr, and O, wherein, The intensity of secondary ions detected by time-of-flight secondary ion mass spectrometry included Li₂F. + The strength relative to Li + The strength is above 0.1%.

2. The solid electrolyte according to claim 1, wherein, By using O 2- When sputtering as an ion source is used for depth-direction analysis, the intensity of secondary ions detected in Li₂F reaches at least a depth of 28 nm. + The strength relative to Li + The strength is above 0.1%.

3. The solid electrolyte according to claim 1 or 2, further comprising Mg and Sr.

4. An ion conductor comprising the solid electrolyte of claim 1 or 2 and an electrolyte obtained by dissolving a lithium salt in a non-aqueous solvent.

5. A sheet comprising the solid electrolyte of claim 1 or 2.

6. An electrode comprising the solid electrolyte of claim 1 or 2.

7. An electrode in contact with a protective layer comprising the solid electrolyte of claim 1 or 2.

8. An energy storage device comprising the electrode as described in claim 6.

9. An energy storage device comprising the electrode of claim 7.

10. A membrane comprising the solid electrolyte of claim 1 or 2.

11. A diaphragm in contact with a protective layer comprising the solid electrolyte of claim 1 or 2.

12. An energy storage device comprising the diaphragm of claim 10.

13. An energy storage device comprising the diaphragm as described in claim 11.

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