Method for producing sulfide solid electrolyte

Abstract

The present invention relates to a method for producing a sulfide solid electrolyte, comprising a step for mixing a raw material containing an element Li, a raw material containing an element P, and a raw material containing an element S to obtain a raw material mixture, and a step for heat-treating the raw material mixture, in which Li2Sx (0.05 < = x < = 0.95) is used as the raw material containing the element S, and Li2Sx (0.05 < = x < = 0.95) is used as the raw material containing the element S; the heat treatment is performed by introducing a gas containing an element S, and the cumulative introduction amount Y (mass%) of the element S in the gas containing the element S with respect to the mass of the raw material mixture satisfies the relationship Y > = x2-6.5 x + 5.8 with respect to x in the Li2Sx.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a sulfide solid electrolyte. Background Art

[0002] Lithium ion secondary batteries are widely used in portable electronic devices such as mobile phones and laptop computers.

[0003] Conventionally, liquid electrolytes have been used in lithium ion secondary batteries, but there are concerns such as leakage and fire, and for safety design, it is necessary to increase the size of the case. In addition, there is also a desire to improve the problems of short battery life and narrow operating temperature range.

[0004] In response to this, all-solid-state lithium secondary batteries using a solid electrolyte as the electrolyte of a lithium ion secondary battery have attracted attention in terms of expected improvement in safety, high-speed charge and discharge, and miniaturization of the case.

[0005] Solid electrolytes are roughly classified into sulfide solid electrolytes and oxide solid electrolytes. Sulfide ions constituting the sulfide solid electrolyte have a larger polarizability than oxide ions constituting the oxide solid electrolyte, and exhibit high ionic conductivity. As sulfide solid electrolytes, Li 10 GeP 2 S 12 and other LGPS-type crystals, Li 6 PS 5 Cl and other argyrodite-type crystals, Li 7 P 3 S 11 and other LPS glass-ceramics such as LPS glass-ceramics are known.

[0006] Lithium sulfide, which is a raw material for sulfide solid electrolytes, originally exists as L 2 S, but if it is stored in dry air or a nitrogen atmosphere at room temperature and a dew point of about -60 to -40 °C for about half a year, sulfur will naturally be lost, and the value of x when representing lithium sulfide as L 2 S x is less than 1.

[0007] If such sulfur-deficient lithium sulfide is used as a raw material to manufacture a sulfide solid electrolyte, the crystal constituting the sulfide solid electrolyte also becomes in a sulfur-deficient state, and the lithium ion conductivity decreases.

[0008] On the other hand, in order to achieve high lithium ion conductivity, if high-purity and high-quality L 2 S is used, the sulfide compound itself is expensive and the cost increases.

[0009] In contrast, Non-Patent Document 1 discloses a method using a solution synthesis method, by adding sulfur (S 8)Stabilize Li 2 S to obtain a sulfide solid electrolyte that suppresses the loss of S atoms.

[0010] Prior art documents

[0011] Non-patent literature

[0012] Non-patent literature 1: M-J Kim et al., A Novel Strategy to Overcome the Hurdle for Commercial All-Solid-State Batteries via Low-Cost Synthesis of Sulfide Solid Electrolytes, Small Methods, 2021, 5, 2100793 Summary of the invention

[0013] However, the method described in Non-patent literature 1 is a solution synthesis method, and the synthesis time required for the solution synthesis and drying processes is long. From the perspective of cost, further improvement is needed.

[0014] Therefore, an object of the present invention is to provide a manufacturing method that can obtain a sulfide solid electrolyte with high lithium ion conductivity in a short time even when using low-quality lithium sulfide lacking S atoms as a raw material.

[0015] The inventors of the present invention repeatedly conducted in-depth research and found that in the method of manufacturing a sulfide solid electrolyte by sintering the raw materials by heating or by melting and cooling by heating, when heating, by introducing a gas containing S element with the cumulative introduction amount of S element set within a specific range, the above problems can be solved, and thus the present invention was completed.

[0016] That is, the present invention relates to the following [1] to [9].

[0017] [1] A method for manufacturing a sulfide solid electrolyte, comprising a step of mixing a raw material containing a Li element, a raw material containing a P element, and a raw material containing an S element to obtain a raw material mixture, and a step of heating the raw material mixture.

[0018] As the raw material containing an S element, use Li 2 S x (0.05 ≤ x ≤ 0.95).

[0019] The above heating treatment is carried out by introducing a gas containing an S element.

[0020] The cumulative introduction amount Y (mass %) of the S element in the gas containing the S element with respect to the mass of the above raw material mixture with respect to the above Li 2 S x the above x in satisfies

[0021] Y ≥ x 2 -6.5x + 5.8 relationship.

[0022] [2] The method for manufacturing a sulfide solid electrolyte according to the above [1], wherein, further comprising the following steps:

[0023] Obtain a melt of the above raw material mixture by the above heat treatment, and then cool the above melt to precipitate crystals.

[0024] [3] The method for manufacturing a sulfide solid electrolyte according to the above [1], wherein crystals are precipitated by the above heat treatment.

[0025] [4] The method for manufacturing a sulfide solid electrolyte according to any one of the above [1] to [3], wherein,

[0026] the above raw material mixture further contains a raw material containing the Ha element,

[0027] the above Ha element is at least one element selected from F, Cl, Br, and I,

[0028] the above sulfide solid electrolyte contains a thiogermanate-type crystal structure.

[0029] [5] The method for manufacturing a sulfide solid electrolyte according to the above [4], wherein the lattice constant of the above thiogermanate-type crystal structure is

[0030] [6] The method for manufacturing a sulfide solid electrolyte according to any one of the above [1] to [5], wherein the above Li 2 S x the above x in is 0.05 ≤ x ≤ 0.90.

