Method for producing a solid electrolyte having an argyrodite-type crystal structure

Abstract

To provide a method for manufacturing a solid electrolyte having an argyrodite-type crystal structure which is excellent in mass productivity and reproducibility of synthesis, generates no by-product of the synthesis, and causes no deterioration of conductivity and charge / discharge capacity.SOLUTION: The method includes: mixing a first dispersion liquid for a precursor in which a material including Li element, P element and S element is dispersed in a solvent containing an aprotic polar solvent containing no ether linkage, and a second dispersion liquid for a precursor in which at least LiX is dispersed in a solvent containing a thiol-based solvent; followed by desolvation and heat treatment.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a solid electrolyte having an argyrodite-type crystal structure suitable for use in solid-state batteries, and more particularly to a method for producing a solid electrolyte having an argyrodite-type crystal structure containing Li, P, S, and X (X is a halogen) in a liquid phase without using a milling process, which is a complicated process. [Background technology]

[0002] Lithium-ion batteries are secondary batteries in which lithium ions are released from the positive electrode during charging, migrate to the negative electrode, and are absorbed there. During discharge, lithium ions are inserted back into the positive electrode. Due to their high energy density and long life, lithium-ion batteries have been widely used as power sources for various electronic devices, including personal computers and cameras, portable electronic devices and communication devices, and power tools. Recently, they have also been applied to large-scale batteries installed in electric vehicles (EVs) and hybrid electric vehicles (HEVs). The use of solid electrolytes in lithium-ion batteries, instead of flammable organic solvent-containing electrolytes, not only simplifies safety devices but also offers advantages in manufacturing cost and productivity. Research into various materials has been actively conducted. In particular, sulfide-containing solid electrolytes have been shown to have high electrical conductivity (lithium ion conductivity) and are considered useful for achieving high-power batteries. In particular, solid electrolytes with an argyrodite-type crystal structure, which provide high electrical conductivity, have attracted attention, and various manufacturing methods are known.

[0003] For example, Non-Patent Document 1 describes a method for producing 10 sintered bodies at room temperature by performing heat treatment after mechanical milling. -4It has been disclosed that a solid electrolyte having an argyrodite-type crystal structure with a conductivity on the order of S / cm can be produced. Patent Document 1 discloses a solution-based method for producing a solid electrolyte without a milling process. Specifically, a mixed powder of raw material powders and acetonitrile are placed in a reaction vessel to produce a mixed solution. The acetonitrile is then removed by vacuum drying, and the resulting solution is then heat-treated at 550°C for 5 hours to crystallize the solution, thereby producing a solid electrolyte having an argyrodite-type crystal structure. Non-Patent Document 2 discloses that a precursor solution is produced by mixing a solution in which Li2S and LiX are dissolved in ethanol in a predetermined ratio with a dispersion obtained by dispersing Li2S and P2S5 in THF (tetrahydrofuran) and stirring for 24 hours to produce Li3PS4. The resulting precursor is then dried, calcined, and crystallized to produce a solid electrolyte having an argyrodite-type crystal structure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-064838 [Non-patent literature]

[0005] [Non-Patent Document 1] RP Rao, S. Adams Phys. Status Solidi A, 208, 8, 1804-1807 (2011). “Studies of lithium argyrodite solid electrolytes for all-solid-state batteries” [Non-patent document 2] Laidong Zhou, Kern-Ho Park, Xiaoqi Sun, Fabien Lalere, Torben Adermann, Pascal Hartmann, and Linda F. Nazar, ACS Energy Lett. 4, 1, 265-270 (2019). “Solvent-Engineered Design of Argyrodite Li6PS5X (X = Cl, Br, I) Solid Electrolytes with High Ionic Conductivity” Summary of the Invention [Problem to be solved by the invention]

[0006] However, the milling method described in Non-Patent Document 1 is difficult to scale up, and the recovery of the synthesized product is cumbersome, making it unsuitable for mass production. Furthermore, it has been found that all-solid-state lithium batteries using an electrolyte prepared by milling can suddenly short-circuit during repeated charge-discharge cycles between lithium metal electrodes, resulting in issues with robustness. The method described in Patent Document 1 involves a multi-step synthesis reaction using a single solution, which results in issues with synthesis uniformity and reproducibility. The resulting electrolyte has inferior conductivity compared to solid electrolytes with an argyrodite-type crystal structure prepared by methods such as milling. The solid electrolyte with an argyrodite-type crystal structure prepared by the method described in Non-Patent Document 2 has the problem of producing a by-product (Li3PO4). Therefore, it has been found that the conductivity is poor and the charge-discharge capacity of all-solid-state batteries is also reduced. Furthermore, it has been found that repeated charge-discharge cycles between lithium metal electrodes result in the formation of an insulating reaction layer, resulting in a significant increase in applied voltage.

