Method for producing sulfide solid electrolyte and method for reusing same

Abstract

The purpose of the present invention is to provide a method for, by using a sulfide solid electrolyte, producing a new sulfide solid electrolyte. The present invention pertains to a method for producing a sulfide solid electrolyte, the method comprising: heating and melting a wet-ground first sulfide solid electrolyte to obtain a melt; removing carbon from the melt; and cooling the carbon-removed melt to obtain a second sulfide solid electrolyte.

Description

Method for producing and reusing sulfide solid electrolyte

[0001] The present invention relates to a method for producing a sulfide solid electrolyte and a method for reusing the same.

[0002] Lithium ion secondary batteries are widely used in portable electronic devices such as mobile phones and laptop computers. Conventionally, liquid electrolytes have been used in lithium ion secondary batteries. Meanwhile, all-solid-state lithium ion secondary batteries, which use solid electrolytes as the electrolyte for lithium ion secondary batteries, have been attracting attention in recent years due to their potential for improved safety, high-speed charging and discharging, and compact casings. Examples of solid electrolytes used in all-solid-state lithium ion secondary batteries include sulfide solid electrolytes.

[0003] The performance of inorganic solid electrolytes such as sulfide solid electrolytes can change significantly due to slight deviations in composition. If the deviation in composition exceeds an allowable error, the electrolyte must be discarded as being out of specification. Therefore, there is a need for a method for suppressing the deviation in composition and producing a sulfide solid electrolyte without wasting raw material resources. For example, Patent Document 1 discloses a method for eliminating the deviation in composition by contacting a sulfide solid electrolyte having a deviation in composition with an alkali metal sulfide or the like using a solid-phase method. Furthermore, even in cases other than when a deviation in composition occurs, there is a need to reuse a sulfide solid electrolyte that has been obtained and separately produce a new sulfide solid electrolyte.

[0004] Japanese Patent Application Publication No. 2015-5372

[0005] However, when reusing the sulfide solid electrolyte by the solid-state method to produce a new sulfide solid electrolyte, the solid-state reaction takes a long time and it is difficult to obtain a sulfide solid electrolyte with a homogeneous composition. Furthermore, the solid-state method is a process that is easily affected by the particle size of the original electrolyte being reused, making it difficult to control the particle size of the newly obtained electrolyte. Therefore, although lithium ion conductivity can be restored, it is difficult to reuse the electrolyte if the particle size falls outside the specified range. Furthermore, in the case of dry reactions, residual solvent adheres to the electrolyte, causing reaction inhibition.

[0006] In view of the above circumstances, an object of the present invention is to provide a method for producing a new sulfide solid electrolyte using a sulfide solid electrolyte.

[0007] The present inventors focused on a method for recycling sulfide solid electrolytes using a melting process. Compared to solid-state processes, the melting process can shorten the reaction time and more easily produce sulfide solid electrolytes with a homogeneous composition. Furthermore, because the sulfide solid electrolyte is first melted, the particle size control problem associated with solid-state processes does not arise. On the other hand, when a sulfide solid electrolyte obtained by wet pulverization is melted and reused, carbon derived from the solvent used in the wet pulverization remains in the electrolyte, and this carbon converts to inorganic carbon (elemental carbon) upon melting. It was found that the residual inorganic carbon in the electrolyte results in electronic conductivity, preventing it from fulfilling its role as a separator in a battery. Based on this knowledge, the present inventors conducted extensive research and discovered that a sulfide solid electrolyte with excellent lithium ion conductivity can be obtained by heating and melting a wet-pulverized sulfide solid electrolyte and then removing carbon from the resulting melt, leading to the completion of the present invention.

[0008] That is, the present invention relates to a method for producing a sulfide solid electrolyte, comprising: heating and melting a wet-pulverized first sulfide solid electrolyte to obtain a melt; removing carbon from the melt; and cooling the melt from which the carbon has been removed to obtain a second sulfide solid electrolyte.

[0009] According to the present invention, a sulfide solid electrolyte having excellent lithium ion conductivity can be produced by a melting method using a sulfide solid electrolyte.

[0010] Fig. 1 is a flowchart of a method for producing a sulfide solid electrolyte according to an embodiment of the present invention. Fig. 2 is a diagram showing the results of AC impedance measurements in Examples.

[0011] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified as desired without departing from the spirit of the present invention. Furthermore, the term "to" indicating a range of values ​​is used to mean that the values ​​before and after it are included as the lower and upper limits.

[0012] <Method for Producing Sulfide Solid Electrolyte> A method for producing a sulfide solid electrolyte according to an embodiment of the present invention (hereinafter also referred to as the present production method) is characterized by including heating and melting a wet-pulverized first sulfide solid electrolyte to obtain a melt, removing carbon from the melt, and cooling the melt from which the carbon has been removed to obtain a second sulfide solid electrolyte.

[0013] A flowchart of this production method is shown in Figure 1. In this production method, a wet-pulverized first sulfide solid electrolyte is heated and melted to obtain a melt, carbon is removed from the melt (step S1), and the melt from which the carbon has been removed is cooled to obtain a second sulfide solid electrolyte (step S2).

[0014] [Wet-Pulverized First Sulfide Solid Electrolyte] In the present production method, a wet-pulverized first sulfide solid electrolyte (hereinafter simply referred to as the first sulfide solid electrolyte) is used. As described above, carbon derived from the solvent used in the wet-pulverization remains in the first sulfide solid electrolyte. Therefore, this carbon as an organic component is converted to inorganic carbon during melting, resulting in the resulting electrolyte exhibiting electronic conductivity. Therefore, in the present invention, a step of removing carbon from the melt is performed to suppress the exhibiting electronic conductivity due to inorganic carbon, thereby producing a sulfide solid electrolyte with excellent lithium ion conductivity.

[0015] An example of a method for producing the first sulfide solid electrolyte will be described below, but the present invention is not limited to this method.

