SULFIDE SOLID ELECTROLYTE AND METHOD FOR MANUFACTURING SAME

[0001] This is a bypass continuation of International Patent Application No. PCT/JP2024/029440, filed on Aug. 20, 2024, which claims priority to Japanese Patent Application No. 2023-168394, filed on Sep. 28, 2023. The contents of these applications are hereby incorporated by reference in their entireties.

[0002] The present invention relates to a sulfide solid electrolyte and a method for producing the same.

[0003] Lithium ion secondary batteries are widely used in portable electronic devices such as a mobile phone and a notebook computer, motor vehicles, or the like.

[0004] In the related art, a liquid electrolyte has been used in a lithium ion secondary battery, but there is a concern of liquid leakage, ignition, and the like, and it is necessary to increase the size of a case for safety design. It has also been desired to improve the shortage of battery life and the narrow operating temperature range.

[0005] In contrast, attention has been paid to an all-solid-state lithium secondary battery in which a solid electrolyte is used as an electrolyte of a lithium ion secondary battery, from the viewpoint of improving safety, charging and discharging at a high speed, and reducing the size of the case.

[0006] The solid electrolytes are roughly classified into sulfide solid electrolytes and oxide solid electrolytes. Sulfide ions constituting the sulfide solid electrolytes have higher polarizability and higher ion conductivity than those of oxide ions constituting the oxide solid electrolytes. Known examples of sulfide solid electrolytes include LGPS crystals such as Li₁₀GeP₂S₁₂, argyrodite crystals such as Li₆PS₅Cl, and LPS crystallized glass such as Li₇P₃S₁₁ crystallized glass.

[0007] As a solid electrolyte which is hardly hydrolyzed and has high ion conductivity, Patent Literature 1 discloses a solid electrolyte of sulfide glass in which Li₂S/P₂S₅/LiI=63/21/16 or 52/17/31, or a solid electrolyte of sulfide glass ceramic obtained by crystallizing a part of a sulfide glass in which Li₂S/P₂S₅/LiBr=64/21/14.

• • [0008] Patent Literature 1: JP2018-049834A

[0009] However, in the solid electrolyte of the sulfide glass ceramic as described in Patent Literature 1, since crystals that are thermodynamically stable at a high temperature and have low ion conductivity precipitate, ion conductivity is significantly reduced. The precipitation of the crystals starts when, for example, the crystals are held at a temperature equal to or higher than the lower part of the 200° C. range for 30 minutes or more. Therefore, a battery using a sulfide solid electrolyte containing a glass phase and a crystalline phase has a problem in battery operation in a high temperature range.

[0010] Therefore, an object of the present invention is to provide a sulfide solid electrolyte containing a crystalline phase and a glass phase and having excellent heat resistance as well as excellent lithium ion conductivity, and a method for producing the same.

[0011] As a result of intensive studies, the present inventors have found that in obtaining a sulfide solid electrolyte precursor by melting raw materials by heating and cooling the raw materials, a sulfide solid electrolyte precursor containing a glass phase and a β-Li₃PS₄ crystalline phase can be obtained by performing the heating and cooling under an ambient pressure condition and performing the cooling at a very high cooling rate, and then a part of the glass phase can be crystallized to solve the problems, thereby completing the present invention.

[0012] That is, the present invention relates to the following [1] to [9].

• • [0013] [1] A sulfide solid electrolyte containing:
  • [0014] a crystalline phase and a glass phase, in which
  • [0015] the glass phase includes a phase including a sulfide glass which contains Li, P, and S as constituent elements,
  • [0016] the crystalline phase includes a high-ion-conductive crystalline phase derived from the sulfide glass and a β-Li₃PS₄ crystalline phase,
  • [0017] compositions of the phase including the sulfide glass and the high-ion-conductive crystalline phase satisfy Li: 30 at % to 50 at %, P: 5 at % to 15 at %, and S: 30 at % to 60 at %, and
  • [0018] a content ratio of the β-Li₃PS₄ crystalline phase in the sulfide solid electrolyte is 0.1 mass % to 30 mass %.
  • [0019] [2] The sulfide solid electrolyte according to [1], in which
  • [0020] the phase including the sulfide glass and the high-ion-conductive crystalline phase further contain Ha as a constituent element, and
  • [0021] Ha is at least one selected from the group consisting of F, Cl, Br, and I.
  • [0022] [3] The sulfide solid electrolyte according to [2], in which
  • [0023] Ha, which is the constituent element of the phase including the sulfide glass and the high-ion-conductive crystalline phase, includes at least one selected from the group consisting of Cl, Br, and I.
  • [0024] [4] The sulfide solid electrolyte according to [2] or [3], in which
  • [0025] Ha, which is the constituent element of the phase including the sulfide glass and the high-ion-conductive crystalline phase, includes Br.
  • [0026] [5] The sulfide solid electrolyte according to any one of [1] to [4], in which
  • [0027] the β-Li₃PS₄ crystalline phase has a crystallite diameter of 400 nm or less.
  • [0028] [6] The sulfide solid electrolyte according to any one of [1] to [5], in which
  • [0029] a content ratio of the high-ion-conductive crystalline phase in the sulfide solid electrolyte is 30 mass % or more.
  • [0030] [7] The sulfide solid electrolyte according to any one of [1] to [6], in which
  • [0031] a peak intensity ratio represented by I_B/I_A is 0.01 to 2.0,
  • [0032] where in an XRD pattern obtained by a powder X-ray diffraction measurement, I_A is a peak intensity of a diffraction peak at 2θ=20.2°±0.5° which indicates the high-ion-conductive crystalline phase, and I_B is a largest intensity among peak intensities of diffraction peaks at 2θ=18.2°±0.5° which indicate the β-Li₃PS₄ crystalline phase.
  • [0033] [8] The sulfide solid electrolyte according to any one of [1] to [7], in which
  • [0034] a peak intensity ratio represented by I_B/I_C is 0.01 to 10.0,
  • [0035] where in an XRD pattern obtained by a powder X-ray diffraction measurement, I_C is a peak intensity of a diffraction peak at 2θ=19.1°±0.5° which indicates the high-ion-conductive crystalline phase, and I_B is a largest intensity among peak intensities of diffraction peaks at 2θ=18.2°±0.5° which indicate the β-Li₃PS₄ crystalline phase.
  • [0036] [9] A method for producing a sulfide solid electrolyte containing a crystalline phase and a glass phase, the method including in order:
  • [0037] mixing raw materials to obtain a raw material mixture containing Li, P, and S;
  • [0038] heating the raw material mixture to obtain a melt;
  • [0039] cooling and solidifying the melt to obtain a sulfide solid electrolyte precursor containing a glass phase and a β-Li₃PS₄ crystalline phase; and
  • [0040] heating the sulfide solid electrolyte precursor to obtain a high-ion-conductive crystalline phase obtained by crystallizing a part of the glass phase, in which
  • [0041] the heating for obtaining the melt and the cooling and solidifying are performed under an ambient pressure condition, and
  • [0042] in the cooling and solidifying, a molten liquid which is the melt is allowed to flow out and then allowed to stay in a temperature range of 750° C. to 210° C. for 0.2 seconds to 100 seconds.

[0043] According to the present invention, it is possible to obtain a sulfide solid electrolyte containing a crystalline phase and a glass phase, which has excellent heat resistance as well as excellent lithium ion conductivity. Therefore, when the sulfide solid electrolyte is used in an all-solid-state lithium secondary battery, favorable battery characteristics can be implemented.

[0047] Hereinafter, the present invention is described in detail, but the present invention is not limited to the following embodiment and can be freely modified and implemented without departing from the gist of the present invention. In addition, the symbol “-” or the word “to” that is used to express a numerical range includes the numerical values before and after the symbol or the word as the upper limit value and the lower limit value of the range, respectively.

[0048] A sulfide solid electrolyte according to the present embodiment contains a crystalline phase and a glass phase. The glass phase includes a phase including a sulfide glass (hereinafter, sometimes simply referred to as a “sulfide glass phase”) which contains Li, P, and S as constituent elements, and the crystalline phase includes a high-ion-conductive crystalline phase derived from the sulfide glass and a β-Li₃PS₄ crystalline phase (hereinafter, sometimes simply referred to as a “β-Li₃PS₄ crystalline phase”).

[0049] Here, the high-ion-conductive crystalline phase derived from the sulfide glass means a crystalline phase obtained by crystallizing the sulfide glass by a heat treatment, and constituent elements and composition ratios thereof can be regarded as substantially the same. Here, the constituent elements and composition ratios being substantially the same means that a ratio of a composition determined from a structural analysis of an XRD pattern of the high-ion-conductive crystalline phase with respect to an overall composition is within a range of ±20% for each component.

[0050] A composition of the high-ion-conductive crystalline phase satisfies Li: 30 at % to 50 at %, P: 5 at % to 15 at %, and S: 30 at % to 60 at %. Therefore, a composition of the sulfide glass phase also satisfies Li: 30 at % to 50 at %, P: 5 at % to 15 at %, and S: 30 at % to 60 at %.

[0051] A content ratio of the β-Li₃PS₄ crystalline phase in the sulfide solid electrolyte according to the present embodiment is 0.1 mass % to 30 mass %.

[0052] The sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain Li, P, and S as constituent elements. In addition to these, it is preferable to contain Ha from a viewpoint of lithium ion conductivity. Here, Ha is at least one selected from the group consisting of F, Cl, Br, and I, and from a viewpoint of obtaining good oxidation resistance and water resistance, Ha more preferably contains at least one selected from the group consisting of Cl, Br, and I, still more preferably contains at least one of Cl and Br, and even more preferably contains Br. In addition, it is also even more preferable to contain both Cl and Br.