[0007] An object of the present invention is to provide a method for producing a solid electrolyte having an argyrodite-type crystal structure, which is easy to scale up and recover the synthesized product, has excellent mass productivity, has good synthesis reproducibility, does not produce synthesis by-products, is not subject to a decrease in conductivity or charge / discharge capacity, and is highly robust, free from sudden short circuits and increases in applied voltage due to the formation of an insulating reaction layer even after repeated charge / discharge. [Means for solving the problem]

[0008] The above-mentioned problems of the present invention can be solved by the following means. [1] A method for producing a solid electrolyte having an argyrodite-type crystal structure containing Li, P, S, and X (X is a halogen) in a liquid phase, comprising: a precursor liquid preparation step of preparing a raw material precursor liquid for producing a solid electrolyte; a desolvation step of desolvating the raw material precursor liquid for producing a solid electrolyte prepared in the precursor liquid preparation step; and a heat treatment step of a dried product obtained in the desolvation step, The precursor liquid preparation step includes: a first precursor dispersion preparation step of preparing a first precursor dispersion in which a material containing Li, P, and S elements, the molar ratio of each element being (3+a):1:(4+b) (0≦a≦2, 0≦b≦1), is dispersed in a solvent containing an aprotic polar solvent not containing at least an ether bond; a second precursor dispersion preparation step of preparing a second precursor dispersion in which at least LiX is dispersed in a solvent containing at least a thiol-based solvent; a mixing step of mixing the first precursor dispersion and the second precursor dispersion; 1. A method for producing a solid electrolyte having an argyrodite-type crystal structure, comprising: [2] The method for producing a solid electrolyte having an argyrodite-type crystal structure according to [1], wherein the solvent used in either or both of the first precursor dispersion preparation step and the second precursor dispersion preparation step contains pyridine. [3] A method for producing a solid electrolyte having an argyrodite-type crystal structure according to [1] or [2], characterized in that the first precursor dispersion preparation step comprises a step of mixing a dispersion in which P2S5 is dispersed in a solvent containing at least pyridine with a dispersion in which Li2S is dispersed in a solvent containing at least an aprotic polar solvent not containing an ether bond. [4] A method for producing a solid electrolyte having an argyrodite-type crystal structure according to [1], characterized in that either or both of the first precursor dispersion preparation step and the second precursor dispersion preparation step comprise a step of stirring at a temperature of 40°C or higher and lower than the boiling point of the solvent with the lowest boiling point among the solvents contained. [5] A method for producing a solid electrolyte having an argyrodite-type crystal structure according to any one of [1] to [4], characterized in that each step included in the precursor liquid preparation step does not involve a material or solvent having an ether bond, and does not involve a material or solvent having an alcoholic OH group. [Effects of the Invention]

[0009] The present invention provides a method for producing a solid electrolyte having an argyrodite-type crystal structure, which is easy to scale up and recover the synthesized product, has excellent mass productivity, has good synthesis reproducibility, does not produce synthesis by-products, is free from a decrease in conductivity or charge / discharge capacity, and is highly robust, free from sudden short circuits and increases in applied voltage due to the formation of an insulating reaction layer even after repeated charge / discharge. [Brief explanation of the drawings]

[0010] [Figure 1] X-ray diffraction patterns of SE2(550), SE2(600), SE3(550), and SE3(600) [Figure 2] Temperature dependence of conductivity of SE2(550), SE2(600), SE3(550), and SE3(600) [Figure 3] Schematic cross-sectional view showing a measurement cell for charging tests [Figure 4] Charge and discharge measurement of SE2(550) and SE3(550) [Figure 5]Constant current charge / discharge cycle measurement of SE1(550), SE2(550), and SE3(550) [Figure 6] X-ray diffraction patterns of SE4(600-10), SE4(600―2), SE5(600-10), and SE5(600―2) [Figure 7] Temperature dependence of conductivity of SE4(600-10), SE4(600―2), SE5(600-10), and SE5(600―2) [Figure 8] X-ray diffraction patterns of SE5(550) and SE6(550) [Figure 9] Temperature dependence of conductivity of SE5(550) and SE6(550) DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail. In the following description, the symbol "to" is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit.

[0012] First, the present invention relates to a liquid-phase method for producing a solid electrolyte having an argyrodite-type crystal structure containing elements Li, P, S, and X (X is a halogen). Whether the produced solid electrolyte has an argyrodite-type crystal structure can be confirmed, for example, by X-ray diffraction measurement (hereinafter also referred to as XRD) using CuKα radiation. The argyrodite-type crystal structure has strong diffraction peaks at 2θ=25.2±1.0 deg and 29.7±1.0 deg. Note that diffraction peaks of the argyrodite-type crystal structure may also appear at 2θ=15.3±1.0 deg, 17.7±1.0 deg, 31.1±1.0 deg, 44.9±1.0 deg, or 47.7±1.0 deg, for example. The argyrodite-type solid electrolyte of the present invention may have these peaks.

