Li-MP-based composition, solid electrolyte and method for producing the same

Abstract

The present invention provides a new solid electrolyte suitable as a protective material for positive electrode active materials, which has excellent properties (decomposition resistance) that make it difficult for a reaction to proceed with the sulfide-based solid electrolyte separator when a charging voltage higher than 4.45 V is applied in an all-solid-state battery. [Solution] The present invention provides a Li-MP-based solid electrolyte whose main component is an amorphous oxide, containing Li, M, and P in the following composition ranges: the Li / (Li+M+P) molar ratio is 0.300 or more and 0.650 or less, the M / (Li+M+P) molar ratio is 0.050 or more and 0.200 or less, and the P / (Li+M+P) molar ratio is 0.250 or more and 0.600 or less, where M represents one or more elements selected from W and Zr, and the ratio of the amount (moles) of element X to the total amount (moles) of Li, M, and P is represented as the X / (Li+M+P) molar ratio.

Description

[Technical Field]

[0001] The present invention relates to a Li-MP-based composition useful for forming a solid electrolyte that constitutes a portion in contact with a positive electrode active material of an all-solid-state lithium-ion secondary battery, and to an amorphous Li-MP-based solid electrolyte obtained from the composition, as well as to methods for producing the Li-MP-based composition and the solid electrolyte. [Background technology]

[0002] In all-solid-state lithium-ion secondary batteries (hereinafter sometimes referred to as "all-solid-state batteries"), a solid electrolyte that functions as a separator is placed between the positive electrode active material and the negative electrode active material, and lithium ions are conducted between the two active materials via the solid electrolyte. Various solid electrolytes have been developed, including sulfide-based, oxide-based, and polymer-based ones, but sulfide-based ones are currently considered to be advantageous in terms of ionic conductivity and manufacturing costs.

[0003] All-solid-state lithium-ion secondary batteries have a problem in that the interfacial resistance between the positive electrode active material and the solid electrolyte separator increases, which can lead to a decrease in performance such as battery capacity. This increase in interfacial resistance is mainly caused by the reaction between the positive electrode active material and the solid electrolyte, which forms high-resistance areas on the surface of the positive electrode active material, and is likely to become a problem when a sulfide-based solid electrolyte is used as the separator.

[0004] Therefore, attempts have been made to prevent the positive electrode active material from reacting with the solid electrolyte, which is the separator, by coating the surface of the positive electrode active material with a solid electrolyte made of a lithium ion conductive oxide (lithium niobate LiNbO3 is a well-known representative example) that functions as a protective material. For example, Patent Document 1 discloses a method in which an aqueous lithium salt solution, fine particles of niobium oxide or niobium hydroxide, and positive electrode active material particles are mixed to form a slurry, and a positive electrode active material coated with lithium niobate is obtained from the slurry by evaporation to dryness or spray drying.

[0005] On the other hand, Patent Document 2 describes a compound of the general formula Li as a protective material for covering the surface of a positive electrode active material.x PT y O z The paper discloses a lithium conductive inorganic compound represented by the formula: Several metals are said to be usable as the element T, among which W and Zr are mentioned. This type of compound is said to have good resistance to water that is inevitably contained in electrolytes (mainly electrolytic solutions), and is useful for maintaining excellent battery performance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-67474 [Patent Document 2] WO2006 / 073104 publication Summary of the Invention [Problem to be solved by the invention]

[0007] From the viewpoint of energy density, the higher the charging voltage of a battery, the better. However, excessively high charging voltage can cause the solid electrolyte, which serves as a protective material for the positive electrode active material, to react with the sulfide-based solid electrolyte, which serves as the separator, and decompose. When the protective material decomposes, the positive electrode active material comes into contact with the sulfide-based solid electrolyte, resulting in the formation of high-resistance regions. For example, in the case of lithium niobate (LiNbO3) as disclosed in Patent Document 1, it is known that it reacts with the sulfide-based solid electrolyte when the charging voltage exceeds 4.45 V. Therefore, it has generally been difficult to increase the charging voltage above 4.45 V. Furthermore, the technology described in Patent Document 2 does not teach a composition that is effective in improving the charging voltage.

[0008] An object of the present invention is to provide a new solid electrolyte suitable as a protective material for a positive electrode active material, which has excellent properties (hereinafter referred to as "decomposition resistance") that make it difficult for a reaction to proceed with a sulfide-based solid electrolyte, which is a separator, when a charging voltage higher than 4.45 V is applied in an all-solid-state battery. [Means for solving the problem]

[0009] In order to achieve the above object, the present specification discloses the following invention. [1] A Li-MP composition comprising an aqueous liquid or powder containing Li, M, and P in the following composition ranges: Li / (Li+M+P) molar ratio is 0.300 or more and 0.650 or less, M / (Li+M+P) molar ratio is 0.050 or more and 0.200 or less, and P / (Li+M+P) molar ratio is 0.250 or more and 0.600 or less, where M represents one or more elements selected from W and Zr, and the ratio of the amount (moles) of element X to the total amount (moles) of Li, M, and P is represented as the X / (Li+M+P) molar ratio. [2] A Li-MP-based solid electrolyte whose main component is an amorphous oxide, containing Li, M, and P in the following composition ranges: Li / (Li+M+P) molar ratio is 0.300 or more and 0.650 or less, M / (Li+M+P) molar ratio is 0.050 or more and 0.200 or less, and P / (Li+M+P) molar ratio is 0.250 or more and 0.600 or less, where M represents one or more elements selected from W and Zr, and the ratio of the amount (moles) of element X to the total amount (moles) of Li, M, and P is represented as the X / (Li+M+P) molar ratio. [3] An all-solid-state battery having the solid electrolyte according to [2] above between a conductive material on a positive electrode side and a conductive material on a negative electrode side. [4] An all-solid-state battery using particles of a positive electrode active material coated with the solid electrolyte according to [2] above as a positive electrode material.

