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

Abstract

The present invention provides: a solid electrolyte (19) with which it is possible to reduce the interfacial resistance; an ion conductor (10); a sheet (15); and a power storage device (11). The solid electrolyte has a garnet-type crystal structure that contains Li, La, Zr, and O, and when the spectrum emitted from the surface of the solid electrolyte is measured by X-ray photoelectron spectroscopy, the value obtained by dividing the abundance ratio of C by the sum of the abundance ratio of La and the abundance ratio of Zr is less than 3.1. The ion conductor includes the solid electrolyte and an electrolyte solution (23) which is obtained by dissolving a lithium salt in a nonaqueous solvent. The sheet includes the ion conductor and a binder for binding the solid electrolyte. The power storage device includes the solid electrolyte.

Description

Solid electrolyte, ionic conductor, sheet and electricity storage device

[0001] The present invention relates to a solid electrolyte, an ion conductor, a sheet, and an electricity storage device.

[0002] An oxide-based solid electrolyte having a garnet-type crystal structure containing Li, La, Zr, and O reacts with moisture and carbon dioxide in the air to produce lithium carbonate on the surface (Patent Document 1).

[0003] Patent No. 6735425

[0004] The lithium ion conductivity of lithium carbonate is significantly lower than that of solid electrolytes with a garnet-type crystal structure, so the lithium carbonate on the surface acts as a resistive layer on the solid electrolyte, increasing the interface resistance. To ensure the lithium ion conductivity of oxide-based solid electrolytes, a technology to reduce the interface resistance is required.

[0005] The present invention has been made to meet this demand, and an object of the present invention is to provide a solid electrolyte, an ion conductor, a sheet, and an electricity storage device that can reduce the interfacial resistance.

[0006] A first aspect for achieving this object is a solid electrolyte having a garnet-type crystal structure containing Li, La, Zr, and O, and when a spectrum emitted from the surface of the solid electrolyte is measured by X-ray photoelectron spectroscopy, the value obtained by dividing the abundance ratio of C by the sum of the abundance ratio of La and the abundance ratio of Zr is less than 3.1.

[0007] In a second aspect, the total abundance ratio of F, S, N, B, P and Cl is 10% or more when a spectrum is measured by X-ray photoelectron spectroscopy in the first aspect.

[0008] In a third aspect, in the first or second aspect, when a spectrum is measured by X-ray photoelectron spectroscopy, the sum of the abundance ratios of F, S, N, B, P, and Cl is 71.0% or more of the abundance ratio of C.

[0009] A fourth aspect is any one of the first to third aspects, further comprising Mg and Sr.

[0010] A fifth aspect is an ion conductor, which includes the solid electrolyte according to any one of the first to fourth aspects and an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent.

[0011] A sixth aspect is a sheet comprising the solid electrolyte of any one of the first to fourth aspects, or the ion conductor of the fifth aspect.

[0012] A seventh aspect is an electrode, which includes the solid electrolyte according to any one of the first to fourth aspects, or the ion conductor according to the fifth aspect.

[0013] An eighth aspect is an electrode that is in contact with a protective layer containing the solid electrolyte of any of the first to fourth aspects, or in contact with a protective layer containing the ion conductor of the fifth aspect.

[0014] A ninth aspect is a separator, which includes the solid electrolyte according to any one of the first to fourth aspects, or the ion conductor according to the fifth aspect.

[0015] A tenth aspect is a separator that is in contact with a protective layer containing the solid electrolyte of any one of the first to fourth aspects, or in contact with a protective layer containing the ion conductor of the fifth aspect.

[0016] An eleventh aspect is an electricity storage device, which includes the electrode according to the seventh or eighth aspect, or the separator according to the ninth or tenth aspect.

[0017] According to the solid electrolyte of the present invention, when a spectrum emitted from the surface is measured by X-ray photoelectron spectroscopy, the value obtained by dividing the abundance ratio of C by the sum of the abundance ratio of La and the abundance ratio of Zr is less than 3.1, and therefore the interface resistance can be reduced.

[0018] 1 is a cross-sectional view of an electricity storage device including an ion conductor according to a first embodiment; FIG. 2 is a diagram schematically showing a garnet-type crystal structure; FIG. 3 is a cross-sectional view of a separator; FIG. 4 is a cross-sectional view of an electricity storage device according to a second embodiment; FIG. 5 is a cross-sectional view of an electricity storage device according to a third embodiment; FIG. 6 is a cross-sectional view of an insulator according to a fourth embodiment, FIG. 7 is a cross-sectional view of an electrode according to a fifth embodiment, and FIG. 8 is a cross-sectional view of an electrode according to a sixth embodiment.

[0019] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic cross-sectional view of an electricity storage device 11 including an ion conductor 10 according to a first embodiment. The electricity storage device 11 in this embodiment is a secondary battery that uses lithium ions as charge carriers. The electricity storage device 11 includes, in order, 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 case (not shown).

[0020] The positive electrode layer 12 is formed by stacking a current collecting layer 13 and an active material layer 14. The current collecting layer 13 is a conductive member. Examples of materials for the current collecting layer 13 include a metal selected from Ni, Ti, Fe, and Al, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0021] The active material layer 14 includes an ion conductor 10 and an active material 20. The ion conductor 10 includes a solid electrolyte 19. The active material layer 14 may contain a conductive additive to reduce the resistance of the active material layer 14. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0022] Examples of the active material 20 include a metal oxide containing a transition metal, a sulfur-based active material, and an organic active material. Examples of the metal oxide containing a transition metal include a metal oxide containing Li and one or more elements selected from Mn, Co, Ni, Fe, Cr, and V. Examples of the metal oxide containing a transition metal include LiCoO ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiMn ２ O ４ , LiNiVO ４ , LiNi ０．５ Mn １．５ O ４ , LiNi １／３ Mn １／３ Co １／３ O ２ and LiFePO ４ is exemplified.