[0053] The compositions of the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment preferably satisfy Li: 30 at % to 50 at %, P: 5 at % to 15 at %, and S: 30 at % to 60 at %, and more preferably satisfy Li: 30 at % to 50 at %, P: 5 at % to 15 at %, S: 30 at % to 60 at %, and Ha: 1 at % to 12 at %. In the present description, at % means atom %, that is, atomic %. In addition, as described above, the sulfide glass phase and the high-ion-conductive crystalline phase derived therefrom can be regarded as having substantially the same constituent elements and composition ratios, but do not mean that the sulfide glass phase and the high-ion-conductive crystalline phase are exactly the same, and it is preferable that the composition of each of the sulfide glass phase and the high-ion-conductive crystalline phase satisfies the above range.

[0054] In the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment, Li is an element responsible for ion conduction as a solid electrolyte.

[0055] In the compositions of the sulfide glass phase and the high-ion-conductive crystalline phase, a Li content is preferably 30 at % to 50 at %, more preferably 35 at % to 45 at %, and still more preferably 37 at % to 43 at %. Here, from a viewpoint of increasing the lithium ion conductivity, the Li content is preferably 30 at % or more, more preferably 35 at % or more, and still more preferably 37 at % or more. Further, the Li content is preferably 50 at % or less, more preferably 45 at % or less, and still more preferably 43 at % or less, from a viewpoint of widening a vitrification range.

[0056] In the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment, P is an element that forms a glass network, and among sulfides, a P—S bond has high resistance to both oxidation and reduction. Therefore, the solid electrolyte has a wide potential window and excellent electrochemical stability.

[0057] In the compositions of the sulfide glass phase and the high-ion-conductive crystalline phase, a P content is preferably 5 at % to 15 at %, more preferably 6 at % to 13 at %, and still more preferably 7 at % to 12 at %. Here, from the viewpoint of widening the vitrification range, the P content is preferably 5 at % or more, more preferably 6 at % or more, and still more preferably 7 at % or more. Further, from the viewpoint of increasing the lithium ion conductivity, the P content is preferably 15 at % or less, more preferably 13 at % or less, and still more preferably 12 at % or less.

[0058] In the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment, S is an element that forms the P—S bond with P, and is an essential element for forming the glass phase.

[0059] In the compositions of the sulfide glass phase and the high-ion-conductive crystalline phase, an S content is preferably 30 at % to 60 at %, more preferably 33 at % to 50 at %, and still more preferably 35 at % to 47 at %. Here, from the viewpoint of widening the vitrification range, the S content is preferably 30 at % or more, more preferably 33 at % or more, and still more preferably 35 at % or more. In addition, from the viewpoint of increasing the lithium ion conductivity, the S content is preferably 60 at % or less, more preferably 50 at % or less, and still more preferably 47 at % or less.

[0060] When the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain Ha, Ha is an element that contributes to high lithium ion conductivity.

[0061] In the compositions of the sulfide glass phase and the high-ion-conductive crystalline phase, an Ha content is preferably 1 at % to 12 at %, more preferably 3 at % to 10 at %, and still more preferably 3.5 at % to 8 at %. Here, from the viewpoint of obtaining the high lithium ion conductivity, the Ha content is preferably 1 at % or more, more preferably 3 at % or more, and still more preferably 3.5 at % or more. Furthermore, from a viewpoint of preventing precipitation of lithium halide crystals, the Ha content is preferably 12 at % or less, more preferably 10 at % or less, and still more preferably 8 at % or less. Here, the Ha content means a total content of F, Cl, Br, and I.

[0062] Ha is at least one selected from the group consisting of F, Cl, Br, and I, and preferably contains at least one of Cl and Br, and more preferably contains Br, from a viewpoint of obtaining the good oxidation resistance and water resistance, as described above. It is also more preferable to contain both Cl and Br.

[0063] For example, in the Ha content, a total content of Cl and Br is preferably 50% or more, more preferably 70% or more, and may be 100%, that is, Ha may only include Cl and Br. Further, a content ratio represented by Cl:Br is preferably 100:0 to 0:100, more preferably 80:20 to 0:100, and still more preferably 60:40 to 0:100.

[0064] The sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment may contain elements other than Li, P, S, and Ha as constituent elements. Examples of the other elements that may be contained include Si, B, Ge, Al, O, Na, K, Mg, Ca, Sr, Ba, Y, Zr, Cr, Zn, Ga, Sn, and Sb.

[0065] When the other elements constitute oxides such as SiO₂, B₂O₃, P₂O₅, or Al₂O₃, such oxides are preferably not contained since the oxides serve as a crystal nucleus. A total content of the oxides is preferably 1 mol % or less, more preferably 0.1 mol % or less, and still more preferably 0.05 mol % or less, in terms of mol % based on oxides, and may be zero, that is, 0 mol %.

[0066] The sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment may contain, for example, Si, B, Ge, Al, Sn, or Sb among the other elements. Si, B, Ge, Al, Sn, and Sb are elements that can occupy a position of P when the sulfide glass phase is crystallized.

[0067] When the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain Si as the constituent element, Si has an effect of increasing a viscosity of a molten liquid and promoting vitrification. A Si content is preferably 0 at % to 15 at %, more preferably 0.1 at % to 10 at %, and still more preferably 1 at % to 5 at %. Here, from the viewpoint of suitably obtaining the effect of Si, the Si content is preferably 0.1 at % or more, and more preferably 1 at % or more. From the viewpoint of the electrochemical stability, the Si content is preferably 15 at % or less, more preferably 10 at % or less, and still more preferably 5 at % or less.

[0068] When Si has a Si—O bond or is contained as an oxide of SiO₂, as described above, the Si content is appropriately adjusted such that a total content of the oxides is 1 mol % or less, or such that a total content of oxides serving as a glass network former is 3 mol % to 20 mol %.

[0069] When the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain B as the constituent element, B has an effect of increasing the viscosity of the molten liquid and promoting the vitrification. A B content is preferably 0 at % to 15 at %, more preferably 0.1 at % to 10 at %, and still more preferably 1 at % to 5 at %. Here, from a viewpoint of suitably obtaining an effect of B, the B content is preferably 0.1 at % or more, and more preferably 1 at % or more. From the viewpoint of the lithium ion conductivity, the B content is preferably 15 at % or less, more preferably 10 at % or less, and still more preferably 5 at % or less.

[0070] When B has a B—O bond or is contained as an oxide of B₂O₃, as described above, the B content is appropriately adjusted such that the total content of the oxides is 1 mol % or less, or such that the total content of oxides serving as a glass network former is 3 mol % to 20 mol %.

[0071] When the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain Ge as the constituent element, Ge has an effect of promoting an improvement in the lithium ion conductivity. A Ge content is preferably 0 at % to 15 at %, more preferably 0.1 at % to 10 at %, and still more preferably 1 at % to 5 at %. Here, from the viewpoint of the lithium ion conductivity, the Ge content is preferably 0.1 at % or more, and more preferably 1 at % or more. From a viewpoint of ease of glass formation, the Ge content is preferably 15 at % or less, more preferably 10 at % or less, and still more preferably 5 at % or less.

[0072] However, when Ge has a Ge—O bond or is contained as an oxide of GeO₂, as described above, the Ge content is appropriately adjusted such that the total content of the oxides is 1 mol % or less.

[0073] When the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain Al as the constituent element, Al has an effect of increasing the viscosity of the molten liquid and promoting the vitrification. An Al content is preferably 0 at % to 15 at %, more preferably 0.1 at % to 10 at %, and still more preferably 1 at % to 5 at %. Here, from the viewpoint of suitably obtaining the effect of Al, the Al content is preferably 0.1 at % or more, and more preferably 1 at % or more. From the viewpoint of the lithium ion conductivity, the Al content is preferably 15 at % or less, more preferably 10 at % or less, and still more preferably 5 at % or less.

[0074] When Al has an Al—O bond or is contained as an oxide of Al₂O₃, as described above, the Al content is appropriately adjusted such that the total content of the oxides is 1 mol % or less, or such that the total content of oxides serving as a glass network former is 3 mol % to 20 mol %.

[0075] When the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain Sn as the constituent element, Sn has an effect of increasing the viscosity of the molten liquid and promoting the vitrification. A Sn content is preferably 0 at % to 15 at %, more preferably 0.1 at % to 10 at %, and still more preferably 1 at % to 5 at %. Here, from a viewpoint of suitably obtaining the effect of Sn, the Sn content is preferably 0.1 at % or more, and more preferably 1 at % or more. From the viewpoint of achieving the high lithium ion conductivity, the Sn content is preferably 15 at % or less, more preferably 10 at % or less, and still more preferably 5 at % or less.

[0076] However, when Sn has a Sn—O bond or is contained as an oxide of SnO₂, as described above, the Sn content is appropriately adjusted such that the total content of the oxides is 1 mol % or less.

[0077] When the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment contain Sb as the constituent element, Sb has an effect of increasing the viscosity of the molten liquid and promoting the vitrification. A Sb content is preferably 0 at % to 15 at %, more preferably 0.1 at % to 10 at %, and still more preferably 1 at % to 5 at %. Here, from the viewpoint of suitably obtaining the effect of Sb, the Sb content is preferably 0.1 at % or more, and more preferably 1 at % or more. From the viewpoint of the lithium ion conductivity, the Sb content is preferably 15 at % or less, more preferably 10 at % or less, and still more preferably 5 at % or less.

[0078] When Sb has a Sb—O bond or is contained as an oxide of Sb₂O₃, as described above, the Sb content is appropriately adjusted such that the total content of the oxides is 1 mol % or less, or such that the total content of oxides serving as the glass network former is 3 mol % to 20 mol %.

[0079] In the compositions of the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment, a composition ratio represented by Li/P is preferably 2.5 or more, more preferably 2.5 to 7.0, still more preferably 3.0 to 6.5, and even more preferably 3.2 to 6.2.

[0080] Here, from a viewpoint of obtaining high lithium ion conductivity, the composition ratio is preferably 2.5 or more, more preferably 3.0 or more, and still more preferably 3.2 or more. The above composition ratio is preferably 7.0 or less, more preferably 6.5 or less, and still more preferably 6.2 or less, from the viewpoint of widening the vitrification range.