[0010] [5] A method for producing a Li-MP composition, comprising a step of forming a liquid in which a Li-containing substance, an M-containing substance, and a P-containing substance are dissolved or dispersed in an aqueous liquid medium, such that the Li / (Li+M+P) molar ratio is 0.300 or more and 0.650 or less, the M / (Li+M+P) molar ratio is 0.050 or more and 0.200 or less, and the P / (Li+M+P) molar ratio is 0.250 or more and 0.600 or less (Li-, M-, P-containing liquid formation step), where M represents one or more elements selected from W and Zr, and the ratio of the amount (moles) of element X to the total amount (moles) of Li, M, and P is represented as the X / (Li+M+P) molar ratio. [6] The method for producing a Li-MP composition according to [5] above, further comprising a step of drying the liquid obtained in the Li, M, P-containing liquid formation step to obtain a powder (drying step). [7] The method for producing a Li-MP composition according to the above [6], further comprising a step of pulverizing the powder obtained in the drying step (pulverization step). [8] A method for producing a Li-MP-based solid electrolyte, comprising a step of subjecting the Li-MP-based composition obtained by the production method according to any one of the above [5] to [7] to a heat treatment at a temperature exceeding 100°C and not exceeding 350°C (firing step). [Effects of the Invention]

[0011] The present invention provides a novel solid electrolyte with excellent decomposition resistance. This solid electrolyte contributes to improving the charging voltage of all-solid-state lithium-ion secondary batteries, for example, by using it as a protective material for a positive electrode active material. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an example of an SEM photograph of a powder sample (Li-MP-based solid electrolyte) obtained in Example 6. [Figure 2] EDX (energy dispersive X-ray fluorescence analysis) spectrum for the powder in Figure 1. [Figure 3] FIG. 1 is a diagram illustrating a cross-sectional structure of one embodiment of an all-solid-state battery using a first solid electrolyte made of a Li-MP-based solid electrolyte according to the present invention and a second solid electrolyte containing a sulfide as a main component. [Figure 4] FIG. 1 is a diagram illustrating a cross-sectional structure of one embodiment of an all-solid-state battery using particles of a positive electrode active material coated with a first solid electrolyte made of a Li-MP-based solid electrolyte according to the present invention and a second solid electrolyte containing a sulfide as a main component. [Figure 5] FIG. 1 is a cross-sectional view schematically showing the laminated structure of an electrochemical cell fabricated for evaluating voltage resistance characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0013] [Li-MP solid electrolyte] In this specification, one or more elements selected from W (tungsten) and Zr (zirconium) are represented as M. The ratio of the amount (moles) of element X to the total amount (moles) of Li (lithium), M, and P (phosphorus) is represented as the X / (Li+M+P) molar ratio. Here, element X is either Li, M, or P.

[0014] According to the inventors' research, amorphous solids containing Li, M, and P in the composition ranges of the Li / (Li + M + P) molar ratio of 0.300 to 0.650, the M / (Li + M + P) molar ratio of 0.050 to 0.200, and the P / (Li + M + P) molar ratio of 0.250 to 0.600 are solid electrolytes that exhibit Li-ion conductivity and exhibit significantly superior decomposition resistance compared to lithium niobate, which has traditionally been widely used as a protective material for positive electrode active materials. More preferred composition ranges include the Li / (Li + M + P) molar ratio of 0.350 to 0.600, the M / (Li + M + P) molar ratio of 0.060 to 0.180, and the P / (Li + M + P) molar ratio of 0.300 to 0.540. More preferred composition ranges include a Li / (Li+M+P) molar ratio of 0.360 or more and 0.580 or less, a M / (Li+M+P) molar ratio of 0.060 or more and 0.150 or less, and a P / (Li+M+P) molar ratio of 0.300 or more and 0.520 or less.

[0015] Here, "amorphous solid" refers to a solid substance whose crystallite size, calculated from the maximum diffraction peak (the diffraction peak with the highest peak height) in the 2θ range of 10° to 60° in an X-ray diffraction pattern using CuKα radiation, is 50 nm or less. Crystallite size calculations can be performed using powder X-ray analysis software (e.g., "PDXL2 (Rigaku)") under analysis conditions of a σ cutoff value of 3.00 and refinement ON. Even when no clear diffraction peak is observed in the 2θ range of 10° to 60°, and a so-called halo pattern appears, the crystallite size is considered to be 50 nm or less. A "halo" is a gradual fluctuation in X-ray intensity that is observed as a broad peak in an XRD spectrum, and the half-width of the halo is 2θ° or more.

[0016] Fig. 1 shows an example of an SEM (scanning electron microscope) photograph of a powder sample of the Li-MP-based solid electrolyte obtained in Example 6, which will be described later. In this example, the element M is W. The X-ray diffraction pattern confirmed that this powder was an amorphous solid. FIG. 2 shows an example of an EDX (energy dispersive X-ray fluorescence) spectrum of the powder in FIG. 1. It can be seen that this powder contains O (oxygen) as a constituent element. The Li-MP solid electrolyte according to the present invention can be said to be composed primarily of an amorphous oxide. Here, "composed primarily of an amorphous oxide" means that the Li-MP solid electrolyte is an amorphous solid (described above) and that the total amount of Li, M, and P in the Li-MP solid electrolyte is 80 mass % or more in terms of oxides, as described below.

[0017] Elements other than Li, M (one or more of W and Zr), and P that constitute the Li-MP solid electrolyte according to the present invention include O (oxygen) and N (nitrogen) and C (carbon) derived from the raw materials. The contents of Li, W, Zr, and P in the Li-MP solid electrolyte can be expressed as "oxide-equivalent" contents converted into the contents of Li2O, WO3, ZrO2, and PO5, respectively. The method for determining the oxide-equivalent contents will be described later. The content of Li in the Li-MP solid electrolyte of the present invention, calculated as Li oxide (LiO), is, for example, preferably 5.00% by mass or more and 21.30% by mass or less, more preferably 7.30% by mass or more and 20.00% by mass or less, and even more preferably 7.80% by mass or more and 18.00% by mass or less. The content of W in the Li-MP-based solid electrolyte of the present invention, calculated as W oxide (WO), is, for example, preferably 17.30% by mass or more and 65.00% by mass or less, more preferably 24.00% by mass or more and 57.00% by mass or less, even more preferably 30.00% by mass or more and 55.00% by mass or less, and may be 30.00% by mass or more and 48.50% by mass or less. The content of Zr in the Li-MP-based solid electrolyte of the present invention, calculated as Zr oxide (ZrO), is, for example, preferably 18.60 mass% or more and 66.10 mass% or less, more preferably 20.00 mass% or more and 56.00 mass% or less, and even more preferably 25.00 mass% or more and 40.00 mass% or less. The content of P in the Li-MP-based solid electrolyte of the present invention, calculated as P oxide (P2O5), is, for example, preferably 16.00 mass% or more and 65.50 mass% or less, more preferably 20.00 mass% or more and 62.00 mass% or less, even more preferably 25.00 mass% or more and 55.00 mass% or less, and may be 30.00 mass% or more and 55.00 mass% or less, or 33.50 mass% or more and 55.00 mass% or less.

[0018] The total amount of Li, M, and P in the Li-MP solid electrolyte is preferably 83 mass% or more, and more preferably 85 mass% or more, calculated as oxides. The total amount of Li, M, and P in the Li-MP solid electrolyte may be 100 mass% or 98 mass% or less, calculated as oxides.