[0023] In order to suppress the reaction between the active material 20 and the solid electrolyte 19, a coating layer can be provided on the surface of the active material 20. The coating layer is made of Al ２ O ３ , ZrO ２ , LiNbO ３ , Li ４ Ti ５ O １２ , LiTaO ３ , LiNbO ３ , LiAlO ２ , Li ２ ZrO ３ , Li ２ WO ４ , Li ２ TiO ３ , Li ２ B ４ O ７ , Li ３ P.O. ４ and Li ２ MoO ４ is exemplified.

[0024] The sulfur-based active material is S, TiS ２ , NiS, FeS ２ , Li ２ S, MoS ３ Examples of organic active materials include radical compounds such as 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radialene compounds, tetraciaquinodimethane, and phenazine oxide.

[0025] The separator 15 separates the positive electrode layer 12 and the negative electrode layer 16, electrically insulating them from each other. The separator 15 is made of an ion conductor 10. The ion conductor 10 includes a solid electrolyte 19 and an electrolytic solution (described later). The ion conductor 10 may further include a binder.

[0026] The negative electrode layer 16 is formed by stacking a current collecting layer 17 and an active material layer 18. The current collecting layer 17 is a conductive member. Examples of materials for the current collecting layer 17 include a metal selected from Ni, Ti, Fe, Cu, and Si, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0027] The active material layer 18 includes an ion conductor 10 and an active material 21. To reduce the resistance of the active material layer 18, the active material layer 18 may contain a conductive additive. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag. The active material 21 may be Li, a Li-Al alloy, Li, ４ Ti ５ O １２ , graphite, In, Si, a Si—Li alloy, and SiO. As with the separator 15, the active material layers 14 and 18 may contain a binder.

[0028] The electricity storage device 11 is manufactured, for example, as follows: A slurry is produced by mixing an electrolyte solution, in which a lithium salt is dissolved in a nonaqueous solvent, with the solid electrolyte 19, and then mixing the mixture with 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 the separator 15.

[0029] A slurry is prepared by mixing an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent with a solid electrolyte 19, mixing the active material 20 with the solid electrolyte, and then mixing a solution in which a binder is dissolved in a solvent with the mixed solution. The slurry is applied onto the current collecting layer 13 and then dried to obtain a green sheet (positive electrode sheet) for the positive electrode layer 12.

[0030] A slurry is prepared by mixing an active material 21 with a solid electrolyte 19 and an electrolyte solution prepared by dissolving a lithium salt in a non-aqueous solvent, and then mixing with a solution prepared by dissolving a binder in a solvent. The slurry is applied onto the current collecting layer 17 and then dried to obtain a green sheet (negative electrode sheet) for the negative electrode layer 16.

[0031] The electrolyte sheet, positive electrode sheet, and negative electrode sheet are each cut to a predetermined shape, and then stacked in this order: positive electrode sheet, electrolyte sheet, negative electrode sheet, and then pressed together to form an integrated unit. Terminals (not shown) are connected to the current collecting layers 13 and 17, respectively, and the device is sealed in a case (not shown), resulting in an electricity storage device 11 including a positive electrode layer 12, a separator 15, and a negative electrode layer 16. In this way, a sheet including a solid electrolyte 19 can become an electrolyte sheet, a positive electrode sheet, and a negative electrode sheet by the ion conductor 10.

[0032] The solid electrolyte 19 is a composite oxide containing Li, La, and Zr and having a garnet-type crystal structure. This type of garnet-type crystal structure is represented by the general formula C ３ A ２ B ３ O １２ It is expressed as:

[0033] FIG. 2 is a diagram schematically showing a garnet-type crystal structure. In the garnet-type crystal structure, the C site Sc is dodecahedrally coordinated with an oxygen atom Oa, the A site Sa is octahedrally coordinated with an oxygen atom Oa, and the B site Sb is tetrahedrally coordinated with an oxygen atom Oa. In the solid electrolyte 19, Li can exist in a position that would be octahedrally coordinated with an oxygen atom Oa in a normal garnet-type crystal structure, but that becomes a void V. The void V is, for example, a position sandwiched between the B site Sb1 and the B site Sb2. The Li present in the void V is octahedrally coordinated with an oxygen atom Oa that forms an octahedron including the tetrahedral face Fb1 that forms the B site Sb1 and the tetrahedral face Fb2 that forms the B site Sb2. For example, Li having a garnet-type crystal structure ７ La ３ Zr ２ O １２ In the formula, La can occupy the C site Sc, Zr can occupy the A site Sa, and Li can occupy the B site Sb and the void V.

[0034] The garnet-type crystal structure is X-ray diffraction file No. 422259 (Li) in the Cambridge Structural Database (CSD). ７ La ３ Zr ２ O １２ ) has an XRD pattern similar to that of No. 422259. Compared with No. 422259, the type of constituent elements and the Li concentration of the solid electrolyte 19 may differ, and therefore the diffraction angle and intensity ratio may differ. A typical crystal structure of this type is a cubic system (space group Ia-3d (- indicates an overline that indicates a reversal operation), JCPDS: 84-1753).