[0081] In the present description, the composition ratio means a content ratio (at %) of each element in the sulfide glass phase and the high-ion-conductive crystalline phase.

[0082] In the compositions of the sulfide glass phase and the high-ion-conductive crystalline phase in the present embodiment, a composition ratio represented by S/P is preferably 3.0 to 5.0, more preferably 3.5 to 4.5, and still more preferably 3.8 to 4.2. Here, from the viewpoint of the lithium ion conductivity, the composition ratio is preferably 3.0 or more, more preferably 3.5 or more, and still more preferably 3.8 or more. From the viewpoint of water resistance, the composition ratio is preferably 5.0 or less, more preferably 4.5 or less, and still more preferably 4.2 or less.

[0083] When the sulfide glass phase and the high-ion-conductive crystalline phase further contain Ha, a composition ratio represented by [(F+Cl+Br)/Ha] is preferably 0.5 or more, more preferably 0.5 to 1.0, still more preferably 0.6 to 1.0, and particularly preferably 0.8 to 1.0. Here, from the viewpoint of oxidation resistance, the composition ratio is preferably 0.5 or more, more preferably 0.6 or more, and still more preferably 0.8 or more. An upper limit of the composition ratio is not particularly limited, and the composition ratio may be 1.0. The composition ratio of 1.0 means that I is not contained as Ha. When the sulfide glass phase and the high-ion-conductive crystalline phase contain I as Ha, the composition ratio may be 0.8 or less or 0.6 or less from a viewpoint of obtaining an effect of I.

[0084] When the sulfide glass phase and the high-ion-conductive crystalline phase further contain Ha, in the composition of the sulfide glass phase in the present embodiment, a composition ratio represented by Ha/P is preferably 0.01 to 1.5, more preferably 0.1 to 1.2, and still more preferably 0.2 to 1.0. Here, from the viewpoint of the water resistance, the composition ratio is preferably 0.01 or more, more preferably 0.1 or more, and still more preferably 0.2 or more. From the viewpoint of preventing precipitation of lithium halide crystals, the composition ratio is preferably 1.5 or less, more preferably 1.2 or less, and still more preferably 1.0 or less. Here, Ha means the total of F, Cl, Br, and I.

[0085] The constituent elements of the sulfide glass phase and the high-ion-conductive crystalline phase and the contents thereof are obtained by comparing results of measuring constituent elements of the entire sulfide solid electrolyte and contents (composition ratio) thereof with element ratios constituting a crystal structure obtained from the XRD pattern.

[0086] Methods for determining the constituent elements of the entire sulfide solid electrolyte and the contents thereof differ depending on the elements. For example, P and S are determined by ICP emission spectroscopy, Li is determined by atomic absorption spectroscopy, and Ha is determined by ion chromatography. Details of each analysis will be described later in the section of Examples.

[0087] The sulfide glass phase in the present embodiment is partially crystallized by a heat treatment to become the high-ion-conductive crystalline phase in the present embodiment. The high-ion-conductive crystalline phase may have a Thio-Lisicon type or LGPS type crystal structure. A Thio-Lisicon type or LGPS type high-ion-conductive crystalline phase has high lithium ion conductivity and is useful as the solid electrolyte.

[0088] A total proportion of the sulfide glass phase and the high-ion-conductive crystalline phase in the sulfide solid electrolyte according to the present embodiment is 99.9 mass % or less, preferably 50 mass % to 99.9 mass %, more preferably 70 mass % to 95 mass %, and still more preferably 75 mass % to 90 mass %. Here, the total proportion is preferably 50 mass % or more, more preferably 70 mass % or more, and still more preferably 75 mass % or more, from the viewpoint of the lithium ion conductivity. In addition, the total proportion is 99.9 mass % or less, preferably 95 mass % or less, and more preferably 90 mass % or less from the viewpoint of suitably exhibiting an effect of the β-Li₃PS₄ crystalline phase.

[0089] Respective proportions of the sulfide glass phase and the high-ion-conductive crystalline phase can be determined by powdering the sulfide solid electrolyte, performing the powder X-ray diffraction (XRD) measurement together with a crystal powder serving as an internal standard, and then performing Rietveld analysis. The proportion of the sulfide glass phase can be determined by determining a proportion of crystals by the above analysis and subtracting the proportion from 100 mass %.

[0090] A content ratio of the high-ion-conductive crystalline phase in the present embodiment is preferably 30 mass % or more, more preferably 30 mass % to 90 mass %, still more preferably 50 mass % to 85 mass %, and even more preferably 60 mass % to 80 mass %. Here, the content ratio is preferably 30 mass % or more, more preferably 50 mass % or more, and still more preferably 60 mass % or more, from the viewpoint of the lithium ion conductivity. Furthermore, from the viewpoint of suitably obtaining an effect of the phase including the sulfide glass and the β-Li₃PS₄ crystalline phase, the content ratio is preferably 90 mass % or less, more preferably 85 mass % or less, and still more preferably 80 mass % or less.

[0091] The content ratio of the sulfide glass phase in the present embodiment is preferably 1 mass % to 70 mass %, more preferably 5 mass % to 50 mass %, and still more preferably 8 mass % to 40 mass %. Here, the content ratio is preferably 1 mass % or more, more preferably 5 mass % or more, and still more preferably 8 mass % or more from the viewpoint of achieving softness caused by the glass phase that facilitates formation of an interface between an active material and the solid electrolyte. In addition, from the viewpoint of suitably obtaining effects of the high-ion-conductive crystalline phase or the β-Li₃PS₄ crystalline phase, the content ratio is preferably 70 mass % or less, more preferably 50 mass % or less, and still more preferably 40 mass % or less.

[0092] The high-ion-conductive crystalline phase in the present embodiment may have, for example, a Thio-Lisicon type or LGPS type crystal structure. These have high lithium ion conductivity.

[0093] The high-ion-conductive crystalline phase in the present embodiment exhibits a diffraction peak at least at 2θ=20.2°±0.5°, and preferably further exhibits a diffraction peak at 2θ=23.8°±0.5° in an XRD pattern obtained by a powder X-ray diffraction (XRD) measurement. Further, it is also preferable that the diffraction peak is exhibited at 2θ=19.1°±0.5°. In this case, it may be determined that the high-ion-conductive crystalline phase has a Thio-Lisicon type or LGPS type crystal structure. The XRD measurement is performed using a CuKα radiation (λ=1.5418 Å) as a radiation source, and details of measurement conditions are as described in the section of Examples below.

[0094] The XRD pattern may exhibit diffraction peaks at both 2θ=19.1°±0.5° and 2θ=23.8°±0.5°. Further, the diffraction peak may be exhibited at one or more of 2θ=12.4°±0.5°, 29.7°±0.5°, and 31.4°±0.5°, or may be exhibited at two or more thereof, or may be exhibited at all the three.

<β-Li₃PS₄ Crystalline Phase>

[0095] The sulfide solid electrolyte according to the present embodiment contains 0.1 mass % to 30 mass % of the β-Li₃PS₄ crystalline phase in addition to the sulfide glass phase and the high-ion-conductive crystalline phase.

[0096] The β-Li₃PS₄ crystalline phase is a thermodynamically stable phase, and exhibits a diffraction peak at least at 2θ=18.2°±0.5°, preferably further at 2θ=26.0°±0.5° in an XRD pattern obtained by the powder X-ray diffraction (XRD) measurement. A plurality of diffraction peaks may be exhibited at 2θ=18.2°±0.5°. The XRD measurement is performed using a CuKα radiation (λ=1.5418 Å) as a radiation source, and details of measurement conditions are as described in the section of Examples below.

[0097] The XRD pattern may further exhibit diffraction peaks at one or more of 2θ=17.8°±0.5°, 20.0°±0.5°, 29.8°±0.5°, and 34.0°±0.5°, or may exhibit diffraction peaks at two or more thereof, three or more thereof, or all the four.

[0098] It is found that since the sulfide solid electrolyte according to the present embodiment contains the β-Li₃PS₄ crystalline phase, even when the heat treatment is performed at a temperature exceeding 200° C., thermal decomposition of the high-ion-conductive crystalline phase can be prevented, and the good heat resistance can be achieved.

[0099] A reason for this is not clear, but it is considered that the β-Li₃PS₄ crystalline phase precipitated in the present embodiment is uniformly precipitated throughout a bulk of the sulfide solid electrolyte. Accordingly, it is considered that the high-ion-conductive crystalline phase that precipitates from residual mother glass may also have a different crystal structure from that of a known material in the related art. In addition to the above, when the β-Li₃PS₄ crystalline phase becomes a microcrystal body, it is considered that a crystallite diameter of the high-ion-conductive crystalline phase is also reduced and micro thermodynamic characteristics are changed, so that better heat resistance can be achieved.

[0100] A content ratio of the β-Li₃PS₄ crystalline phase in the sulfide solid electrolyte according to the present embodiment is 0.1 mass % to 30 mass %, preferably 5 mass % to 25 mass %, and more preferably 10 mass % to 20 mass %. Here, from a viewpoint of suitably obtaining the effect of the heat resistance of the β-Li₃PS₄ crystalline phase, the content ratio is 0.1 mass % or more, preferably 5 mass % or more, and more preferably 10 mass % or more. Further, from the viewpoint of the lithium ion conductivity, the content ratio is 30 mass % or less, preferably 25 mass % or less, and more preferably 20 mass % or less.

[0101] A proportion of the β-Li₃PS₄ crystalline phase can be determined by powdering the sulfide solid electrolyte, performing the powder X-ray diffraction (XRD) measurement together with the crystal powder serving as the internal standard, and performing Rietveld analysis.

