Solid solution, electrode active material, electrode, and secondary battery

Abstract

The present invention addresses the problem of providing an electrode active material having a novel structure, said electrode active material enabling Li2S to be used as an electrode. The above-described problem is solved by a solid solution which contains Li, Cu and S as main constituents, while having an anti-fluorite crystal structure.

Description

Technical Field

[0001] The present invention relates to a solid solution, an electrode active material, an electrode, an electrode composite, and a secondary battery. More specifically, the present invention relates to a new solid solution having an inverse fluorite crystal structure, and an electrode active material, an electrode, an electrode composite, and a secondary battery containing the same.

Background Art

[0002] In recent years, the demand for lithium-ion secondary batteries for storing electric power has been increasing in automobiles such as electric vehicles and hybrid vehicles, and power generation devices such as solar cells and wind power generation. Also, from the viewpoint of ensuring safety, all-solid-state batteries using solid electrolytes instead of liquids in the electrolyte layer have been actively studied. These lithium-ion secondary batteries and all-solid-state batteries are required to have further higher performance. Among these, Li2S has attracted attention as a material for the electrode active material constituting the electrode.

[0003] Li2S has a high theoretical capacity of 1167 mAh g -1 and is expected as a next-generation electrode active material. However, Li2S has a very low electrical conductivity of 10 -9 S cm -1 or less and cannot be used alone as an electrode active material. Attempts have been made to utilize this Li2S. For example, International Publication WO2016 / 063877 (Patent Document 1) discloses a solid solution obtained by mixing and treating Li2S with LiI or LiBr. Also, Hayashi, Tatsumi et al., The Annual Meeting of the Chemical Society of Japan (2017) (Non-Patent Document 1) discloses a positive electrode active material obtained by adding and mixing V2S3 and S to Li2S.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005] [[Non - Patent Document 1]] Hayashi, Tatsumi et al., The Annual Meeting of The Chemical Society of Japan (2017) [[Summary of the Invention]] [[Problems to be Solved by the Invention]]

[0006] Although electrode active materials using Li2S have been developed by the methods listed above, for further performance improvement, the development of new electrode active materials using Li2S has been demanded. [[Means for Solving the Problems]]

[0007] As a result of intensive studies, the inventors of the present invention found that a solid solution containing Li, Cu, and S as main constituent components unexpectedly has an inverse fluorite - type structure. And it was found that this solid solution has high electronic conductivity and, when used as an electrode active material, brings excellent effects on electrode characteristics and secondary battery characteristics, leading to the present invention. Thus, according to the present invention, a solid solution having an inverse fluorite - type crystal structure containing Li, Cu, and S as main constituent components is provided. Also, according to the present invention, an electrode active material containing the above solid solution is provided. Furthermore, according to the present invention, an electrode containing the above electrode active material is provided. Furthermore, according to the present invention, an electrode composite in which the above electrode and a current collector are combined is provided. Furthermore, according to the present invention, a secondary battery containing the above electrode or the above electrode composite is provided. [[Effects of the Invention]]

[0008] According to the present invention, a novel Li - containing metal sulfide - based solid solution is provided. Also, when having any of the following configurations, a solid solution having higher electronic conductivity and / or excellent charge - discharge capacity can be provided. (1) The solid solution has the following formula Li4-x-y-z Cu x-y M y S 2-z A z (1) (wherein M is a divalent metal cation, A is a monovalent anion, x is 0.1 or more and 2.0 or less, y is 0 or more and less than 1.0, z is 0 or more and 1.0 or less, and x > y) It is represented by (2) In the above formula, M is selected from Cu, Mg, Ca, Ni, Zn, Fe, Co, Sn, Pb or Mn. (3) In the above formula, A is selected from F, Cl, Br or I. (4) The lattice constant of the a-axis of the solid solution is in the range of 5.60 to 5.80 Å.

Brief Description of the Drawings

[0009]

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Embodiments for Carrying Out the Invention

[0010] (Solid solution) The solid solution of the present invention has an inverse fluorite-type crystal structure containing Li, Cu, and S as main constituent components. Here, the solid solution referred to is the same as what those skilled in the art generally recognize as a solid solution, and refers to a state in which two or more elements are melted and homogenized. The inverse fluorite crystal structure is a structure in which cations with positive charges enter the central parts of eight small cubes obtained by cutting each side in half in a tetrahedron of a face-centered cubic lattice composed of anions with negative charges. The inverse fluorite crystal structure of the solid solution of the present invention is shown in FIG. 1. As shown in FIG. 1, the solid solution of the present invention has a structure in which a part of Li in Li2S is substituted with Cu. Showing a structure in which a part of Li2S is substituted with Cu is what the present inventor has first found. The solid solution of the present invention exhibits high conductivity that could not be obtained with only Li2S due to a part of Li in Li2S being substituted with Cu. The present inventor believes that this substitution occurs randomly regardless of the position of Li. It is possible to take a structure of a solid solution of the Li2S-Cu2S system in which a region represented by Li2S and a region that can also be represented by Cu2S are mixed. Including as a main constituent means that the total molar content of Li, Cu, and S in the solid solution is 50% or more with respect to the total molar content constituting the solid solution. This is preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, particularly preferably 90% or more, and particularly more preferably 95% or more.

[0011] The solid solution of the present invention may further contain divalent and / or trivalent metal cations and / or monovalent anions in addition to Li, Cu, and S. Here, the metal cation is not particularly limited, but is preferably selected from metal cations in the 3rd to 5th periods, more preferably selected from metal cations in the 3rd to 4th periods. Also, for example, it is preferably selected from metal cations in Groups 2 to 14, more preferably selected from metal cations in Groups 2, 6 to 14, even more preferably selected from metal cations in Groups 2, 7 to 14, and even more preferably selected from metal cations in Groups 2, 7 to 12. Furthermore, the metal cation is preferably selected from metal cations in the 3rd to 5th periods and Groups 2 to 14, and more preferably selected from metal cations in the 3rd to 4th periods and Groups 2, 7 to 12. These metal cations may be one type of metal cation or a combination of a plurality of metal cations. The anion is not particularly limited, and examples thereof include Group 17 anions. The Group 17 anion is preferably selected from F, Cl, Br, or I. The anion may be one type of anion, or a plurality of monovalent anions may be combined. By including a metal cation selected from metal cations of Groups 2 to 14 in the 3rd to 5th periods and / or a monovalent anion, a solid solution having better electrical conductivity can be provided.

[0012] In a specific embodiment, the solid solution of the present invention has, for example, the following formula Li 4-x-y(1)-2y(2)-z Cu x-y(1)-y(2) M(II) y(1) M(III) y(2) S 2-z A z (2) (In the formula, M(II) is a divalent metal cation, M(III) is a trivalent metal cation, A is a monovalent anion, x is 0.1 or more and 2.0 or less, y(1) + y(2) is 0 or more and less than 1, z is 0 or more and 1.0 or less, and x > y(1) + y(2)) It can be represented by.