[0019] (How to calculate the oxide-equivalent content of Li, M, and P) The respective contents (mass%) of Li, M, and P in the Li-MP solid electrolyte are measured, for example, by ICP (inductively coupled plasma) atomic emission spectroscopy. M is one or both of W and Zr. By regarding Li as a monovalent cation, W as a hexavalent cation, Zr as a tetravalent cation, and P as a pentavalent cation, the contents (mass%) of Li, W, Zr, and P are converted into the contents of Li2O, WO3, ZrO2, and P2O5, respectively, to determine the oxide-equivalent contents.

[0020] For example, in Example 1 described below, the respective contents of Li, M, and P in the Li-MP solid electrolyte were determined by ICP emission spectroscopy to be Li: 5.50 mass%, W: 41.40 mass%, and P: 11.90 mass%. When these were converted into the respective contents of Li2O, WO3, and PO5, the oxide-equivalent contents were calculated as follows: Li oxide content = 5.50 mass% × (formula weight of LiO / 2) / atomic weight of Li = 5.50 mass% × (29.88 / 2) / 6.94 = 11.84 mass% W oxide content = 41.40 mass% × (formula weight of WO3) / atomic weight of W = 41.40 mass% × 231.84 / 183.84 = 52.21 mass% P content converted to oxide = 11.90 mass% × (formula weight of P2O5 / 2) / atomic weight of P = 11.90 mass% × (141.94 / 2) / 30.97 = 27.27 mass% In this case, the total amount of Li, M, and P in the Li-MP solid electrolyte is 11.84 mass % + 52.21 mass % + 27.27 mass % = 91.32 mass % in terms of oxides.

[0021] The Li content (content as elemental Li, not as an oxide) of the Li-MP solid electrolyte of the present invention is, for example, preferably from 2.40 mass% to 13.00 mass%, more preferably from 2.80 mass% to 11.00 mass%, even more preferably from 3.00 mass% to 9.00 mass%, and may be from 3.00 mass% to 8.50 mass%. The M content (not calculated as an oxide but as the total content of W and Zr elements) of the Li-MP-based solid electrolyte of the present invention is, for example, preferably 8.00 mass% or more and 51.00 mass% or less, more preferably 10.00 mass% or more and 47.00 mass% or less, even more preferably 15.00 mass% or more and 43.00 mass% or less, and may be 20.00 mass% or more and 40.00 mass% or less, or 20.00 mass% or more and 38.50 mass% or less. The P content (content as P element, not calculated as oxide) of the Li-MP-based solid electrolyte of the present invention is, for example, preferably 7.00 mass% or more and 38.00 mass% or less, more preferably 9.00 mass% or more and 36.00 mass% or less, even more preferably 10.00 mass% or more and 32.00 mass% or less, and may be 13.00 mass% or more and 28.00 mass% or less, or 15.00 mass% or more and 24.00 mass% or less.

[0022] The Li-MP solid electrolyte according to the present invention exhibits excellent resistance to decomposition when a voltage higher than 4.45 V is applied. This is presumably because the presence of covalent P-O and M-O bonds in the structure makes it difficult for oxygen to be released when a high potential is applied.

[0023] [Li-MP composition] A Li-MP composition containing Li, M, and P adjusted to the desired Li-MP solid electrolyte composition can be used as a precursor for synthesizing the Li-MP solid electrolyte. This composition can be in the form of an aqueous liquid or powder. That is, this composition is specified as a Li-MP composition consisting of an aqueous liquid or powder containing Li, M, and P in the following compositional ranges: the Li / (Li + M + P) molar ratio is 0.300 to 0.650, the M / (Li + M + P) molar ratio is 0.050 to 0.200, and the P / (Li + M + P) molar ratio is 0.250 to 0.600. As in the case of the Li-MP-based solid electrolyte described above, more preferred composition ranges include a Li / (Li+M+P) molar ratio of 0.350 or more and 0.600 or less, a M / (Li+M+P) molar ratio of 0.080 or more and 0.160 or less, and a P / (Li+M+P) molar ratio of 0.270 or more and 0.550 or less.

[0024] Here, "aqueous liquid containing Li, M, and P" is a general term for a liquid (aqueous solution) in which all of Li, M, and P are dissolved in an aqueous liquid medium, a liquid in which some of Li, M, and P are dissolved in an aqueous liquid medium and the remainder are dispersed as solid particles, and a liquid in which all of Li, M, and P are dispersed as solid particles in an aqueous liquid medium. Among aqueous liquids containing Li, M, and P, liquids other than aqueous solutions are called "dispersions." Dispersions typically exhibit cloudiness due to the solid particles dispersed in the liquid. An aqueous liquid medium refers to a liquid medium consisting of water or a mixture of water and other solvent components, where the water content is 50% by mass or more. The liquid medium preferably contains 80% by mass or more of water, and may contain 90% by mass or more of water, 95% by mass or more of water, or 100% by mass of water.

[0025] The Li-MP composition, which is an aqueous liquid, is easy to use as a coating liquid when forming a protective coating layer (for example, the first solid electrolyte 1 in Figure 4 described later) on the surface of positive electrode active material particles. From the viewpoint of forming a highly uniform coating layer, it is more effective for the aqueous liquid to be an aqueous solution. Coating methods include the spray dryer method, the tumbling flow method, and the immersion method, but the spray dryer method is superior in terms of productivity. On the other hand, powdered Li-MP-based compositions are useful for synthesizing Li-MP-based solid electrolytes used in protective layers (e.g., the first solid electrolyte 1 in Figure 3 below) for isolating a positive electrode active material from a sulfide-based solid electrolyte. They are also useful as coating materials for dry-forming protective coating layers (e.g., the first solid electrolyte 1 in Figure 4 below) on the surfaces of positive electrode active material particles. Dry coating methods include mixing positive electrode active material particles with the powdered Li-MP-based composition using a powder processing device such as Nobilta (manufactured by Hosokawa Micron Co., Ltd.) or a ball mill grinder to form a coating layer.

[0026] The total content of Li, M, and P in the aqueous Li-MP composition is preferably 0.50% by mass or more and 10.00% by mass or less, and may be 0.80% by mass or more and 5.00% by mass or less. If the total content of Li, M, and P is too low, the amount of Li-MP composition must be increased to achieve the required coating layer thickness, which increases the amount of aqueous liquid medium, resulting in a cost disadvantage. If the total content of Li, M, and P in the aqueous Li-MP composition is too high, the viscosity of the aqueous Li-MP composition increases, increasing the risk of clogging the narrow pipes in the coating treatment equipment.

[0027] [All-solid battery] The Li-MP solid electrolyte according to the present invention has a capacitance of 1.0×10 -11 It exhibits lithium ion conductivity of 1000 S / cm or more, and is useful as a protective material to be interposed between the positive electrode active material and the solid electrolyte (e.g., one mainly composed of sulfide) that serves as the separator in all-solid-state batteries.