[0013] The argyrodite-type solid electrolyte of the present invention contains Li, P, S, and X elements (X is a halogen). Examples of X elements include F, Cl, Br, and I. The X elements may be one type or two or more types. In the composition formula, for example, X may be used to distinguish the type of X element. 1 Element (halogen 1), X 2 As an element (halogen 2), Li 7-y-z PS 6-y-z X 1 y X 2 z (y≧0, z≧0, 1≦y+z≦1.8). Specifically, Li6PS5Cl, Li6PS5Br, Li6PS5I, Li6PS5Cl 0.75 Br 0.25 , Li6PS5Cl 0.5 Br 0.5 , Li 5.75 PS 4.75 Cl 1.25 , Li 5.5 PS 4.5 Cl 1.5 These can include, but are not limited to:

[0014] The method for producing a solid electrolyte of the present invention includes a precursor liquid preparation step of preparing a raw material precursor liquid for producing a solid electrolyte, a desolvation step of desolvating the raw material precursor liquid for producing a solid electrolyte prepared in the precursor liquid preparation step, and a heat treatment step of a dried product obtained in the desolvation step.

[0015] In the method for producing a solid electrolyte of the present invention, it is preferable that no material or solvent having an ether bond, or no material or solvent having an alcoholic OH group is used in any of the steps.

[0016] The precursor liquid preparation step will be described. The precursor liquid preparation step includes a first precursor dispersion preparation step of preparing a first precursor dispersion in which a material containing Li, P, and S elements, with the molar ratio of each element being (3+a):1:(4+b) (0≦a≦2, 0≦b≦1), is dispersed in a solvent containing at least an aprotic polar solvent that does not contain an ether bond; a second precursor dispersion preparation step of preparing a second precursor dispersion in which at least LiX is dispersed in a solvent containing at least a thiol-based solvent; and a mixing step of mixing the first precursor dispersion and the second precursor dispersion.

[0017] In Patent Document 1, the raw materials are synthesized by mixing them all at once, but in the present invention, dispersions are prepared in separate steps, a first precursor dispersion preparation step and a second precursor dispersion preparation step, and then the materials are mixed. For example, when the raw materials are Li2S, P2S5, and LiCl, the synthesis can be expressed as 5Li2S + P2S5 + 2LiCl → 2Li6PS5Cl, but Li6PS5Cl is synthesized via 2Li3PS4 + 2Li2S + 2LiCl. In the present invention, Li3PS4 or its precursor is formed in advance in the first precursor dispersion preparation step, thereby significantly improving the stability of the synthesis compared to the method of synthesizing by mixing the materials all at once.

[0018] In the first precursor dispersion preparation step, an aprotic polar solvent that does not contain at least an ether bond is used. Other solvents may be used in combination as long as they do not inhibit the production of the argyrodite-type solid electrolyte. In this case, however, it is preferable not to use a solvent with an ether bond or an alcoholic OH group. In the present invention, an aprotic polar solvent that does not contain an ether bond refers to a solvent that does not contain an ether bond in the molecule, does not have proton-donating properties, and has a relative dielectric constant of 20 or more, such as acetonitrile, propionitrile, benzonitrile, dimethylformamide, and dimethyl sulfoxide. Of these, acetonitrile is preferred. Non-Patent Document 2 uses tetrahydrofuran that contains an ether bond, but the ether bond opens and reacts with P2S5 to form PO4. 3-In the present invention, the use of an aprotic polar solvent containing no ether bond results in the formation of PO4 3- The aprotic polar solvent not containing an ether bond preferably accounts for 30 mass % or more of the solvent used in the first precursor dispersion, more preferably 60 mass % or more, and most preferably 80 mass % or more, except when pyridine, which will be described later, is used in combination.

[0019] In the first precursor dispersion preparation step, a material containing Li, P, and S elements, with the molar ratio of each element being (3 + a):1:(4 + b) (0≦a≦2, 0≦b≦1), is dispersed in a solvent containing at least an aprotic polar solvent that does not contain an ether bond. As mentioned above, in the first precursor dispersion, the molar ratio of Li, P, and S elements is preferably 3:1:4 because Li3PS4 or its precursor is formed. However, because the second precursor dispersion also contains Li and P, a portion of these elements may be included in the first precursor dispersion preparation step as long as they do not inhibit the formation of Li3PS4 or its precursor. The range is 0≦a≦2, 0≦b≦1.

[0020] Examples of raw materials used for the material containing Li, P, and S include, but are not limited to, lithium (Li), sulfur (S), phosphorus (P), lithium sulfide (LiS), diphosphorus pentasulfide (P2S5), and Li3PS4. However, it is preferable not to include materials having an ether bond or an alcoholic OH group. Among these, lithium sulfide (LiS) and diphosphorus pentasulfide (P2S5) are preferred. Materials containing other elements may also be included as long as they do not inhibit the production of the argyrodite-type solid electrolyte. The concentration of the raw materials in the first precursor dispersion is not particularly limited, but is preferably 0.1 to 30% by mass, and more preferably 0.5 to 15% by mass. A low concentration reduces productivity. A high concentration may result in the formation of unreacted materials.