[0031] [7] The method for manufacturing a sulfide solid electrolyte according to any one of the above [1] to [6], wherein, as the gas containing the S element, it contains sulfur gas.

[0032] [8] The method for manufacturing a sulfide solid electrolyte according to any one of the above [1] to [7], wherein the cumulative introduction amount Y of the S element in the gas containing the S element is 2 mass % or more.

[0033] [9] The manufacturing method of the sulfide solid electrolyte according to any one of [1] to [8] above, wherein the lithium ion conductivity of the sulfide solid electrolyte is 2 mS / cm or more.

[0034] According to the present invention, even when using low-quality lithium sulfide lacking S atoms as a raw material, a high lithium ion conductivity can be obtained in a short time. Therefore, a high-quality sulfide solid electrolyte can be obtained at low cost. The manufacturing cost of the all-solid-state lithium secondary battery containing the sulfide solid electrolyte can also be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a flowchart showing the manufacturing method of the sulfide solid electrolyte according to the present embodiment.

[0036] Figure 2 is a flowchart showing the manufacturing method of the sulfide solid electrolyte according to the present embodiment.

[0037] Figure 3 is a flowchart showing the manufacturing method of the sulfide solid electrolyte according to the present embodiment.

[0038] Figure 4 is the XRD pattern of the sulfide solid electrolyte of Example 1.

[0039] Figure 5 is the XRD pattern of the sulfide solid electrolyte of Example 2.

[0040] Figure 6 is the XRD pattern of the sulfide solid electrolyte of Example 3.

[0041] Figure 7 is the XRD pattern of the sulfide solid electrolyte of Example 4.

[0042] Figure 8 is the XRD pattern of the sulfide solid electrolyte of Example 5.

[0043] Figure 9 is the XRD pattern of the sulfide solid electrolyte of Example 6.

[0044] Figure 10 is a graph showing the cumulative introduction amount Y (mass%) of the S element in the gas containing the S element with respect to the mass of the raw material mixture, and the value of x in Li 2 S x wherein, showing the quadratic function curve of Y = x 2 -6.5x + 5.8, showing the quadratic function curve of Y = x 2 -6.5x + 6.2, and the graphs of each example and comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0045] Hereinafter, the present invention will be described in detail. However, the present invention is not limited to the following embodiments and can be arbitrarily modified and implemented without departing from the gist of the present invention. In addition, "~" indicating a numerical range is used to mean including the numerical values described before and after it as the lower limit value and the upper limit value.

[0046] In this specification, "mass" has the same meaning as "weight", and "mass%" is synonymous with "weight%".

[0047] <Manufacturing Method of Sulfide Solid Electrolyte>

[0048] As Figure 1 shown, the manufacturing method of the sulfide solid electrolyte according to this embodiment sequentially includes the following steps S1 and S2.

[0049] Step S1 is a step of mixing a raw material containing Li element, a raw material containing P element, and a raw material containing S element to obtain a raw material mixture.

[0050] Step S2 is a step of heat-treating the raw material mixture obtained in step S1.

[0051] As Figure 2 shown, step S2 may be a step of performing heat melting as heat treatment in step S2a to obtain a melt of the raw material mixture obtained in step S1. In this case, next, as step S3a, it further includes a step of cooling the above melt and precipitating crystals, thereby obtaining the sulfide solid electrolyte of this embodiment.

[0052] As Figure 3 shown, step S2 may also be a step of obtaining a sintered body of the raw material mixture obtained in step S1 through heat treatment in step S2b, and this sintered body becomes the sulfide solid electrolyte of this embodiment.

[0053] Each step will be described.

[0054] · Step S1

[0055] In step S1, a raw material containing Li element, a raw material containing P element, and a raw material containing S element are mixed to obtain a raw material mixture.

[0056] In the case of obtaining a sulfide solid electrolyte having a thiogermanate-type crystal structure, in addition to the raw material containing Li element, the raw material containing P element, and the raw material containing S element, the raw material further includes a raw material containing Ha element. In this specification, the Ha element refers to at least one element selected from F, Cl, Br, and I.

[0057] Depending on the composition of the desired sulfide solid electrolyte, raw materials containing other elements may also be included. By "depending on the composition of the sulfide solid electrolyte" is meant that, for example, when a part of elements such as Li element, P element, S element, etc. is replaced by other elements, raw materials containing the replaced other elements may also be included.

[0058] As other elements, for example, Si element, Al element, Sn element, In element, Cu element, Sb element, Ge element, O element, etc. can be cited.

[0059] Among the raw materials, as the raw material containing S element, lithium sulfide represented by Li 2 S x (0.05 ≤ x ≤ 0.95) is used. This lithium sulfide is a lithium sulfide in which a part of S element is missing relative to lithium sulfide represented by x = 1, i.e., Li 2 S, and the value of x is 0.95 or less.

[0060] Li 2 S x For the lithium sulfide represented by, the value of x can be 0.05 to 0.95, preferably 0.10 to 0.93, more preferably 0.20 to 0.90. Here, from the viewpoint of suppressing the cost of raw materials, the value of x is preferably 0.93 or less, more preferably 0.90 or less. In addition, from the viewpoint of improving the efficiency of compensating for S deficiency, the value of x is preferably 0.10 or more, more preferably 0.20 or more.

[0061] When using Li 2 S x as the raw material containing S element, as the raw material containing Li element, the raw material containing P element, and as the raw material containing Ha element according to the desire, and the raw material containing other elements, conventionally known substances can be used. In addition, a raw material containing S element other than Li 2 S x can also be used.

[0062] Specifically, Li metal, Li-containing compounds, P metal, P-containing compounds, S-containing compounds according to the desire, and Ha-containing compounds according to the desire can be appropriately combined and used. Furthermore, when O element is contained, oxides can also be used as the above compounds.