[0013] M(II) is not particularly limited as long as it is a divalent metal cation, but a divalent metal cation selected from the above metal cations is preferable. As the divalent metal cation, for example, it can be selected from Cu, Mg, Ca, Ni, Zn, Fe, Co, Sn, Pb, or Mn. M(II) may be one type of divalent metal cation, or a plurality of divalent metal cations may be combined. M(III) is not particularly limited as long as it is a trivalent metal cation, but a trivalent metal cation selected from the above metal cations is preferable. As the trivalent metal cation, for example, it can be selected from Al, Fe, or Cr. M(III) may be one type of trivalent metal cation, or a plurality of trivalent metal cations may be combined. A is not particularly limited as long as it is a monovalent anion, but for example, group 17 anions are preferred. Examples of the monovalent anion include halogens such as F, Cl, Br, or I. A may be a single type of monovalent anion or a combination of multiple monovalent anions.

[0014] In the formula, x can take any value, for example, from 0.1 or more to 2.0 or less. The range of x can be represented by, for example, any combination of upper and lower limit values selected from values such as 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, or 0.10. Among these, the range of 0.5 to 1.0 is preferable. When x is in the range of 0.5 to 1.0, a solid solution with more excellent electrical conductivity can be provided. y(1) and y(2) are any values that satisfy 0 ≦ y(1) + y(2) < 1 and y(1) + y(2) < x. Preferably, y(1) and y(2) are values that satisfy y(1) + y(2) ≦ 0.5x. In the formula, y(1) can take any value, for example, from 0 or more to 0.5 or less. The range of y(1) can be represented by, for example, any combination of upper and lower limit values selected from values such as 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.10, 0.05, 0.01, or 0. In the formula, y(2) can take any value, for example, from 0 or more to 0.5 or less. The range of y(2) can be represented by, for example, any combination of upper and lower limit values selected from values such as 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.10, 0.05, 0.01, or 0. In the formula, z can take any value, for example, a value between 0 and 1.0. The range of z can be represented by any combination of upper and lower limit values selected from values such as 1.0, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.10, 0.05, 0.01 or 0. Among these, preferably, it is in the range of 0 to 0.5, more preferably in the range of 0 to 0.4, still more preferably in the range of 0 to 0.3, and even more preferably in the range of 0 to 0.2. Also, it is preferable that z ≤ x. When z is in the range of 0 to 0.5, the charge-discharge capacity can be further improved.

[0015] In a more specific embodiment, the solid solution of the present invention has the following formula Li 4-x-y-z Cu x-y M y S 2-z A z (1) (In the formula, M is a divalent metal cation, A is a monovalent anion, x is from 0.1 to 2.0, y is from 0 to less than 1.0, z is from 0 to 1.0, and x > y) and can be represented by.

[0016] For M in formula (1), the same divalent metal cation as M(II) in the above formula (2) can be used. For A in formula (1), the same monovalent anion as A in the above formula (2) can be used. For x and z in formula (1), they are the same as x and z in the above formula (2). In formula (1), y can take any value, for example, greater than or equal to 0 and less than 1.0. The range of y can be represented by, for example, any combination of upper and lower limit values selected from the values of 0.99, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.10, 0.05, 0.01 or 0. Among these, the range of 0 to 0.5 is preferable, the range of 0 to 0.4 is more preferable, the range of 0 to 0.3 is still more preferable, and the range of 0 to 0.2 is even more preferable. Also, it is preferable that y ≤ 0.5x. When y is in the range of 0 to 0.5, the charge-discharge capacity can be further improved. As described above, when a divalent metal cation or a monovalent anion enters the inverse fluorite-type crystal structure of the solid solution, defects are generated in a part of the crystal structure, and the conductivity can be further improved.

[0017] In the solid solution of the present invention, when x is in the range of 0.5 to 1.0, and y = 0 and z = 0, for example, it can be represented by Li3CuS2, Li5CuS 3, Li8Cu2S5 or Li7CuS4, etc. The solid solution of the present invention has an XRD pattern measured using the CuKα of the solid solution as an X-ray source, and the lattice constant calculated by Fox software (vincefn) based on the real space method, etc., based on the space group JPEG0007693226000001.jpg6141 and lattice constant (a = 5.76 Å) preferably has a range of 5.60 Å to 5.80 Å. More preferably, it has a range of 5.65 Å to 5.75 Å. It is preferable from the viewpoints of high capacity and high output that the solid solution has a lattice constant in this range.

[0018] The solid solution of the present invention preferably has an electronic conductivity of 1.0×10 -6 S cm -1 or more. When the solid solution has an electronic conductivity of 1.0×10 -6 S cm -1Since it has the above electron conductivity, a secondary battery excellent in performance such as charge and discharge capacity can be manufactured. The solid solution of the present invention has 1.0×10 -4 S cm -1 or more of electron conductivity is more preferable, and 1.0×10 -3 S cm -1 or more of electron conductivity is even more preferable, and 5.0×10 -3 S cm -1 or more of electron conductivity is particularly preferable. Since the solid solution has the above electron conductivity, the solid solution of the present invention can be used as an electrode without adding a conductive assistant.

[0019] The solid solution of the present invention preferably has 1.0×10 -8 S cm -1 or more of ion conductivity. Since the solid solution has 1.0×10 -8 S cm -1 or more of ion conductivity, a secondary battery excellent in performance such as charge and discharge capacity can be manufactured. The solid solution of the present invention preferably has 1.0×10 -7 S cm -1 or more of ion conductivity, more preferably 1.0×10 -6 S cm -1 or more of ion conductivity, even more preferably 3.0×10 -6 S cm -1 or more of electron conductivity is particularly preferable. Since the solid solution has the above ion conductivity, the solid solution of the present invention can be used as an electrode without adding a solid electrolyte.

[0020] (Method for manufacturing solid solution) The method for producing a solid solution is not particularly limited as long as it can produce an antifluorite-type solid solution containing all of the elements Li, Cu, and S by combining raw materials containing at least one of the elements Li, Cu, and S. Examples of raw materials containing at least one of the elements Li, Cu, and S include elemental Li, Li2S, elemental Cu, elemental S, CuS, and the like. S may be cyclic sulfur such as S8 or chain-like S. In addition to the raw materials containing at least one of the above-mentioned elements Li, Cu, and S, raw materials containing divalent metal cations, trivalent metal cations, and / or monovalent anions as described above may be further added. The method for solid solution formation is not particularly limited, but for example, since it can be uniformly reacted, mechanochemical treatment can be mentioned. As the processing apparatus, a ball mill can be used. A ball mill is preferable because a large amount of mechanical energy can be obtained. Among ball mills, a planetary ball mill is preferable because the pot rotates and revolves while the base rotates in the direction opposite to the rotation direction, so that high impact energy can be efficiently generated. The processing conditions can be appropriately set according to the processing apparatus used. For example, when using a planetary ball mill, conditions such as a rotation speed of 160 to 400 revolutions per minute and a processing time of 0.1 to 120 hours can be mentioned. When using a lithium salt as a raw material, in order to prevent the lithium salt from reacting with water and oxygen, it is preferably processed in an inert atmosphere (for example, an argon atmosphere) using a glove box or the like under an environment where the water concentration is 1000 ppm or less and the oxygen concentration is 1000 ppm or less. As an example of the production of an antifluorite-type solid solution containing Li, Cu, and S as main constituent components, it can be obtained by adding elemental Cu and elemental S to Li2S and mixing them under an argon atmosphere, and then performing mechanochemical treatment on this mixture.

