Sulfide solid electrolyte and method for manufacturing same

Abstract

The present invention relates to a sulfide solid electrolyte which has a crystal phase containing a crystal that is represented by composition formula RaMbScXd (wherein 3 ≤ a ≤ 7, b = 1, 3 ≤ c ≤ 6, 0.1 ≤ d ≤ 2, R represents an alkali metal element, M represents an element which can have a four-coordinate structure with S and is selected from among the elements in groups 2 to 15 of the periodic table, S represents elemental sulfur, and X represents a halogen element), wherein: in the composition formula, M is composed of a total of three or more elements that include a group 14 element MA and a group 15 element MB; the crystal has three or more tetrahedral structures that include MAS4 and MBS4; and the proportion of Ge in M in the crystal is 0.05 or less in terms of the molar ratio.

Description

Sulfide solid electrolyte and method for producing same

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

[0002] Lithium-ion secondary batteries are widely used in portable electronic devices such as mobile phones and laptop computers. While liquid electrolytes have traditionally been used in lithium-ion secondary batteries, solid-state lithium-ion secondary batteries, which use solid electrolytes as the electrolyte, have attracted attention due to their potential for improved safety, high-speed charging and discharging, and compact housings.

[0003] Among the solid electrolytes used in all-solid-state lithium ion secondary batteries, sulfide solid electrolytes exhibit high lithium ion conductivity. Among them, argyrodite-type solid electrolytes exhibit even higher lithium ion conductivity. For example, Patent Document 1 discloses a sulfide solid electrolyte having the composition formula: Li ７－ｘ P.S. ６－Ｘ Ha Ｘ (Ha is Cl or Br) (x=0.2 to 1.8), and ＊ a ＊ b ＊ An argyrodite-type sulfide solid electrolyte having a colorimetric lightness L value of 60.0 or more is disclosed.

[0004] However, conventionally known argyrodite-type sulfide solid electrolytes easily react with water to form hydrogen sulfide (H ２ S) is easily generated, and when exposed to a moisture-containing atmosphere, the lithium ion conductivity drops significantly.

[0005] In contrast, Patent Document 2 discloses an argyrodite-type Li ６．５ [P ０．２５ Si ０．２５ Ge ０．２５ Sb ０．２５ ]S ５ I is disclosed. ２ It is said that the amount of sulfur generated is reduced, and the decrease in lithium ion conductivity when exposed to a moisture-containing atmosphere is suppressed, resulting in excellent water resistance.

[0006] International Publication No. 2015 / 012042 International Publication No. 2023 / 111138

[0007] However, when the sulfide solid electrolyte described in Patent Document 2 is implemented in a lithium ion battery, it has been found to have problems with oxidation resistance and reduction resistance, and improvements in cycle characteristics are expected.

[0008] Therefore, an object of the present invention is to provide a sulfide solid electrolyte that has good water resistance and good cycle characteristics, and a method for producing the same.

[0009] According to the study by the present inventors, MS in which a specific element M forms a tetracoordinate structure with S ４ By including a tetrahedral structure in the crystal, H ２ The amount of sulfur generated can be reduced, and the decrease in ionic conductivity when exposed to a moisture-containing atmosphere can be suppressed, resulting in good water resistance. ４ The inclusion of three or more types of tetrahedral structures improves the water resistance. In addition, the inventors discovered that reducing the Ge content in the crystal or forming a crystal that does not contain Ge can improve oxidation resistance and reduction resistance, as well as achieve good cycle characteristics, and have completed the present invention.

[0010] That is, the present invention relates to the following [1] to

[18] . [1] A sulfide solid electrolyte having a crystalline phase, wherein the crystalline phase is represented by the composition formula: R ａ M ｂ S ｃ X ｄ In the composition formula, R represents an alkali metal element, M represents an element of Groups 2 to 15 of the periodic table that can form a tetracoordinate structure with S, and M is an element of Group 14 of the periodic table. Ａ and M, an element of Group 15 Ｂ wherein S represents a sulfur element, and X represents a halogen element, and in the composition formula, 3≦a≦7, b=1, 3≦c≦6, and 0.1≦d≦2 are satisfied, and the crystal is Ａ S ４ Tetrahedral structure and M Ｂ S ４[2] A sulfide solid electrolyte according to [1], wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. [3] A sulfide solid electrolyte according to [1], wherein M in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. [4] A sulfide solid electrolyte according to [1], wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. [5] A sulfide solid electrolyte according to [1], wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. [6] A sulfide solid electrolyte according to [1], wherein M in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. [7] A sulfide solid electrolyte according to [1], wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. [8] A sulfide solid electrolyte according to [1], wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. [9] A sulfide solid electrolyte according to [1], wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K.