[0063] In addition, the above compounds may also be compounds containing two or more of Li, P, S, and Ha at the same time.

[0064] For example, the above Li 2 S xIt is a compound containing both raw materials containing S and raw materials containing Li. This does not exclude the further use of Li 2 S x As a raw material containing the Li element, a compound other than Li can be used, and a single substance of Li and other compounds containing Li can also be used in combination.

[0065] As another example, phosphorus pentasulfide (P) is a compound that serves as both a raw material containing the P element and a raw material containing the S element. 2 S 5 ) etc. In addition, as a compound serving as both a raw material containing a Li element and a raw material containing a Ha element, lithium halide can be cited.

[0066] As the Li-containing compound, Li 2 S x , but in addition, lithium carbonate (Li 2 CO 3 ), lithium sulfate (Li 2 SO 4 ), lithium oxide (Li 2 O), lithium hydroxide (LiOH), lithium halide (LiHa) and the like. These may be used alone or in combination of two or more.

[0067] As the compound containing S, Li 2 S x In addition, phosphorus trisulfide (P 2 S 3 ), phosphorus pentasulfide (P 2 S 5 ) and other phosphorus sulfides, other sulfur compounds containing phosphorus, and compounds containing sulfur. Examples of compounds containing sulfur include H 2 S.C.S. 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－x S, etc.). When using Li 2 S x In the case of a compound containing S other than sulfide, phosphorus sulfide is preferred, and phosphorus pentasulfide (P 2 S 5) These can be used alone or in combination of two or more. It should be noted that phosphorus sulfide is a compound that serves as both a compound containing S and a compound containing P.

[0068] As the compound containing P, for example, diphosphorus trisulfide (P 2 S 3 ), phosphorus pentasulfide (P 2 S 5 ) and other phosphorus sulfides, sodium phosphate (Na 3 PO 4 ) and other phosphorus compounds can be cited. Among them, from the viewpoint of preventing elements other than those constituting the target sulfide solid electrolyte, phosphorus sulfide is preferred, and phosphorus pentasulfide (P 2 S 5 ) is more preferred. These can be used alone or in combination of two or more.

[0069] As the compound containing Ha, for example, lithium halides (LiHa) such as lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), phosphorus halides, phosphoryl halides, sulfur halides, sodium halides, boron halides, etc. can be cited. Among them, from the viewpoint of preventing elements other than those constituting the target sulfide solid electrolyte, lithium halide is preferred, LiCl, LiBr, LiI are more preferred, and LiCl, LiBr are further preferred. These compounds can be used alone or in combination of two or more.

[0070] As an optional component, as a raw material containing Si element, for example, SiO 2 , SiS 2 can be cited. Among them, from the viewpoint of lithium ion conductivity, SiO 2 is more preferred. These compounds can be used alone or in combination of two or more.

[0071] As an optional component, as a raw material containing Al element, for example, AL 2 S 3 , AL 2 O 3 , AlCL 3 can be cited. Among them, from the viewpoint of lithium ion conductivity, AL 2 S 3 , AlCL 3 are preferred, and AL 2 S 3 is more preferred. These compounds can be used alone or in combination of two or more.

[0072] As an optional component, as a raw material containing Sn element, for example, SnS, SnS 2 , SnO, SnO2 、SnCL 2 。Among them, from the perspective of lithium ion conductivity, SnS is preferred 2 、SnCL 2 , and SnS is more preferred 2 。These compounds can be used alone or in combination of two or more

[0073] As an optional component, as a raw material containing In element, for example, In 2 O 3 、In 2 S 3 、InCL 3 。Among them, from the perspective of lithium ion conductivity, In 2 S 3 、InCL 3 , and In 2 S 3 。These compounds can be used alone or in combination of two or more

[0074] As an optional component, as a raw material containing Cu element, for example, Cu 2 O, CuO, Cu 2 S, CuS, CuCL 2 。Among them, from the perspective of lithium ion conductivity, CuS, CuCL 2 , and CuS is more preferred. These compounds can be used alone or in combination of two or more

[0075] As an optional component, as a raw material containing Sb element, for example, Sb 2 O 3 、Sb 2 S 3 、SbCL 3 。Among them, from the perspective of lithium ion conductivity, Sb 2 S 3 、SbCL 3 , and Sb 2 S 3 。These compounds can be used alone or in combination of two or more

[0076] As an optional component, as a raw material containing Ge element, for example, GeO 2 、GeS、GeS 2 、GeCL 2 。Among them, from the perspective of lithium ion conductivity, GeS 2 、GeCL 2 , and GeS is more preferred 2These compounds can be used alone or in combination of two or more.

[0077] Mixing of raw materials can be carried out, for example, by mixing in a mortar, mixing using a medium such as a planetary ball mill, a needle mill, a powder blender, or medium-free mixing such as pneumatic mixing. The raw materials can be made amorphous by mixing before heating.

[0078] In particular, in the case of obtaining a sulfide solid electrolyte by heat sintering in step S2b described later, it is preferable to make it amorphous by mixing in step S1 as a precursor of the sulfide solid electrolyte. In this case, it is preferable to mix the raw materials using a medium, and among them, a planetary ball mill, an attritor (a grinder, registered trademark), or a bead mill with higher mixing force and grinding force is more preferable.

[0079] · Step S2

[0080] In step S2, the raw material mixture obtained in step S1 is heat-treated. In step S2, as step S2a, a melt can be obtained by heat melting, and as step S2b, a sulfide solid electrolyte can be obtained by heat sintering. However, in either case, the heat treatment is carried out by introducing a gas containing an S element. Among the above, from the viewpoint of obtaining the desired sulfide solid electrolyte in a shorter time, it is preferable to adopt the method of obtaining a melt by heat melting as step S2a.