[0035] Returning to FIG. 1, the solid electrolyte 19 is typically Li ７ La ３ Zr ２ O １２The solid electrolyte 19 may have some of its constituent elements substituted with other elements, or may have a small amount of other elements added without substituting the constituent elements. Examples of the other elements include 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 lanthanides (excluding La).

[0036] The solid electrolyte 19 is, for example, Li ６ La ３ Zr １．５ W ０．５ O １２ , Li ６．１５ La ３ Zr １．７５ Ta ０．２５ Al ０．２ O １２ , Li ６．１５ La ３ Zr １．７５ Ta ０．２５ Ga ０．２ O １２ , Li ６．２５ La ３ Zr ２ Ga ０．２５ O １２ , Li ６．４ La ３ Zr １．４ Ta ０．６ O １２ , Li ６．５ La ３ Zr １．７５ Te ０．２５ O １２ , Li ６．７５ La ３ Zr １．７５ Nb ０．２５ O １２ , Li ６．９ La ３ Zr １．６７５ Ta ０．２８９ Bi ０．０３６ O １２ , Li ６．４６ Ga ０．２３ La ３ Zr １．８５ Y ０．１５ O １２ , Li ６．８ La ２．９５ Ca ０．０５ Zr１．７５ 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 ０．０５ Zr ２ O １２ Examples include:

[0037] The solid electrolyte 19 preferably contains at least one of Mg and element A (A is at least one element selected from the group consisting of Ca, Sr, and Ba), with the molar ratio of each element satisfying all of the following (1) to (3), or contains both Mg and element A, with the molar ratio of each element satisfying all of the following (4) to (6). Element A is preferably Sr, in order to increase the ionic conductivity of the solid electrolyte 19. (1) 1.33≦Li / (La+A)≦3 (2) 0≦Mg / (La+A)≦0.5 (3) 0≦A / (La+A)≦0.67 (4) 2.0≦Li / (La+A)≦2.6 (5) 0.01≦Mg / (La+A)≦0.14 (6) 0.04≦A / (La+A)≦0.17

[0038] The median diameter of the circle-equivalent diameter of the solid electrolyte 19 appearing in the cross section of the separator 15 is preferably 0.5 to 10 μm, and more preferably 0.5 to 6 μm, in order to ensure that the surface area of ​​the solid electrolyte 19 is of an appropriate size and to ensure the amount of Li ions that can move between the solid electrolyte 19 and the electrolytic solution present on the surface of the solid electrolyte 19.

[0039] To determine the median diameter of the solid electrolyte 19, first, an image of the solid electrolyte 19 that appears on the cross section of the separator 15 (a polished surface, a surface obtained by irradiating with a focused ion beam (FIB), or a surface obtained by ion milling) is analyzed using a scanning electron microscope (SEM), and the circle-equivalent diameter is calculated from the area of ​​each particle of the solid electrolyte 19, and a volume-based particle size distribution is determined. The median diameter is the circle-equivalent diameter at which the cumulative frequency in the particle size distribution is 50%. To ensure accuracy, the image for determining the particle size distribution is taken from 400 μm particles of the separator 15. ２ The area shall be equal to or greater than this.

[0040] The ionic conductor 10 may contain one or more other solid electrolytes in addition to the solid electrolyte 19 having a garnet-type crystal structure containing Li, La, Zr, and O. Examples of the other solid electrolytes include crystalline or amorphous oxide-based solid electrolytes such as perovskite-type, NASICON-type, and LISICON-type, and hydride-based solid electrolytes.

[0041] The perovskite-type solid electrolyte is an oxide containing at least Li, Ti, and La, for example, La ２／３－Ｘ Li ３Ｘ TiO ３ The NASICON type solid electrolyte is an oxide containing at least Li, M (M is one or more elements selected from Ti, Zr, and Ge), and P, such as Li(Al,Ti) ２ (P.O. ４ ) ３ and Li(Al,Ge) ２ (P.O. ４ ) ３ The LISICON type solid electrolyte is Li １４ Zn(GeO ４ ) ４ Examples of hydride-based solid electrolytes include hydrides of alkali metals or alkaline earth metals, and include at least one element from Group 13 of the Periodic Table of Elements (e.g., B, Al, Ga, In, and Ta). For example, LiBH ４ , LiAlH ４ Examples include:

[0042] FIG. 3 is a cross-sectional view of the separator 15 (see FIG. 1). The ionic conductor 10 contains an electrolyte solution 23 (non-aqueous electrolyte solution) in which a lithium salt is dissolved in a non-aqueous solvent. The lithium salt is a compound used for exchanging cations between the positive electrode layer 12 and the negative electrode layer 16. The anion of the lithium salt is a halide ion (I － , Cl － ,Br － etc.), SCN － , BF ４ － , BF ３ (CF ３ ) － , BF ３ (C２ F ５ ) － , P.F. ６ － , ClO ４ － , SbF ６ － , N(SO ２ F) ２ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C ２ F ５ ) ２ － , B(C ６ H ５ ) ４ － , B(O ２ C ２ H ４ ) ２ － , C(SO ２ F) ３ － , C(SO ２ CF ３ ) ３ － , C.F. ３ COO － , C.F. ３ SO ２ O － , C ６ F ５ SO ２ O － , B(O ２ C ２ O ２ ) ２ － , RCOO － (R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0043] The anion of the lithium salt is a sulfonyl group -S(=O) ２ - N(SO ２ F) ２ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C２ F ５ ) ２ － Sulfonylimides such as PF ６ － The sulfonylimide anion is less susceptible to increases in the viscosity of the electrolyte and decreases in ionic conductivity even when the salt concentration is high, while the halophosphate ion has a high degree of dissociation. Both are preferred because they can form a highly stable and low-resistance coating (SEI) that reduces the reductive decomposition of the non-aqueous solvent and widens the reduction-side potential window.