[0028] FIG. 3 shows a schematic cross-sectional view of one embodiment of an all-solid-state battery using a first solid electrolyte made of the Li-MP-based solid electrolyte according to the present invention and a second solid electrolyte containing a sulfide as a main component. The cell structure is simplified in this figure to explain the relative positions of the components. This all-solid-state battery has a first solid electrolyte 1 (Li-MP-based solid electrolyte) between a conductive material 3 on the positive electrode side and a conductive material 4 on the negative electrode side. The first solid electrolyte 1 is interposed between the conductive material 3 on the positive electrode side and a second solid electrolyte 2, which serves as a separator. There is a contact area between the conductive material 3 on the positive electrode side and the first solid electrolyte 1, and there is also a contact area between the first solid electrolyte 1 and the second solid electrolyte 2.

[0029] In all-solid-state batteries, the conductive material 3 on the positive electrode side is typically composed of a positive electrode active material 31 and a conductive material 32 other than the positive electrode active material. The positive electrode active material 31 is both an ion-conductive material and a conductive material. The conductive material 32 other than the positive electrode active material is a current collector, a conductive additive, etc. In actual all-solid-state batteries, the positive electrode material is typically composed of conductive materials such as a current collector, multiple particles of positive electrode active material, and multiple particles of conductive additive, as well as particles of an ion-conductive material (solid electrolyte) mixed to fill the spaces between the particles of the positive electrode active material. The conductive material 3 on the positive electrode side in Figure 3 is a schematic diagram illustrating the positional relationship of the portion of such a positive electrode material composed of the conductive material relative to the first solid electrolyte 1, and one particle of the positive electrode active material 31 is shown as a representative.

[0030] Known materials can be used as the positive electrode active material 31, and newly developed positive electrode active materials can also be used. Representative known positive electrode active materials include LiCoO2 (LCO type), LiNiO2 (LNO type), LiMn2O4 (LMO type), LiNiCoAlO2 (NCA type), LiNiCoMnO2 (NCM type), Li2MnO3-LiNiCoMnO2 (solid solution type), LiNiMnO4 (spinel type), LiMnFePO4 (phosphate type), and Li2FeSiO4 (silicate type).

[0031] The first solid electrolyte 1 is made of the Li-MP-based solid electrolyte according to the present invention described above. The second solid electrolyte 2 is composed mainly of sulfide and functions as a separator. "Composed mainly of sulfide" means that the total sulfide content of the constituent materials of the second solid electrolyte 2 is 50 mass % or more. One type of sulfide may be used alone, or two or more types of sulfides may be used in combination. The total sulfide content of the constituent materials of the second solid electrolyte 2 is preferably 80 mass % or more, and more preferably 90 mass % or more.

[0032] The sulfides mentioned above can be any known substance with ionic conductivity, and if a new ionic conductive substance is developed, it may also be applicable. Representative known ionic conductive substances include Li6PS5Cl (crystal (argyrodite)), Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 (crystal), Li 10 GeP2S 12 (crystal), 30Li2S·26B2S3·44LiI (glass), 63Li2S·36SiS2·1Li3PO4 (glass), 57Li2S·38SiS2·5Li4SiO4 (glass), 70Li2S·30P2S5 (glass), Li7P3S 11 (glass ceramic), Li 3.25 P 0.95 S4 (glass ceramic) and others.

[0033] In the embodiment of FIG. 3, a first solid electrolyte 1 (a Li-MP-based solid electrolyte) is interposed between a positive electrode active material 31 and a second solid electrolyte 2 (containing a sulfide as a main component), thereby preventing direct contact between the positive electrode active material 31 and the second solid electrolyte 2.

[0034] In all-solid-state batteries, the conductive material 4 on the negative electrode side is typically composed of a negative electrode active material 41 and a conductive material 42 other than the negative electrode active material. In actual all-solid-state batteries, the negative electrode material is generally composed of conductive materials such as a current collector and numerous particles of negative electrode active material, as well as particles of an ion-conducting material (solid electrolyte) mixed to fill the spaces between the particles of the negative electrode active material. The conductive material 4 on the negative electrode side in FIG. 3 is a schematic illustration of the positional relationship of the conductive material portion of such a negative electrode material relative to the second solid electrolyte 2. The configuration of the conductive material 4 on the negative electrode side and the positional relationship between the conductive material 4 and the second solid electrolyte 2 can be similar to those of known all-solid-state batteries. If necessary, a third solid electrolyte may be disposed between the second solid electrolyte 2 and the conductive material 4 on the negative electrode side.

[0035] Figure 4 shows a schematic cross-sectional view of one embodiment of an all-solid-state battery using particles of positive electrode active material coated with the first solid electrolyte, which is the Li-MP-based solid electrolyte according to the present invention, and a second solid electrolyte containing a sulfide as a main component. The cell structure is also simplified in this figure to explain the positional relationship of each component, and only one particle of the positive electrode active material 31 is shown. The structure of this embodiment of the all-solid-state battery is essentially the same as that shown in Figure 3, except that the first solid electrolyte 1 is used as a coating material for the particles of the positive electrode active material 31.

[0036] 4 also has a first solid electrolyte 1 (Li-MP solid electrolyte) between a conductive material 3 on the positive electrode side and a conductive material 4 on the negative electrode side, and the first solid electrolyte 1 is interposed between a positive electrode active material 31 and a second solid electrolyte 2 (mainly composed of sulfide), thereby preventing direct contact between the positive electrode active material 31 and the second solid electrolyte 2. Note that by making the thickness of the coating layer of the first solid electrolyte 1 (Li-MP solid electrolyte) sufficiently thin, a tunnel effect is exerted, ensuring electrical conduction between the positive electrode active material 31 and a conductive material 32 other than the positive electrode active material, even if they are not in direct contact with each other.

[0037] [Method of producing Li-MP-based composition] The Li-MP composition described above can be produced, for example, by the following steps.

[0038] (Li, M, P containing liquid formation process) An aqueous liquid is formed in which a Li-containing substance, an M-containing substance, and a P-containing substance are dissolved or dispersed in an aqueous liquid medium so that the total amount of Li, M, and P falls within the above-mentioned composition range. As the M-containing substance, one or more of a W-containing substance and a Zr-containing substance are used.

[0039] Examples of the Li-containing substance include lithium hydroxide, lithium oxide, lithium chloride, lithium citrate, lithium acetate, lithium sulfate, lithium phosphate, lithium borate, lithium carbonate, etc. From the viewpoint of reducing impurities in the Li-MP solid electrolyte and increasing lithium ion conductivity, it is preferable to use lithium hydroxide, lithium phosphate, lithium oxide, or lithium carbonate as the Li-containing substance, and among these, lithium hydroxide is more preferable.