[0081] The cumulative introduction amount Y (mass%) of the S element in the gas containing the S element with respect to the mass of the raw material mixture satisfies Y≥x 2 S x in Li 2 -6.5x + 5.8 relationship.

[0082] The inventors have found that even when using the heat melting method or the solid-phase synthesis method without using the solution synthesis method disclosed in Non-Patent Document 1, by carrying out heat treatment by introducing a gas containing an S element in which the cumulative introduction amount Y of the S element with respect to the mass of the raw material mixture satisfies the above relational expression, a sulfide solid electrolyte excellent in lithium ion conductivity with suppressed S atom deficiency can be obtained in a short time.

[0083] The appropriate range of the cumulative introduction amount Y (mass%) of the S element in the gas containing the S element with respect to the mass of the raw material mixture varies depending on the degree of S atom deficiency in lithium sulfide L 2 S x as the raw material. Therefore, the inventors have determined according to lithium sulfide L 2 S xBased on the degree of S atom deficiency, the cumulative introduction amount Y (mass%) of S element in the gas containing S element relative to the mass of the raw material mixture, and the results of the lithium ion conductivity of the obtained sulfide solid electrolyte, the above relational expression is derived. Here, since the S element is introduced through the gas, from the perspective of vapor pressure and gas-liquid equilibrium, it is considered appropriate to represent the relationship between the value of x and the cumulative introduction amount Y (mass%) by a quadratic function. Then, each coefficient of the above relational expression is determined according to the actual evaluation results.

[0084] In addition to the above relational expression, the cumulative introduction amount Y (mass%) of S element in the gas containing S element relative to the mass of the raw material mixture and the Li of the raw material containing S element 2 S x The value of x in preferably satisfies Y≥x 2 -6.5x + 6.2 relationship. This is the same as the relational expression of Y≥x 2 -6.5x + 5.8, and each coefficient above is determined according to the actual evaluation results.

[0085] In addition, the cumulative introduction amount Y (mass%) of S element in the gas containing S element relative to the mass of the raw material mixture and the Li as the raw material containing S element 2 S x The value of x in preferably satisfies Y≤x 2 -6.5x + 16.5 relationship, and more preferably satisfies Y≤x 2 -6.5x + 12.5 relationship. In the above formula, each coefficient is determined according to the actual evaluation results and from the perspective of the reduction of lithium ion conductivity caused by impurity generation.

[0086] That is, although the value of x in L 2 S x is 0.05 - 0.95, for example, when x is 0.90, the cumulative introduction amount Y of S element in the gas containing S element relative to the mass of the raw material mixture can be 0.76 mass% or more, and from the perspective of further suppressing the deficiency of S atoms, it is more preferably 1.16 mass% or more. From the perspective of the reduction of lithium ion conductivity caused by impurity generation, the cumulative introduction amount Y is preferably 11.46 mass% or less, and more preferably 7.46 mass% or less.

[0087] When x is 0.50, the cumulative introduction amount Y of S element in the gas containing S element relative to the mass of the raw material mixture can be 2.80 mass% or more, and from the perspective of further suppressing the deficiency of S atoms, it is more preferably 3.20 mass% or more. From the perspective of the reduction of lithium ion conductivity caused by impurity generation, the cumulative introduction amount Y is preferably 13.50 mass% or less, and more preferably 9.50 mass% or less.

[0088] When x is 0.30, the cumulative introduction amount Y of the S element in the gas containing the S element with respect to the mass of the raw material mixture can be 3.94% by mass or more, and from the viewpoint of further suppressing the loss of S atoms, it is more preferably 4.34% by mass or more. From the viewpoint of reducing the lithium ion conductivity due to the generation of impurities, the cumulative introduction amount Y is preferably 14.64% by mass or less, and more preferably 10.64% by mass or less. Here, the value of x is not limited to the above, and can also be in the range of 0.05 to 0.95.

[0089] As the S element source in the gas containing the S element, for example, sulfur gas, hydrogen sulfide gas, sulfur dioxide gas, etc. can be cited, and sulfur gas is preferably included from the viewpoint of reactivity with the raw materials.

[0090] In addition, as the gas containing the S element, elemental sulfur powder can be added, and the elemental sulfur powder is vaporized by heating during the heat treatment to introduce the gas containing the S element. The elemental sulfur powder can be added together with the raw materials when mixing the raw materials to obtain the raw material mixture, or can be added separately after obtaining the raw material mixture. Here, when the elemental sulfur powder is added together with the raw materials when obtaining the raw material mixture, the mass of the obtained raw material mixture does not include the mass of the above elemental sulfur powder.

[0091] As the gas containing the S element, the above-mentioned sulfur gas, hydrogen sulfide gas, sulfur dioxide gas, etc. can also be introduced, and at the same time, the gas containing the S element obtained by adding elemental sulfur powder is introduced together.

[0092] The gas containing the S element is preferably a mixed gas of the gas serving as the S element source and, for example, an inert gas as described above. The mixing ratio with the inert gas is arbitrary, and as long as the cumulative introduction amount Y of the S element with respect to the mass of the raw material mixture reaches the desired value, there is no particular limitation.

[0093] For example, when the raw material mixture obtained by mixing the raw materials is 100 g, a gas containing the S element in which the sulfur component is Y g is introduced. At this time, the gas containing the S element is introduced in a specified amount so that the cumulative introduction amount of the S element with respect to the mass of the raw material mixture becomes the desired value, rather than the concentration of the S element in the mixed gas or the concentration of the S element in the container for heat treatment.

[0094] As the inert gas, for example, nitrogen, argon, helium, etc. can be cited, and these gases can be used alone or in combination of two or more.