[0016] (Sulfide solid electrolyte raw material) As the first sulfide solid electrolyte, a commercially available sulfide solid electrolyte may be used, or one produced from a sulfide solid electrolyte raw material may be used. As the sulfide solid electrolyte raw material, a commercially available sulfide solid electrolyte raw material may be used, or a sulfide solid electrolyte raw material produced from a material may be used. Furthermore, the sulfide solid electrolyte raw material may be subjected to a conventionally known pretreatment.

[0017] The sulfide solid electrolyte raw material will be specifically described below. The sulfide solid electrolyte raw material usually contains an alkali metal element (R) and a sulfur element (S).

[0018] Examples of the alkali metal element (R) include lithium (Li), sodium (Na), and potassium (K), among which lithium (Li) is preferred. As the alkali metal element (R) source, an appropriate combination of substances (components) containing an alkali metal element, such as an alkali metal element alone or a compound containing an alkali metal element, can be used. Among these, as the lithium element, an appropriate combination of substances (components) containing Li, such as Li alone or a compound containing Li, can be used.

[0019] Examples of substances containing lithium element (Li) include lithium sulfide (Li ２ S), lithium iodide (LiI), lithium carbonate (Li ２ CO ３ ), lithium sulfate (Li ２ SO ４ ), lithium oxide (Li ２ Examples of the material include lithium compounds such as lithium ion (LiO) and lithium hydroxide (LiOH), metallic lithium, etc. As the material containing lithium element (Li), it is preferable to use lithium sulfide from the viewpoint of obtaining a sulfide material.

[0020] As the sulfur element (S) source, an appropriate combination of substances (components) containing S, such as simple S or compounds containing S, can be used. One type of substance containing sulfur element (S) may be used, or two or more types may be used in combination.

[0021] Examples of substances containing sulfur (S) include diphosphorus trisulfide (P ２ S ３ ), diphosphorus pentasulfide (P ２ S ５ ), other sulfur compounds containing phosphorus, elemental sulfur, and compounds containing sulfur. ２ S, CS ２ , iron sulfide (FeS, Fe ２ S ３ , FeS ２ , Fe １－ｘ S, etc.), bismuth sulfide (Bi ２ S ３ ), copper sulfide (CuS, Cu ２ S, Cu １－ｘFrom the viewpoint of obtaining a sulfide material, the substance containing sulfur element (S) is preferably phosphorus sulfide, and diphosphorus pentasulfide (P ２ S ５ These may be used alone or in combination of two or more. Phosphorus sulfide can be considered as a compound that serves both as a substance containing S and a substance containing P, which will be described later.

[0022] From the viewpoint of improving the ionic conductivity of the resulting sulfide solid electrolyte, the sulfide solid electrolyte raw material preferably further contains phosphorus (P). As a source of phosphorus (P), an appropriate combination of P-containing substances (components), such as simple P or P-containing compounds, can be used.

[0023] Examples of substances containing phosphorus (P) include diphosphorus trisulfide (P ２ S ３ ), diphosphorus pentasulfide (P ２ S ５ ) and other phosphorus sulfides, sodium phosphate (Na ３ P.O. ４ As a substance containing elemental phosphorus (P), from the viewpoint of further exerting the effects of the present invention, phosphorus sulfide, which is highly volatile, is preferred, and diphosphorus pentasulfide (P ２ S ５ These may be used alone or in combination of two or more.

[0024] The sulfide solid electrolyte raw material may be obtained as a mixed raw material by, for example, appropriately mixing the above-mentioned substances according to the composition of the target sulfide solid electrolyte. The mixing ratio is not particularly limited. For example, the molar ratio S / R of sulfur (S) to alkali metal (R) in the sulfide solid electrolyte raw material is preferably 0.65 / 0.35 or less, more preferably 0.5 / 0.5 or less, from the viewpoint of improving the ionic conductivity of the resulting sulfide solid electrolyte. Furthermore, the mixed raw material is preferably obtained by mixing in a predetermined stoichiometric ratio according to the substances used for mixing. Examples of the mixing method include mixing in a mortar, mixing using media such as a planetary ball mill, and medialess mixing such as a pin mill, a powder mixer, or airflow mixing.

[0025] An example of a preferred combination of an alkali metal element and a sulfur element contained in the sulfide solid electrolyte raw material is Li ２ S and P ２ S ５ The combination of Li ２ S and P ２ S ５ When these are combined, the molar ratio of Li to P, Li / P, is preferably 40 / 60 or more, more preferably 50 / 50 or more. The molar ratio of Li to P, Li / P, is preferably 88 / 12 or less. The molar ratio of Li to P, Li / P, is preferably 40 / 60 to 88 / 12, more preferably 50 / 50 to 88 / 12. P ２ S ５ Li ２ By adjusting the mixture ratio so that the amount of Li is relatively small compared to S, ２ Melting point of S is ２ S ５ Since the boiling point of the sulfur component is low, it is easy to suppress the volatilization of the sulfur component and the phosphorus component during the heat treatment.

[0026] On the other hand, lithium compounds other than lithium sulfide, metallic lithium, etc. may be used. Specifically, in this case, the sulfide solid electrolyte raw material may be a material containing Li, such as metallic lithium, lithium iodide (LiI), or lithium carbonate (Li ２ CO ３ ), lithium sulfate (Li ２ SO ４ ), lithium oxide (Li ２ It is preferable that the solvent contains at least one selected from the group consisting of lithium hydroxide (LiOH) and lithium nitrate (LiO). These may be used alone or in combination of two or more.

[0027] The sulfide solid electrolyte raw material may contain further substances (compounds, etc.) in addition to the above substances depending on the composition of the target sulfide solid electrolyte or as additives, etc.