[0040] Examples of W-containing substances include tungsten oxide, tungstic acid, metatungstic acid, paratungstic acid, phosphotungstic acid, ammonium phosphotungstate, sodium phosphotungstate, etc. As the W-containing substance, from the viewpoints of reducing impurities in the Li-MP-based solid electrolyte and increasing lithium ion conductivity, tungsten oxide, tungstic acid, and phosphotungstic acid are preferably used, and among these, tungsten oxide is more preferred.

[0041] Examples of Zr-containing substances include zirconium chloride, zirconium sulfate, zirconyl nitrate dihydrate, zirconium oxide chloride, zirconium carbonate hydrate, zirconium propoxide, zirconium butoxide, zirconium ethoxide, zirconium acetylacetonate, and zirconium acrylate.

[0042] Examples of the P-containing substance include phosphoric acid, metaphosphoric acid, polyphosphoric acid, pyrophosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphonate, calcium phosphate, calcium phosphonate, lithium phosphate, lithium phosphonate, magnesium phosphate, magnesium phosphinate, sodium phosphate, sodium phosphinate, sodium phosphonate, sodium metaphosphate, etc. As the P-containing substance, from the viewpoints of reducing impurities in the Li-MP-based solid electrolyte and increasing lithium ion conductivity, phosphoric acid, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, metaphosphoric acid, and lithium phosphate are preferably used, and among these, phosphoric acid and ammonium dihydrogen phosphate are more preferred.

[0043] When obtaining an aqueous solution as a water-based liquid, water-soluble substances are used as the Li-containing substance, M-containing substance, and P-containing substance. Examples of water-soluble Li-containing substances include lithium hydroxide, lithium phosphate, and lithium carbonate. Examples of water-soluble W-containing substances include tungsten oxide and tungstic acid. Examples of water-soluble Zr-containing substances include zirconium chloride, zirconium chloride oxide, and zirconyl nitrate dihydrate. Examples of water-soluble P-containing substances include phosphoric acid and ammonium dihydrogen phosphate.

[0044] This Li-MP composition containing Li, M, and P is obtained as an aqueous solution, which can be used as a coating solution for coating the surfaces of positive electrode active material particles.

[0045] (drying process) Furthermore, a powder Li-MP composition can be obtained by removing (i.e., drying) the liquid medium component from the Li-MP composition composed of the aqueous liquid obtained in the Li-, M-, and P-containing liquid formation step. The liquid medium component can be removed, for example, by evaporation to dryness. When evaporation to dryness is used, the temperature is preferably set in the range of 120°C to 220°C, and the atmosphere can be air. The inclusion of a drying step can further facilitate the removal of impurities (e.g., nitrogen compounds derived from ammonium dihydrogen phosphate) that are mixed in from the raw materials in the firing step described below.

[0046] (Crushing process) When using the powder obtained in the drying process to produce a Li-MP-based solid electrolyte, it is preferable to perform pulverization in order to obtain a sintered body with a highly uniform and dense structure. The particle size after pulverization is preferably adjusted so that the cumulative 50% particle size D50 in the volume-based particle size distribution measured by laser diffraction / scattering method is, for example, 1.5 μm to 4.0 μm. Ball mill pulverization, bead mill pulverization, etc. can be used as the pulverization method.

[0047] [Method of manufacturing Li-MP solid electrolyte] (Firing process) The Li-MP composition after the drying or pulverization step exhibits lithium ion conductivity and can therefore be considered a Li-MP solid electrolyte. However, to remove impurities (e.g., nitrogen compounds derived from ammonium dihydrogen phosphate) from the raw materials and enhance lithium ion conductivity, it is effective to perform a heat treatment at a temperature greater than 100°C and less than 350°C, more preferably greater than 200°C and less than 350°C, and even more preferably greater than 220°C and less than 350°C. This heat treatment is referred to as "sintering" in this specification. The firing atmosphere can be air. The firing time can be set, for example, in the range of 1 hour to 15 hours. The firing time can also be set in the range of 3 hours or more, or 7 hours or more. The Li-MP composition composed of the aqueous solution obtained in the Li-, M-, and P-containing solution formation step can also be directly subjected to a firing process at a temperature greater than 100°C and less than 350°C. [Example]

[0048] [Example 1] (Li, M, P containing liquid formation process) 2.318 g of tungsten trioxide (WO) was added all at once to a room-temperature (25°C) lithium aqueous solution prepared by dissolving 1.678 g of lithium hydroxide monohydrate (LiOH·H2O) in 250 mL of pure water. The solution was then heated to a temperature range of 50–80°C and stirred. After the tungsten trioxide was completely dissolved, the solution was cooled to room temperature (25°C). Next, the entire amount of a phosphoric acid solution prepared by dissolving 2.301 g of ammonium dihydrogen phosphate (NH4H2PO4) in 15.17 g of pure water was added to the solution and stirred for 15 minutes to obtain an aqueous solution (a Li-MP composition consisting of an aqueous solution containing Li, W, and P). In this example, the element M is W, and the Li / (Li + M + P) molar ratio based on the raw material blend is 0.571, the M / (Li + M + P) molar ratio is 0.143, and the P / (Li + M + P) molar ratio is 0.286. These molar ratios are listed in Table 1 as the "nominal composition based on the raw material blend" (the same applies to the following examples except for Comparative Example 1).

[0049] (drying process) The entire amount of the obtained aqueous liquid (Li-MP-based composition consisting of an aqueous solution containing Li, W, and P) was evaporated to dryness in an air atmosphere at 200°C for 5 hours, yielding a powdered Li-MP-based composition (dried solid).

[0050] (Crushing process) 1 g of the resulting dried solid and 17 zirconia beads with a diameter of 10 mm were placed in a 45 mL zirconia pot and pulverized for 5 hours using a planetary ball mill. After pulverization, the zirconia beads were separated from the contents of the pot to obtain a Li-MP composition (pulverized material) consisting of pulverized powder.

[0051] (Firing process) The obtained pulverized material was calcined in an air atmosphere at 120°C for 12 hours to obtain a calcined powder (Li-MP solid electrolyte). This fired powder was used as a test material for the following investigation.

[0052] (X-ray diffraction) X-ray diffraction patterns were measured using an X-ray diffractometer (Shimadzu, XRD-6100) with CuKα radiation under the following conditions: tube voltage 40 kV, tube current 30 mA, divergence slit 1.0°, scattering slit 1.0°, receiving slit 0.3 mm, scan speed 2.0° / min, step width 0.02° / step, measurement time 0.25 s, and 2θ range 10–60°. Peak search was performed using the analysis software provided with the instrument (XRD-6100) under the following conditions: smoothing: automatic, background processing: 100, Kα1-α2 ratio: 50, peak search: automatic. As a result, no diffraction peaks were detected in the peak search, confirming that the sintered powder obtained in this example was amorphous.