[0044] 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. Among molecular solvents, aprotic solvents are preferred because they broaden the potential window of non-aqueous electrolytes. Examples of aprotic solvents include cyclic esters, chain esters, aliphatic carboxylic acid esters, phosphate esters, nitriles, amides, sulfur compounds, ketones, ethers, nitro compounds, fluorous solvents, and sulfone-based solvents. Mixtures of these solvents are also acceptable.

[0045] Examples of cyclic esters include carbonate esters such as propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, and fluoroethylene carbonate, and lactones such as β-propiolactone, γ-butyrolactone, δ-valerolactone, α-pyrone, and coumarin. Examples of chain esters include carbonate esters such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of aliphatic carboxylic acid esters include methyl formate, methyl acetate, and ethyl propionate. Examples of phosphate esters include trimethyl phosphate. Examples of nitriles include acetonitrile, propionitrile, butyronitrile, and benzonitrile.

[0046] Examples of amides include formamide, N-methylformamide, dimethylformamide, N-methylacetamide, dimethylacetamide, N-methylpropioamide, hexamethylphosphoramide, and N-methylpyrrolidone. Examples of sulfur compounds include dimethyl sulfoxide, sulfolane, dimethylthioformamide, and N-methylthiopyrrolidone. Examples of ketones include acetone, 4-methyl-2-pentanone, and acetylacetone. Examples of ethers include tetrahydrofuran and monoglyme. Examples of nitro compounds include nitromethane and nitrobenzene. Fluorous solvents are compounds in which the hydrogen atoms of hydrocarbons are replaced with fluorine atoms, and derivatives thereof.

[0047] Examples of sulfone solvents include trimethylene sulfone, sulfolane, difluorosulfolane, dimethyl sulfolane, monofluorosulfolane, 3-methyl sulfolane, ethyl methyl sulfone, and ethyl isopropyl sulfone. Sulfone solvents are preferred because of their high thermal stability.

[0048] The reaction in which an electrolyte dissolves in a molecular solvent and dissociates into free ions proceeds more easily the higher the relative dielectric constant of the solvent and the more easily ions are solvated, so a solvent with a relatively high relative dielectric constant εr (εr>20) is preferred. Examples of molecular solvents with a relative dielectric constant greater than 20 include cyclic esters, nitriles, amides, sulfur compounds, acetone, acetylacetone, and nitro compounds. It is of course possible to mix a solvent with a relative dielectric constant greater than 20 with a solvent with a relative dielectric constant of 20 or less in order to adjust the viscosity of the solvent, etc.

[0049] Ionic liquids are compounds consisting of cations and anions, and are liquid at room temperature and normal pressure. If the solvent of a non-aqueous electrolyte is an ionic liquid, the flame retardancy of the non-aqueous electrolyte can be improved. The ionic liquid preferably contains one or more cation species selected from the group consisting of ammonium, imidazolium, pyrrolidinium, and piperidinium.

[0050] The anion component of the ionic liquid is not particularly limited. ４ － , N(SO ２ F) ２ －inorganic anions such as B(C ６ H ５ ) ４ － , CH ３ SO ３ － , C.F. ３ SO ３ － , N(SO ２ CF ３ ) ２ － , N(SO ２ C ４ F ９ ) ２ － Examples of organic anions include:

[0051] The ionic liquid may be a solvated ionic liquid, such as a sulfone-based solvent such as sulfolane or a sulfolane derivative, or a glyme-based solvent such as tetraglyme, in which a lithium salt is dissolved.

[0052] The ionic conductor 10 may contain a binder that binds the solid electrolyte 19. Examples of the binder include fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, styrene-butadiene rubber, and other rubber-like polymers. Examples of the fluorinated resin include vinylidene fluoride polymers, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

[0053] Examples of vinylidene fluoride polymers include homopolymers of vinylidene fluoride and copolymers of vinylidene fluoride and copolymerizable monomers. Examples of copolymerizable monomers include halogen-containing monomers (excluding vinylidene fluoride) and non-halogen copolymerizable monomers. Examples of halogen-containing monomers include chlorine-containing monomers such as vinyl chloride; and fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ether. Examples of non-halogen copolymerizable monomers include olefins such as ethylene and propylene; acrylic monomers such as acrylic acid, methacrylic acid, and esters or salts thereof; and vinyl monomers such as acrylonitrile, vinyl acetate, and styrene. One or more copolymerizable monomers are polymerized with vinylidene fluoride to form a copolymer.

[0054] Because the solid electrolyte 19 is highly reactive, even when the solid electrolyte 19 is handled in an inert gas atmosphere, it reacts with small amounts of moisture and carbon dioxide present in the inert gas, forming a resistive layer of lithium hydroxide, lithium carbonate, or the like on the surface of the solid electrolyte 19. Therefore, it is effective to subject the solid electrolyte 19 to a surface treatment process before the resistive layer forms, thereby providing the surface of the solid electrolyte 19 with a component effective in reducing resistance. Examples of surface treatment processes include vapor deposition and plating.