[0028] For example, when producing a sulfide solid electrolyte containing a halogen element such as F, Cl, Br, or I, the sulfide solid electrolyte raw material preferably contains a halogen element (Ha). In this case, the sulfide solid electrolyte raw material preferably contains a compound containing a halogen element. Examples of compounds containing a halogen element include lithium halides such as lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), and lithium iodide (LiI), phosphorus halides, phosphoryl halides, sulfur halides, sodium halides, and boron halides. From the viewpoint of the reactivity of the raw material, lithium halides are preferred as the compound containing a halogen element, and LiCl, LiBr, and LiI are more preferred. These compounds may be used alone or in combination of two or more.

[0029] Note that an alkali metal halide such as lithium halide is also a compound containing an alkali metal element such as Li. When the sulfide solid electrolyte raw material contains an alkali metal halide, part or all of the alkali metal element such as Li in the sulfide solid electrolyte raw material may be derived from the alkali metal halide such as lithium halide.

[0030] When the sulfide solid electrolyte raw material contains a halogen element (Ha) and an element phosphorus (P), the molar equivalent of Ha relative to P in the sulfide solid electrolyte raw material is preferably 0.2 molar equivalents or more, and more preferably 0.5 molar equivalents or more, from the viewpoint of improving the ionic conductivity of the resulting sulfide solid electrolyte. Furthermore, from the viewpoint of the stability of the resulting sulfide solid electrolyte, the molar equivalent of Ha is preferably 4 molar equivalents or less, and more preferably 3 molar equivalents or less.

[0031] The obtained sulfide solid electrolyte may be an amorphous sulfide solid electrolyte depending on the purpose. From the viewpoint of improving the ease of forming an amorphous phase, it is preferable that the sulfide solid electrolyte raw material is SiS. ２ , B ２ S ３ , GeS ２ , Al ２ S ３It is also preferable to contain sulfides such as those mentioned above. By facilitating the formation of an amorphous phase, an amorphous sulfide solid electrolyte can be obtained even if the cooling rate is reduced when an amorphous phase is obtained by rapid cooling, and the load on the equipment can be reduced.

[0032] In addition, from the viewpoint of imparting moisture resistance to the sulfide solid electrolyte, SiO ２ , B ２ O ３ , GeO ２ , Al ２ O ３ , P ２ O ５ It is also preferable to contain oxides such as the following. These may be used alone or in combination of two or more.

[0033] (Solid-phase method, melting method) Examples of methods for producing the first sulfide solid electrolyte using the sulfide solid electrolyte raw material include a solid-phase method and a melting method. Hereinafter, the solid-phase method and the melting method will be described separately.

[0034] (Solid-Phase Method) In the solid-phase method, first, the sulfide solid electrolyte raw materials are mixed. Although a conventionally known method can be used for mixing, mixing by mechanical milling is preferred. When using a mechanical milling method using a ball mill, examples include a rotary ball mill that imparts rotational motion to a container, a vibration ball mill that imparts vibrational motion, a planetary ball mill that imparts revolution and rotational motion, a bead mill, and Attritor (registered trademark). Among these, planetary ball mills and bead mills, which have higher mixing and pulverizing power, are preferred. The ball mill may be either dry mixing or wet mixing using a dispersion medium, but dry mixing is preferred from the viewpoint of efficient energy transmission.

[0035] The raw materials are mixed by the above-mentioned mixing to form a raw material mixture. This raw material mixture becomes a precursor of the sulfide solid electrolyte. The precursor may be a homogeneous amorphous intermediate compound that has been made amorphous by adopting much stricter mixing conditions than conventionally. An amorphous intermediate compound means that no XRD peaks originating from the raw materials are observed.

[0036] In this production method, the raw material mixture is wet-pulverized to produce a first sulfide solid electrolyte. Wet-pulverization may be performed together with the above-described mixing (wet mixing) of the sulfide solid electrolyte raw materials. Wet-pulverization involves pulverization in a solvent using media such as alumina or zirconia, a bead mill, or the like, and any method known in the art can be used. In this production method, a carbon-containing solvent is used, and examples of the solvent include alkanes such as heptane, hexane, and octane; aromatic hydrocarbons such as benzene, toluene, and xylene; and ether compounds having 2 to 20 carbon atoms, such as diethyl ether, dibutyl ether, and anisole.

[0037] In wet pulverization, the average particle size of the pulverized sulfide solid electrolyte is preferably 1 to 100 μm. Here, the average particle size refers to the median diameter (D50) at which 50% by volume of the particles are equal to or smaller than this value, which is determined from a volume-based particle size distribution chart obtained by measuring the particle size distribution using a particle size distribution analyzer using a laser diffraction method.

[0038] In the melting method, the sulfide solid electrolyte raw materials are first mixed to obtain a raw material mixture. The mixing may be performed, for example, by mixing in a mortar, mixing using a media such as a planetary ball mill, or medialess mixing such as a pin mill, a powder mixer, or airflow mixing.

[0039] Next, the raw material mixture obtained above is heated to obtain a melt. The specific method for heating and melting the raw material mixture is not particularly limited, and for example, the raw material mixture is placed in a heat-resistant container and heated in a heating furnace. The raw material mixture may be sealed in a heat-resistant container. Alternatively, the melting may be carried out in an atmosphere containing elemental sulfur. Examples of the atmosphere containing elemental sulfur include a mixed gas atmosphere of a gas containing elemental sulfur, such as sulfur gas, hydrogen sulfide gas, or sulfur dioxide gas, and an inert gas.

[0040] The heat-resistant container may be a heat-resistant container made of carbon, a heat-resistant container containing an oxide such as quartz, quartz glass, borosilicate glass, aluminosilicate glass, alumina, zirconia, or mullite, a heat-resistant container containing a nitride such as silicon nitride or boron nitride, or a heat-resistant container containing a carbide such as silicon carbide, etc. Furthermore, these heat-resistant containers may be formed in bulk from the above-mentioned materials, or may be containers on which a layer of carbon, oxide, nitride, carbide, or the like is formed, such as a carbon-coated quartz tube.