[0053] (composition analysis) A 0.1 g sample taken from the calcined powder was weighed, and 15 mL of pure water and 5 mL of 36% by weight hydrochloric acid were added. The mixture was heated and allowed to cool. 2 mL of 35% by weight hydrogen peroxide solution was then added. After cooling, the solution was diluted to 100 mL. The concentrations of Li, M (W in this example), and P in the diluted solution were measured using an inductively coupled plasma atomic emission spectrometry (ICP-AES, Agilent Technologies, CP-720). Based on the analytical values, the respective oxide-equivalent contents of Li, M (W in this example), and P were calculated according to the "Method for Determining the Equivalent Oxide Contents of Li, M, and P" described above. The results were: Li oxide content 11.84 mass%, W oxide content 52.21 mass%, and P oxide content 27.27 mass%. The total oxide-equivalent amounts of Li, M (W in this example), and P were 91.32 mass%. The analytical compositions of the calcined powders used as test materials are shown in Tables 1 and 2 (the same applies to the following examples).

[0054] (SEM-EDX analysis) A sample taken from the sintered powder was observed using a scanning electron microscope (SEM-EDX, JEOL JSM-7800F), and the sintered powder was subjected to energy dispersive X-ray spectroscopy (EDX) using an EDX device attached to the SEM. As a result of elemental mapping and measurement, it was confirmed that the sintered powder contained O (oxygen) as a constituent element, and it is believed that the sintered powder is an oxide (the same applies to each of the following Examples and Comparative Examples 2 and 3).

[0055] (particle size distribution) The cumulative 50% particle diameter D50 on a volume basis was measured at a dispersion pressure of 5 bar using a laser diffraction / scattering particle size distribution analyzer (SYMPATEC's HELOS particle size distribution analyzer, HELOS & RODOS (airflow dispersion module)). As a result, the D50 of the calcined powder in this example was 2.0 μm.

[0056] (Ionic conductivity measurement) A 0.15 g powder sample of the test material was placed in a 10 mm diameter cylindrical container and pressed at 300 MPa using a press to obtain a green compact. The ionic conductivity of the resulting green compact was measured using an AC impedance method at 25°C under a nitrogen atmosphere using a potentio-galvanostat (Prinston, VersaSTAT4) in the range of 0.1 Hz to 1 MHz with an amplitude voltage of 100 mV. The resistance of the sample was determined from a Cole-Cole plot (complex impedance plane plot) of the measured values, and the ionic conductivity of the test material was calculated from the obtained resistance value. As a result, the specimen in this example was 1.0 x 10 -11 It was confirmed that the solid electrolyte exhibited a lithium ion conductivity of 100 S / cm or more (the same applies to the following Examples and Comparative Examples 2 and 3).

[0057] From the above, it can be said that the sintered powder of the test material of this example is a solid electrolyte containing amorphous oxide as the main component (the same applies to the following examples and comparative examples 2 and 3).

[0058] (Evaluation of decomposition resistance) An all-solid-state electrochemical cell was fabricated in which a solid electrolyte (first solid electrolyte) made of the sintered powder used as the test material was placed next to a sulfide-based solid electrolyte (second solid electrolyte), and the current generated due to the oxidation reaction of the test material was investigated. Specifically, the experiment was conducted as follows.

[0059] An electrochemical cell having the stacked structure shown in FIG. 5 was fabricated as follows. Argyrodite-type sulfide (Li6PS5Cl) was prepared as the second solid electrolyte. 57 mg of this sulfide was placed in an insulating outer cylinder (inner diameter 9.5 mm) made of alumina and pressed at a pressure of 80 MPa to form a layer of the second solid electrolyte. Next, 20 mg of the powder sample was placed on the second solid electrolyte layer and pressed at a pressure of 80 MPa to form a first solid electrolyte layer. Next, 20 mg of a composite material obtained by mixing the powder of the test material and the powder of stainless steel (SUS316) in a mortar in a volume ratio of 50:50 was poured onto the first solid electrolyte layer and pressed at a pressure of 360 MPa to form a composite layer. Next, a 200 μm thick sheet of metallic In (indium), a 300 μm thick sheet of metallic Li, and a 200 μm thick sheet of metallic In were stacked in the above order under the second solid electrolyte layer (opposite the first solid electrolyte), and then press-molded at a pressure of 80 MPa to form a counter electrode layer consisting of an In / Li / In three-phase structure. Next, current collectors made of stainless steel (SUS316) plates were placed on the composite layer and under the counter electrode layer, respectively, and current collecting leads were attached to each current collector.

[0060] In this way, an electrochemical cell fabricated having a working electrode consisting of a layer of a first solid electrolyte, a layer of composite material, and a current collector disposed on the composite material layer, a counter electrode consisting of a layer of a counter electrode member and a current collector disposed below the layer of the counter electrode member, and a layer of a second solid electrolyte formed between the working electrode and the counter electrode was placed in a thermostatic chamber at 25 ° C. The composite layer side was the working electrode, which was the high-potential electrode, and the counter electrode layer side was the counter electrode, which was the low-potential electrode. The voltage was swept from the open circuit voltage of the electrochemical cell to 5.0 V vs. Li (synonymous with 4.4 V vs. In-Li) at a sweep rate of 1 mV / s, and the current fluctuation was measured. Hereinafter, unless otherwise specified, the potential value (V) represents the potential relative to Li (V vs. Li). The potential relative to Li (V vs. Li) is the value obtained by adding 0.6 V to the potential relative to In-Li (V vs. In-Li).

[0061] Solid electrolytes are inherently insulating materials with respect to electrical conduction based on electron transfer. However, when the applied voltage to the working electrode of the electrochemical cell is swept toward a higher potential, a small current initially flows, which is thought to be due to non-Faraday reactions that do not involve the transfer of electrons, or side reactions of adsorbed water or impurities. Then, when the first solid electrolyte undergoes an oxidative decomposition reaction and its properties change, a current associated with the decomposition reaction begins to be observed. In this test, decomposition resistance was evaluated based on the current value at the time the applied voltage was increased to 5.0 V. In Comparative Example 1 (described below), which uses lithium niobate (LiNbO), a commonly used protective material, the current value at 5.0 V was 35.0 μA. Since the present invention aims to significantly improve decomposition resistance compared to lithium niobate, a strict standard was set, and a current value of 6.0 μA or less at 5.0 V in this test was deemed acceptable. The current value of the sintered powder of this example at 5.0 V was 5.8 μA. Therefore, by applying the sintered powder of this example to the first solid electrolyte 1 in an all-solid-state battery having an arrangement structure such as that shown in FIGS. 3 and 4, excellent decomposition resistance is exhibited, and the reliability of the all-solid-state battery when the charging voltage is increased is improved (the same applies to the sintered powders of the following examples).