[0021] In the first precursor dispersion preparation step, it is preferable to use pyridine as a solvent in combination or to heat and stir the mixture in order to improve the stability of the synthesis.

[0022] The case where pyridine is used in combination as a solvent will be described. It is believed that the use of pyridine in combination improves the solubility of the raw materials and further improves the stability of the synthesis. When pyridine is used in combination in the first precursor dispersion preparation step, it is more preferable to mix a dispersion in which P2S5 is dispersed in a solvent containing at least pyridine with a dispersion in which Li2S is dispersed in a solvent containing at least an aprotic polar solvent not containing an ether bond. The concentrations of the raw materials in each dispersion are not particularly limited, but are preferably 0.1 to 30% by mass, more preferably 0.5 to 15% by mass. Furthermore, the amounts of pyridine and aprotic polar solvent used in each dispersion are preferably 60% by mass or more, more preferably 80% by mass or more, of the total solvent in each dispersion.

[0023] The case of heating and stirring in the first precursor dispersion preparation step will be described. It is believed that heating and stirring improves the solubility of the raw materials and further improves the stability of the synthesis. Conventional stirring methods can be used, such as a method using a magnetic stirrer or a method using a motor to rotate stirring blades such as a propeller, paddle, or disk turbine attached to a shaft. The heating temperature is preferably 40°C or higher and lower than the boiling point of the lowest boiling point of the solvents contained. In the case of acetonitrile, the temperature is preferably 40°C or higher and 80°C or lower, and more preferably 60°C or higher and 80°C or lower. The stirring time is preferably 1 to 100 hours, more preferably 12 to 72 hours.

[0024] The second precursor dispersion preparation step is a step of dispersing at least LiX in a solvent containing at least a thiol-based solvent. The thiol-based solvent is a solvent having a structure represented by R-SH (R is an organic group), and examples thereof include ethanethiol, 1-propanethiol, 2-propanethiol, 1-butanethiol, and 2-butanethiol. Of these, 1-propanethiol and 2-propanethiol are preferred. In Non-Patent Document 2, an alcohol-based solvent is used, but the alcohol-based solvent reacts with P2S5 to produce PO4 3- Thiol-based solvents have properties similar to alcohol-based solvents, but in the present invention, even if a thiol-based solvent reacts with P2S5, only PS4 is produced. 3- This is a constituent element of the solid electrolyte having an argyrodite-type crystal structure produced by the present invention and does not adversely affect it. Furthermore, other solvents may be used in combination to the extent that they do not inhibit the production of the argyrodite-type solid electrolyte. In this case, however, it is preferable that the solvent does not contain a solvent having an ether bond or a solvent having an alcoholic OH group.

[0025] The raw materials used in the second precursor dispersion preparation step include LiX and, as needed, LiS, lithium, and sulfur. Other materials may be included as long as they do not inhibit the production of the argyrodite-type solid electrolyte. In this case, however, it is preferable to exclude materials having ether bonds and materials having alcoholic OH groups. The amount of additives added is determined to achieve the desired composition of the solid electrolyte having the argyrodite-type crystal structure, taking into account the mixing ratio of the dispersion obtained in the first precursor dispersion preparation step and the dispersion obtained in the second precursor dispersion preparation step, as well as the concentration of each element contained in the dispersion obtained in the first precursor dispersion preparation step. The concentration of the dispersion obtained in the second precursor dispersion preparation step is not particularly limited, but is preferably 0.1 to 30% by mass, and more preferably 0.5 to 15% by mass. A low concentration reduces productivity. A high concentration may result in the generation of unreacted materials. The thiol-based solvent preferably accounts for 30% by mass or more of the solvent used in the second precursor dispersion, more preferably 60% by mass or more, and most preferably 80% by mass or more, except when pyridine, which will be described later, is used in combination.

[0026] In the second precursor dispersion preparation step, it is preferable to use pyridine as a solvent in combination or to heat and stir the mixture in order to improve the stability of the synthesis.

[0027] The case where pyridine is used in combination as a solvent will be described below. It is believed that the use of pyridine in combination improves the solubility of the raw materials, or that the trapping of halogen atoms by the basicity of pyridine contributes to the stability of synthesis. When pyridine is used in combination, the pyridine solvent and the thiol-based solvent are each preferably 30% by mass or more, and more preferably 40% by mass or more, of the total solvent in the second precursor dispersion.

[0028] The case of heating and stirring in the second precursor dispersion preparation step will be described. It is believed that heating and stirring improves the solubility of the raw materials and further improves the stability of the synthesis. Conventional stirring methods can be used, such as using a magnetic stirrer or a motor-driven rotation of stirring blades such as a propeller, paddle, or disk turbine attached to a shaft. The heating temperature is preferably 40°C or higher and lower than the boiling point of the lowest boiling point of the solvents contained. In the case of 1-propanethiol, the temperature is preferably 40°C or higher and 65°C or lower, more preferably 45°C or higher and 55°C or lower. The stirring time is preferably 1 to 100 hours, more preferably 12 to 72 hours.