[0041] The heating temperature when the raw material mixture is heated and melted varies depending on the raw materials used and the composition of the raw material mixture, but is, for example, preferably 550 to 1000°C, more preferably 600 to 950°C, even more preferably 630 to 900°C, and particularly preferably 650 to 800°C. Here, the heating temperature is preferably 550°C or higher, more preferably 600°C or higher, even more preferably 630°C or higher, and particularly preferably 650°C or higher, from the viewpoint of increasing the meltability of the raw materials and homogenizing the melt in a short period of time. Furthermore, the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, even more preferably 900°C or lower, and particularly preferably 800°C or lower, from the viewpoint of suppressing deterioration of components due to heating, suppressing compositional deviation due to volatilization of components, and further suppressing decomposition.

[0042] The heat-melting time varies depending on the scale, but is preferably 10 minutes to 10 hours, more preferably 30 minutes to 9.5 hours, even more preferably 45 minutes to 9 hours, and particularly preferably 1 to 9 hours. From the viewpoint of smoothly progressing the reaction, the heat-melting time is preferably 10 minutes or more, more preferably 30 minutes or more, even more preferably 45 minutes or more, and particularly preferably 1 hour or more. From the viewpoint of productivity, the heat-melting time is preferably 10 hours or less, more preferably 9.5 hours or less, and even more preferably 9 hours or less.

[0043] The pressure during heat melting is not particularly limited, but for example, normal pressure or slight pressure is preferred, and normal pressure is more preferred. The dew point during heat melting is preferably −20°C or lower, and although there is no particular lower limit, it is usually about −80°C. The oxygen concentration is preferably 1000 ppm by volume or lower. Complete dissolution of the molten material can be confirmed by the absence of peaks derived from crystals in high-temperature X-ray diffraction measurement.

[0044] The molten material is then cooled. The cooling method may be any known method, and is not particularly limited, but in order to increase the cooling rate, it is preferable to use twin rollers, which are generally considered to have the fastest quenching rate.

[0045] From the viewpoint of maintaining the composition obtained by heat melting, the cooling rate is preferably 0.01°C / sec or more, more preferably 0.05°C / sec or more, and even more preferably 0.1°C / sec or more. Although there is no particular upper limit to the cooling rate, the cooling rate of the twin rollers is, for example, 1,000,000°C / sec or less.

[0046] Furthermore, when it is desired to obtain an amorphous sulfide solid electrolyte, it is preferable to rapidly cool the melt obtained by heating and melting. The cooling rate in the case of rapid cooling is preferably 10°C / sec or more, more preferably 100°C / sec or more, even more preferably 500°C / sec or more, and particularly preferably 700°C / sec or more. The upper limit of the cooling rate is not particularly limited, but the cooling rate of the twin rollers is, for example, 1,000,000°C / sec or less.

[0047] On the other hand, slow cooling during the cooling step can be performed to crystallize at least a portion of the solid, thereby obtaining a sulfide solid electrolyte having a specific crystal structure or a sulfide solid electrolyte composed of a crystalline phase and an amorphous phase. When slowly cooling, the cooling rate is preferably 0.01°C / sec or more, more preferably 0.05°C / sec or more. Furthermore, the cooling rate is preferably 500°C / sec or less, more preferably 450°C / sec or less. The cooling rate may be 10°C / sec or less, or may be 5°C / sec or less. The cooling rate may be adjusted appropriately depending on the crystallization conditions. Here, the crystal of the sulfide solid electrolyte is preferably an ion-conductive crystal. Specifically, the ion-conductive crystal is one having a lithium ion conductivity of 1.0 x 10 －４ S / cm, more preferably 1.0×10 －３ The crystals are larger than S / cm.

[0048] Furthermore, when it is desired that the solid obtained after cooling be a sulfide solid electrolyte containing a crystalline phase, it is preferable to include a compound that will become a crystal nucleus in the melt obtained in the heat-melting step. This makes it easier for crystals to precipitate in the cooling step. The method for including the compound that will become a crystal nucleus in the melt is not particularly limited, but examples include adding the compound that will become a crystal nucleus to a raw material or an intermediate, or adding the compound that will become a crystal nucleus to the melt during heat-melting.

[0049] Examples of compounds that can become crystal nuclei include oxides, oxynitrides, nitrides, carbides, other chalcogen compounds, and halides. Compounds that can become crystal nuclei are preferably compounds that have a certain degree of compatibility with the melt. Note that compounds that are completely incompatible with the melt cannot become crystal nuclei.

[0050] When the solid obtained after cooling is intended to be a sulfide solid electrolyte containing a crystalline phase, the content of the compound that will become the crystal nuclei in the melt is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and even more preferably 1% by mass or more. On the other hand, from the viewpoint of suppressing a decrease in lithium ion conductivity, the content of the compound that will become the crystal nuclei in the melt is preferably 20% by mass or less, and more preferably 10% by mass or less.

[0051] When the solid obtained after cooling is to be an amorphous sulfide solid electrolyte, the melt preferably does not contain a compound that will become a crystal nucleus, or the content of the compound is preferably a predetermined amount or less. Specifically, the content of the compound that will become a crystal nucleus in the melt is preferably 1 mass% or less, more preferably 0.1 mass% or less. The content of the compound that will become a crystal nucleus in the melt may be 0.01 mass% or less.

[0052] The cooling is preferably carried out under atmospheric pressure. "Under atmospheric pressure" means that the pressure is not controlled during cooling. Specifically, the pressure is about 0.8 to 1.2 atm.

[0053] In this production method, the raw material mixture is wet-pulverized to form the first sulfide solid electrolyte. The wet-pulverization can be performed in the same manner as described in the solid-phase method.

[0054] (First sulfide solid electrolyte) The crystal structure of the wet-pulverized first sulfide solid electrolyte obtained through the above steps is not particularly limited. For example, ７ P ３ S １１ Sulfide solid electrolytes having a crystal structure containing Li, P, and S elements, called LPS systems, such as Li １０ GeP ２ S １２ Examples of such sulfide solid electrolytes include sulfide solid electrolytes having a crystalline structure containing Li, Ge, P, and S elements, which are called LGPS-based, such as those described above, sulfide solid electrolytes having an argyrodite-type crystalline structure containing Li, P, S, and Ha, sulfide solid electrolytes made of Li-P-S-Ha-based crystallized glass, and sulfide solid electrolytes having other crystalline structures. The above-mentioned sulfide solid electrolytes may include a crystalline phase and an amorphous phase.