[0055] In this embodiment, the solid electrolyte 19 is wet-polished (wet-pulverized) in an inert gas atmosphere in the presence of a nonaqueous solvent or nonaqueous electrolyte. The nonaqueous solvent or nonaqueous electrolyte immediately comes into contact with the surface of the solid electrolyte 19 from which the resistive layer has been removed by polishing, and the components of the nonaqueous solvent or nonaqueous electrolyte react with the solid electrolyte 19 to form a surface layer 22. Immediately after polishing, the nonaqueous solvent or nonaqueous electrolyte used in the wet polishing remains on the surface layer 22 of the solid electrolyte 19. When the adhered nonaqueous solvent or nonaqueous electrolyte is removed, the surface layer 22 of the solid electrolyte 19 appears.

[0056] The surface layer 22 can be analyzed by X-ray photoelectron spectroscopy (XPS). XPS involves irradiating the surface layer 22 of the solid electrolyte 19 with X-rays using AlKα rays in a vacuum, and measuring the spectrum of the kinetic energy of electrons emitted by photoionization, thereby obtaining information about the abundance ratio of elements constituting the surface layer 22. Because the mean free path in a solid of photoelectrons having energy equivalent to the kinetic energy of electrons emitted from the surface layer 22 is 2 nm or less, the thickness of the surface layer 22 analyzed by XPS is estimated to be about 10 nm.

[0057] When the spectrum of the kinetic energy of electrons emitted from the surface (surface layer 22) of the solid electrolyte 19 is measured by XPS, the value V obtained by dividing the abundance ratio of C by the sum of the abundance ratio of La and the abundance ratio of Zr is less than 3.1. The value V can be determined by calculating the area intensities of La, Zr, and C at the peaks of the XPS spectrum, and calculating the abundance ratios (atm %) of La, Zr, and C by a relative sensitivity factor method using a sensitivity coefficient specific to the device, and then calculating the ratio.

[0058] Although it depends on the type of lithium salt contained in the nonaqueous solvent and nonaqueous electrolyte used when polishing the solid electrolyte 19, C is an element mainly derived from lithium carbonate, and La and Zr are elements derived from the solid electrolyte 19. A value V of less than 3.1 means that the proportion of lithium carbonate on the surface of the solid electrolyte 19 is low, and therefore the interfacial resistance of the solid electrolyte 19 can be reduced.

[0059] When the spectrum is measured by XPS, the sum S of the abundance ratios of F, S, N, B, P, and Cl is preferably 10% or more. F, S, N, B, P, and Cl are elements that constitute the non-aqueous solvent and the lithium salt. The fact that the sum S of the abundance ratios of these elements is 10% or more means that the reaction between the components of the non-aqueous solvent and the lithium salt and the solid electrolyte 19 has progressed.

[0060] When measuring a spectrum by XPS, the sum S of the abundance ratios of F, S, N, B, P, and Cl is preferably 71% or more of the abundance ratio of C. This means that the proportion of lithium carbonate on the surface of the solid electrolyte 19 is relatively small, and therefore the interfacial resistance of the solid electrolyte 19 can be further reduced.

[0061] The lithium salt concentration of the nonaqueous electrolyte is preferably 4.0 mol / kg or less. This is because if the salt concentration of the nonaqueous electrolyte exceeds 4.0 mol / kg, the viscosity of the nonaqueous electrolyte increases, which significantly reduces the lithium ion conductivity. The salt concentration of the nonaqueous electrolyte is determined, for example, as follows. Here, the ionic conductor 10 constituting the separator 15 will be described, but the ionic conductor 10 constituting the active material layers 14 and 18 can also be determined in the same manner.

[0062] First, the separator 15 is crushed and immersed in a solvent, and the nonaqueous electrolyte contained in the separator 15 is dissolved in the solvent, and then the separator 15 is separated into a solid component and a liquid component using a centrifuge. The Li content of the separated liquid component is determined by high-frequency inductively coupled plasma spectroscopy (ICP).

[0063] The type of nonaqueous solvent contained in separator 15 is identified by, for example, gas chromatography-mass spectrometry (GC-MS). The nonaqueous solvent of which type has been identified (hereinafter referred to as a "standard substance") and separator 15 are analyzed by thermogravimetric differential thermal analysis (TG-DTA). The analysis results of the standard substance and the analysis results of separator 15 are compared to identify the content of nonaqueous solvent contained in separator 15. The molar concentration (mol / kg) of the lithium salt in the nonaqueous electrolyte is calculated based on the Li content in the liquid component and the content of the nonaqueous solvent in separator 15.

[0064] In the ion conductor 10, the ratio of the volume of the solid electrolyte 19 to the total volume of the solid electrolyte 19 and the non-aqueous electrolyte solution is preferably 52% or more and less than 100%, and more preferably 61% or more and less than 100%. The combination of the solid electrolyte 19 and the non-aqueous electrolyte solution can reduce the interfacial resistance of the solid electrolyte 19, thereby increasing the operational stability of the electricity storage device 11 in which the ion conductor 10 is disposed.

[0065] The contents (volume %) of the solid electrolyte 19 and the non-aqueous electrolyte solution are determined by freezing the separator 15 or embedding the separator 15 in a tetrafunctional epoxy resin or the like and then analyzing a randomly selected cross section of the separator 15 at a magnification of 5000 times using an SEM equipped with an energy dispersive X-ray spectrometer (EDS). The analysis involves identifying the distribution of La, Zr, and S and performing image analysis of the contrast of a backscattered electron image to identify the areas of the solid electrolyte 19 and the non-aqueous electrolyte solution, and the proportions of these areas in the cross section of the separator 15 are regarded as the proportions of the volume of the ion conductor 10 in the separator 15 to obtain the contents (volume %) of the solid electrolyte 19 and the non-aqueous electrolyte solution.