[0029] The mixing step of mixing the dispersion obtained in the first precursor dispersion preparation step and the dispersion obtained in the second precursor dispersion preparation step will be described. The dispersion obtained in the first precursor dispersion preparation step and the dispersion obtained in the second precursor dispersion preparation step are mixed in a ratio that results in the desired composition of a solid electrolyte having an argyrodite-type crystal structure. The mixed solution is preferably stirred to uniformly mix the two dispersions. The stirring method described for the first precursor dispersion preparation step can be utilized. When pyridine is not used in either the first precursor dispersion preparation step or the second precursor dispersion preparation step, the mixture is preferably heated during stirring in the mixing step. The temperature is preferably 40°C or higher and lower than the boiling point of the lowest boiling point of the solvents contained. When 1-propanethiol is used, the temperature is preferably 40°C or higher and 65°C or lower, and more preferably 45°C or higher and 55°C or lower. The stirring time is preferably 1 to 100 hours, and more preferably 12 to 72 hours.

[0030] The solvent removal step, which removes the solvent from the raw material precursor liquid for producing a solid electrolyte prepared in the precursor liquid preparation step, will now be described. The solvent removal step is a step of removing the solvent contained in the raw material composition for producing a solid electrolyte, and can be carried out by using heating, reduced pressure, or the like. The temperature for heating is preferably 60°C to 180°C, more preferably 70°C to 150°C. For reduced pressure, a conventionally known method using a vacuum drying device can be used. A method using both heating and reduced pressure can also be preferably used.

[0031] The step of heat-treating the dried product obtained by the solution removal step will now be described. This step involves heat-treating the dried product obtained by the solution removal step to crystallize it and produce a solid electrolyte having an argyrodite-type crystal structure. It is also preferable to subject the dried product obtained by the solution removal step to pressure molding before the heat treatment. Pressure conditions are, for example, 20 to 150 MPa. Any conditions can be used for the heat treatment as long as they result in a solid electrolyte having an argyrodite-type crystal structure, but the temperature is preferably 400 to 700°C, more preferably 500 to 650°C, and the heat treatment time is preferably 0.5 to 24 hours, more preferably 1 to 15 hours.

[0032] The atmosphere used in each step of the manufacturing method of the present invention is not particularly limited, but is preferably a dry inert gas such as dry nitrogen gas or dry argon gas, or dry air. [Example]

[0033] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to these examples. 1. Manufacturing raw materials The raw materials used in the production of the solid electrolyte are as follows: 1-1. Phosphorus pentasulfide (P2S5) powder Merck's "P2S5" (trade name) was used, with a purity of 99%. 1-2. Lithium sulfide (Li2S) powder The product used was "Li2S" (trade name) manufactured by Mitsuwa Chemical Co., Ltd. The purity was 99.9%. 1-3. Lithium chloride (LiCl) powder "LiCl" (trade name) manufactured by Sigma-Aldrich was used. The purity was 99%. 1-4. Acetonitrile The product used was "Acetonitrile (ultra-dehydrated)" (trade name) manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. The purity was 99.9%. 1-5.Tetrahydrofuran "Tetrahydrofuran (anhydrous)" (trade name) manufactured by Sigma-Aldrich was used. The purity was 99.9%. 1-6. Ethanol The product used was "Ethanol (ultra-dehydrated)" (product name) manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. The purity was 99.5%. 1-7,1-Propanethiol "1-Propanethiol" (trade name) manufactured by Tokyo Chemical Industry Co., Ltd. was used. The purity was 98%. 1-8. Pyridine "Pyridine (anhydrous)" (trade name) manufactured by Sigma-Aldrich was used. The purity was 99.8%.

[0034] (Comparative Example 1: Production of solid electrolyte SE1 by milling treatment) Lithium sulfide powder, diphosphorus pentasulfide powder, and lithium chloride powder were weighed out in a molar ratio of 5:1:2, totaling 1 g, and mixed in an agate mortar. The mixed powder was then placed in a Frisch planetary ball mill (zirconia container) along with 10 mm diameter zirconia balls and subjected to mechanical milling (600 rpm, 20 hours). The milled powder was recovered. The powder adhered strongly to the zirconia balls and container, making recovery extremely time-consuming and difficult, and it was not possible to recover the entire amount. The recovered powder was pelletized by uniaxial pressing at 127 MPa. The resulting pellets were heat-treated at 550°C for 10 hours to obtain solid electrolyte SE1(550). The solid electrolyte in this example was produced in a dry argon gas atmosphere.

[0035] (Comparative Example 2: Production of solid electrolyte SE2 by a liquid phase method using tetrahydrofuran and ethanol) 0.275 g of lithium sulfide powder and 0.141 g of diphosphorus pentasulfide powder were weighed out and added to 10 mL of tetrahydrofuran solvent so that the molar ratio of Li, P, and S was 3:1:4. The mixture was then stirred at 25 ° C. for 24 hours using a magnetic stirrer to obtain a first precursor dispersion. Next, 0.171 g of lithium sulfide powder and 0.158 g of lithium chloride powder were weighed out and added to 10 mL of ethanol solvent. The mixture was then stirred at 25 ° C. for 24 hours using a magnetic stirrer to obtain a second precursor dispersion. The resulting first precursor dispersion and second precursor dispersion were mixed and then stirred at 50 ° C. for 24 hours to obtain a raw material precursor liquid for solid electrolyte production.