[0055] Of the above, the argyrodite-type crystal structure is a compound having the composition formula Ag ８ GeS ６ The sulfide solid electrolyte is not limited to the above crystal structure, and some elements may be substituted with other elements.

[0056] When the sulfide solid electrolyte has an argyrodite-type crystal structure, it is more preferable that the Ha element contains at least one element, and even more preferable that the Ha element contains two or more elements, selected from the group consisting of Cl, Br, and I. Furthermore, it is more preferable that the sulfide solid electrolyte contains at least one of Cl and Br, and even more preferable that the Ha element contains Cl and Br.

[0057] The argyrodite-type crystal structure preferably has the above structure, but the composition formula is Li α P.S. β Ha γ It is preferable that the element ratios satisfy the relationships of 5≦α≦7, 4≦β≦6, and 1.3≦γ≦2. It is more preferable that the element ratios satisfy the relationships of 5.1<α<6.3, 4<β<5.3, and 1.4≦γ≦1.9, and even more preferable that the element ratios satisfy the relationships of 5.2<α<6.2, 4.1<β<5.2, and 1.5≦γ≦1.8. That is, α is preferably 5 or more, more preferably greater than 5.1, even more preferably greater than 5.2, and preferably 7 or less, more preferably less than 6.3, and even more preferably less than 6.2. β is preferably 4 or more, more preferably greater than 4, even more preferably greater than 4.1, and preferably 6 or less, more preferably less than 5.3, and even more preferably less than 5.2. γ is preferably 1.3 or more, more preferably 1.4 or more, even more preferably 1.5 or more, and preferably 2 or less, more preferably 1.9 or less, and even more preferably 1.8 or less.

[0058] In the argyrodite-type crystal structure, some of the S element is replaced by Ha element, O element, Se element, Te element, BH element, etc. ４ , CN, etc. Also, a portion of the P element may be substituted with Si element, Al element, Sn element, In element, Cu element, Sb element, Ge element, etc.

[0059] As described above, the first sulfide solid electrolyte in this production method contains carbon as an organic component derived from the solvent used in wet pulverization. The amount of carbon is, for example, preferably 3% by mass or less, more preferably 1.5% by mass or less, and even more preferably 1% by mass or less, relative to the first sulfide solid electrolyte. The amount of carbon refers to the amount of elemental carbon in the first sulfide solid electrolyte, and can be measured, for example, by oxygen stream combustion-infrared absorption spectroscopy using a sulfur-carbon analyzer (CS844, manufactured by LECO Japan, LLC).

[0060] [Heat Melting] In this production method, the wet-pulverized first sulfide solid electrolyte is heated and melted to obtain a melt. The heat melting method can be the same as the heat melting method in the melting method described above.

[0061] The first sulfide solid electrolyte may be heated and melted together with the sulfide solid electrolyte raw material. In this case, a mixture obtained by mixing the first sulfide solid electrolyte and the sulfide solid electrolyte raw material may be heated and melted, or the sulfide solid electrolyte raw material may be added to a melt obtained by heating and melting the first sulfide solid electrolyte, or the first sulfide solid electrolyte may be added to a melt obtained by heating and melting the sulfide solid electrolyte raw material. From another perspective, this means that when a sulfide solid electrolyte is produced by a melting method using a sulfide solid electrolyte raw material, the sulfide solid electrolyte raw material may be heated and melted together with the wet-pulverized first sulfide solid electrolyte. The sulfide solid electrolyte raw material to be heated and melted together with the first sulfide solid electrolyte may be the same as the sulfide solid electrolyte raw material described in the above section [Wet-Pulverized First Sulfide Solid Electrolyte].

[0062] The temperature for heat-melting the first sulfide solid electrolyte is preferably 600°C or higher, more preferably 700°C or higher, from the viewpoint of homogeneity of the melt. The temperature for heat-melting is preferably 1000°C or lower, more preferably 950°C or lower, even more preferably lower than 900°C, and particularly preferably lower than 800°C, from the viewpoint of suppressing deterioration or decomposition of components in the melt due to heating. The temperature is preferably 600°C or higher but lower than 1000°C.

[0063] [Carbon Removal] In this production method, carbon is removed from a melt obtained by heating and melting the first sulfide solid electrolyte. Here, "removing" carbon from the melt means reducing the amount of carbon contained in the melt, and is not limited to completely eliminating carbon from the melt. For example, the amount of carbon in the melt before and after removal may be reduced by 0.5% by mass or more, or may be reduced by 0.1% by mass or more. The method for removing carbon from the melt is not particularly limited, and examples thereof include the following Method 1 and Method 2.

[0064] (Method 1) As Method 1, a method of removing carbon from the melt by introducing a sulfur source into the melt can be mentioned. By introducing the sulfur source into the melt, carbon in the melt reacts with sulfur in the sulfur source, and the carbon in the melt is converted into CS. ２ The sulfur source may be, for example, elemental sulfur, hydrogen sulfide, organic sulfur compounds such as carbon disulfide, iron sulfide (FeS, Fe ２ S ３ , FeS ２ , Fe １－ｘ S, etc.), bismuth sulfide (Bi ２ S ３ ), copper sulfide (CuS, Cu ２ S, Cu １－ｘ Examples of the sulfur source include sulfur dioxide, sulfur dioxide gas, sulfur vulcanization rubber, sulfur dioxide gas ...

[0065] The sulfur source may be introduced into the melt as a gas, or as a powder of the sulfur source. A conventionally known method can be used to introduce the sulfur source into the melt as a gas, and examples of such a method include a method in which sulfur powder is heated at 200° C. or higher and 400° C. or lower to vaporize the sulfur and introduce the resulting gas into the melt.