[0102] The crystallite diameter of the β-Li₃PS₄ crystalline phase is preferably 400 nm or less, more preferably 20 nm to 400 nm, still more preferably 20 nm to 350 nm, even more preferably 50 nm to 300 nm, and particularly preferably 80 nm to 260 nm. The crystallite diameter is preferably 400 nm or less, more preferably 350 nm or less, still more preferably 300 nm or less, and even more preferably 260 nm or less, because the presence of the β-Li₃PS₄ crystalline phase in the form of the microcrystal body has the effect of achieving good heat resistance even when the high-ion-conductive crystalline phase is precipitated. Further, it is presumed that the smaller the crystallite diameter, the better, but in practice, the crystallite diameter is preferably 20 nm or more, more preferably 50 nm or more, and still more preferably 80 nm or more from a viewpoint of a crystal growth rate.

[0103] The crystallite diameter is determined by a Scherrer equation using a half-width of a peak in an XRD pattern.

[0104] In obtaining the sulfide solid electrolyte precursor by melting the raw materials by heating and cooling the raw materials, the β-Li₃PS₄ crystalline phase as described above is obtained in an appropriate amount by performing the heating and cooling under an ambient pressure condition and performing the cooling at a very high cooling rate.

[0105] Even when the obtained β-Li₃PS₄ crystalline phase is heated to crystallize a part of the sulfide glass phase in the sulfide solid electrolyte precursor to form the high-ion-conductive crystalline phase, no remarkable crystal growth is confirmed, and the content ratio and the crystallite diameter do not change.

[0106] In the XRD pattern obtained by the powder XRD measurement, a peak intensity of the diffraction peak at 2θ=20.2°±0.5° which indicates the high-ion-conductive crystalline phase is defined as I_A. In addition, the largest intensity among peak intensities of the diffraction peaks at 2θ=18.2°±0.5° which indicate the β-Li₃PS₄ crystalline phase is defined as I_B. In this case, a peak intensity ratio represented by I_B/I_A is preferably 0.01 to 2.0, more preferably 0.05 to 1.0, and still more preferably 0.07 to 0.7. Here, the intensity ratio is preferably 0.01 or more, more preferably 0.05 or more, and still more preferably 0.07 or more, from the viewpoint of the heat resistance. From the viewpoint of the lithium ion conductivity, the ratio is preferably 2.0 or less, more preferably 1.0 or less, and still more preferably 0.7 or less.

[0107] Two or more diffraction peaks indicating the β-Li₃PS₄ crystalline phase may be exhibited in the range of 2θ=18.2°±0.5°. In this case, a peak intensity of a peak having the highest intensity among the plurality of diffraction peaks is defined as I_B.

[0108] In the XRD pattern obtained by the powder XRD measurement, a peak intensity of the diffraction peak at 2θ=19.1°±0.5° which indicates the high-ion-conductive crystalline phase is defined as I_C. Furthermore, the largest intensity among the peak intensities of the diffraction peaks at 2θ=18.2°±0.5° which indicate the β-Li₃PS₄ crystalline phase is defined as I_B. In this case, a peak intensity ratio represented by I_B/I_C is preferably 0.01 to 10.0, more preferably 0.3 to 5.0, and still more preferably 0.5 to 2.5. Here, the above intensity ratio is preferably 0.01 or more, more preferably 0.3 or more, and still more preferably 0.5 or more, from the viewpoint of the heat resistance. Further, from the viewpoint of the lithium ion conductivity, the ratio is preferably 10.0 or less, more preferably 5.0 or less, and still more preferably 2.5 or less.

[0109] In a case in which a peak is observed at 2θ=18.6° to 18.7° in the XRD pattern obtained by the powder XRD measurement, when there is a diffraction peak within the range of 2θ=18.2°±0.5° and on a lower angle side than the peak, the peak is attributed to the diffraction peak at 2θ=19.1°±0.5°, which indicates the high-ion-conductive crystalline phase. Further, in the above case, when there is a diffraction peak within the range of 2θ=19.1°±0.5° and on a higher angle side than the peak, the peak is attributed to the diffraction peak at 2θ=18.2°±0.5°, which indicates the β-Li₃PS₄ crystalline phase.

[0110] The sulfide solid electrolyte according to the present embodiment may also include another phase other than the sulfide glass phase, the high-ion-conductive crystalline phase, and the β-Li₃PS₄ crystalline phase. Examples of the another phase include crystalline phases obtained together when the sulfide glass phase and the β-Li₃PS₄ crystalline phase are produced.

[0111] A proportion of such a phase in the sulfide solid electrolyte is preferably 0 mass % to 30 mass %, more preferably 0 mass % to 20 mass %, and still more preferably 0 mass % to 15 mass %. Here, from the viewpoint of suitably obtaining the effects of the sulfide glass phase, the high-ion-conductive crystalline phase, and the β-Li₃PS₄ crystalline phase, the proportion of the another phase is preferably 30 mass % or less, more preferably 20 mass % or less, and still more preferably 15 mass % or less. In addition, the another phase may not be contained.

[0112] When the sulfide solid electrolyte according to the present embodiment is formed into a green compact under 380 MPa, the lithium ion conductivity at 25° C. is preferably 1 mS/cm or more. This is due to the high lithium ion conductivity of the sulfide glass phase and the high-ion-conductive crystalline phase derived therefrom. However, since it is difficult to extract only the sulfide glass phase or the high-ion-conductive crystalline phase from the sulfide solid electrolyte, the lithium ion conductivity of the sulfide glass phase and the high-ion-conductive crystalline phase is evaluated based on the lithium ion conductivity of the sulfide solid electrolyte.

[0113] The lithium ion conductivity of the sulfide solid electrolyte according to the present embodiment is preferably 1 mS/cm or more, and higher lithium ion conductivity is more preferable, but a value thereof changes depending on the composition or the proportion of the sulfide glass phase, the high-ion-conductive crystalline phase, or the like.

[0114] For example, the lithium ion conductivity when the content of Br in Ha is 90 at % or more is preferably 2 mS/cm or more, and the lithium ion conductivity when the content of Cl in Ha is 90 at % or more is preferably 1.5 mS/cm or more.

[0115] The lithium ion conductivity in the present description can be obtained by an electrochemical impedance spectroscopy using, as a measurement sample, a green compact obtained by compacting a powder of the sulfide solid electrolyte at 380 MPa. Specifically, the electrochemical impedance spectroscopy of the measurement sample is performed at a measurement frequency of 100 Hz to 1 MHz, a measurement voltage of 100 mV, and a measurement temperature of 25° C., and a value obtained from the obtained Nyquist plot is regarded as the lithium ion conductivity.

[0116] The heat resistance of the sulfide solid electrolyte according to the present embodiment can be evaluated by the lithium ion conductivity after performing a heat treatment on the sulfide solid electrolyte in a nitrogen atmosphere at 240° C. for 1 hour.

[0117] Specifically, the lithium ion conductivity after the heat treatment is preferably 0.8 mS/cm or more, and higher lithium ion conductivity is more preferable, but the value thereof changes depending on the composition or the proportion of the sulfide glass phase, the high-ion-conductive crystalline phase, or the like.

[0118] When a difference in lithium ion conductivity before and after the heat treatment is 2 mS/cm or less, it can be said that the heat resistance of the sulfide solid electrolyte is good, and the difference is more preferably 1.8 mS/cm or less and still more preferably 1.5 mS/cm or less, and the smaller the difference, the more preferable.

[0119] A generation amount of H₂S when the sulfide solid electrolyte according to the present embodiment is held in an atmosphere having a dew point of −20° C. for 5 hours is preferably 10 mL/g or less, more preferably 1 mL/g or less, still more preferably 0.1 mL/g or less, and the smaller the better.

[0120] More specific measurement conditions of the generation amount of H₂S are as follows.

[0121] First, the sulfide solid electrolyte is passed through a 100 μm mesh to obtain a powder having an average particle diameter of 10 μm to 20 μm. Further, a generation amount of hydrogen sulfide (H₂S) that is measured when 10 mg of such a powder is exposed to nitrogen (N₂) gas humidified to a dew point of −20° C. at a flow rate of 0.5 L/min for 5 hours is monitored, and the total amount thereof is defined as the generation amount of H₂S.

[0122] The generation amount of H₂S is a value serving as an index of hydrolysis resistance, that is, the water resistance of the sulfide solid electrolyte, and can be achieved by the presence of the sulfide glass phase or the high-ion-conductive crystalline phase or by containing Ha as the constituent element. In particular, it is preferable to contain at least one of Cl and Br as Ha, and it is more preferable to contain Br. It is also more preferable to contain both Cl and Br.

[0123] An average particle diameter (D50) of the sulfide solid electrolyte according to the present embodiment is preferably 0.1 μm to 300 μm, more preferably 0.3 μm to 50 μm, and still more preferably 0.5 μm to 5 μm. Here, from a viewpoint of reducing a load in a fine pulverization step when the sulfide solid electrolyte is used in a secondary battery, the average particle diameter is preferably 300 μm or less, more preferably 50 μm or less, and still more preferably 5 μm or less. On the other hand, from a viewpoint of ease of handling the powder, the average particle diameter is preferably 0.1 μm or more, more preferably 0.3 μm or more, and still more preferably 0.5 μm or more.

[0124] The average particle diameter (D50) can be adjusted by coarsely pulverizing the obtained sulfide solid electrolyte. The coarse pulverization can be adopted by a known method in the related art, such as a cutter mill, a planetary ball mill, or a jet mill, and dry pulverization is preferred. Thereafter, by a wet or dry pulverization method such as a bead mill or a jet mill, particles having a finer particle diameter can be used as a solid electrolyte of a positive electrode, a negative electrode, or a separator layer as a battery material.

[0125] The sulfide solid electrolyte according to the present embodiment is suitable as an electrolyte of a lithium ion secondary battery. When the sulfide solid electrolyte is used for the lithium ion secondary battery, the sulfide solid electrolyte forms a solid electrolyte layer together with other components such as a binder as necessary. As the binder or other components, known substances according to the related art can be used.

[0126] A content of the sulfide solid electrolyte according to the present embodiment with respect to the entire solid electrolyte layer is preferably 80 mass % or more, and more preferably 90 mass % or more.