[0036] The raw material precursor liquid for producing the solid electrolyte was then dried under reduced pressure (80°C, 12 hours) using a diaphragm pump (Buchi, V-700 vacuum pump) to remove the solvent and obtain a dried product. The obtained dried product was uniaxially pressed at 127 MPa to form pellets. The obtained pellets were heat-treated at 550°C for 10 hours to obtain solid electrolyte SE2(550). Solid electrolyte SE2(600) was obtained in the same manner, except that the heat treatment temperature was 600°C. The solid electrolyte of this example was produced in a dry argon gas atmosphere.

[0037] (Example 1; Present invention; Production of solid electrolyte SE3 by liquid phase method using acetonitrile and 1-propanethiol) 0.275 g of lithium sulfide powder and 0.141 g of diphosphorus pentasulfide powder were weighed out and added to 10 mL of acetonitrile solvent so that the molar ratio of Li, P, and S was 3:1:4. The mixture was then stirred at 75°C for 24 hours using a magnetic stirrer to obtain a first precursor dispersion. Next, 0.171 g of lithium sulfide powder and 0.158 g of lithium chloride powder were weighed out and added to 10 mL of 1-propanethiol solvent. The mixture was then stirred at 50°C for 24 hours using a magnetic stirrer to obtain a second precursor dispersion. The resulting first precursor dispersion and second precursor dispersion were mixed and then stirred at 50°C for 24 hours to obtain a raw material precursor liquid for solid electrolyte production.

[0038] The raw material precursor liquid for producing the solid electrolyte was then dried under reduced pressure (80°C, 12 hours) using a diaphragm pump (Buchi, V-700 vacuum pump) to remove the solvent and obtain a dried product. The obtained dried product was uniaxially pressed at 127 MPa to form pellets. The obtained pellets were heat-treated at 550°C for 10 hours to obtain solid electrolyte SE3(550). Solid electrolyte SE3(600) was obtained in the same manner, except that the heat treatment temperature was 600°C. The solid electrolyte of this example was produced in a dry argon gas atmosphere.

[0039] (X-ray diffraction measurement) X-ray diffraction measurements (diffraction angle 2θ = 10° to 70°, sampling width 0.02°, scan rate 5° / min) were performed on SE2(550), SE2(600), SE3(550), and SE3(600) using an X-ray diffractometer (Rigaku Smart Lab), and the X-ray diffraction patterns shown in Figure 1 were obtained. It can be seen that all of them have the Argyrodiform crystal structure of Li6PS4Cl. However, while SE2(550) and SE2(600) also contain the Li3PO4 structure, SE3(550) and SE3(600) produced by the present invention do not contain Li3PO4.

[0040] (Conductivity measurement) For SE2(550), SE2(600), SE3(550), and SE3(600), disc-shaped test specimens (size: 5 mm radius x 0.6 mm height) were cut using a uniaxial hydraulic press and placed in a measurement unit (glass container) under an argon gas atmosphere. A ribbon heater connected to a thermostat and insulation material were wrapped around the measurement unit (glass container). The test specimens were gradually heated from room temperature (25°C) using a SOLATRON Impedance Analyzer "S1260" (model name) and their electrical conductivity was measured at 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 110°C, and 130°C. The electrical conductivity was measured after leaving the test specimens at each of the above temperatures for 1 hour from the low temperature side.

[0041] A graph of the temperature dependence of conductivity is shown in Figure 2. The conductivity at room temperature (25°C) was 1.80 mS / cm for SE2(550) and 1.47 mS / cm for SE2(600). On the other hand, the conductivity was 2.75 mS / cm for SE3(550) and 2.13 mS / cm for SE3(600). SE3 produced by the manufacturing method of the present invention exhibited higher conductivity than SE2.

[0042] (Charge / discharge test) Titanium sulfide (TiS2) and SE2 (550) were weighed out to a mass ratio of 1:1 and mixed in an agate mortar to obtain a composite for a positive electrode.

[0043] Next, SE2 (550) was pressure-molded using a uniaxial hydraulic press to form a disk-shaped preform (radius: 5 mm, thickness: 0.5 mm). This electrolyte layer preform was then housed inside a polyetheretherketone (PEEK) cylindrical body, and approximately 5 mg of the cathode composite obtained above was filled onto the entire surface of one side of the body, followed by pressure molding using a uniaxial hydraulic press. Furthermore, Li-In alloy foil (thickness: 0.1 mm, diameter: 5 mm) was attached to the other side of the electrolyte layer preform, resulting in an all-solid-state lithium-sulfur battery 20 comprising a cathode electrode 21 (thickness: approximately 30 μm) made of the cathode composite, an electrolyte layer 25 made of SE2 (550), and an anode electrode 23 made of a Li-In alloy.