[0066] The Li ion conductivity of the ionic conductor 10 is determined by the type and salt concentration of the solid electrolyte 19 and the non-aqueous electrolyte. The lithium ion conductivity of the ionic conductor 10 at 25° C. is 1.0×10 －５ The ionic conductivity is preferably 25 S / cm or more in order to ensure the output density of the electricity storage device 11 including the ionic conductor 10.

[0067] A second embodiment will be described with reference to Fig. 4. In the first embodiment, an electricity storage device 11 using a solid electrolyte 19 as the electrolyte was described. In the second embodiment, a case where an ion conductor 10 is used in a liquid-based lithium-ion battery using a nonaqueous electrolyte solution as the electrolyte will be described. The same parts as those described in the first embodiment are given the same reference numerals, and the following description will be omitted. Fig. 4 is a cross-sectional view of an electricity storage device 24 in the second embodiment.

[0068] The power storage device 24 includes, in order, a positive electrode layer 12, a separator 25, and a negative electrode layer 16. These are housed in a case (not shown). The separator 25 is made of a porous material that is durable against the active materials 20, 21 and electrolyte contained in the positive electrode layer 12 and the negative electrode layer 16, and that allows lithium ions to pass through but does not have electronic conductivity. Examples of the separator 25 include nonwoven fabrics and porous membranes made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc. The nonaqueous electrolyte is the same as that described in the first embodiment, so its description will be omitted.

[0069] In the electricity storage device 24 of the second embodiment, the ion conductor 10 is contained in the positive electrode layer 12 and the negative electrode layer 16, and therefore, the stability of operation is increased, similar to the electricity storage device 11 of the first embodiment.

[0070] A third embodiment will be described with reference to Fig. 5. In the first and second embodiments, the positive electrode layer 12, the separator 15, and the negative electrode layer 16 contain the ion conductor 10. In the third embodiment, the protective layers 29 and 32 contain the ion conductor 10. The same parts as those described in the first and second embodiments are designated by the same reference numerals, and the following description will be omitted. Fig. 5 is a cross-sectional view of an electricity storage device 26 in the third embodiment.

[0071] The power storage device 26 includes, in order, a positive electrode layer 27, a separator 25, and a negative electrode layer 30. These are housed in a case (not shown). The power storage device 26 is a liquid-based lithium-ion battery that uses a non-aqueous electrolyte solution as the electrolyte.

[0072] The positive electrode layer 27 is formed by stacking the current collecting layer 13 and an active material layer 28. The active material layer 28 contains an active material 20. In order to reduce the resistance of the active material layer 28, the active material layer 28 may contain a conductive additive such as carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, or Ag.

[0073] 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.

[0074] The negative electrode layer 30 is formed by stacking an active material layer 31, a protective layer 32, and a current collecting layer 17 in this order. The active material layer 31 is made 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 arranged by stacking sheet-like compacts made of a slurry containing the ion conductor 10, or by applying a slurry containing the ion conductor 10 to the separator 25 or the current collecting layer 17, or the like.

[0075] The solid electrolyte 19 contained in the ion conductor 10 has a garnet-type crystal structure containing Li, La, Zr, and O, and is resistant to reduction by the metallic lithium of the active material layer 31, so the protective layer 29 increases the operational stability of the electricity storage device 26. Furthermore, the protective layer 29 suppresses short circuits caused by dendrite growth of metallic lithium. The protective layer 32 interposed between the active material layer 31 and the current collecting layer 17 suppresses deterioration of the current collecting layer 17.

[0076] Fourth to sixth embodiments will be described with reference to Fig. 6. Note that the same parts as those described in the first to third embodiments are given the same reference numerals, and the description thereof will be omitted. Fig. 6(a) is a cross-sectional view of an insulator 33 in the fourth embodiment.

[0077] The insulator 33 includes a separator 25 and a protective layer 29 in contact with the separator 25. The separator 25 includes a first interface 34 and a second interface 35 opposite the first interface 34, with the protective layer 29 disposed at the first interface 34 and the second interface 35. The protective layer 29 disposed on the separator 25 can reduce short circuits caused by dendritic growth of metallic lithium contained in the electricity storage device. Even if a short circuit occurs in the electricity storage device and the separator 25 attempts to thermally deform, the presence of the protective layer 29 allows the shape of the separator 25 to be maintained, thereby suppressing the occurrence of thermal runaway in the electricity storage device.

[0078] 6(b) is a cross-sectional view of an 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 electrode 36 has the protective layer 29 disposed at an interface 37 of the active material layer 14 opposite the surface on which the current collecting 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 electricity storage device.

[0079] 6( c) is a cross-sectional view of an electrode 38 according to a 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 electrode 38 has the protective layer 29 disposed at an interface 39 opposite the surface of the active material layer 18 on which the current collecting layer 17 is disposed. The protective layer 29 disposed at the interface 39 of the active material layer 18 can reduce dendrite growth from the negative electrode layer 16 of the electricity storage device.

[0080] The insulator 33 is disposed in the electric storage device in place of the separator 25 of the electric storage device 24 of the second embodiment or the electric storage device 26 of the third embodiment. The insulator 33 may omit one of the two protective layers 29 disposed at the interfaces 34, 35 of the separator 25.