[0127] The sulfide solid electrolyte according to the present embodiment may be used as a positive electrode layer or a negative electrode layer by being mixed with a positive electrode active material or a negative electrode active material. As the positive electrode active material or negative electrode active material used for the positive electrode layer or negative electrode layer, a current collector, a binder, a conductive aid, and the like, known materials according to the related art can be used.

[0128] The lithium ion secondary battery includes the positive electrode layer, the solid electrolyte layer, and the negative electrode layer, and the sulfide solid electrolyte according to the present embodiment is preferably contained in at least one layer selected from the group consisting of the positive electrode layer, the solid electrolyte layer, and the negative electrode layer.

[0129] As a material of an outer casing of the lithium ion secondary battery, known materials according to the related art may also be used. As a shape of the lithium ion secondary battery, known shapes according to the related art may be used, and examples of the shape of the lithium ion secondary battery include a coin shape, a sheet shape (film shape), a folded shape, a wound cylindrical shape with a bottom, and a button shape. The shape thereof can be appropriately selected according to the application.

[0130] A method for producing a sulfide solid electrolyte according to the present embodiment includes the following step 1 to step 4 in order as step S1 to step S4 as shown in FIG. 1.

• • [0131] Step S1: step 1 of mixing raw materials to obtain a raw material mixture containing Li, P, and S
  • [0132] Step S2: step 2 of heating the raw material mixture obtained in step 1 to obtain a melt
  • [0133] Step S3: step 3 of cooling and solidifying the melt obtained in step 2 to obtain a sulfide solid electrolyte precursor containing a glass phase and a β-Li₃PS₄ crystalline phase
  • [0134] Step S4: step 4 of heating the sulfide solid electrolyte precursor obtained in step 3 to obtain a high-ion-conductive crystalline phase obtained by crystallizing a part of the glass phase

[0135] The heating in step 2 and the cooling and solidifying in step 3 are performed under an ambient pressure condition, and in the cooling and solidifying, a molten liquid which is the melt is allowed to flow out and then stay in a temperature range of 750° C. to 210° C. for 0.2 seconds to 100 seconds. As a result, a sulfide solid electrolyte which contains the phase including the sulfide glass, the high-ion-conductive crystalline phase derived therefrom, and the β-Li₃PS₄ crystalline phase and in which the content ratio of the β-Li₃PS₄ crystalline phase is 0.1 mass % to 30 mass %, is obtained. In the present description, “under an ambient pressure condition” means a pressure range of about (gauge pressure±15 kPa).

[0136] In step 1, the raw materials are mixed such that compositions of the sulfide glass phase to be obtained and the high-ion-conductive crystalline phase satisfy Li: 30 at % to 50 at %, P: 5 at % to 15 at %, and S: 30 at % to 60 at %, whereby the sulfide solid electrolyte described in the <<Sulfide Solid Electrolyte>> is obtained.

[0137] Each step will be described below.

[0138] Step S1 in the present embodiment is step 1 of mixing the raw materials to obtain the raw material mixture containing Li, P, and S.

[0139] Specifically, the raw material mixture is obtained by mixing a raw material containing a Li element, a raw material containing a P element, and a raw material containing an S element. When it is desired to obtain a glass phase further containing Ha as the constituent element, in addition to the raw material containing the Li element, the raw material containing the P element, and the raw material containing the S element, a raw material containing an Ha element is further mixed to obtain a raw material mixture containing Li, P, S, and Ha. Here, the Ha element is at least one element selected from the group consisting of F, Cl, Br, and I.

[0140] Examples of the raw material containing the Li element include lithium compounds such as lithium sulfide (Li₂S), lithium carbonate (Li₂CO₃), lithium sulfate (Li₂SO₄), lithium oxide (Li₂O), and lithium hydroxide (LiOH), and metallic lithium. The substances containing the Li element may be used alone or in combination of two or more kinds thereof.

[0141] As the raw material containing the Li element, lithium sulfide is preferably used from the viewpoint of obtaining the sulfide glass phase. In the case in which the sulfide solid electrolyte to be obtained contains a halogen element, a lithium halide (LiHa, Ha is a halogen element) is also preferred as the raw material containing the Li element. The lithium halide is to be described later.

[0142] Examples of the raw material containing the P element include phosphorus sulfides such as phosphorus pentasulfide (P₂S₅) and phosphorus trisulfide (P₂S₃), phosphorus compounds such as sodium phosphate (Na₃PO₄), and elemental phosphorus. The substances containing the P element may be used alone or in combination of two or more kinds thereof.

[0143] The raw material containing the P element is preferably phosphorus sulfides, and more preferably phosphorus pentasulfide (P₂S₅), from the viewpoint of preventing the inclusion of elements other than the elements constituting the target sulfide solid electrolyte.

[0144] Examples of the raw material containing the S element include lithium sulfide (Li₂S), phosphorus sulfides such as phosphorus trisulfide (P₂S₃), and phosphorus pentasulfide (P₂S₅), other sulfur compounds containing phosphorus, and a sulfur-containing compound. Examples of the sulfur-containing compound include H₂S, CS₂, iron sulfides (such as FeS, Fe₂S₃, FeS₂, and Fe_1-xS), bismuth sulfide (Bi₂S₃), and copper sulfides (such as CuS, Cu₂S, and Cu_1-xS). The substances containing the S element may be used alone or in combination of two or more kinds thereof.

[0145] The raw material containing the S element is preferably lithium sulfide or phosphorus sulfides, and more preferably phosphorus pentasulfide (P₂S₅) as the phosphorus sulfide, from the viewpoint of preventing the inclusion of elements other than the elements constituting the target sulfide solid electrolyte. Lithium sulfide is a compound serving as both the raw material containing the Li element and the raw material containing the S element, and the phosphorus sulfide is a compound serving as both the raw material containing the S element and the raw material containing the P element.

[0146] Examples of the raw material containing the Ha 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. The substances containing the Ha element may be used alone or in combination of two or more kinds thereof.

[0147] From the viewpoint of reactivity, the raw material containing the Ha element is preferably the lithium halides, more preferably LiCl, LiBr, or LiI, and still more preferably LiCl or LiBr.

[0148] A raw material mixture may be obtained by mixing raw materials further containing other elements in accordance with a composition of a desired sulfide solid electrolyte.

[0149] As described above, examples of the other elements include Si, B, Ge, Al, O, Na, K, Mg, Ca, Sr, Ba, Y, Zr, Cr, Zn, Ga, Sn, and Sb. Among the other elements, for example, Si, B, Ge, Al, Sn, and Sb may be contained. Si, B, Ge, Al, Sn, and Sb are elements that can occupy the position of P when the sulfide glass phase is crystallized.

[0150] As the raw material containing the other elements, a known material in the related art can be used.

[0151] Examples of a raw material containing the Si element include SiO₂ and SiS₂. Among them, SiO₂ is more preferred from the viewpoint of the lithium ion conductivity. These compounds may be used alone or in combination of two or more kinds thereof.

[0152] Examples of a raw material containing the B element include B₂O₃ and B₂S₃. Among them, B₂O₃ is more preferred from the viewpoint of the water resistance of the glass. These compounds may be used alone or in combination of two or more kinds thereof.

[0153] Examples of a raw material containing the Ge element include GeO₂, GeS, GeS₂, and GeCl₂. Among them, from the viewpoint of the lithium ion conductivity, GeS₂ and GeCl₂ are preferred, and GeS₂ is more preferred. These compounds may be used alone or in combination of two or more kinds thereof.

[0154] Examples of a raw material containing the Al element include Al₂S₃, Al₂O₃, and AlCl₃. Among them, Al₂S₃ and AlCl₃ are preferred, and Al₂S₃ is more preferred, from the viewpoint of the lithium ion conductivity and the water resistance. These compounds may be used alone or in combination of two or more kinds thereof.

[0155] Examples of a raw material containing the Sn element include SnS, SnS₂, SnO, SnO₂, and SnCl₂. Among them, from the viewpoint of the lithium ion conductivity, SnS₂ and SnCl₂ are preferred, and SnS₂ is more preferred. These compounds may be used alone or in combination of two or more kinds thereof.

[0156] Examples of a raw material containing the Sb element include Sb₂S₃, Sb₂O₃, Sb₂O₅, SbCl₃, and SbCl₅. Among them, from the viewpoint of the lithium ion conductivity and the water resistance, Sb₂S₃ and SbCl₃ are preferred, and Sb₂S₃ is more preferred. These compounds may be used alone or in combination of two or more kinds thereof.

[0157] These raw materials are appropriately blended according to compositions of a desired sulfide glass phase and a high-ion-conductive crystalline phase derived therefrom. Specifically, in order to obtain the sulfide solid electrolyte described in the above <<Sulfide Solid Electrolyte>>, the raw materials are mixed such that the compositions of the glass phase to be obtained and the high-ion-conductive crystalline phase derived therefrom satisfy Li: 30 at % to 50 at %, P: 5 at % to 15 at %, and S: 30 at % to 60 at %. Further, it is preferable that the glass phase and the high-ion-conductive crystalline phase further contain Ha as the constituent element, and further satisfy Ha: 1 at % to 12 at % in addition to the above compositions.

[0158] A crystal structure of the high-ion-conductive crystalline phase to be obtained is also determined by blending these raw materials.

[0159] In addition to the above, preferred aspects of the glass phase to be obtained and the high-ion-conductive crystalline phase derived therefrom are the same as preferred aspects of the <Phase Including Sulfide Glass and High-ion-conductive Crystalline Phase> of the above <<Sulfide Solid Electrolyte>>.

[0160] It is preferable to reduce a particle size of each raw material from the viewpoint of shortening a holding time in the heating and melting in the subsequent step 2. When the particle size of the raw material is too large, the homogeneity of the sulfide solid electrolyte precursor and the sulfide solid electrolyte may be affected, and it is preferable that the particle size of the raw material is small to some extent from such a viewpoint. The production method according to the present embodiment is excellent in composition controllability. Therefore, for example, even when a raw material having a particle size that can reduce the homogeneity in a production method in the related art is used, a more homogeneous sulfide solid electrolyte precursor and a more homogeneous sulfide solid electrolyte can be produced in the production method according to the present embodiment.