[0066] Examples of methods for introducing the sulfur source powder into the melt include adding the powder directly above the melt using a metered feeder. The average particle size of the powder is preferably 1 to 5,000 μm from the viewpoint of homogenization. The average particle size is preferably 1 μm or more, more preferably 10 μm or more, and even more preferably 100 μm or more, and is preferably 5,000 μm or less, more preferably 2,500 μm or less, and even more preferably 1,000 μm or less. Here, the average particle size refers to the median diameter (D50) at which 50 volume percent of the particles are equal to or less than this value, as determined from a volume-based particle size distribution chart obtained by measuring the particle size distribution using a particle size distribution analyzer using a laser diffraction method.

[0067] The carbon amount A in the first sulfide solid electrolyte, i.e., the sulfur amount B introduced relative to the carbon amount A in the melt, preferably satisfies a weight ratio of A / B≦0.2, more preferably A / B≦0.1. Here, the carbon amount A refers to the elemental carbon amount in the first sulfide solid electrolyte or the melt. The sulfur amount B refers to the elemental sulfur amount in the introduced sulfur source. A / B is more preferably 0.05 or less, even more preferably 0.03 or less, and particularly preferably 0.01 or less. The carbon amount A and the sulfur amount B can be measured, for example, by oxygen stream combustion-infrared absorption spectroscopy using a sulfur-carbon analyzer (CS844, manufactured by LECO Japan, LLC).

[0068] In Method 1, by introducing a sulfur source into the melt, carbon in the melt can be removed, and the introduction of sulfur can also suppress changes in the sulfur composition due to volatilization.

[0069] (Method 2) As Method 2, a method of removing carbon from the melt by introducing a sulfur source into the first sulfide solid electrolyte and heating and melting the first sulfide solid electrolyte can be mentioned. By introducing a sulfur source into the first sulfide solid electrolyte, the carbon in the melt reacts with the sulfur in the sulfur source during subsequent heating and melting, and the carbon in the melt becomes CS. ２The sulfur source and the method for introducing it can be the same as those in the above-mentioned Method 1. In particular, in Method 2, it is preferable to use sulfur powder as the sulfur source, and it is also preferable to introduce (add) the sulfur source powder to the first sulfide solid electrolyte and then mix it.

[0070] In Method 2, the weight ratio of the amount of sulfur B introduced relative to the amount of carbon A in the first sulfide solid electrolyte preferably satisfies A / B≦0.2, and more preferably A / B≦0.1. Here, the carbon amount A refers to the amount of carbon in the first sulfide solid electrolyte or in the melt. The sulfur amount B refers to the amount of sulfur in the sulfur source introduced. A / B is more preferably 0.05 or less, even more preferably 0.03 or less, and particularly preferably 0.01 or less. The carbon amount A and the sulfur amount B can be measured, for example, by oxygen stream combustion-infrared absorption spectroscopy using a sulfur-carbon analyzer (CS844, manufactured by LECO Japan, LLC).

[0071] In Method 2, similarly to Method 1, by introducing a sulfur source into the first sulfide solid electrolyte, carbon can be removed from the melt during the subsequent heating and melting. In addition, the introduction of sulfur can suppress changes in the sulfur composition due to volatilization.

[0072] [Cooling] This production method includes a step of cooling the melt from which carbon has been removed by the above method. This results in the production of the second sulfide solid electrolyte described below. The melt can be cooled by the same method as described in the above section [Wet-pulverized first sulfide solid electrolyte].

[0073] [Second sulfide solid electrolyte] As described above, the melt from which carbon has been removed is cooled to obtain a second sulfide solid electrolyte. Examples of the second sulfide solid electrolyte include those described in the section [Wet-pulverized first sulfide solid electrolyte] above.

[0074] The lithium ion conductivity of the second sulfide solid electrolyte is preferably 1.0 × 10 from the viewpoint of improving the battery characteristics when used in a lithium ion secondary battery. －４ S / cm or more is preferable, and 5.0 × 10 －４S / cm or more is more preferable, and 1.0×10 －３ S / cm or more is more preferable, and 5.0 × 10 －３ The lithium ion conductivity is measured using an AC impedance measuring device (for example, Potentiostat / Galvanostat VSP manufactured by Bio-Logic Sciences Instruments) under the following measurement conditions: measurement frequency: 100 Hz to 1 MHz, measurement voltage: 100 mV, and measurement temperature: 25°C.

[0075] The obtained second sulfide solid electrolyte can be identified by analyzing its crystal structure using X-ray diffraction (XRD) measurement, or by analyzing its elemental composition using various methods such as ICP optical emission spectrometry, atomic absorption spectrometry, and ion chromatography. For example, P and S can be measured by ICP optical emission spectrometry, Li can be measured by atomic absorption spectrometry, and Ha can be measured by ion chromatography.

[0076] [Annealing Treatment] When the second sulfide solid electrolyte obtained by cooling the melt from which carbon has been removed is an amorphous sulfide solid electrolyte or a sulfide solid electrolyte containing an amorphous phase, the present manufacturing method may further include annealing the second sulfide solid electrolyte. Furthermore, annealing a sulfide solid electrolyte containing sulfide solid electrolyte crystals can rearrange ions within the crystal structure and increase lithium ion conductivity. The annealing treatment refers to at least one of annealing the second sulfide solid electrolyte obtained by cooling to crystallize it and rearranging ions within the crystal structure. Here, the heating treatment of these amorphous sulfide solid electrolytes or sulfide solid electrolytes containing an amorphous phase is referred to as the annealing treatment, including the crystallization treatment.

[0077] The annealing treatment is preferably carried out at 340° C. or higher, more preferably at 380° C. or higher, but may also be carried out at 550° C. or lower, or at 500° C. or lower.