[0021] (Electrode active material) The electrode active material of the present invention may consist only of the solid solution of the present invention. The solid solution of the present invention has high ionic conductivity and electronic conductivity by itself. Therefore, the amount of the solid solution in the electrode active material can be increased. However, it is not to be construed that the electrode active material must not contain substances other than the solid solution, and it may contain any of the solid electrolytes, binders, or conductive agents described below.

[0022] The amount of the solid electrolyte contained in the electrode active material is not particularly limited, but is preferably 0 to 200 parts by weight with respect to 100 parts by weight of the solid solution in the electrode active material. If it is more than 200 parts by weight, the positive electrode using the electrode active material may not function as a positive electrode. The amount of the solid electrolyte is preferably 0 to 100 parts by weight with respect to 100 parts by weight of the solid solution in the electrode active material.

[0023] (Electrode) The electrode of the present invention contains the solid solution of the present invention or the electrode active material of the present invention. The electrode of the present invention may consist only of the solid solution of the present invention or the electrode active material of the present invention, or may be mixed with a binder, a conductive material, an electrolyte, etc. The solid solution of the present invention or the electrode active material of the present invention has sufficiently high ionic conductivity and electronic conductivity by itself. Therefore, the amount of the solid electrolyte or the conductive material contained in the conventional electrodes and electrode active materials may be reduced, or the solid electrolyte or the conductive material may not be necessary. The proportion of the electrode active material in the electrode can be 70% by mass or more, 85% by mass or more, or 100% by mass. However, the use of the solid electrolyte or the conductive material is not denied, and these may be used as necessary. The solid solution of the present invention or the electrode active material of the present invention can be used as a positive electrode or a negative electrode depending on the combination with the counter electrode.

[0024] The binder is not particularly limited, and examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyvinyl acetate, polymethyl methacrylate, polyethylene, styrene-butadiene rubber, acrylonitrile-butadiene rubber, and copolymers thereof. When mixing the binder and the solid solution of the present invention using a solvent, it is not particularly limited, but a solvent that does not cause side reactions with the solid solution of the present invention is preferable.

[0025] The conductive material is not particularly limited, and examples thereof include natural graphite, artificial graphite, acetylene black, ketjen black, denka black, carbon black, vapor-grown carbon fiber (VCGF), etc.

[0026] The solid electrolyte contained in the electrode is not particularly limited, and the solid electrolyte used in the production of the secondary battery described later can be used.

[0027] A known electrode active material may be added to the electrode of the present invention as necessary. As the known electrode active material, for example, if the electrode is a positive electrode, Li4Ti5O 12 , LiCoO2, LiMnO2, LiVO2, LiCrO2, LiNiO2, Li2NiMn3O8, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, FeS, Ti2S, LiFeO2, Li3V2(PO 4)3 or LiMn2O4, etc. are mentioned. If the electrode is a negative electrode, as the negative electrode active material, for example, carbon-based materials such as natural graphite, artificial graphite, acetylene black, ketjen black, denka black, carbon black or VCGF, Si, Li alloy, Na alloy, Au, Pt, Pd, Ag, Al, Bi, Sn, Sb, Zn, Mg, K, Ca or Na and other metals, Li 4 / 3 Ti 5 / 3 O4, Li3V2(PO4)3 or various transition metal oxides such as SnO are mentioned. These negative electrode active materials may be used alone or in combination of two or more. The electrode may be coated with a material such as, for example, LiNbO3, NaNbO3, Al2O3 or NiS. These electrodes may be used individually, or two or more of them may be combined and used.

[0028] The electrode can be obtained in pellet form, for example, by mixing an electrode active material and optionally a binder, a conductive material or an electrolyte, etc., and pressing the resulting mixture.

[0029] (Electrode composite) The present invention also provides an electrode composite in which a positive electrode and a current collector are combined. The electrode combined with the current collector is the electrode of the present invention described above. The current collector can be combined with the electrode of the present invention, and the material, shape, etc. are not particularly limited as long as it can function as a current collector. The shape of the current collector may be like a uniform alloy plate or may have a shape with holes. Also, it may be in the form of a foil, sheet or film.

[0030] Examples of the material of the current collector include Ni, Cu, Ti, Fe, Co, Ge, Cr, Mo, W, stainless steel, or steel. In addition to the above materials, the current collector may be coated with either gold or aluminum. The thickness of the coating is not particularly limited, but is preferably 10 nm - 100 μm. Also, the coating preferably has a uniform thickness. The coating method is not particularly limited as long as the current collector can be coated. For example, it can be formed by vapor deposition on the surface using a sputter coater. The electrode composite of the present invention may be formed by combining the electrode and the current collector respectively as an electrode composite, or the electrode may be directly formed on the current collector. When directly forming, the electrode active material may be applied to the surface of the current collector using a known method.

[0031] (Secondary battery) The present invention provides a secondary battery including an electrode or an electrode composite of the present invention. The secondary battery may be a general lithium-ion secondary battery or an all-solid-state secondary battery. The electrode of the present invention can be used for either the positive electrode or the negative electrode. When the electrode of the present invention is used as the positive electrode, during charge and discharge, if Li can act as a mobile ion and exchange between the positive electrode and the negative electrode, it is not particularly limited and it may be used in combination with a known negative electrode. As the known negative electrode, those with a low redox potential are preferred, and those having an average charge-discharge potential of 0.7 V or less with respect to the redox potential of Li are more preferred. Also, when the electrode of the present invention is used as the negative electrode, during charge and discharge, if Li can act as a mobile ion and exchange between the positive electrode and the negative electrode, it is not particularly limited and it may be used in combination with a known positive electrode. As the known positive electrode, those with a high redox potential are preferred, and those having an average charge-discharge potential of 3.5 V or more with respect to the redox potential of Li are more preferred. The positive electrode active materials used in the known positive electrode and negative electrode are composed of the above-mentioned known positive electrode active materials and the like. The known positive electrode and negative electrode may consist only of the electrode active material, or may be mixed with a binder, a conductive material, an electrolyte, etc.

[0032] The electrolyte layer used in the secondary battery can be roughly classified into a type mainly composed of an electrolytic solution and a type composed of a solid electrolyte.