[0062] [Example 2] In the firing step, a fired powder was prepared under the same conditions as in Example 1, except that the firing temperature was set to 300° C., and the same investigation was carried out. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 13.86 mass%, the W oxide content was 48.93 mass%, and the P oxide content was 33.00 mass%. The total amount of Li, M (W in this example), and P, calculated as oxides, was 95.79 mass%. D50 was 2.0 μm. In the evaluation of decomposition resistance, the current value at 5.0 V was 4.3 μA.

[0063] [Example 3] (Li, M, P containing liquid formation process) To a room-temperature (25°C) lithium aqueous solution prepared by dissolving 1.556 g of lithium hydroxide monohydrate (LiOH·HO) in 250 mL of pure water, 2.149 g of tungsten trioxide (WO) was added all at once, and the resulting solution was stirred at a temperature between 50 and 80°C. After the tungsten trioxide was completely dissolved, the solution was cooled to room temperature (25°C). Next, the entire amount of a phosphoric acid solution obtained by dissolving 2.665 g of ammonium dihydrogen phosphate (NH4H2PO4) in 15.17 g of pure water was added to the resulting solution, and the mixture was stirred for 15 minutes to obtain an aqueous solution (a Li-MP composition consisting of an aqueous solution containing Li, W, and P). In this example, the element M is W, and the Li / (Li+M+P) molar ratio based on the raw material composition is 0.533, the M / (Li+M+P) molar ratio is 0.133, and the P / (Li+M+P) molar ratio is 0.333.

[0064] A fired powder was prepared under the same conditions as in Example 1, except that the composition was adjusted to the above in the Li, M, P-containing liquid formation step, and subjected to the same investigation. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 11.07 mass%, the W oxide content was 47.42 mass%, and the P oxide content was 34.19 mass%. The total amount of Li, M (W in this example), and P, calculated as oxides, was 92.68 mass%. D50 was 1.9 μm. In the decomposition resistance evaluation, the current value at 5.0 V was 3.6 μA.

[0065] [Example 4] In the firing step, a fired powder was prepared under the same conditions as in Example 3, except that the firing temperature was set to 300° C., and the same investigation was carried out. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 12.14 mass%, the W oxide content was 48.30 mass%, and the P oxide content was 36.00 mass%. The total amount of Li, M (W in this example), and P, calculated as oxides, was 96.44 mass%. D50 was 2.1 μm. In the evaluation of decomposition resistance, the current value at 5.0 V was 2.5 μA.

[0066] [Example 5] (Li, M, P containing liquid formation process) To a room-temperature (25°C) lithium aqueous solution prepared by dissolving 1.128 g of lithium hydroxide monohydrate (LiOH·HO) in 250 mL of pure water, 1.558 g of tungsten trioxide (WO) was added all at once. The resulting solution was then stirred at a temperature between 50 and 80°C. After the tungsten trioxide was completely dissolved, the solution was cooled to room temperature (25°C). Next, a phosphoric acid solution prepared by dissolving 3.865 g of ammonium dihydrogen phosphate (NH4H2PO4) in 15.17 g of pure water was added in its entirety to the resulting solution, and the mixture was stirred for 15 minutes to obtain an aqueous solution (a Li-MP composition consisting of an aqueous solution containing Li, W, and P). In this example, the element M is W, and the Li / (Li+M+P) molar ratio based on the raw material composition is 0.400, the M / (Li+M+P) molar ratio is 0.100, and the P / (Li+M+P) molar ratio is 0.500.

[0067] A fired powder was prepared under the same conditions as in Example 1, except that the composition was adjusted to the above in the Li, M, P-containing liquid formation step, and subjected to the same investigation. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 7.84 mass%, the W oxide content was 33.04 mass%, and the P oxide content was 44.69 mass%. The total amount of Li, M (W in this example), and P, calculated as oxides, was 85.57 mass%. D50 was 3.3 μm. In the evaluation of decomposition resistance, the current value at 5.0 V was 4.1 μA.

[0068] [Example 6] In the firing step, a fired powder was prepared under the same conditions as in Example 5, except that the firing temperature was set to 300° C., and the same investigation was carried out. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 8.85 mass%, the W oxide content was 34.81 mass%, and the P oxide content was 51.97 mass%. The total amount of Li, M (W in this example), and P, calculated as oxides, was 95.63 mass%. D50 was 3.4 μm. In the evaluation of decomposition resistance, the current value at 5.0 V was 3.2 μA.

[0069] [Example 7] (Li, M, P containing liquid formation process) 1.00 g of lithium hydroxide monohydrate (LiOH·H2O) and 1.92 g of zirconium oxide chloride octahydrate (ZrOCl2·8H2O) were added to 38 mL of pure water and stirred at room temperature (25°C) to completely dissolve these substances. Next, 1.72 g of an 85% by weight aqueous solution of phosphoric acid (H3PO4) was added to this solution at room temperature (25°C), resulting in a cloudy solution (dispersion). In this example, the element M is Zr, and the Li / (Li + M + P) molar ratio based on the raw material composition is 0.533, the M / (Li + M + P) molar ratio is 0.133, and the P / (Li + M + P) molar ratio is 0.333.

[0070] Except for using the aqueous liquid of the above composition prepared in the Li, M, P-containing liquid formation process, the drying process, the grinding process, and the firing process were carried out under the same conditions as in Example 1 to prepare a fired powder, which was then subjected to the same investigation. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 14.57 mass%, the Zr oxide content was 30.08 mass%, and the P oxide content was 43.31 mass%. The total amount of Li, M (Zr in this example), and P, calculated as oxides, was 87.96 mass%. D50 was 2.0 μm. In the decomposition resistance evaluation, the current value at 5.0 V was 4.0 μA.

[0071] [Example 8] In the firing step, a fired powder was prepared under the same conditions as in Example 7, except that the firing temperature was set to 300°C, and the same investigation was carried out. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 16.08 mass%, the Zr oxide content was 33.16 mass%, and the P oxide content was 47.76 mass%. The total amount of Li, M (Zr in this example), and P, calculated as oxides, was 97.00 mass%. D50 was 2.3 μm. In the evaluation of decomposition resistance, the current value at 5.0 V was 3.0 μA.

[0072] [Comparative Example 1] In this example, as a reference, a test material of lithium niobate (LiNbO3), which is a common protective material that has been widely used in the past, was prepared as follows and subjected to the same investigation as in Example 1.