[0161] From the viewpoint, specifically, the particle size of each raw material is preferably 1 mm or less, more preferably 500 μm or less, still more preferably 250 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less. The smaller the particle size, the more preferred, a lower limit thereof is actually about 0.1 μm, and the particle size may be 1 μm or more or 5 μm or more.

[0162] As described above, according to the production method of the present embodiment, even when a raw material having a relatively large particle size is used, a more homogeneous sulfide solid electrolyte precursor and a more homogeneous sulfide solid electrolyte are easily obtained. In consideration of this, for example, from the viewpoint of reducing a production cost, the particle size of each raw material may be 10 μm or more, 100 μm or more, or 250 μm or more.

[0163] From them, the particle size of each raw material is preferably 0.1 μm to 1 mm, more preferably 1 μm to 500 μm, still more preferably 5 μm to 250 μm, even more preferably 5 μm to 100 μm, and particularly preferably 5 μm to 50 μm. From the viewpoint of the production cost, the particle size of each raw material is preferably 10 μm to 1 mm, more preferably 100 μm to 1 mm, and still more preferably 250 μm to 500 μm.

[0164] In the present description, the particle size of each raw material refers to an average particle diameter (D50) represented by a median diameter determined from a chart of a volume-based particle size distribution obtained by measuring a particle size distribution using a laser diffraction particle size distribution analyzer MT 3300 EXII manufactured by Microtrac.

[0165] The raw materials can be mixed by, for example, mixing in a mortar, mixing using a medium such as a planetary ball mill, and medium-less mixing such as a pin mill, a powder stirrer, and air flow mixing. The raw materials may be made amorphous by mixing before heating.

[0166] Step S2 in the present embodiment is step 2 of heating the raw material mixture obtained in step 1 to obtain the melt.

[0167] A specific method for heating and melting the raw material mixture is not particularly limited except that heating is performed under the ambient pressure condition.

[0168] In the production method according to the present embodiment, by performing the heating for melting the raw material mixture under the ambient pressure condition, a composition deviation between a composition in the raw material mixture and the compositions of the sulfide glass phase to be obtained and the high-ion-conductive crystalline phase derived therefrom or the composition of the sulfide solid electrolyte is small. Further, by performing the heating in a gas atmosphere containing an S element, the composition deviation can be further prevented.

[0169] Here, the ambient pressure condition means that a pressure in a container at the time of heating and melting is within a range of (gauge pressure±15 kPa).

[0170] That is, in the production method according to the present embodiment, the heating and melting in step 2 are performed under the ambient pressure condition of (gauge pressure±15 kPa). A composition ratio represented by [P/Li] of the glass phase to be obtained or the high-ion-conductive crystalline phase with respect to a composition ratio represented by [P/Li] of the raw material mixture at this time is preferably, for example, 95% to 105%.

[0171] The pressure during the heating and melting may be under the ambient pressure condition of (gauge pressure±15 kPa), and is preferably (gauge pressure±10 kPa), more preferably (gauge pressure±5 kPa), and still more preferably (gauge pressure±2 kPa).

[0172] Further, the composition ratio represented by [P/Li] of the glass phase to be obtained or the high-ion-conductive crystalline phase with respect to the composition ratio represented by [P/Li] of the raw material mixture at this time is more preferably 95% to 105%, still more preferably 97% to 103%, even more preferably 99% to 101%, and the closer to 100%, the more preferable. Here, the above composition ratio is preferably 95% or more, more preferably 97% or more, and still more preferably 99% or more, and is preferably 105% or less, more preferably 103% or less, and still more preferably 101% or less.

[0173] Regarding the composition ratio represented by [P/Li] of the glass phase to be obtained or the high-ion-conductive crystalline phase with respect to the composition ratio represented by [P/Li] of the raw material mixture, Li is an element that is difficult to volatilize, and P is an element that is easy to volatilize. Therefore, comparison of the composition ratios represented by [P/Li] between the raw material mixture and the glass phase or the high-ion-conductive crystalline phase is appropriate as an index of the composition deviation between the composition in the raw material mixture and the composition of the sulfide glass phase to be obtained or the high-ion-conductive crystalline phase or the composition of the sulfide solid electrolyte.

[0174] Examples of a heat-resistant container containing the raw material mixture include 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, and mullite, a heat-resistant container containing a nitride such as silicon nitride and boron nitride, and a heat-resistant container containing a carbide such as silicon carbide. These heat-resistant containers may have a bulk formed of the above material, or may be a container having a layer made of carbon, an oxide, a nitride, a carbide, or the like, such as a carbon-coated quartz tube.

[0175] A melting temperature when the raw material mixture is heated varies depending on the raw materials to be used and the composition of the raw material mixture, and is, for example, preferably 600° C. to 950° C., more preferably 630° C. to 850° C., and still more preferably 650° C. to 750° C. Here, the melting temperature is preferably 600° C. or higher, more preferably 630° C. or higher, and still more preferably 650° C. or higher, from the viewpoint of the reaction rate. From the viewpoint of preventing the composition deviation due to volatilization of the components, the melting temperature is preferably 950° C. or lower, more preferably 850° C. or lower, and still more preferably 750° C. or lower.

[0176] A time for the heating and melting varies depending on a scale, but 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, the time for the heating and melting 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, from the viewpoint of allowing the reaction to proceed smoothly. The time for the heating and melting is preferably 10 hours or shorter, more preferably 9.5 hours or shorter, and still more preferably 9 hours or shorter, from the viewpoint of productivity.

[0177] The atmosphere in the container during the heating and melting is not particularly limited. For example, the atmosphere may be a nitrogen atmosphere, an argon atmosphere, or a gas atmosphere containing the S element. Among them, the gas atmosphere containing the S element is preferred from the viewpoint of more suitably preventing the composition deviation.

[0178] When the atmosphere is the gas atmosphere containing the S element, examples of an S element source include a sulfur gas, a hydrogen sulfide gas, and a sulfur dioxide gas, and it is preferable to contain a sulfur gas from the viewpoint of reactivity with the raw materials.

[0179] As the gas containing the S element, an elemental sulfur powder may be added, and the elemental sulfur powder may be vaporized by heating during the heating and melting to introduce the gas containing the S element. The elemental sulfur powder may be added together with the raw materials when the raw materials are mixed to obtain the raw material mixture, or may be separately added after the raw material mixture is obtained. However, when the elemental sulfur powder is added together with the raw materials when the raw material mixture is obtained, a mass of the elemental sulfur powder is not included in the mass of the obtained raw material mixture.

[0180] As the gas containing the S element, the sulfur gas, the hydrogen sulfide gas, the sulfur dioxide gas, or the like may be introduced, and the gas containing the S element obtained by adding the elemental sulfur powder may be introduced together.

[0181] As the gas containing the S element, a gas serving as the S element source is preferably used as a mixed gas with, for example, an inert gas. A mixing ratio with the inert gas can be freely selected, and is not particularly limited as long as a cumulative introduction amount of the S element with respect to the mass of the raw material mixture can be a desired value.

[0182] Examples of the inert gas include a nitrogen gas, an argon gas, and a helium gas, which may be used alone or in combination of two or more kinds thereof.

[0183] A dew point during the heating and melting is preferably −20° C. or lower, and a lower limit thereof is not particularly limited and is usually about −80° C. An oxygen concentration is preferably 1,000 ppm by volume or less.

[0184] Complete dissolution of the melt of the raw material mixture in step 2 can be confirmed by the absence of a peak derived from a crystal in a high-temperature X-ray diffraction measurement. It can also be confirmed by heating at a predetermined temperature and tilting the melt to confirm the presence or absence of fluidity.

[0185] Step S₃ in the present embodiment is step 3 of cooling and solidifying the melt obtained in step 2 to obtain the sulfide solid electrolyte precursor containing the glass phase and the β-Li₃PS₄ crystalline phase.

[0186] The cooling and solidifying is performed under an ambient pressure condition. Here, the ambient pressure condition means an atmosphere under a pressure of about (gauge pressure±15 kPa) as described above.

[0187] A pressure at the time of cooling and solidifying may be under the ambient pressure condition of (gauge pressure±15 kPa), and is preferably (gauge pressure±10 kPa), more preferably (gauge pressure±5 kPa), and still more preferably (gauge pressure±2 kPa).

[0188] In the cooling and solidifying, a molten liquid which is the melt obtained in step 2 is allowed to flow out of the container used for the heating and melting in step 2. After the molten liquid flows out, the molten liquid is allowed to stay in the temperature range of 750° C. to 210° C. for 0.2 seconds to 100 seconds. Accordingly, it is found that the sulfide solid electrolyte precursor containing, as constituent elements, the sulfide glass phase containing Li, P, and S as the constituent elements and the β-Li₃PS₄ crystalline phase is obtained. In addition to the above, a content ratio of the β-Li₃PS₄ crystalline phase in the sulfide solid electrolyte precursor can be set to a suitable amount of, for example, 0.1 mass % to 30 mass %. That is, the content ratio of the β-Li₃PS₄ crystalline phase in the sulfide solid electrolyte obtained through step 4 can also be a suitable amount of, for example, 0.1 mass % to 30 mass %.

[0189] During the cooling to the temperature range of 750° C. to 210° C., depending on the composition, the melt may already be a solid rather than a molten liquid (liquid), but for the sake of convenience, the temperature will be referred to as a temperature of a molten liquid here.

[0190] In the cooling and solidifying, a time for which the poured molten liquid is allowed to stay in the temperature range of 750° C. to 210° C. is 0.2 seconds to 100 seconds, preferably 0.5 seconds to 60 seconds, and more preferably 1 second to 40 seconds. Here, the time is 0.2 seconds or more, preferably 0.5 seconds or more, and more preferably 1 second or more from the viewpoint of precipitation of the β-Li₃PS₄ crystalline phase. In addition, the time is 100 seconds or less, but is preferably 60 seconds or less, and more preferably 40 seconds or less from the viewpoint of preventing excessive precipitation of the β-Li₃PS₄ crystalline phase.