[0033] (1) Non-aqueous electrolyte layer The non-aqueous electrolyte layer used in the present invention can be composed of a mixture of a known electrolyte and a non-aqueous solvent. Examples of the electrolyte include LiClO4, LiPF6, LiBF4, LiCF3SO3, LiAsF6, LiB(C6H5)4, LiCl, LiBr, CH3SO3Li, CF3SO3Li, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, or LiN(SO3CF3)2, etc. The non-aqueous solvent is not particularly limited, and examples thereof include carbonates, ethers, ketones, sulfolane-based compounds, lactones, nitriles, chlorinated hydrocarbons, amines, esters, amides, phosphate ester compounds, etc. Representative examples of these include 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, ethylene carbonate, vinylene carbonate, methyl formate, dimethyl sulfoxide, propylene carbonate, acetonitrile, γ-butyrolactone, dimethylformamide, dimethyl carbonate, diethyl carbonate, sulfolane, ethyl methyl carbonate, 1,4-dioxane, 4-methyl-2-pentanone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methyl sulfolane, propionitrile, benzonitrile, butyronitrile, valeronitrile, 1,2-dichloroethane, trimethyl phosphate, triethyl phosphate, etc. These can be used alone or in combination of two or more.

[0034] (2) Solid electrolyte layer The solid electrolyte constituting the solid electrolyte layer is not particularly limited, and known ones that can be used in all-solid-state secondary batteries can be used. The solid electrolyte is composed of, for example, a sulfide-based solid electrolyte material or an oxide-based solid electrolyte material. Examples of the sulfide-based solid electrolyte material include Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-LiI-LiBr, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-GeS2, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, Li 10 GeP2S 12 、Li7P3S 11 、Li3PS4, or Li 3.25 P 0.75Examples include S4. These sulfide-based solid electrolyte materials may be used individually or in combination of two or more. Examples of oxide-based solid electrolyte materials include, for example, Li2O-B2O3-P2O3, Li2O-SiO2, Li2O-P2O5, Li5La3Ta2O 12 , Li7La3Zr2O 12 , Li6BaLa2Ta2O 12 , Li 3.6 SI 0.6 P 0.4 O4, or Li3BO3-Li2SO4-Li2CO3, etc. These oxide-based solid electrolyte materials may be used individually or in combination of two or more.

[0035] In addition to the above solid electrolyte materials, the solid electrolyte layer may contain other components used in all-solid-state secondary batteries. For example, metal oxides such as P, As, Ti, Fe, Zn, or Bi, and binder materials such as polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyvinyl acetate, polymethyl methacrylate, or polyethylene. The solid electrolyte may be in a glassy state or a glass-ceramic state. Here, the glassy state means a substantially amorphous state. Here, substantially includes not only a 100% amorphous state but also a case where crystalline solid electrolytes are finely dispersed. The glass-ceramic state means a state produced by heating a glassy solid electrolyte at a temperature above the glass transition point. The glass-ceramic solid electrolyte may be in a state where crystalline parts are dispersed in an amorphous glass component. The ratio of the crystalline parts can be measured by transmission electron microscope observation, crystal structure analysis by the Rietveld method, etc. Furthermore, the glass-ceramic solid electrolyte may not have the glass transition point that existed in the corresponding glassy solid electrolyte.

[0036] The solid electrolyte can be made into a solid electrolyte layer, for example, by pressing it to a predetermined thickness. The pressure for pressing may be selected from pressures in the range of 50 - 2000 MPa.

[0037] The method for manufacturing the solid electrolyte layer is not particularly limited as long as it is a method capable of mixing the materials of the solid electrolyte. As the materials of the solid electrolyte, the materials of the solid electrolyte described above can be used. From the viewpoint of mixing each component more uniformly, mechanochemical treatment is preferable as a method capable of mixing. The basic elements regarding the mechanochemical treatment are the same as those described above. The treatment conditions can be appropriately set according to the treatment apparatus used. For example, when using a ball mill, the higher the rotation speed and / or the longer the treatment time, the more uniformly the raw materials can be mixed. Specifically, when using a planetary ball mill, for example, conditions such as a rotation speed of 50 - 600 revolutions per minute, a treatment time of 0.1 - 100 hours, and 1 - 100 kWh per 1 kg of raw material can be mentioned.

[0038] (Method for manufacturing a secondary battery) The present invention also provides a method for manufacturing a secondary battery using the electrode or electrode composite of the present invention. (I) Lithium-ion secondary battery When manufacturing a lithium-ion secondary battery, for example, a laminate of the positive electrode of the present invention and a negative electrode for a known lithium-ion secondary battery is inserted into a battery can, and a mixture of an electrolyte and a non-aqueous solvent is poured into the battery can to obtain a lithium-ion secondary battery. A separator may be used between the positive electrode and the negative electrode. In this case, it is preferable to use a microporous polymer film. Specifically, a separator made of a polyolefin polymer such as nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, or polybutene can be used. The positive electrode, separator, and negative electrode may be laminated or wound. Instead of the positive electrode and the negative electrode, the electrode composite of the present invention may be used. (II) All-solid-state battery A all-solid-state battery can be obtained, for example, by laminating and pressing a positive electrode of the present invention, a solid electrolyte layer, a known negative electrode, and a current collector, and fixing this in a container. A metal layer selected from Au, Pt, In, Al, Sn, Si, etc. may be provided between the negative electrode and the solid electrolyte layer. Further, the metal layer may be provided between the positive electrode and the solid electrolyte layer. The thickness of the metal layer is preferably 10 nm - 100 μm.

Examples

[0039] Hereinafter, the present invention will be described more specifically by way of examples and comparative examples, but the present invention is not limited thereto. In the following examples and comparative examples, Li2S was manufactured by Mitsuwa Chemical Co., Ltd. (purity > 99.9%), Cu was manufactured by Fujifilm Wako Pure Chemical Industries, S was manufactured by Aldrich (99.998%), ZnS was manufactured by Fujifilm Wako Pure Chemical Industries (95+%), and LiCl was manufactured by Aldrich (99.998%). Also, in the following examples and comparative examples, a Rigaku fully automatic multi-purpose X-ray diffractometer SmartLab was used as the X-ray diffractometer. A Nagano charge-discharge measurement device (BTS-2004) was used for the measurement of constant current charge-discharge measurement. A Toei Technica potentiostat (SI-1287) was used for the DC polarization measurement. As the scanning electron microscope (SEM), a JSM-5300 manufactured by JEOL Ltd. was used, and as the energy dispersive X-ray analysis (EDS), a JED-2300 manufactured by JEOL Ltd. was used.

[0040] Example 1 (Preparation of solid solution) A solid solution of one embodiment of the present invention was prepared by the following procedure. Li2S, Cu, and S8 were placed in a mortar in a molar ratio of 3:2:1 and mixed. This was put into a zirconia pot (45 mL) together with zirconia balls (4 mm in diameter, 500 pieces), and a solid solution was prepared by performing mechanochemical treatment using a planetary ball mill device PULVERISETTE 7 (P-7) manufactured by Fritsch. The conditions for the mechanochemical treatment were a base rotation speed of 370 rpm and a treatment time of 90 - 100 hours. No pretreatment of the raw materials was carried out other than mixing in the mortar. The charging into the pot was carried out in a glove box under an argon atmosphere. Since lithium salts are chemically unstable, they react with moisture, oxygen, etc. in the glove box. Therefore, the environmental management of the glove box was strictly carried out, and the work was carried out in an environment where the moisture concentration was 0.2 ppm or less and the oxygen concentration was 5 ppm or less. As a result, a solid solution of Li3CuS2 was prepared.