[0095] When the heating treatment in step S2 is the heat melting in step S2a, the specific method for heat melting the raw material mixture is not particularly limited. For example, when adding the raw materials into a heat-resistant container and heating in a heating furnace, heat melting can be carried out in a state where the gas containing S element is introduced into the heating furnace, or the gas containing S element can be introduced into the heating furnace after obtaining the melt. When the raw material mixture is sealed in a heat-resistant container and melted, the gas containing S element is introduced by sealing in a state of adding an atmosphere of the gas containing S element and elemental sulfur powder.

[0096] As the heat-resistant container, a heat-resistant container made of carbon, a heat-resistant container containing oxides such as quartz, quartz glass, borosilicate glass, aluminosilicate glass, alumina, zirconia, and mullite, a heat-resistant container containing nitrides such as silicon nitride and boron nitride, a heat-resistant container containing carbides such as silicon carbide, etc. can be used. In addition, these heat-resistant containers can be a body formed of the above materials, or can be a container formed with layers of carbon, oxides, nitrides, carbides, etc. like a quartz tube coated with carbon.

[0097] The heating temperature when heat melting the raw material mixture varies depending on the raw materials used and the composition of the raw material mixture. For example, it is preferably 550 to 950 °C, more preferably 600 to 900 °C, further preferably 650 to 850 °C, and particularly preferably 650 to 800 °C. Here, from the viewpoints of melting of the raw materials and introduction of the gas containing S element, the heating temperature is preferably 550 °C or higher, more preferably 600 °C or higher, and further preferably 650 °C or higher. In addition, from the viewpoint of suppressing the compositional deviation caused by volatilization of components, the heating temperature is preferably 950 °C or lower, more preferably 900 °C or lower, further preferably 850 °C or lower, and particularly preferably 800 °C or lower.

[0098] The heating and melting time varies depending on the scale, and is preferably 10 minutes to 10 hours, more preferably 30 minutes to 9.5 hours, still more preferably 45 minutes to 9 hours, and particularly preferably 1 hour to 9 hours. Here, from the viewpoint of better performing the reaction, the heating and melting time is preferably 10 minutes or more, more preferably 30 minutes or more, still more preferably 45 minutes or more, and particularly preferably 1 hour or more. In addition, from the viewpoint of productivity, the heating and melting time is preferably 10 hours or less, more preferably 9.5 hours or less, still more preferably 9 hours or less. In addition, although a gas containing S element is introduced, the length of the above time is adjusted according to the concentration of S element in the atmosphere during the heat treatment. That is, when a gas containing S element is introduced as a mixed gas with an inert gas or the like, the melting time can be extended when the amount of inert gas is large and the concentration of S element in the mixed gas is low. In addition, when the amount of inert gas is small and the concentration of S element in the mixed gas is high, the melting time can be shortened.

[0099] The dew point during heating and melting is preferably -20°C or lower, and the lower limit is not particularly limited and is usually about -80°C. The oxygen concentration is preferably 1000 ppm or lower.

[0100] In step S2a, it can be confirmed that the melt is completely dissolved by the absence of peaks from crystals in high-temperature X-ray diffraction measurement.

[0101] When the heat treatment in step S2 is the heat sintering in step S2b, the specific method of heat sintering the raw material mixture is not particularly limited. For example, when the raw materials are added to a heat-resistant container and heated in a heating furnace, the above-mentioned gas containing S element is heat-sintered in a state of being introduced into the heating furnace. When the raw material mixture is sealed in a heat-resistant container and heated and melted, the gas containing S element is introduced by sealing in an atmosphere of a gas containing S element and elemental sulfur powder.

[0102] For the raw material mixture, the heating temperature during heat sintering of the amorphized raw material mixture is preferably 350°C or higher and less than 600°C, more preferably 400 - 575°C, still more preferably 450 - 575°C. Here, from the viewpoint of promoting the solid-phase reaction, that is, crystallization, the heating temperature is preferably 350°C or higher, more preferably 400°C or higher, still more preferably 450°C or higher. In addition, from the viewpoint of suppressing thermal decomposition, the heating temperature is preferably less than 600°C, more preferably 575°C or lower.

[0103] The heating and sintering time is preferably 1 to 100 hours, more preferably 2 to 50 hours, and further preferably 4 to 24 hours. Here, from the viewpoint of promoting solid-phase reaction, i.e., crystallization, the heating time is preferably 1 hour or more, more preferably 2 hours or more, and further preferably 4 hours or more. In addition, from the viewpoint of suppressing thermal decomposition, the heating time is preferably 100 hours or less, more preferably 50 hours or less, and further preferably 24 hours or less. In addition, although a gas containing S element is introduced, the length of the above time can also be adjusted according to the concentration of S element in the atmosphere during the heat treatment. That is, when a gas containing S element is introduced as a mixed gas with an inert gas or the like, when the amount of inert gas is large and the concentration of S element in the mixed gas is low, the melting time can be extended. In addition, when the amount of inert gas is small and the concentration of S element in the mixed gas is high, the melting time can be shortened.

[0104] Through the above step S2b, a sulfide solid electrolyte is obtained.

[0105] · Step S3a

[0106] In the case of going through step S2a as step S2, next, by further including a process of cooling the melt and precipitating crystals as step S3a, the sulfide solid electrolyte in the present embodiment can be obtained.

[0107] The cooling conditions in step S3a vary depending on the composition, target crystallization rate, etc.

[0108] The cooling rate is not particularly limited as long as the desired crystals are precipitated, and is preferably 5 to 2000 °C / min, more preferably 10 to 1000 °C / min, and further preferably 30 to 300 °C / min. Here, from the viewpoint of productivity, the cooling rate is preferably 5 °C / min or more, more preferably 10 °C / min or more, and further preferably 30 °C / min or more. In addition, from the viewpoint of increasing the crystallization rate, the cooling rate is preferably 2000 °C / min or less, more preferably 1000 °C / min or less, and further preferably 300 °C / min or less.