[0044] Then, stainless steel-nickel conductive parts were inserted into both sides of the cylindrical body housing the lithium-sulfur battery 20 and fixed with a jig to obtain the measurement cell 10 shown in Figure 3. The measurement cell 10 was then sealed in a glass container (not shown), the gas inside the glass container was replaced with argon gas, and a charging test was performed. The charging test was performed using a Nagano charge-discharge device "BTS-2004H" (model name) under conditions of 1.0-2.4 V vs. Li-In and a C rate of 0.1 C. A similar test was also performed on SE3(550). The results are shown in Figure 4.

[0045] The charging capacity of SE2(550) is 182.8mAh·g per unit weight of TiS2 -1 , discharge capacity is 197.86mAh g -1 On the other hand, the charging capacity of SE3(550) was 221.73mAh g -1 , discharge capacity is 222.38mAh g -1 The theoretical capacity of SE3 produced by the manufacturing method of the present invention is 236 mAh g -1 The charge / discharge capacity was close to

[0046] (Constant current charge / discharge cycle test) Using electrode SE2(550), a cell with a symmetrical structure was obtained in the same manner as in the charge-discharge test, except that the positive electrode 21 made of the positive electrode composite and the negative electrode 23 made of the Li-In alloy were replaced with Li foil. A charge-discharge device "BTS-2004H" (model name) manufactured by NAGANO was used, and the current intensity was ±0.1 mA cm up to 100 cycles. ―2 , 1 hour charge / discharge, ±0.2mA cm after 100 cycles ―2 The test was conducted as follows. SE1(550) and SE3(550) were also tested in the same way. The results are shown in Figure 5.

[0047] SE1(550) short-circuited after 10 hours. SE2(550) showed an initial voltage of 21 mV, which increased to 154 mV after 200 hours. This is thought to be due to the formation of an insulating reaction layer. SE3(550) manufactured by the present invention showed an initial voltage of 6 mV, and even after 150 hours, the voltage only increased to 13 mV.

[0048] (Example 2; Present invention; Production of solid electrolytes SE4 and SE5 by a liquid phase method using acetonitrile, 1-propanethiol, and pyridine in combination) 0.141 g of diphosphorus pentasulfide powder was weighed and added to 5 mL of pyridine solvent, followed by stirring using a magnetic stirrer at room temperature (25°C) for 1 hour. 0.428 g of lithium sulfide powder was weighed and added to 5 mL of acetonitrile solvent, followed by stirring using a magnetic stirrer at room temperature (25°C) for 1 hour. These were mixed and further stirred using a magnetic stirrer at room temperature (25°C) for 24 hours to obtain a first precursor dispersion. The molar ratio of Li, P, and S in the first precursor dispersion was 5:1:5. Next, 0.158 g of lithium chloride powder was weighed and added to a mixed solvent of 5 mL of pyridine solvent and 5 mL of 1-propanethiol solvent, followed by stirring using a magnetic stirrer at room temperature (25°C) for 24 hours to obtain a second precursor dispersion. The first precursor dispersion and the second precursor dispersion were mixed and stirred at room temperature (25°C) for 24 hours to obtain a raw material precursor liquid for producing a solid electrolyte, in which the molar ratio of Li, P, S, and Cl was 6:1:5:1.

[0049] The solid electrolyte precursor liquid was then dried under reduced pressure (80°C, 12 hours) using a diaphragm pump (Buchi, V-700 vacuum pump) to remove the solvent and obtain a dried product. The dried product was uniaxially pressed at 127 MPa to form pellets. The pellets were heat-treated at 600°C for 10 hours to obtain solid electrolyte SE4 (600-10). Solid electrolyte SE4 (600-2) was obtained in the same manner, except that the heat treatment time was 2 hours. The solid electrolyte of this example was produced in a dry argon gas atmosphere.

[0050] Furthermore, solid electrolyte SE5(600-10) was obtained in the same manner except that the raw materials were weighed and adjusted so that the molar ratio of Li, P, S, and Cl in the raw material precursor liquid for solid electrolyte production was 5.5:1:4.5:1.5. Furthermore, solid electrolyte SE5(600-2) was obtained in the same manner except that the heat treatment time was 2 hours.

[0051] (X-ray diffraction measurement) X-ray diffraction measurements (diffraction angle 2θ = 10° to 70°, sampling width 0.02°, scan rate 5° / min) were performed on SE4(600-10), SE4(600-2), SE5(600-10), and SE5(600-2) using an X-ray diffraction measuring device (Rigaku Smart Lab), and the X-ray diffraction patterns shown in Figure 6 were obtained. It can be seen that all electrolytes have an Argyrodiform crystal structure and do not contain Li3PO4.