[0041] Example 2 (Preparation of a mixture containing a solid electrolyte) To the solid solution of Li3CuS2 in Example 1 above, Li3PS4 glass solid electrolyte powder (LPS) was further mixed. This LPS was prepared as follows.

[0042] (Preparation of Li3PS4 glass solid electrolyte powder) Commercially available single Li3PS4 was put into a zirconia pot (45 mL) together with zirconia balls (4 mm in diameter, 500 pieces), and LPS was prepared by performing mechanochemical treatment using the above planetary ball mill device P-7. The conditions for the mechanochemical treatment were a base rotation speed of 510 rpm and a treatment time of 40 hours.

[0043] The LPS prepared by the above method was mixed with the Li3CuS2 solid solution using a mortar. This mixture was subjected to mechanochemical treatment using the above planetary ball mill device P-7 to prepare a Li3CuS2 - LPS mixture. The conditions for the mechanochemical treatment were a base rotation speed of 160 rpm and a treatment time of 1 hour. The mixing ratio of Li3CuS2 and LPS was Li3CuS2: LPS wt% = 65:35 wt% (Li3CuS2 - LPS[65:35] mixture).

[0044] Example 3 A Li3CuS2-LPS[50:50] mixture of Example 3 was prepared in the same manner as above, except that no mechanochemical treatment was performed during the mixing of the solid solution of LPS and Li3CuS2 in Example 2, and the mixing ratio was Li3CuS2: LPS wt% = 50:50 wt%.

[0045] (X-ray diffraction measurement of the solid solutions and mixtures of Examples 1 to 3) X-ray diffraction (XRD) measurement was performed to analyze the structures of the prepared solid solutions and mixtures. As the X-ray diffractometer, SmartLab was used, and with CuKα radiation (= 1.54056×10 -10 m), at a tube voltage of 45 kV, a tube current of 200 mA, a scanning angle of 2θ = 10° to 80°, a sampling interval of 0.02°, and a scanning speed of 10° min -1 structural analysis was carried out. Since the prepared solid solutions and mixtures are unstable in air, all the operations of packing the sample onto a slide glass and enclosing the sample in an airtight sample stage were performed inside a glove box under an argon atmosphere. For comparison, XRD measurements were also performed on Li2S, Cu, and S8 samples in the same manner. The XRD patterns of the respective solid solutions and mixtures are shown in Fig. 2. Fig. 2 shows, in addition to the measurement data of the above-mentioned respective samples, the predicted pattern of Li3CuS2 with an inverse fluorite structure. As shown in Fig. 2, the XRD pattern of the Li3CuS2 solid solution of Example 1 prepared was in agreement with the predicted pattern of the solid solution of Li3CuS2. Therefore, it was shown that the prepared solid solution of Li3CuS2 has an inverse fluorite structure. Also, when comparing the XRD patterns of the Li3CuS2-LPS mixtures of Examples 2 and 3 with the XRD pattern of the Li3CuS2 solid solution of Example 1, no clear difference was observed in the XRD patterns. From this, it was shown that no side reaction occurred between Li3CuS2 and LPS in the mixing and mechanochemical treatment of the solid solution of Li3CuS2 and LPS.

[0046] (Calculation of the lattice constant of the solid solution of Li3CuS2) The XRD pattern of the solid solution of Li3CuS2 in Example 1 above and the space group of Li2S Based on JPEG0007693226000002.jpg6142 and the lattice constant (a = 5.76 Å) of Li2S, the lattice constant of Li3CuS2 was calculated using Fox software based on the real space method. As a result, the lattice constant of the solid solution of Li3CuS2 was found to be a = 5.706 Å, which is smaller than that of Li2S. This indicates that Cu2S was dissolved in Li2S to form a solid solution, resulting in a reduction in the lattice constant. The formation of the solid solution shows that when Li and Cu are in an arbitrary composition ratio, a substance with a continuously changing structure can be obtained according to the composition ratio.

[0047] Example 4 A solid solution represented by Li7CuS4 was prepared in the same manner as in Example 1, except that the molar ratio of Li2S, Cu, and S8 in Example 1 was 7:2:1.

[0048] Example 5 A solid solution represented by Li5CuS3 was prepared in the same manner as in Example 1, except that the molar ratio of Li2S, Cu, and S8 in Example 1 was 5:2:1.

[0049] The results of X-ray diffraction measurements similar to the above X-ray diffraction measurements on the solid solutions of Examples 4 and 5 are shown in Figure 3. Figure 3 also shows the predicted patterns of Li7CuS4 and Li5CuS3 with an inverse fluorite structure in addition to the measurement data of each solid solution above. As shown in Fig. 3, the XRD patterns of the solid solutions of Li7CuS4 of Example 4 and Li5CuS3 of Example 5 prepared were consistent with the predicted patterns. Also, when comparing the XRD patterns of the solid solutions of Li7CuS4 of Example 4 and Li5CuS3 of Example 5 with the XRD pattern of the solid solution of Li3CuS2 of Example 1, no clear difference was observed in the XRD patterns. Therefore, it was shown that the prepared solid solutions of Li7CuS4 and Li5CuS3 also have an inverse fluorite structure. The lattice constant a of the solid solution of Li7CuS4 was 5.711 Å. The lattice constant a of the solid solution of Li5CuS3 was 5.725 Å. As described above, it was found that Li and Cu can form a solid solution in any composition ratio.

[0050] (Preparation of Pellets of Li3CuS2 Solid Solution) Using the Li3CuS2 solid solution obtained in Example 1 as an electrode active material, pellets for conductivity measurement were prepared. The pellets for electronic conductivity measurement were obtained by uniaxially pressing 20 mg of the above solid solution at room temperature (25 °C) and 360 MPa. The pellets for ionic conductivity measurement were prepared by sandwiching both sides of 43 mg of the above solid solution with 40 mg (total 80 mg) of Li3PS4 glass powder, performing uniaxial pressing at room temperature (about 25 °C) and 360 MPa, and further attaching a lithium-indium alloy to both sides.

[0051] (Measurement of Conductivity of Pellets of Li3CuS2 Solid Solution) Using the prepared pellets, the electronic conductivity and ionic conductivity were measured. The measurement was performed using an electrochemical measurement cell in which the above pellets were sandwiched between current collectors made of SUS and a potentiostat SI-1287. The measurement results are shown in Table 1 below.

[0052]

Table 1

[0053] (Fabrication of All-Solid-State Secondary Battery Using Li3CuS2 Pellets) Using the fabricated pellets as electrodes, an all-solid-state secondary battery was fabricated. The fabrication method is as follows. Approximately 5 mg of Li3CuS2 pellets obtained by the above pellet fabrication process was used as the positive electrode, and 80 mg of Li3PS4 glass solid electrolyte powder fabricated by the above method for fabricating Li3PS4 glass solid electrolyte powder was pressed at a pressure of 360 MPa to obtain a two-layer pellet of a positive electrode layer and a solid electrolyte layer (SE layer) with a diameter of 10 mm and a thickness of about 0.7 mm. As the negative electrode, a Li-In alloy was laminated, sandwiched between current collectors made of stainless steel, and pressed again at a pressure of 120 MPa to obtain a Li3CuS2 all-solid-state secondary battery.