[0078] [Solid-State Reaction] In the present production method, the composition of the second sulfide solid electrolyte may be further adjusted by appropriately adding other sulfide solid electrolytes or sulfide solid electrolyte raw materials to the second sulfide solid electrolyte. As a method for adjusting the composition, for example, the method described in JP 2015-5352 A or the like can be adopted.

[0079] [Continuous Process] In this production method, it is preferable that the heating and melting of the first sulfide solid electrolyte and the cooling of the melt from which carbon has been removed are performed as a continuous process. For example, the melt from which carbon has been removed obtained in the furnace is continuously discharged and transferred to a cooling step to continuously produce the second sulfide solid electrolyte. This allows a series of steps, including the introduction of the first sulfide solid electrolyte, heating and melting, discharging the melt, and cooling, to be performed continuously, allowing the second sulfide solid electrolyte to be produced in large quantities more efficiently in a short period of time. Specifically, it is preferable that a series of steps including the heating and melting of the first sulfide solid electrolyte raw material in the furnace to obtain a melt, removing carbon from the melt, and discharging the melt from which carbon has been removed from the furnace and cooling the melt to obtain the second sulfide solid electrolyte be performed continuously.

[0080] <Method for Reusing Sulfide Solid Electrolyte> A method for reusing a sulfide solid electrolyte according to an embodiment of the present invention (hereinafter also referred to as the present recycling method) is characterized by comprising heating and melting a wet-pulverized sulfide solid electrolyte to obtain a melt, removing carbon from the melt, and cooling the melt from which the carbon has been removed to obtain a recycled sulfide solid electrolyte.

[0081] This recycling method includes the same steps as the above-described production method, and the "wet-pulverized sulfide solid electrolyte" in this recycling method corresponds to the "wet-pulverized first sulfide solid electrolyte" in this production method, and the "recycled sulfide solid electrolyte" corresponds to the "second sulfide solid electrolyte."

[0082] The present invention is not limited to the above-described embodiments, and various modifications can be adopted within the scope of the present invention. For example, the present invention is not limited to the above-described embodiments, and modifications and improvements can be made as appropriate. In addition, the material, shape, size, number, and location of each component in the above-described embodiments are arbitrary and not limited as long as the present invention can be achieved.

[0083] As described above, the present specification discloses the following: [1] A method for producing a sulfide solid electrolyte, comprising: heating and melting a wet-pulverized first sulfide solid electrolyte to obtain a melt, removing carbon from the melt, and cooling the melt from which the carbon has been removed to obtain a second sulfide solid electrolyte. [2] A method for producing a sulfide solid electrolyte according to [1] above, in which carbon is removed from the melt by introducing a sulfur source to the melt. [3] A method for producing a sulfide solid electrolyte according to [1] or [2] above, in which carbon is removed from the melt by introducing a sulfur source to the wet-pulverized first sulfide solid electrolyte and heating and melting it. [4] A method for producing a sulfide solid electrolyte according to [2] or [3] above, in which an amount of sulfur B introduced relative to an amount of carbon A in the wet-pulverized first sulfide solid electrolyte satisfies a weight ratio of A / B≦0.2. [5] A method for producing a sulfide solid electrolyte according to any one of [1] to [4] above, wherein the wet-pulverized first sulfide solid electrolyte is heated and melted at 600°C or higher to obtain the melt. [6] A method for producing a sulfide solid electrolyte according to any one of [1] to [5] above, wherein the wet-pulverized first sulfide solid electrolyte is heated and melted together with a sulfide solid electrolyte raw material to obtain the melt. [7] A method for producing a sulfide solid electrolyte according to [6] above, wherein the wet-pulverized first sulfide solid electrolyte and the sulfide solid electrolyte raw material are mixed, and the resulting mixture is heated and melted to obtain the melt. [8] A method for recycling a sulfide solid electrolyte, comprising: heating and melting a wet-pulverized sulfide solid electrolyte to obtain a melt, removing carbon from the melt, and cooling the melt from which the carbon has been removed to obtain a recycled sulfide solid electrolyte.

[0084] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these. Example 1 is a comparative example, Examples 2 and 3 are examples of the present production method, and Example 4 is a reference example.

[0085] (Lithium Ion Conductivity Evaluation) The powder obtained in each example was further crushed in a mortar and passed through a 100 μm mesh to obtain a sulfide solid electrolyte powder having a D50 of 10 μm according to the volumetric particle size distribution. The powder was then pressed under a pressure of 380 MPa to form a measurement sample. The lithium ion conductivity was measured using an AC impedance measuring device (Bio-Logic Sciences Instruments, potentiostat / galvanostat VSP) and evaluated according to the following criteria. The measurement conditions were: measurement frequency: 100 Hz to 1 MHz, measurement voltage: 100 mV, measurement temperature: 25°C. The results of each example are shown in Table 1. Note that the "-" in Table 1 indicates that, as shown in FIG. 2, in Example 1, there is no rise in the imaginary component Z' on the low frequency side in the AC impedance measurement, and electron blocking is not occurring at the electrolyte-electrode interface. Therefore, it is highly likely that the electrical conductivity function includes not only ionic conduction but also electronic conduction, and accurate measurement was not possible.

[0086] (Carbon content measurement) The carbon content of the wet-pulverized first sulfide solid electrolyte and the finally obtained sulfide solid electrolyte was measured by oxygen stream combustion-infrared absorption method using a sulfur-carbon analyzer (CS844, manufactured by LECO Japan, LLC). The results for each example are shown in Table 1.

[0087] (Appearance Color) The appearance color of the finally obtained sulfide solid electrolyte was visually inspected. The results for each example are shown in Table 1.