[0052] (Conductivity measurement) Ionic conductivity measurements were performed on SE4(600-10), SE4(600-2), SE5(600-10), and SE5(600-2) in the same manner as for SE2(550). Figure 7 shows a graph of the temperature dependence of conductivity. The conductivity at room temperature (25°C) was 4.76 mS / cm for SE4(600-10), 2.11 mS / cm for SE4(600-2), 5.41 mS / cm for SE5(600-10), and 1.82 mS / cm for SE5(600-2), demonstrating higher conductivity than SE2. In these cases, slight electronic conductivity was also observed in addition to ionic conductivity. It is possible that some solvent remained during the desolventization process and carbonized.

[0053] (Example 3; Present Invention; Production of Solid Electrolytes SE5 and SE6 with Different Addition of Lithium Sulfide from Example 1) A solid electrolyte SE5(550) was obtained in the same manner as in Example 1 (conditions for the heat treatment step were 550°C for 10 hours), except that the amount of lithium sulfide added in the first precursor dispersion was 0.324 g so that the molar ratio of Li, P, and S in the first precursor dispersion was 4:1:4.5, and the amount of lithium sulfide added in the second precursor dispersion was 0.086 g (the molar ratio of Li, P, S, and Cl in the raw material precursor liquid for production of a solid electrolyte was 6:1:5:1).

[0054] Furthermore, in Example 1, the amount of lithium sulfide added was 0.428 g so that the molar ratio of Li, P, and S in the first precursor dispersion was 5:1:5, and the amount of lithium sulfide added in the second precursor dispersion was 0 g (the molar ratio of Li, P, S, and Cl in the raw material precursor liquid for production of a solid electrolyte was 6:1:5:1), but the same procedure was followed (conditions for the heat treatment step were 550°C for 10 hours) to obtain a solid electrolyte SE6(550).

[0055] (X-ray diffraction measurement) X-ray diffraction measurements (diffraction angle 2θ = 10° to 70°, sampling width 0.02°, scan rate 5° / min) were performed on SE5(550) and SE6(550) using an X-ray diffraction measurement device (Rigaku Smart Lab), and the X-ray diffraction patterns shown in Figure 8 were obtained. It can be seen that both electrolytes have an Argyrodiform crystal structure and do not contain Li3PO4.

[0056] (Conductivity measurement) The ionic conductivity of SE5(550) and SE6(550) was measured in the same manner as for SE2(550). A graph of the temperature dependence of conductivity is shown in Figure 9. The conductivity at room temperature (25°C) was 2.21 mS / cm for SE5(550) and 2.37 mS / cm for SE6(550), which showed higher conductivity than SE2. [Explanation of symbols]

[0057] 10: Measurement cell for charging test 20: Lithium-sulfur battery 21: Positive electrode 23: Negative electrode 25: Electrolyte layer 27: PEEK cylindrical body 31: Retaining plate 33: Presser pin 35: Clamping screw 37: Kapton (registered trademark) tape

Claims

1. A method for producing a solid electrolyte having an argyrodite-type crystal structure containing Li, P, S, and X (X is a halogen) in a liquid phase, comprising: a precursor liquid preparation step of preparing a raw material precursor liquid for producing a solid electrolyte; a desolvation step of desolvating the raw material precursor liquid for producing a solid electrolyte prepared in the precursor liquid preparation step; and a heat treatment step of a dried product obtained in the desolvation step, The precursor liquid preparation step includes: a first precursor dispersion preparation step of preparing a first precursor dispersion in which a material containing Li, P, and S elements, the molar ratio of the elements being (3+a):1:(4+b) (0≦a≦2, 0≦b≦1), is dispersed in a solvent containing an aprotic polar solvent not containing at least an ether bond; a second precursor dispersion preparation step of preparing a second precursor dispersion in which at least LiX is dispersed in a solvent containing at least a thiol-based solvent; a mixing step of mixing the first precursor dispersion and the second precursor dispersion; 1. A method for producing a solid electrolyte having an argyrodite-type crystal structure, comprising:

2. 2. The method for producing a solid electrolyte having an argyrodite-type crystal structure according to claim 1, wherein the solvent used in either or both of the first precursor dispersion preparation step and the second precursor dispersion preparation step contains pyridine.

3. The first precursor dispersion preparation step comprises dissolving P in a solvent containing at least pyridine. ２ S ５ and a solvent containing at least an aprotic polar solvent not containing an ether bond. ２ 3. The method for producing a solid electrolyte having an argyrodite-type crystal structure according to claim 1, further comprising a step of mixing a dispersion liquid in which S is dispersed with the solid electrolyte.

4. 2. The method for producing a solid electrolyte having an argyrodite-type crystal structure according to claim 1, wherein either or both of the first precursor dispersion preparation step and the second precursor dispersion preparation step include a step of stirring at a temperature of 40° C. or higher and lower than the boiling point of the solvent with the lowest boiling point among the solvents contained.

5. 5. The method for producing a solid electrolyte having an argyrodite-type crystal structure according to claim 1 , wherein each step included in the precursor liquid preparation step does not include a material or solvent having an ether bond, and does not include a material or solvent having an alcoholic OH group.

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