[0054] (Measurement of Charge and Discharge Capacity) Using the obtained Li3CuS2 all-solid-state secondary battery, the charge and discharge capacity was measured. The measurement conditions were a current density of 0.13 mA / cm 2 at 25°C. One cycle of charge and discharge consists of performing discharge and charge once each, and in this experiment, it was performed twice. The obtained measurement results are shown in Fig. 4. As shown in Fig. 4, it can be seen that the solid solution of the present invention can be charged and discharged even without including a solid electrolyte.

[0055] Instead of the solid solution of Example 1, a Li3CuS2-LPS[65:35] mixture of Example 2 was used as the electrode active material to fabricate a Li3CuS2-LPS(65:35) all-solid-state secondary battery, and the charge and discharge capacity was measured. The measurement conditions were also the same. Charge and discharge were performed for 3 cycles. The obtained measurement results are shown in Fig. 5. Further, using the Li3CuS2-LPS[50:50] mixture of Example 3 as the electrode active material, a Li3CuS2-LPS(50:50) all-solid-state secondary battery was fabricated, and experiments were conducted in the same manner. The charge and discharge capacities of these were recalculated to the charge and discharge capacity per weight of the Li3CuS2 solid solution in the Li3CuS2-LPS[50:50] mixture and are shown in Fig. 6. Li3CuS2: 50wt% in Fig. 6 shows the results of the Li3CuS2-LPS(50:50) all-solid-state secondary battery of Example 3, and Li3CuS2: 65wt% shows the results of the Li3CuS2-LPS(65:35) all-solid-state secondary battery. As shown in Fig. 5, the charge and discharge capacity was improved by including a solid electrolyte in the solid solution. When this value was converted as shown in Fig. 6, the charge and discharge capacity of the mixture was recalculated to the charge and discharge capacity per weight of the Li3CuS2 solid solution contained in the mixture, and 300 mAh g -1 It was found that the above high charge and discharge capacity was obtained. From the above, it was found that the solid solution of the present invention is excellent as a novel electrode material.

[0056] (Cycle test of charge and discharge capacity) Example 6 A Li3CuS2-LPS[70:30] mixture of Example 6 was prepared in the same manner as in Example 2, except that the mixing ratio of the Li3CuS2 solid solution and LPS in Example 2 was Li3CuS2: LPS wt% = 70:30 wt%. Using this mixture, a Li3CuS2-LPS(70:30) all-solid-state secondary battery was fabricated in the same manner as the method for fabricating the above Li3CuS2 all-solid-state secondary battery. Using this Li3CuS2-LPS(70:30) all-solid-state secondary battery, charge and discharge were repeated 18 cycles to measure the charge and discharge capacity. The cut-off voltage during measurement was set to 1.9 V. The measurement results are shown in Fig. 7A, and the cycle characteristics are shown in Fig. 7B. As shown in Fig. 7A, the fabricated Li3CuS2-LPS(70:30) all-solid-state secondary battery shows a charge and discharge capacity of 250 mAh g -1 or more. Also, as shown in Fig. 7B, it was found that the charge and discharge capacity was stable even when the cycles were repeated.

[0057] (Mixing test of conductive assistant) Example 7 In Example 2, except that acetylene black (AB) was further mixed as a conductive additive at a ratio of Li3CuS2: LPS: AB wt% = 65: 35: 6 wt%, the Li3CuS2-LPS-AB mixture of Example 7 was prepared in the same manner as in Example 2. Using this mixture as an electrode active material, a Li3CuS2-LPS-AB all-solid-state secondary battery was prepared in the same manner as the method for preparing the above Li3CuS2 all-solid-state secondary battery. Using this Li3CuS2-LPS-AB all-solid-state secondary battery, the above charge-discharge capacity cycle test was measured in the same manner except that the number of cycles was set to 20. The measurement results are shown in Fig. 8A, and the cycle characteristics are shown in Fig. 8B, respectively. As shown in Fig. 8A, an excellent charge-discharge capacity was obtained in the secondary battery with the conductive additive added. Also, as shown in Fig. 8B, the charge-discharge capacity did not change even after repeated cycling.

[0058] Example 8 (Preparation of Li3CuS2 solid solution with ZnS substitution) In addition to Li2S, Cu and S8, ZnS was mixed in the same manner as in Example 1 except that the molar ratio was Li2S:Cu:S:ZnS = 29:18:9:2 to prepare the Li 2.9 Cu 0.9 Zn 0.1 3CuS2 solid solution. Using this solid solution as an electrode active material, pellets were prepared in the same manner as the above pellet preparation method. For this pellet, an XRD pattern and electron conductivity were measured, and the lattice constant was calculated.

[0059] Example 9 (Preparation of Li3CuS2 solid solution with LiCl substitution) In addition to Li2S, Cu and S8, LiCl was mixed in the same manner as in Example 1 except that the molar ratio was Li2S:Cu:S:LiCl = 13:10:5:2 to prepare the Li 2.8 CuS 1.8 Cl 0.2A solid solution was prepared. Using this solid solution as an electrode active material, pellets were prepared in the same manner as the method for preparing the above pellets. For these pellets, an XRD pattern and electronic conductivity were measured, and the lattice constant was calculated.

[0060] Example 10 Example 10 Li was prepared in the same manner as Example 9, except that the molar ratio of LiCl in Example 9 was mixed so that Li2S:Cu:S:LiCl = 14:10:5:1. 2.9 CuS 1.9 Cl 0.1 A solid solution was prepared. Using this solid solution as an electrode active material, pellets were prepared in the same manner as the method for preparing the above pellets. For these pellets, an XRD pattern and electronic conductivity were measured.

[0061] The XRD patterns obtained from the solid solutions of Examples 8 to 10 are shown in Fig. 9. From Fig. 9, it was found that the same pattern as that of the Li3CuS2 solid solution was observed for all the solid solutions, and the inverse fluorite structure was maintained. Li of Example 8 2.9 Cu 0.9 Zn 0.1 The lattice constant a of the S2 solid solution was 5.674 Å. Li of Example 9 2.8 CuS 1.8 Cl 0.2 The lattice constant a of the solid solution was 5.720 Å. As described above, it was found that by substituting a part of Li3CuS2 having a lattice constant of 5.706 Å with ZnS, the lattice constant decreased, and by substituting a part of Li3CuS2 with LiCl, an increase in the lattice constant occurred. It was found that the lattice constant can be changed by substituting a part of the Li2S-Cu2S-based solid solution of the present invention. From these, it was found that a part of the Li2S-Cu2S-based solid solution can be substituted with various types of cations and anions.

[0062] The measurement results of the electronic conductivity obtained from the solid solutions of Examples 8 and 9 are shown in Table 2.