[0073] (Preparation of LiNbO3 test powder) A hydrogen peroxide solution was prepared by adding 7.7 g of 35% hydrogen peroxide to 19.6 g of pure water. 4.4 g of niobic acid (NbO·nH2O) containing 58.0% NbO was added to this hydrogen peroxide solution. The solution was then maintained at 20-30°C, and 3.5 g of 28% ammonia water was added. The mixture was thoroughly stirred in an air atmosphere to obtain a clear solution. Next, 0.9 g of lithium hydroxide monohydrate (LiOH·H2O) was added to the solution in a nitrogen atmosphere to obtain a clear solution containing lithium and peroxo complexes. The solution was then heated to dryness on a hot plate at 100°C in a nitrogen atmosphere, resulting in a dried solid. The dried solid was then heat-treated at 200°C for 12 hours to obtain amorphous LiNbO3 powder. 1 g of the resulting dried solid and 17 10 mm diameter zirconia beads were placed in a 45 mL zirconia pot and milled for 5 hours using a planetary ball mill. After milling, the zirconia beads were separated from the contents of the pot, yielding a milled LiNbO3 powder (milled material). The milled material was then calcined in air at 120°C for 12 hours to obtain the LiNbO3 test powder (LiNbO3 solid electrolyte). No X-ray diffraction peaks were detected in the X-ray diffraction pattern of this powder, confirming that the LiNbO3 test powder was amorphous.

[0074] The decomposition resistance of the obtained LiNbO3 test powder was evaluated in the same manner as in Example 1. As a result, the current value at 5.0 V was 35.0 μA.

[0075] Comparative Example 2 (Li, M, P containing liquid formation process) 2.777 g of tungsten trioxide (WO) was added all at once to a room-temperature (25°C) lithium aqueous solution prepared by dissolving 2.010 g of lithium hydroxide monohydrate (LiOH·HO) in 250 mL of pure water. The solution was then heated to a temperature range of 50–80°C and stirred. After the tungsten trioxide was completely dissolved, the solution was cooled to room temperature (25°C). Next, a phosphoric acid solution prepared by dissolving 1.378 g of ammonium dihydrogen phosphate (NH4H2PO4) in 15.17 g of pure water was added to the solution and stirred for 15 minutes to obtain an aqueous solution (a Li-MP composition consisting of an aqueous solution containing Li, W, and P). In this example, the element M is W, and the Li / (Li+M+P) molar ratio based on the raw material composition is 0.667, the M / (Li+M+P) molar ratio is 0.167, and the P / (Li+M+P) molar ratio is 0.167.

[0076] A fired powder was prepared under the same conditions as in Example 1, except that the composition was adjusted to the above in the Li, M, P-containing liquid formation step, and subjected to the same investigation. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 15.16 mass%, the W oxide content was 58.83 mass%, and the P oxide content was 18.01 mass%. The total amount of Li, M (W in this example), and P, calculated as oxides, was 92.00 mass%. D50 was 3.2 μm. In the evaluation of decomposition resistance, the current value at 5.0 V was 8.0 μA, which was higher than that of each example.

[0077] Comparative Example 3 In the firing step, a fired powder was prepared under the same conditions as in Comparative Example 2, except that the firing temperature was set to 300° C., and the same investigation was carried out. As a result, no X-ray diffraction peaks were detected in the X-ray diffraction pattern by peak search, confirming that the sintered powder obtained in this example was amorphous. The Li oxide content was 15.82 mass%, the W oxide content was 61.39 mass%, and the P oxide content was 18.79 mass%. The total amount of Li, M (W in this example), and P, calculated as oxides, was 96.00 mass%. D50 was 3.1 μm. In the evaluation of decomposition resistance, the current value at 5.0 V was 7.1 μA, which was higher than that of each example. The above results are shown in Tables 1 and 2.

[0078] [Table 1]

[0079] [Table 2] [Explanation of symbols]

[0080] 1. First solid electrolyte 2. Second solid electrolyte 3 Conductive material on the positive electrode side 4. Conductive material on the negative electrode side 31 Cathode active material 32 Conductive materials other than positive electrode active materials 41 Negative electrode active material 42 Conductive materials other than negative electrode active materials

Claims

1. A Li-M-P-based composition comprising an aqueous liquid or powder containing Li, M, and P in the following composition ranges: the Li / (Li+M+P) molar ratio is 0.300 or more and 0.650 or less, the M / (Li+M+P) molar ratio is 0.050 or more and 0.200 or less, and the P / (Li+M+P) molar ratio is 0.250 or more and 0.600 or less, where M represents one or more elements selected from W and Zr, and the ratio of the amount (moles) of element X to the total amount (moles) of Li, M, and P is represented as the X / (Li+M+P) molar ratio.

2. A Li-M-P based solid electrolyte containing an amorphous oxide as a main component, which contains Li, M, and P in the following composition ranges: the Li / (Li+M+P) molar ratio is 0.300 or more and 0.650 or less, the M / (Li+M+P) molar ratio is 0.050 or more and 0.200 or less, and the P / (Li+M+P) molar ratio is 0.250 or more and 0.600 or less, where M represents one or more elements selected from W and Zr, and the ratio of the amount (moles) of element X to the total amount (moles) of Li, M, and P is represented as the X / (Li+M+P) molar ratio.

3. An all-solid-state battery comprising the solid electrolyte according to claim 2 between a conductive material on a positive electrode side and a conductive material on a negative electrode side.

4. An all-solid-state battery using the particles of the positive electrode active material coated with the solid electrolyte according to claim 2 as a positive electrode material.

5. A method for producing a Li-M-P based composition, comprising a step of forming a liquid in which a Li-containing substance, an M-containing substance, and a P-containing substance are dissolved or dispersed in an aqueous liquid medium, such that the Li / (Li+M+P) molar ratio is 0.300 or more and 0.650 or less, the M / (Li+M+P) molar ratio is 0.050 or more and 0.200 or less, and the P / (Li+M+P) molar ratio is 0.250 or more and 0.600 or less (Li-M-P-containing liquid formation step), where M represents one or more elements selected from W and Zr, and the ratio of the amount (moles) of element X to the total amount (moles) of Li, M, and P is represented as the X / (Li+M+P) molar ratio.

6. The method for producing a Li-M-P based composition according to claim 5, further comprising a step of drying the liquid obtained in the Li, M, P-containing liquid formation step to obtain a powder (drying step).

7. The method for producing a Li-MP-based composition according to claim 6, further comprising a step of pulverizing the powder obtained in the drying step (pulverization step).

8. A method for producing a Li-M-P based solid electrolyte, comprising: a step of subjecting the Li-M-P based composition obtained by the production method according to any one of claims 5 to 7 to a heat treatment at a temperature higher than 100°C and not higher than 350°C (firing step).

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