[0109] The atmosphere during cooling is not particularly limited. Similar to the heat treatment, it can be a gas atmosphere containing S element or an inert atmosphere. In the case of obtaining a melt in a vacuum sealed tube, the cooling can also be directly carried out in the state of the vacuum sealed tube.

[0110] Through the above step S3a, a sulfide solid electrolyte is obtained.

[0111] The sulfide solid electrolyte obtained through the above step S2b and step S3a can also be further subjected to stabilization treatment using heating, pulverization treatment, etc. as needed.

[0112] <Sulfide solid electrolyte>

[0113] The sulfide solid electrolyte obtained in the above <Method for producing sulfide solid electrolyte> preferably contains a thiargite-type crystal structure. Here, the thiargite-type crystal structure refers to a crystal structure having a compound group of minerals represented by the composition formula Ag 8 GeS 6 The crystal structure represented by.

[0114] As the thiargite-type crystal structure, for example, in the case of a sulfide solid electrolyte represented by Li 6 PS 5 Cl, when using Li 2 S without S atom deficiency as a raw material, the following reaction is carried out.

[0115] 5 / 2·Li 2 S+1 / 2·P 2 S 5 +LiCl→Li 6 PS 5 Cl

[0116] In contrast, if Li 2 S x (0.05≤x≤0.95) with S atom deficiency is used as a raw material,

[0117] As shown in 5 / 2·Li 2 S x +1 / 2·P 2 S 5 +LiCl→Li 6 PS 5－1 / 2x Cl, a thiargite-type sulfide solid electrolyte with 1 / 2 of the x value of Li 2 S x missing is manufactured.

[0118] However, in the manufacturing method according to this embodiment, by introducing a gas containing S element, even if Li 2 S x (0.05≤x≤0.95) with S atom deficiency is used as a raw material, a thiargite-type sulfide solid electrolyte represented by Li 6 PS 5 Cl or a sulfide solid electrolyte similar thereto with suppressed S atom deficiency can be obtained.

[0119] In the above thiargite-type sulfide solid electrolyte, as the Ha element, it is more preferably to contain at least one element selected from Cl, Br, and I, and further preferably to contain at least one of Cl and Br.

[0120] When the composition formula of the argyrodite-type sulfide solid electrolyte is represented by Li a PS b Ha c It is preferably satisfied that the relationship of 5 ≤ a ≤ 7, 4 ≤ b ≤ 6 and 1.3 ≤ c ≤ 2. This element ratio is more preferably satisfied with the relationship of 5.1 < a < 6.3, 4 < b < 5.3 and 1.4 ≤ c ≤ 1.9, and further preferably satisfied with the relationship of 5.2 < a < 6.2, 4.1 < b < 5.2 and 1.5 ≤ c ≤ 1.8.

[0121] That is, regarding a, it is preferably 5 or more, more preferably greater than 5.1, and further preferably greater than 5.2. In addition, it is preferably 7 or less, more preferably less than 6.3, and further preferably less than 6.2. Regarding b, it is preferably 4 or more, more preferably greater than 4, and further preferably greater than 4.1. In addition, it is preferably 6 or less, more preferably less than 5.3, and further preferably less than 5.2. Regarding c, it is preferably 1.3 or more, more preferably 1.4 or more, and further preferably 1.5 or more. In addition, it is preferably 2 or less, more preferably 1.9 or less, and further preferably 1.8 or less.

[0122] The lattice constant of the above-mentioned argyrodite-type crystal structure is preferably More preferably Further preferably Even more preferably Particularly preferably Here, from the viewpoint of maintaining high lithium ion conductivity, the lattice constant is preferably or more, more preferably or more. In addition, from the same viewpoint, the lattice constant is preferably or less, more preferably or less, further preferably or less, even more preferably or less, particularly preferably or less.

[0123] In the sulfide solid electrolyte obtained by the manufacturing method according to this embodiment, since the loss of S atoms is suppressed, the decrease in lithium ion conductivity is suppressed. Specifically, the lithium ion conductivity at 25 °C is preferably 2 mS / cm or more, more preferably 5 mS / cm or more, and further preferably 8 mS / cm or more, and the higher the better. The upper limit of the lithium ion conductivity is not particularly limited, and is usually 1×10 －1 S / cm or less.

[0124] It should be noted that the lithium ion conductivity can be measured by the AC impedance method.

[0125] The sulfide solid electrolyte obtained by the manufacturing method according to this embodiment is suitable for use in electrode binders and solid electrolyte layers used in lithium ion secondary batteries, and is particularly suitable for all-solid-state lithium secondary batteries.

[0126] That is, the above-mentioned electrode binder can be used in lithium ion secondary batteries and contains the above-mentioned solid electrolyte and active material.

[0127] In addition, the above-mentioned solid electrolyte layer can be used in lithium ion secondary batteries and contains the above-mentioned solid electrolyte.

[0128] In addition, the above-mentioned all-solid-state lithium secondary battery contains the above-mentioned solid electrolyte.

[0129] The above-mentioned electrode binder, solid electrolyte layer, and all-solid-state lithium secondary battery may further contain other solid electrolytes. The other solid electrolytes are not particularly limited. For example, solid electrolytes having a thiogermanate-type crystal structure known in the past, Li 3 PS 4 、Li 4 P 2 S 6 、Li 2 S, LiHa, etc. can be cited.

[0130] The active material contained in the electrode binder can use substances known in the past.

[0131] For example, as the positive electrode active material, as long as it can reversibly occlude and release lithium ions, deintercalate and intercalate (insert) lithium ions, or dope and de-dope the counter anion of the lithium ions, there is no particular limitation. Specifically, lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel manganate, composite metal oxides, polyanion olivine-type positive electrodes, etc. can be cited.