[0063] [Table 2] From Table 2, it was found that the solid solutions of Examples 8 and 9 both exhibited higher electronic conductivity than the Li3CuS2 solid solution.

[0064] For the pellets of Examples 8 and 9, Li 2.9 Cu 0.9 Zn 0.1 S2 all-solid-state secondary batteries and Li 2.8 CuS 1.8 Cl 0.2 all-solid-state secondary batteries were fabricated in the same manner as the fabrication method of the above Li3CuS2 all-solid-state secondary battery. The charge-discharge capacity was measured using these all-solid-state secondary batteries. The charge-discharge was performed for 5 cycles. The respective measurement results are shown in FIGS. 10 and 11. As shown in FIG. 10, the Li 2.9 Cu 0.9 Zn 0.1 S2 all-solid-state secondary battery is charge-dischargeable. As shown in FIG. 11, it was found that the Li 2.8 CuS 1.8 Cl 0.2 all-solid-state secondary battery exhibited a charge-discharge capacity superior to that of the Li3CuS2 all-solid-state secondary battery.

[0065] Example 11 A Li 2.9 Cu 0.9 Zn 0.1 S2 solid solution was used, and in the same manner as in Example 2, a Li 2.9 Cu 0.9 Zn 0.1 S2-LPS mixture of Example 11 was fabricated.

[0066] Example 12 Li 2.8 CuS 1.8 Cl 0.2 A Li 2.8 CuS 1.8 Cl 0.2 -LPS mixture of Example 12 was fabricated in the same manner as in Example 2, except that the solid solution and LPS were mixed at a molar ratio (Li 2.8 CuS 1.8 Cl 0.2 : LPS = 70:30).

[0067] Example 13 Li 2.9 CuS 1.9 Cl 0.1 A solid solution and LPS were mixed at a molar ratio (Li 2.9 CuS 1.9 Cl 0.1 : LPS = 70:30), and in the same manner as in Example 2, an Li 2.9 CuS 1.9 Cl 0.1 -LPS mixture was prepared.

[0068] Using the mixtures of Examples 11 to 13 as electrode active materials, three types of all-solid-state secondary batteries (Li 2.9 Cu 0.9 Zn 0.1 S2-LPS all-solid-state secondary battery, Li 2.8 CuS 1.8 Cl 0.2 -LPS all-solid-state secondary battery, and Li 2.9 CuS 1.9 Cl 0.1 -LPS all-solid-state secondary battery) were fabricated. The charge-discharge capacity was measured using these all-solid-state secondary batteries. Charging and discharging were performed for 5 cycles in the Li 2.9 Cu 0.9 Zn 0.1 S2-LPS all-solid-state secondary battery of Example 11, 14 cycles in the Li 2.8 CuS 1.8 Cl 0.2 -LPS all-solid-state secondary battery of Example 12, and 13 cycles in the Li 2.9 CuS 1.9 Cl 0.1 -LPS all-solid-state secondary battery of Example 13. The cut-off value was 2.0 V in the Li 2.9 Cu 0.9 Zn 0.1 S2-LPS all-solid-state secondary battery of Example 11, and 1.9 V in the Li 2.8 CuS 1.8 Cl 0.2 -LPS all-solid-state secondary battery of Example 12 and the Li 2.9 CuS 1.9 Cl 0.1 -LPS all-solid-state secondary battery of Example 13. These results are shown in FIGS. 12 to 14. From these results, it was found that each all-solid-state secondary battery using the mixtures of Examples 11 to 13 exhibited excellent charge-discharge capacity and the capacity was stable even when charge-discharge was repeated.

[0069] (SEM-EDS Measurement of Li3CuS2 Solid Solution and Li3CuS2-LPS Mixture) The solid solution of Li3CuS2 and the Li3CuS2-LPS [70:30] mixture prepared in Example 1 and Example 6 were subjected to SEM-EDS.

[0070] The results of measuring the Li3CuS2 solid solution at an acceleration voltage of 15 kV and a magnification of 3000 times are shown in FIGS. 15A to C, and the results of measuring at an acceleration voltage of 15 kV and a magnification of 10000 times are shown in FIGS. 16A to C, respectively. FIGS. 15A and 16A are SEM images, and FIGS. 15B or C and FIGS. 16B or C are the results of mapping the images of FIGS. 15A and 16A to the elements of Cu and S, respectively. As shown in FIGS. 15A to C and FIGS. 16A to C, it was found that in the solid solution of Li3CuS2, the elements of Cu and S were uniformly dispersed without any bias.

[0071] Similar to the Li3CuS2 solid solution, the results of measuring the Li3CuS2-LPS [70:30] mixture at an acceleration voltage of 15 kV and a magnification of 3000 times are shown in FIGS. 17A to D, and the results of measuring at an acceleration voltage of 15 kV and a magnification of 10000 times are shown in FIGS. 18A to D, respectively. FIGS. 17A and 18A are SEM images, and FIGS. 17B to D and FIGS. 18B to D are the results of mapping the images of FIGS. 17A and 18A to the elements of Cu, S, and P, respectively. As shown in FIGS. 17A to C and FIGS. 18A to C, it was found that in the Li3CuS2-LPS [70:30] mixture, the elements of Cu and S were uniformly dispersed without any bias. On the other hand, it was found that P derived from the solid electrolyte was unevenly distributed in the mixture.

[0072] Examples of the results of observing the elemental composition at each measurement point during the SEM-EDS measurement of this Li3CuS2-LPS [70:30] mixture are shown in FIGS. 19A to D. From A to D in Fig. 19, it was found that the elemental distributions in the Li3CuS2-LPS[70:30] mixture showed almost the same shape, indicating that this mixture showed the same crystal structure as a whole.

[0073] (Cycle test of charge-discharge capacity 2) Example 14 A Li3CuS2-LPS[70:30] mixture of Example 14 was prepared in the same manner as in Example 2, except that the mixing ratio of the Li3CuS2 solid solution and LPS in Example 2 was Li3CuS2: LPS wt% = 70:30 wt%. Using this mixture, a Li3CuS2-LPS(70:30) all-solid-state secondary battery was prepared in the same manner as the manufacturing method of the above Li3CuS2 all-solid-state secondary battery. Using this Li3CuS2-LPS(70:30) all-solid-state secondary battery, charge and discharge were repeated 30 cycles to measure the charge-discharge capacity. The cut-off voltage during measurement was set to 3.0V. The measurement results are shown in Fig. 20A, and the cycle characteristics are shown in Fig. 20B, respectively. As shown in Fig. 20A, it can be seen that the prepared Li3CuS2-LPS(70:30) all-solid-state secondary battery shows a high charge-discharge capacity of 350 mAh g -1 as described above. Also, as shown in Fig. 20B, it was found that the charge-discharge capacity was stable even when the cycles were repeated.

[0074] Example 15 A solid solution represented by Li8Cu2S5 was prepared in the same manner as in Example 1, except that the molar ratio of Li2S, Cu, and S8 in Example 1 was 4:2:1.