[0081] The electrode 36 is disposed in the electricity storage device in place of the positive electrode layers 12, 27 of the electricity storage device 24 in the second embodiment or the electricity storage device 26 in the third embodiment. The electrode 38 is disposed in the electricity storage device in place of the negative electrode layers 16, 30 of the electricity storage device 24 in the second embodiment or the electricity storage device 26 in the third embodiment.

[0082] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0083] (Preparation of solid electrolyte) Li ６．９５ Mg ０．１５ La ２．７５ Sr ０．２５ Zr ２．０ O １２ So that Li ２ CO ３ , MgO, La(OH) ３ , SrCO ３ , ZrO ２ was weighed. ２ CO ３Considering the volatilization of Li during firing, the amount of Li was in excess of about 15 mol% in elemental terms. The weighed raw materials and ethanol were placed in a nylon pot together with zirconia balls and ground and mixed in a ball mill for 15 hours. The slurry removed from the pot was dried, then placed on an MgO plate and pre-fired at 1200°C for 10 hours. A binder was added to the pre-fired material, and the material was ground and mixed in an inert solvent for 60 hours in a ball mill. The slurry removed from the pot was dried to obtain a raw material powder for the solid electrolyte.

[0084] The raw material powder was placed in a mold with an inner diameter of 32.5 mm and pressed to obtain a disk with a diameter of 32.5 mm and a thickness of about 2.5 mm. The disk was pressed with a cold isostatic press at a pressure of 1.5 t / cm ２ The compact was covered with the raw material powder and sintered in the air at 1100° C. for 4 hours to obtain a sintered body of the solid electrolyte.

[0085] (Example 1) A non-aqueous electrolyte solution was prepared by dissolving lithium bis(fluorosulfonyl)imide (LiFSI) in sulfolane to a lithium salt concentration of 2.7 mol / L. The non-aqueous electrolyte solution was dropped onto waterproof abrasive paper to which silicon carbide abrasive (grain size 320 as specified in JIS R6010:2000) was fixed, and each circular surface of the sintered body was wet-polished under an argon atmosphere. This produced the solid electrolyte of Example 1.

[0086] Example 2 A solid electrolyte in Example 2 was obtained in the same manner as in Example 1, except that a nonaqueous electrolyte prepared by dissolving LiFSI in fluoroethylene carbonate to a lithium salt concentration of 3.0 mol / L was dropped onto a waterproof abrasive paper, and the sintered body was wet-polished.

[0087] Comparative Example A solid electrolyte in Comparative Example was obtained in the same manner as in Example 1, except that each circular surface of the sintered body was dry-polished under an argon atmosphere using waterproof abrasive paper to which a silicon carbide abrasive (grain size 320 as specified in JIS R6010:2000) was fixed.

[0088] (Measurement of Interface Resistance) A solid electrolyte with both sides polished was sandwiched between insulating cylinders with a gasket between them. A nonaqueous electrolyte of the same type as that used for polishing was placed in the cylinders on both sides of the solid electrolyte, and a working electrode and a reference electrode made of metallic lithium were immersed in the nonaqueous electrolyte, and AC impedance was measured using a four-terminal method. The nonaqueous electrolyte used in Example 1 was poured into the solid electrolyte of the comparative example that had been dry-polished, and AC impedance was measured. The AC impedance measurement conditions were a temperature of 25°C, a voltage of 10 mV, and a frequency of 1 MHz-10 mHz. The impedance of the solid electrolyte / nonaqueous electrolyte interface was separated using a Nyquist plot, and the interface resistance was determined.

[0089] (XPS Spectral Measurement) The solid electrolyte for which the interfacial resistance was measured was washed by shaking in dimethyl carbonate and then stored in a transfer vessel under an argon atmosphere. After being introduced into an XPS analysis device, the circular surface of the solid electrolyte was analyzed. The XPS conditions for all three types were: X-ray: monochromated AlKα radiation, pass energy: 140 eV, and analysis area: 100 μmΦ. The area intensity was calculated from the spectrum obtained by XPS, and the abundance ratio (atom %) of each element was measured using the relative sensitivity factor method using the sensitivity coefficient specific to the device. The abundance ratio of each element and the interfacial resistance are shown in Table 1.

[0090]

[0091] As shown in Table 1, the interface resistance in Example 1 was 18 Ωcm ２ and Example 2 is 11 Ωcm ２ In contrast, the comparative example was 79 Ωcm ２ It was clear that the solid electrolytes in Examples 1 and 2, which were wet-polished with a nonaqueous electrolyte in an argon atmosphere, had reduced interfacial resistance compared to the solid electrolyte in the comparative example, which was dry-polished in an argon atmosphere.

[0092] The value V obtained by dividing the abundance ratio of C by the sum of the abundance ratio of La and the abundance ratio of Zr was 17.1 / (2.4+3.3)=3.0 in Example 1 and 13.5 / (3.7+2.9)=2.0 in Example 2, while it was 25.9 / (0.4+0.9)=19.9 in the Comparative Example. It can be estimated that in Examples 1 and 2, where the value V was less than 3.1, the amount of lithium carbonate present on the surface of the solid electrolyte was reduced compared to the Comparative Example, and therefore it is considered that the difference in the amount of lithium carbonate on the surface reduced the interfacial resistance of the solid electrolyte.

[0093] The sum S of the abundance ratios of F, S, N, B, P, and Cl was 3.4 + 6.3 + 2.5 = 12.2 atm% in Example 1, 7.1 + 5.5 + 2.9 = 15.5 atm% in Example 2, and 0.0 atm% in the comparative example. In Examples 1 and 2, where the sum S was 10 atm% or more, it can be assumed that a surface layer containing elements constituting the nonaqueous electrolyte was formed on the solid electrolyte. In Examples 1 and 2, it is believed that the surface layer reduced the interfacial resistance of the solid electrolyte.