[0132] The negative electrode active material also has no particular limitation as long as it can reversibly occlude and release lithium ions, deintercalate and intercalate (insert) lithium ions, or dope and de-dope the counter anion of the lithium ions. Specifically, lithium metal, carbon-based materials such as graphite, hard carbon, and soft carbon, metals that can form alloys with lithium such as aluminum, silicon, and tin, amorphous oxides such as silicon oxide and tin oxide, lithium titanate, etc. can be cited.

[0133] The solid electrolyte layer only needs to contain the solid electrolyte according to this embodiment. In addition, additives such as adhesives can be further contained.

[0134] The adhesive can use substances known in the past. For example, butadiene rubber, acrylate butadiene rubber, styrene butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, etc. can be cited. The content of the adhesive in the solid electrolyte layer is also within the range known in the past.

[0135] In the all-solid-state lithium secondary battery, except for the solid electrolyte according to this embodiment, as long as a positive electrode and a negative electrode are included, there is no particular limitation. In addition, the positive electrode and the negative electrode may also be electrode mixtures containing the solid electrolyte according to this embodiment.

[0136] As the active material of the positive electrode, the same materials as the positive electrode active materials described in the electrode mixture can be used, and the positive electrode may further contain a positive electrode current collector, a binder, a conductive assistant, etc. as needed. As the positive electrode current collector, a metal thin plate such as aluminum or its alloy, stainless steel, etc. can be used.

[0137] As the active material of the negative electrode, the same materials as the negative electrode active materials described in the electrode mixture can be used, and the negative electrode may further contain a negative electrode current collector, a binder, a conductive assistant, etc. as needed. As the negative electrode current collector, a metal thin plate such as copper, aluminum, etc. can be used.

[0138] Examples

[0139] Examples are given below to specifically illustrate the present invention, but the present invention is not limited thereto.

[0140] Examples 1 to 4 are examples, and Examples 5 and 6 are comparative examples.

[0141] [Example 1]

[0142] In a dry nitrogen atmosphere, lithium sulfide powder (manufactured by Sigma, purity 99.98%), diphosphorus pentasulfide powder (manufactured by Sigma, purity 99%), lithium chloride powder (manufactured by Sigma, purity 99.99%), and lithium bromide powder (manufactured by Sigma, purity 99.995%) were weighed in a composition ratio to become Li 5.4 PS 4.4 CL 0.8 Br 0.8 , and mixed in a mortar to obtain a raw material mixture. It should be noted that the lithium sulfide powder was measured using an X-ray powder diffraction (XRD) apparatus before use, and its composition was not Li 2 S but Li 2 S 0.33 , and it was confirmed that S atoms were missing due to long-term storage.

[0143] The obtained raw material mixture was added to a heat-resistant container in the same atmosphere and heated in an electric furnace at 750 °C for 1 hour to obtain a completely dissolved melt. In this state, a gas containing S element was introduced, and after storing for 1 hour, it was cooled to room temperature at 150 °C / minute to obtain a sulfide solid electrolyte.

[0144] The gas containing S element was set as a mixed gas of sulfur gas and nitrogen gas, and the cumulative introduction amount of S element relative to the mass of the raw material mixture was set as 4% by mass.

[0145] The obtained XRD (powder X-ray diffraction) pattern of the sulfide solid electrolyte is shown in Figure 4 , and the measurement conditions of XRD are as follows.

[0146] Radiation source: CuKα ray Tube voltage: 45 kV, tube current: 200 mA, scanning angle: 10 - 100°, scanning speed: 5° / min, step size: 0.01° / step.

[0147] According to the XRD pattern, it was confirmed that the obtained sulfide solid electrolyte has a thiogermanate-type crystal structure.

[0148] [Example 2]

[0149] The cumulative introduction amount of S element relative to the mass of the raw material mixture was set to 8% by mass, and except for this, a sulfide solid electrolyte having a thiogermanate-type crystal structure was obtained in the same manner as in Example 1.

[0150] The obtained XRD pattern of the sulfide solid electrolyte is shown in Figure 5 .

[0151] [Example 3]

[0152] As the lithium sulfide powder, lithium sulfide having a composition of Li 2 S 0.81 was used, and except for this, a sulfide solid electrolyte having a thiogermanate-type crystal structure was obtained in the same manner as in Example 1.

[0153] The obtained XRD pattern of the sulfide solid electrolyte is shown in Figure 6 .

[0154] [Example 4]

[0155] As the lithium sulfide powder, lithium sulfide having a composition of Li 2 S 0.86 was used, and except for this, a sulfide solid electrolyte having a thiogermanate-type crystal structure was obtained in the same manner as in Example 1.

[0156] The obtained XRD pattern of the sulfide solid electrolyte is shown in Figure 7 .

[0157] [Example 5]

[0158] Nitrogen gas without S element was used instead of the mixed gas containing S element, and except for this, a sulfide solid electrolyte having a thiogermanate-type crystal structure was obtained in the same manner as in Example 1.

[0159] The obtained XRD pattern of the sulfide solid electrolyte is shown in Figure 8 .

[0160] [Example 6]

[0161] Using nitrogen gas without S element instead of the mixed gas containing S element, a sulfide solid electrolyte having a thiargite-type crystal structure was obtained in the same manner as in Example 3 except for this.

[0162] The XRD pattern of the obtained sulfide solid electrolyte is shown in Figure 9 .

[0163] The sulfide solid electrolytes obtained in Examples 1 to 6 were pulverized in a mortar, and coarse particles were removed with a sieve having a mesh size of 100 μm. Then, 100 mg was weighed and pressed at 80 MPa to form an area with a diameter of 10 mm as a measurement sample.

[0164] The lithium ion conductivity of this measurement sample was measured using an alternating current impedance measurement device (manufactured by Bio-Logic Sciences Instruments, potentiostat / galvanostat VSP).