[0191] When the heating temperature for obtaining the melt in step 2 is 750° C. or less, the time for which the molten liquid is allowed to stay in the temperature range of 750° C. to 210° C. may be read as the time for which the molten liquid is allowed to stay in a temperature range of the heating temperature to 210° C.

[0192] An average cooling rate while the molten liquid is allowed to stay in the temperature range of 750° C. to 210° C. in the cooling and solidifying is preferably 5.4° C./sec or more, more preferably 5.4° C./sec to 2,700° C./sec, still more preferably 50° C./sec to 2,000° C./sec, and even more preferably 100° C./sec to 1,000° C./sec.

[0193] By controlling the cooling rate within a suitable range and performing quenching, the sulfide solid electrolyte precursor containing, as the constituent elements, the sulfide glass phase containing Li, P, and S as the constituent elements and the β-Li₃PS₄ crystalline phase is easily obtained. More specifically, from a viewpoint of achieving a small crystallite diameter, the average cooling rate is preferably 5.4° C./sec or more, more preferably 50° C./sec or more, and still more preferably 100° C./sec or more. In addition, from a viewpoint of suitably growing the crystals, the average cooling rate is preferably 2,700° C./sec or less, more preferably 2,000° C./sec or less, and still more preferably 1,000° C./sec or less.

[0194] In the cooling and solidifying, after a temperature of the poured molten liquid reaches 210° C., the molten liquid is further cooled to, for example, 150° C. or less, and then subjected to the next step 4.

[0195] A cooling rate until the temperature of the molten liquid reaches 210° C. and a cooling rate at which the molten liquid is further cooled to 150° C. or lower may be the same or different.

[0196] By the cooling, the content ratio of the sulfide glass phase in the sulfide solid electrolyte precursor can be, for example, 60 mass % or more, and the content ratio of the β-Li₃PS₄ crystalline phase can be 0.1 mass % to 30 mass %. Furthermore, a total content ratio of the sulfide glass phase and the β-Li₃PS₄ crystalline phase can be, for example, 80 mass % or more.

[0197] The sulfide glass phase can achieve the higher lithium ion conductivity than the sulfide glass in the related art. Further, by performing the cooling and solidifying under the ambient pressure condition, the composition deviation between the composition in the raw material mixture and the compositions of the sulfide glass phase to be obtained and the high-ion-conductive crystalline phase derived therefrom or the composition of the sulfide solid electrolyte can also be reduced.

[0198] Step S4 according to the present embodiment is step 4 of heating the sulfide solid electrolyte precursor obtained in step 3 to obtain the high-ion-conductive crystalline phase obtained by crystallizing a part of the glass phase in the sulfide solid electrolyte precursor.

[0199] The heating in step 4, that is, a temperature at the time of crystallization can be determined depending on a crystallization temperature at which the glass phase becomes the high-ion-conductive crystalline phase.

[0200] Specifically, from a viewpoint of effectively promoting the crystallization, the temperature is preferably (crystallization temperature−15° C.) or higher, may be (crystallization temperature−10° C.) or higher, may be a temperature equal to or higher than the crystallization temperature, or may be (crystallization temperature+1° C.) or higher. Further, it is found that when heating at a high temperature is performed, other heterogeneous phases, which are crystals having low ion conductivity, are precipitated after precipitation of the high-ion-conductive crystalline phase. Therefore, from the viewpoint of preventing the precipitation of the crystals having the low ion conductivity, the crystallization temperature is preferably equal to or lower than a temperature at which the crystals having the low ion conductivity are precipitated, more preferably (crystallization temperature+20° C.) or lower, and still more preferably (crystallization temperature+15° C.) or lower.

[0201] That is, the temperature at the time of crystallization is preferably (crystallization temperature−15° C.) or higher, and more preferably (crystallization temperature−15° C.) or higher and equal to or lower than the temperature at which the crystals having the low ion conductivity are precipitated. Further, the temperature may be (crystallization temperature−10° C.) or higher and (crystallization temperature+20° C.) or lower, may be crystallization temperature or higher and (crystallization temperature+20° C.) or lower, or may be (crystallization temperature+1° C.) or higher and (crystallization temperature+15° C.) or lower. Within the above range, the high-ion-conductive crystalline phase is obtained from the glass phase, and at the same time, other crystalline phases are obtained.

[0202] The crystallization temperature in the present description refers to a temperature at a peak top of an exothermic peak observed when differential scanning calorimetry (DSC) is performed on the sulfide solid electrolyte precursor obtained in step 3, and the temperature is increased at a heating rate of 5° C./min.

[0203] The heating time for crystallization is, for example, preferably 1 minute to 180 minutes, more preferably 5 minutes to 120 minutes, and still more preferably 10 minutes to 100 minutes. Here, from a viewpoint of a stable quality, the heating time is preferably 1 minute or more, more preferably 5 minutes or more, and still more preferably 10 minutes or more. From the viewpoint of productivity, the heating time is preferably 180 minutes or shorter, more preferably 120 minutes or shorter, and still more preferably 100 minutes or shorter.

[0204] By adjusting the heating temperature and the heating time during the crystallization, the content ratio of the high-ion-conductive crystalline phase can be adjusted.

[0205] Examples of a heating atmosphere during the crystallization include a nitrogen atmosphere, an argon atmosphere, and a dry air atmosphere. Among these, a nitrogen atmosphere or an argon atmosphere is preferable from a viewpoint of not impairing the ion conductivity.

[0206] A dew point during the crystallization is preferably −30° C. or lower.

[0207] An oxygen concentration during the crystallization may be in a dry air environment, but is preferably 5% by volume or less from a viewpoint of safety.

[0208] After the heating, the sulfide solid electrolyte according to the present embodiment is obtained by cooling to room temperature at 1° C./min to 10,000° C./min, for example.

[0209] The sulfide solid electrolyte obtained as described above may be used as it is in at least one or more selected from the group consisting of the positive electrode layer, the solid electrolyte layer, and the negative electrode layer, or may be used after pulverization, drying, or the like according to an application or the like.

[0210] Hereinafter, the present invention is specifically described with reference to Examples, but the present invention is not limited thereto.

[0211] Examples 3 to 5 and Example 7 are Inventive Examples, and Examples 1, 2, and 6 are Comparative Examples.

[0212] Under a dry nitrogen gas atmosphere, a lithium sulfide powder (manufactured by Sigma, purity 99.98%), a phosphorus pentasulfide powder (manufactured by Sigma, purity 99%), and a lithium bromide powder (manufactured by Sigma, purity 99.995%) were weighed so as to have a composition ratio of Li_3.4P_1.0S_4.0Br_0.3 (Li: 38.9 at %, P: 11.4 at %, S: 45.9 at %, and Br: 3.8 at %), and mixed in a mortar to obtain a raw material mixture (step 1).

[0213] The obtained raw material mixture was put into a carbon container in an S element-containing nitrogen atmosphere having a dew point of −50° C. or lower and containing a sulfur powder (manufactured by Sigma, purity 99.998%) as an S element source, the container was put into an electric furnace inside a glove box, and the container was heated at a pressure of gauge pressure+1 kPa and a temperature of 750° C. for 1 hour to obtain a completely dissolved melt (step 2).

[0214] Next, a front end of the container was heated to melt the melt, and a molten liquid which is the melt was poured onto twin rolls and cooled to room temperature at an average cooling rate of 5,400° C./sec to obtain a sulfide solid electrolyte precursor (step 3). The average cooling rate was adjusted by an outflow rate of the molten liquid, a roll gap of the twin rolls, and a rotation speed. A pressure at this time was (gauge pressure+1 kPa), and a time during which the poured molten liquid stays in a temperature range of 750° C. to 210° C. was 0.1 seconds.

[0215] The sulfide solid electrolyte precursor obtained as described above was heated at 210° C. for 10 minutes in a nitrogen atmosphere and cooled to room temperature at a cooling rate of 300° C./min to obtain a sulfide solid electrolyte.

[0216] The sulfide solid electrolyte precursor obtained as described above was subjected to a DSC measurement at a heating rate of 5° C./min, and it was confirmed that a temperature at a peak top of an exothermic peak, that is, the crystallization temperature was 223° C.

[0217] Under a dry nitrogen gas atmosphere, a lithium sulfide powder (manufactured by Sigma, purity 99.98%), a phosphorus pentasulfide powder (manufactured by Sigma, purity 99%), and a lithium bromide powder (manufactured by Sigma, purity 99.995%) were weighed so as to have a composition ratio of Li_3.4P_1.0S_4.0Br_0.3, and mixed in a mortar to obtain a raw material mixture (step 1).

[0218] The obtained raw material mixture was further mixed using a planetary ball mill (manufactured by Fritsch, P-7) to obtain a sulfide solid electrolyte precursor (steps 2′+3′). The mixing by the planetary ball mill was performed by mechanical milling at 400 rpm for 20 hours in a nitrogen atmosphere using zirconia balls having a particle size of 4 mm and a zirconia pot. A dew point during operation was maintained at −70° C. or lower.

[0219] The sulfide solid electrolyte obtained as described above was heated at 210° C. for 10 minutes in a nitrogen atmosphere and then cooled to room temperature at a cooling rate of 300° C./min to obtain a sulfide solid electrolyte (step 4). A crystallization temperature of the sulfide solid electrolyte precursor was 210° C.

[0220] A sulfide solid electrolyte was obtained in the same manner as in Example 1 except that a cooling rate of the melt obtained by the heating was changed and the time for which the molten liquid stays in the temperature range of 750° C. to 210° C. was changed to 0.5 seconds (Example 3), 3.2 seconds (Example 4), 15 seconds (Example 5), and 150 seconds (Example 6). The crystallization temperatures of the sulfide solid electrolyte precursor were all around 225° C.