[0094] The sum S of the abundance ratios of F, S, N, B, P, and Cl relative to the abundance ratio of C was 12.2 / 17.1=71.3% in Example 1 and 15.5 / 13.5=114.8% in Example 2, while it was 0% in the comparative example. In Examples 1 and 2, where the ratio of the sum S to the abundance ratio of C was 71.0% or more, it is presumed that the surface layer containing elements that constitute the nonaqueous electrolyte solution was able to relatively reduce the proportion of lithium carbonate.

[0095] In the examples, a case where a sintered body of a solid electrolyte is wet-polished in an argon atmosphere in the presence of a nonaqueous electrolyte solution to form a low-resistance surface layer on the surface of the solid electrolyte is described, but the present invention is not limited to this. It is clear that a low-resistance surface layer can also be formed on the surface of a solid electrolyte (particle) by wet-pulverizing particles of a solid electrolyte in an argon atmosphere in the presence of a nonaqueous electrolyte solution.

[0096] Oxide-based solid electrolytes with a garnet-type crystal structure containing Li, La, and Zr have the excellent characteristics of having bulk conductivity comparable to that of nonaqueous electrolyte solutions and being electrochemically stable against metallic lithium. However, because oxide-based solid electrolytes have high interfacial resistance, it is difficult for energy storage devices employing compacts formed by pressure molding of solid electrolyte particles to achieve practical battery characteristics. In contrast, the examples demonstrate that solid electrolytes capable of reducing interfacial resistance can be obtained. Therefore, energy storage devices employing compacts formed by pressure molding of solid electrolyte particles can be obtained that achieve practical battery characteristics without forming and sintering the solid electrolyte particles into a high-density sintered body.

[0097] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0098] In the embodiment, the electricity storage device 11 has been described as including a positive electrode layer 12 in which an active material layer 14 is provided on one side of a current collecting layer 13, and an negative electrode layer 16 in which an active material layer 18 is provided on one side of a current collecting layer 17, but this is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to an electricity storage device including electrode layers in which an active material layer 14 and an active material layer 18 are provided on both sides of a current collecting layer 13 (so-called bipolar electrodes). If bipolar electrodes and separators 15 are alternately stacked and housed in a case (not shown), an electricity storage device with a so-called bipolar structure can be obtained.

[0099] In the embodiment, the active material layers 14, 18 and the separator 15 all contain the ion conductor 10, but this is not necessarily limited to this. It is sufficient for the power storage device that at least one of the active material layers 14, 18 and the separator 15 contains the ion conductor 10.

[0100] In the embodiments, the power storage devices 11, 24, and 26 are described as being composed of lithium ion batteries, but this is not necessarily limited to this. It is clear that other power storage devices may include the solid electrolyte 19. Examples of other power storage devices include electrochemical capacitors. Examples of electrochemical capacitors include redox capacitors that utilize redox reactions and hybrid capacitors that are asymmetric cells that combine an electric double layer capacitor with the solid electrolyte 19.

[0101] Although not described in the embodiment, it is of course possible to dispose a protective layer 29 between the active material layer 18 and the separator 15, 25, or between the current collecting layer 17 and the active material layer 18. Disposing a protective layer 29 between the active material layer 18 and the separator 15, 25 can reduce short circuits caused by dendrites. Disposing a protective layer 29 between the current collecting layer 17 and the active material layer 18 can reduce deterioration of the current collecting layer 17.

[0102] REFERENCE SIGNS LIST 10 ion conductor 11, 24, 26 electricity storage device 12 positive electrode layer (sheet, electrode) 15 separator (sheet) 16 negative electrode layer (sheet, electrode) 19 solid electrolyte 23 non-aqueous electrolyte (electrolyte) 25 separator 29, 32 protective layer

Claims

1. A solid electrolyte having a garnet-type crystal structure containing Li, La, Zr, and O, wherein, when a spectrum emitted from the surface of the solid electrolyte is measured by X-ray photoelectron spectroscopy, the value obtained by dividing the abundance ratio of C by the sum of the abundance ratio of La and the abundance ratio of Zr is less than 3.

1.

2. The solid electrolyte according to claim 1, wherein the total content of F, S, N, B, P and Cl is 10% or more when the spectrum is measured by X-ray photoelectron spectroscopy.

3. A solid electrolyte according to claim 1 or 2, wherein the sum of the abundance ratios of F, S, N, B, P and Cl is 71.0% or more of the abundance ratio of C when the spectrum is measured by X-ray photoelectron spectroscopy.

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

5. An ionic conductor comprising the solid electrolyte according to claim 1 or 2 and an electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent.

6. A sheet comprising the solid electrolyte according to claim 1 or 2.

7. An electrode comprising the solid electrolyte according to claim 1 or 2.

8. An electrode in contact with a protective layer containing the solid electrolyte according to claim 1 or 2.

9. An electricity storage device comprising the electrode according to claim 7.

10. An electricity storage device comprising the electrode according to claim 8.

11. A separator comprising the solid electrolyte according to claim 1 or 2.

12. A separator in contact with a protective layer containing the solid electrolyte according to claim 1 or 2.

13. An electricity storage device comprising the separator according to claim 11.

14. An electricity storage device comprising the separator according to claim 12.

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