[0165] The measurement conditions were set as follows: measurement frequency: 100 Hz to 1 MHz, measurement voltage: 100 mV, measurement temperature: 25 °C.

[0166] In Table 1, the composition of lithium sulfide powder in the raw materials is designated as "raw material Li 2 S x ", the cumulative introduced amount of S element in the gas containing S element relative to the mass of the raw material mixture is designated as "cumulative introduced amount of S element Y (mass%)", the value represented by the relational expression (x 2 S x ) in the composition of lithium sulfide powder used (Li 2 - 6.5x + 5.8) is designated as "relational expression (x 2 - 6.5x + 5.8)", and the lithium ion conductivity at 25 °C is designated as "σ Li+ (mS / cm)", and they are summarized.

[0167] [Table 1]

[0168]

[0169] Based on the above results, regarding the cumulative introduced amount Y (mass%) of S element in the gas containing S element relative to the mass of the raw material mixture and the value of x in Li 2 S x which is the raw material containing S element, Figure 10 in which the quadratic function curve representing Y = x 2 - 6.5x + 5.8 is represented by a solid line, and the curve representing Y = x 2The quadratic function curve of -6.5x + 6.2 is represented by a dashed line. In addition, the curves of the x and Y values for each example are plotted, and the lithium-ion conductivity of each example is shown near the curve.

[0170] From the above results, it can be seen that by satisfying Y≥x 2 -6.5x + 5.8 relationship, even if low-quality lithium sulfide lacking S atoms is used as the raw material, a sulfide solid electrolyte with high lithium-ion conductivity can be obtained in a short time. Especially in view of using Li 2 S without deficiency and with an x value of 1 as the raw material, the lithium-ion conductivity of the obtained sulfide solid electrolyte is 9 - 9.5 mS / cm, and it can be seen that the results of Example 2 and Example 4 are very good.

[0171] In addition, comparing Figures 4 to 7 the XRD pattern with Figure 8 , Figure 9 the XRD pattern, it can be seen that the intensity ratio of the peak of the (220) plane observed near 2θ = 25° to the peak of the (311) plane observed near 2θ = 30° is different. As Figures 4 to 7 shown, when the cumulative introduction amount Y of the S element in the gas containing the S element with respect to the mass of the raw material mixture satisfies Y≥x 2 -6.5x + 5.8 relationship, it becomes a relationship where the peak intensity of the (220) plane > the peak intensity of the (311) plane. On the other hand, when Y≥x 2 -6.5x + 5.8 relationship is not satisfied Figure 8 and Figure 9 , it becomes a relationship where the peak intensity of the (220) plane < the peak intensity of the (311) plane, and it can be seen that S is missing in the obtained sulfide solid electrolyte.

[0172] Therefore, it is considered that if a sulfide solid electrolyte is obtained using a low-quality sulfide raw material lacking S atoms like in Example 5 and Example 6, the S atoms in the sulfide solid electrolyte are also missing, and as a result, the lithium-ion conductivity decreases.

[0173] In contrast, for the sulfide solid electrolytes of Example 1 and Example 2, although the same sulfide raw material as in Example 5 is used, a high lithium-ion conductivity of more than 10 times in Example 1 and more than 18 times in Example 2 is achieved. It is considered that this is because a gas containing the S element is introduced for heat treatment, so that even when using a low-quality sulfide raw material lacking S atoms, the missing of S atoms in the obtained sulfide solid electrolyte can be suppressed, and as a result, high lithium-ion conductivity can be achieved.

[0174] Although the present invention has been described in detail with reference to specific embodiments, various changes and modifications can be made without departing from the spirit and scope of the present invention, which will be apparent to those skilled in the art. This application is based on a Japanese patent application filed on November 7, 2022 (Japanese Patent Application No. 2022-178441), the content of which is incorporated herein by reference.

Claims

1. A method for manufacturing a sulfide solid electrolyte, comprising the steps of mixing a raw material containing Li element, a raw material containing P element, and a raw material containing S element to obtain a raw material mixture, and heating the raw material mixture. As the raw material containing S element, Li 2 S x , Wherein, 0.05≤x≤0.95， The heating treatment is carried out by introducing a gas containing S element. The cumulative introduction amount Y of the S element in the gas containing the S element with respect to the mass of the raw material mixture satisfies Y≥x 2 S x for the x in 2 -6.5x + 5.8, where the unit of Y is mass%.

2. The method for manufacturing a sulfide solid electrolyte according to claim 1, Wherein, It further includes the following steps: Obtaining a melt of the raw material mixture through the heating treatment, and then cooling the melt to precipitate crystals.

3. The method for manufacturing a sulfide solid electrolyte according to claim 1, Wherein, Crystals are precipitated through the heating treatment.

4. The method for manufacturing a sulfide solid electrolyte according to claim 2 or 3, Wherein, The raw material mixture further contains a raw material containing Ha element, The Ha element is at least one element selected from F, Cl, Br, and I, The sulfide solid electrolyte has a thiogermanate-type crystal structure.

5. The method for manufacturing a sulfide solid electrolyte according to claim 4, Wherein, The lattice constants of the crystal structure of the argyrodite type are 6. The method for manufacturing a sulfide solid electrolyte according to claim 2 or 3, Wherein, The Li 2 S x wherein x in 2 x satisfies 0.05 ≤ x ≤ 0.

90.

7. The method for manufacturing a sulfide solid electrolyte according to claim 2 or 3, Wherein, As the gas containing S element, it contains sulfur gas.

8. The method for manufacturing a sulfide solid electrolyte according to claim 2 or 3, Wherein, The cumulative introduction amount Y of the S element in the gas containing S element is 2% by mass or more.

9. The method for manufacturing a sulfide solid electrolyte according to claim 2 or 3, Wherein, The lithium ion conductivity of the sulfide solid electrolyte is 2 mS / cm or more.

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