[0221] A sulfide solid electrolyte was obtained in the same manner as in Example 1 except that a raw material mixture was obtained by weighing and mixing raw materials so as to have a composition ratio of Li_3.4P_1.0S_4.0Br_0.15I_0.15, and that the time for which the molten liquid stays in the temperature range of 750° C. to 210° C. was set to 55 seconds. A crystallization temperature of the sulfide solid electrolyte precursor was 215° C.

[0222] In each example, the sulfide solid electrolyte precursor obtained in step 3 or steps 2′+3′ was weighed in a glove box and dissolved in an alkaline aqueous solution, and a composition analysis was performed for each element.

[0223] Specifically, P and S were subjected to ICP emission spectrometry (apparatus: model number PS3520UV DDII, manufactured by Hitachi High-Tech Science Corporation).

[0224] Li was analyzed by atomic absorption spectroscopy (apparatus: model number ZA3300, manufactured by Hitachi High-Tech Corporation, CsCl was added at a solution concentration of 0.1% in a Li measurement).

[0225] Br and I were analyzed by ion chromatography (apparatus: model number ICS-2100 (column: AS11HC) manufactured by Thermo Fisher Scientific Inc, and a small amount of H₂O₂ was added and diluted with ultrapure water for a measurement).

[0226] As described above, an overall composition of the sulfide solid electrolyte precursor was confirmed.

[0227] In addition to the above, each of the sulfide solid electrolyte precursors was pulverized in a mortar and passed through a 100 μm sieve to obtain a powder of a solid electrolyte precursor having an average particle diameter D50 of about 20 μm, which was used as a sample.

[0228] Using an X-ray diffraction (XRD) device (manufactured by Rigaku Corporation, Smart Lab), measurements were performed under the following conditions: radiation source: CuKα radiation (λ=1.5418 Å), tube voltage: 45 kV, tube current: 200 mA, scan angle: 10° to 100°, scan speed: 5°/min, step number: 0.01°/step, under an air-free environment. In addition, when a silicon powder was added as an internal standard when XRD is measured and a peak was observed in an XRD pattern, crystallinity (wt %) was obtained by Rietveld analysis.

[0229] The presence or absence of a crystal peak was confirmed in the obtained XRD pattern, and the presence or absence of the β-Li₃PS₄ crystalline phase was confirmed. When the β-Li₃PS₄ crystalline phase was present, the content ratio and the crystallite diameter thereof were obtained. The crystallite diameter was determined by a Scherrer equation using a half-width of the crystal peak. More specifically, the crystallite diameter was calculated using analysis software PDXL attached to the apparatus, analyzing a peak at 2θ=18.2±0.5 (derived from plane 011).

[0230] In addition, the content (wt %) of the glass phase can be calculated by subtracting the crystallinity (wt %) in the sulfide solid electrolyte precursor from 100 wt %.

[0231] The results are shown in Table 1.

[0232] The sulfide solid electrolyte obtained in each of Examples was pulverized in a mortar and passed through a 100 μm sieve to obtain a powder of a solid electrolyte precursor having an average particle diameter D50 of about 20 μm, which was used as a sample.

[0233] Further, the XRD measurement was performed under the same conditions as in the case of using the sulfide solid electrolyte precursor as the sample.

[0234] By analyzing the obtained XRD pattern, the composition ratio and the content ratio of each of the high-ion-conductive crystalline phase and the β-Li₃PS₄ crystalline phase were obtained, and the content ratio of the phase including the sulfide glass was also obtained from the crystallinity.

[0235] Here, the composition ratio of the phase including the sulfide glass can be determined to be the same as the composition ratio of the high-ion-conductive crystalline phase, but just to be sure, the composition ratio was compared with the composition ratio obtained in the evaluation with respect to the sulfide solid electrolyte precursor. As a result, it was confirmed that the composition ratio of the sulfide solid electrolyte precursor with respect to the composition ratio of the high-ion-conductive crystalline phase was within a range of ±20% for all components.

[0236] Regarding the β-Li₃PS₄ crystalline phase, it was confirmed that there was no change in crystallinity, content ratio, and crystallite diameter between the sulfide solid electrolyte precursor and the sulfide solid electrolyte after the crystallization of the phase including the sulfide glass by the heating.

[0237] The results are shown in Table 1. Although the content ratios of the sulfide glass phases in Examples 2 and 5 to 7 and the high-ion-conductive crystalline phases in Examples 2 and 7 were not analyzed, it was confirmed that at least the sulfide solid electrolytes in Examples 5 to 7 include the sulfide glass phase and the high-ion-conductive crystalline phase derived therefrom.

[0238] In the analysis of the XRD pattern, when the peak intensity of the diffraction peak at 2θ=20.2°±0.5° which indicates the high-ion-conductive crystalline phase was defined as I_A and the largest intensity among the peak intensities of the diffraction peaks at 2θ=18.2°±0.5° which indicate the β-Li₃PS₄ crystalline phase was defined as I_B, the peak intensity ratio represented by I_B/I_A was also obtained. The results are shown in Table 1.

[0239] Further, when the peak intensity of the diffraction peak at 2θ=19.1°±0.5° which indicates the high-ion-conductive crystalline phase was defined as I_C and the largest intensity among the peak intensities of the diffraction peaks at 2θ=18.2°±0.5° which indicate the β-Li₃PS₄ crystalline phase was defined as I_B, the peak intensity ratio represented by I_B/I_C was also obtained. The results are shown in Table 1.

[0240] FIGS. 2 and 3 show the XRD patterns of the sulfide solid electrolyte precursor obtained in Example 4 and the sulfide solid electrolyte obtained in Example 4, respectively.

[0241] The obtained sulfide solid electrolyte was pulverized in a mortar and then passed through a 100 μm sieve to obtain a powder having an average particle diameter (D50) of about 20 μm, which was used as a sample.

[0242] The sample was compressed under a pressure of 380 MPa to prepare a green compact as a measurement sample, and measured by using an electrochemical impedance spectroscopy device (potentiostat/galvanostat VSP, manufactured by Bio-Logic Sciences Instruments). The measurement conditions were a measurement frequency of 100 Hz to 1 MHz, a measurement voltage of 100 mV, and a measurement temperature of 25° C., and the lithium ion conductivity was obtained from the obtained Nyquist plot. Results are shown in “σ^Li+ (mS/cm)” of “before heat treatment” in “heat resistance” in Table 1.

[0243] The obtained sulfide solid electrolyte was subjected to a heat treatment at 240° C. for 1 hour in a nitrogen atmosphere. The lithium ion conductivity of the sulfide solid electrolyte after the heat treatment was determined in the same manner as in the method described in <Lithium Ion Conductivity> above. Results are shown in “σ^Li+ (mS/cm)” of “after heat treatment” in “heat resistance” in Table 1, and the difference in lithium ion conductivity before and after the heat treatment is shown in “Δσ^Li+ (mS/cm)” of “heat resistance”.

[0244] Here, when the difference between the lithium ion conductivity before the heat treatment and the lithium ion conductivity after the heat treatment is 2 mS/cm or less, it can be said that the heat resistance is good. A smaller difference in lithium ion conductivity before and after the heat treatment is more preferable.

	TABLE 1
	Sulfide solid electrolyte
	Production method
	Content ratio (wt %)		Staying
	Composition		High-ion-				time at
	Sulfide glass phase and		conductive	β-Li3PS4	Precursor	Cooling	750° C. to
	high-ion-conductive	Sulfide	crystalline	crystalline	production	rate	210° C.
	crystalline phase	glass phase	phase	phase	method	(° C./min)	(seconds)
Example	Li3.4P1.0S4.0Br0.3	<20%	80%	0%	Melt	5,400	0.1
1					quenching
					method
Example	Li3.4P1.0S4.0Br0.3	Unanalyzed	Unanalyzed	0%	Mechanical	—	—
2					milling
					method
Example	Li3.4P1.0S4.0Br0.3	<20%	80%	1% to 5%	Melt	1,080	0.5
3					quenching
					method
Example	Li3.7P1.0S4.0Br0.3	<20%	70%	10%	Melt	169	3.2
4					quenching
					method
Example	Li3.7P1.0S4.0Br0.3	Unanalyzed	30% to 50%	20%	Melt	36	15
5					quenching
					method
Example	Li3.7P1.0S4.0Br0.3	Unanalyzed	<30%	60%	Melt	4	150
6					quenching
					method
Example	Li3.7P1.0S4.0Br0.15I0.15	Unanalyzed	Unanalyzed	10%	Melt	10	55
7					quenching
					method
	Heat resistance
	Before heat	After heat
		XRD	treatment	treatment
	Crystallite diameter	Peak intensity ratio	σLi+	σLi+	ΔσLi+
	(β-Li3PS4) (nm)	IB/IA	IB/IC	(mS/cm)	(mS/cm)	(mS/cm)
Example	No crystal peak	—	—	3.2	0.7	2.5
1
Example	No crystal peak	—	—	2.4	0.1	2.3
2
Example	120 nm	0.07	0.45	3.2	2.8	0.4
3
Example	250 nm	0.39	2.26	2.0	1.6	0.4
4
Example	340 nm	0.55	4.06	1.1	0.8	0.3
5
Example	450 nm	1.06	10.50	0.4	0.2	0.2
6
Example	250 nm	0.45	2.73	2.7	2.2	0.5
7

[0245] From the above results, the sulfide solid electrolyte according to the present embodiment has the high lithium ion conductivity due to the presence of the phase including the sulfide glass and the high-ion-conductive crystalline phase derived therefrom. Further, although it is considered that it is better not to contain the β-Li₃PS₄ crystalline phase due to the low lithium ion conductivity thereof, by containing the β-Li₃PS₄ crystalline phase in the range of 0.1 mass % to 30 mass %, it is possible to achieve good heat resistance without significantly decreasing the lithium ion conductivity of the sulfide solid electrolyte.

[0246] Although the present invention has been described in detail with reference to specific embodiments, it is 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. The present application is based on a Japanese Patent Application (Japanese Patent Application No. 2023-168394) filed on Sep. 28, 2023, the content of which is incorporated herein by reference.