[0075] Example 16 Example 16's Li8Cu2S5-LPS[70:30] mixture was prepared in the same manner as Example 2, except that the mixing ratio of the Li8Cu2S5 solid solution and LPS in Example 15 was set to Li8Cu2S5: LPS wt% = 70:30 wt%. Using this mixture, a Li8Cu2S5-LPS(70:30) all-solid-state secondary battery was fabricated in the same way as the method for fabricating the above Li3CuS2 all-solid-state secondary battery. Using this Li8Cu2S5-LPS(70:30) all-solid-state secondary battery, charge and discharge were repeated 30 cycles to measure the charge and discharge capacity. The cut-off voltage during measurement was set to 3.0 V. The measurement results are shown in Fig. 21A, and the cycle characteristics are shown in Fig. 21B, respectively. As shown in Fig. 21A, it can be seen that the fabricated Li8Cu2S5-LPS(70:30) all-solid-state secondary battery exhibits a high charge and discharge capacity of 350 mAh g -1 or more. Also, as shown in Fig. 21B, it was found that the charge and discharge capacity remained stable even when the cycles were repeated.

[0076] Example 17 Example 17's Li5CuS3-LPS[70:30] mixture was prepared in the same manner as Example 2, except that the mixing ratio of the Li5CuS3 solid solution and LPS in Example 5 was set to Li5CuS3: LPS wt% = 70:30 wt%. Using this mixture, a Li5CuS3-LPS(70:30) all-solid-state secondary battery was fabricated in the same way as the method for fabricating the above Li3CuS2 all-solid-state secondary battery. Using this Li5CuS3-LPS(70:30) all-solid-state secondary battery, charge and discharge were repeated 18 cycles to measure the charge and discharge capacity. The cut-off voltage during measurement was set to 3.0 V. The measurement results are shown in Fig. 22A, and the cycle characteristics are shown in Fig. 22B, respectively. As shown in Fig. 22A, it can be seen that the fabricated Li5CuS3-LPS(70:30) all-solid-state secondary battery exhibits a high charge and discharge capacity of 350 mAh g -1 or more. Also, as shown in Fig. 22B, it was found that the charge and discharge capacity remained stable even when the cycles were repeated.

[0077] Example 18 A Li7CuS4-LPS[70:30] mixture of Example 18 was prepared in the same manner as in Example 2, except that the mixing ratio of the Li7CuS4 solid solution and LPS in Example 4 was Li7CuS4: LPS wt% = 70:30 wt%. Using this mixture, a Li7CuS4-LPS(70:30) all-solid-state secondary battery was fabricated in the same manner as the fabrication method of the above Li3CuS2 all-solid-state secondary battery. Using this Li7CuS4-LPS(70:30) all-solid-state secondary battery, charge and discharge were repeated 5 cycles to measure the charge and discharge capacity.

[0078] The theoretical capacity, first charge capacity, and fifth discharge capacity of the Li3CuS2-LPS(70:30) all-solid-state secondary battery of Example 14, the Li8Cu2S5-LPS(70:30) all-solid-state secondary battery of Example 16, the Li5CuS3-LPS(70:30) all-solid-state secondary battery of Example 17, and the Li7CuS4-LPS(70:30) all-solid-state secondary battery of Example 18 are shown in Table 3 below. From Table 3, it was shown that the fabricated all-solid-state secondary battery had excellent charge and discharge capacity.

[0079]

Table 3

[0080] (Preparation of Li5CuS3 solid solution with substitution of LiI) A Li5CuS3 solid solution of Example 19 was prepared in the same manner as in Example 1, except that in addition to Li2S, Cu, and S8, LiI was mixed so that the molar ratio was Li2S:Cu:S:LiI = 24:10:5:1. 4.9 CuS 2.9 I 0.1 solid solution was prepared.

[0081] Li of Example 19 4.9 CuS 2.9 I 0.1 The mixing ratio of the solid solution and LPS was Li 4.9 CuS 2.9 I 0.1 : LPS wt% = 70:30 wt% and in the same manner as in Example 2, Li of Example 20 4.9 CuS 2.9 I0.1 - An LPS[70:30] mixture was prepared, and using this mixture, Li was prepared in the same manner as the method for manufacturing the above Li3CuS2 all-solid-state secondary battery. 4.9 CuS 2.9 I 0.1 - An LPS(70:30) all-solid-state secondary battery was manufactured.

[0082] The Li of Example 19 4.9 CuS 2.9 I 0.1 The solid solution and the Li of Example 20 4.9 CuS 2.9 I 0.1 - The XRD pattern obtained from the LPS[70:30] mixture is shown in Fig. 23. Also, using the Li 4.9 CuS 2.9 I 0.1 (70:30) all-solid-state secondary battery, charge and discharge were repeated 7 cycles to measure the charge and discharge capacity. The results are shown in Fig. 24. From Fig. 24, it can be seen that the manufactured Li 4.9 CuS 2.9 I 0.1 - LPS(70:30) all-solid-state secondary battery shows a high charge and discharge capacity of 350 mAh g -1 or more.

[0083] From the above, it was shown that the solid solution containing Li, Cu, and S as main constituent components of the present invention has an inverse fluorite-type crystal structure and can be used as an electrode for a secondary battery.

Claims

1. A solid solution having an inverse fluorite crystal structure containing Li, Cu, and S as main constituent components, wherein the solid solution has the following formula (1) Li 4-x-y-z Cu x-y M y S 2 -z A z (1) (In the formula, M is a divalent metal cation selected from Mg, Ca, Ni, Zn, Fe, Co, Sn, Pb, or Mn, A is a monovalent anion selected from F, Cl, Br, or I, x is 0.1 or more and 2.0 or less, y is 0 or more and less than 1.0, z is 0 or more and 1.0 or less, and x > y. However, the case where both y and z are 0 is excluded) The solid solution represented by

2. The solid solution according to claim 1, wherein in the formula (1), x is in the range of 0.5 to 1.

0.

3. The solid solution according to claim 1 or 2, wherein in the formula (1), y is in the range of 0 to 0.

5.

4. The solid solution according to any one of claims 1 to 3, wherein in the formula (1), z is in the range of 0 to 0.

5.

5. The solid solution according to any one of claims 1 to 4, wherein in the formula (1), x is in the range of 0.5 to 1.0, and y = 0 and z = 0.

6. The solid solution according to any one of claims 1 to 5, wherein the lattice constant of the a-axis of the solid solution is in the range of 5.60 Å to 5.80 Å.

7. The electronic conductivity of the solid solution is 1.0×10 -6 S cm -1 or more, and the solid solution according to any one of claims 1 to 6.

8. An electrode active material containing the solid solution according to any one of claims 1 to 7.

9. The electrode active material according to claim 8, further comprising a solid electrolyte.

10. An electrode comprising the solid solution according to any one of claims 1 to 7 or the electrode active material according to claim 8 or 9.

11. An electrode composite in which the electrode according to claim 10 is combined with a current collector.

12. A secondary battery comprising the electrode according to claim 10 or the electrode composite according to claim 11.

13. The secondary battery according to claim 12, wherein the secondary battery is an all-solid-state secondary battery.

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