[0088] (Production of sulfide solid electrolyte) Examples 1 to 3 (1) First, sulfide solid electrolyte pellets (argyrodite-type crystal structure, composition: Li ５．４ P.S. ４．４ Cl ０．８ Br ０．８(AGC, no composition deviation) was crushed in a cutter mill to obtain a dry powder of coarse particles, which was then crushed in a mortar (coarse grinding) by hand. The particles were then passed through a 100 μm mesh sieve to select sulfide solid electrolyte particles of 100 μm or less. (2) Next, a liquid medium for fine grinding was prepared. The liquid medium was a mixed solvent (8 g) of 5 g of heptane (Kanto Chemical, special grade (dehydrated - Super-)) and 3 g of dibutyl ether (Tokyo Chemical Industry, stabilized with BHT). The mixed solvent was prepared by adding molecular sieves 4A 1 / 16 (Fujifilm Wako Pure Chemical Industries, Ltd.) and leaving it overnight or longer. This pre-dehydrated product was used. (3) Next, 8 g of the mixed solvent, 2 g of the selected sulfide solid electrolyte, and 40 g of high-purity alumina balls (AL9 series, manufactured by AS ONE Corporation) with a diameter of 1 mm were placed in a 45 mL zirconia container (manufactured by Ito Seisakusho Co., Ltd.). The container was then placed in a small planetary ball mill LP-M4 manufactured by Ito Seisakusho Co., Ltd., and wet-pulverized for 60 minutes at a rotation speed of 200 rpm. Thereafter, in a glove box, the slurry and the alumina balls were separated using a stainless steel sieve (100 μm mesh). Furthermore, the mixed solvent was poured over the balls remaining on the sieve several times to wash away the slurry adhering to the ball surface and collect it. (4) After that, the mixture was left to stand for 3 to 5 hours, and the supernatant liquid was removed. The resulting slurry was placed in a bottle and placed in a separable flask (500 mL) inserted into a mantle heater in a glove box. Then, N ２While flowing gas at 1.5 L / min, the mixture was heated at 150°C for 2 hours, and then further heated at 190°C for 4 hours for drying. (5) The above (2) to (4) were performed a total of four times, and the resulting pulverized material was crushed together in a cutter mill to obtain a dry powder of coarse particles. The dry powder was then crushed in a hand mortar (coarse crushing). A sulfide solid electrolyte having a particle size of 100 μm or less was selected using a 100 μm mesh sieve, yielding a wet-pulverized first sulfide solid electrolyte. (6) 2 g of the resulting electrolyte was placed in a quartz test tube, and elemental sulfur powder (Hosoi Chemical Co., Ltd., sulfur powder 16-60 Mesh) was added to the electrolyte in the amount shown in Table 1 and mixed. (7) The quartz test tube was heated and melted at 730°C for 1 hour. (8) After the heating and melting, the melt was allowed to cool naturally to obtain a second sulfide solid electrolyte. (9) The obtained second sulfide solid electrolyte was annealed for 30 minutes at 450° C. In this manner, the sulfide solid electrolytes of Examples 1 to 3 were obtained.

[0089] Example 4 The sulfide solid electrolyte pellets used in Examples 1 to 3 (argyrodite-type crystal structure, composition: Li ５．４ P.S. ４．４ Cl ０．８ Br ０．８ , manufactured by AGC) was used as the sulfide solid electrolyte of Example 4.

[0090]

[0091] In Examples 2 and 3, which are working examples, a step of removing carbon from the melt obtained by heating and melting the wet-pulverized first sulfide solid electrolyte was performed. Therefore, the amount of carbon derived from the solvent used during wet-pulverization was reduced. As a result, the lithium ion conductivity of the resulting second sulfide solid electrolyte was high. Furthermore, in Examples 2 and 3, due to the removal of carbon, the external color was white or yellow, rather than the black color derived from carbon (soot). On the other hand, in Example 1, which is a comparative example, the step of removing carbon from the melt was not performed. Therefore, the amount of carbon derived from the solvent used during wet-pulverization was high, and the lithium ion conductivity of the finally obtained sulfide solid electrolyte was low. Furthermore, since carbon was not removed from Example 1, the external color was black, derived from carbon (soot). Note that Example 4 is a reference example, and since wet-pulverization was not performed in Example 4, the carbon amount in the sulfide solid electrolyte was low.

[0092] According to the present invention, a sulfide solid electrolyte having excellent lithium ion conductivity can be produced by a melting method using a sulfide solid electrolyte.

[0093] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-112311) filed on July 7, 2023, the contents of which are incorporated herein by reference.

Claims

1. A method for producing a sulfide solid electrolyte, comprising: heating and melting a wet-ground first sulfide solid electrolyte to obtain a melt, and removing carbon from the melt; and cooling the melt from which the carbon has been removed to obtain a second sulfide solid electrolyte.

2. The method for producing a sulfide solid electrolyte according to claim 1, further comprising removing carbon from the melt by introducing a sulfur source into the melt.

3. The method for producing a sulfide solid electrolyte according to claim 1, further comprising the steps of: introducing a sulfur source into the wet-ground first sulfide solid electrolyte and heating and melting the first sulfide solid electrolyte, thereby removing carbon from the melt.

4. The method for producing a sulfide solid electrolyte according to claim 2 or 3, wherein an amount of sulfur B to be introduced relative to an amount of carbon A in the wet-pulverized first sulfide solid electrolyte satisfies a weight ratio of A / B≦0.

2.

5. The method for producing a sulfide solid electrolyte according to claim 1 or 2, wherein the wet-ground first sulfide solid electrolyte is heated and melted at 600° C. or higher to obtain the molten liquid.

6. The method for producing a sulfide solid electrolyte according to claim 1 or 2, wherein the wet-ground first sulfide solid electrolyte is heated and melted together with a sulfide solid electrolyte raw material to obtain the melt.

7. The method for producing a sulfide solid electrolyte according to claim 6, further comprising heating and melting a mixture obtained by mixing the wet-pulverized first sulfide solid electrolyte with the sulfide solid electrolyte raw material to obtain the molten liquid.

8. A method for reusing a sulfide solid electrolyte, comprising: heating and melting a wet-pulverized sulfide solid electrolyte to obtain a melt, and removing carbon from the melt; and cooling the melt from which the carbon has been removed to obtain a regenerated sulfide solid electrolyte.

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