[10] A sulfide solid electrolyte according to [1], wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K.

[11] A sulfide solid electrolyte according to

[12] , wherein M in the composition formula contains at least one element selected from the group consisting of Li, Na, and K.

[12] A sulfide solid electrolyte according to

[13] , wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na, and K.

[13] A sulfide solid electrolyte according to

[14] , wherein M in the composition formula contains at least one element selected from the group consisting of Li, Na, and K. Ａ As the element, one or more elements selected from Si, Ge, Sn, and Pb are included, and M included in M ​​in the composition formula Ｂ [4] The sulfide solid electrolyte according to [1] or [2], wherein M included in M ​​in the composition formula contains one or more elements selected from P, As, Sb, and Bi. Ｂ [5] The sulfide solid electrolyte according to any one of [1] to [3], wherein the proportion of Sb in M ​​in the crystal is 0.01 to 0.9 in terms of molar ratio. [6] The sulfide solid electrolyte according to [4], wherein M included in M ​​in the composition formula Ａ [7] The sulfide solid electrolyte according to any one of [1] to [5], wherein M is contained as Si. [7] The sulfide solid electrolyte according to [6], wherein in the crystal, the proportion of Si in M ​​is 0.01 to 0.6 in terms of molar ratio. [8] The sulfide solid electrolyte according to [6], wherein M is contained as M in the composition formula. Ｂ [9] The sulfide solid electrolyte according to any one of [1] to [7], wherein P is contained as M. [9] The sulfide solid electrolyte according to [8], wherein in the crystal, the proportion of P in M ​​is greater than 0 and 0.7 or less in molar ratio.

[10] M in the composition formula is M, which is an element of Group 14. Ａ and M, an element of Group 15 Ｂ

[11] The sulfide solid electrolyte according to any one of the above [1] to

[10] , wherein M in the composition formula is composed of a total of four or more elements.

[12] The sulfide solid electrolyte according to any one of the above [1] to

[11] , wherein the crystal has a crystal structure of the F-43m space group.

[13] The sulfide solid electrolyte according to the above

[12] , wherein the content of the crystal having the crystal structure of the F-43m space group in the sulfide solid electrolyte is 50 mass % or more.

[14] The sulfide solid electrolyte according to any one of the above [1] to

[13] , wherein the value of an ionic radius parameter, expressed by the following formula using the ionic radii of the elements constituting M in the crystal, is 0.47 to 0.55: Ionic radius parameter=(b １ ×r １ ) + (b ２ ×r ２ ) + ... + (b ｎ ×r ｎ ) In the composition formula, M is M １ ~M ｎ n elements (n is a natural number) in the formula １ ~b ｎ are the M １ ~M ｎ represents the composition ratio of each element, and (b １ +b ２ +...+b ｎ )=1, and r １ ~r ｎ are the M １ ~M ｎ represents the ionic radius (Å) of the tetracoordinated valence of each element in the crystal.

[15] The lithium ion conductivity at 25°C is 0.1 × 10 －３ The sulfide solid electrolyte according to any one of [1] to

[14] , wherein the sulfide solid electrolyte has a conductivity of 1.5 S / cm or more.

[0011]

[16] A method for producing the sulfide solid electrolyte according to any one of [1] to

[15] above, comprising the steps of: mixing raw materials containing elements R, M, S, and X to obtain a raw material mixture; obtaining a melt of the raw material mixture by heat treatment; and cooling the melt to precipitate crystals, wherein, among the elements, R represents an alkali metal element, M represents an element that can form a tetracoordination structure with S among elements of Groups 2 to 15 of the periodic table, and M is an element of Group 14, M Ａ and M, an element of Group 15 Ｂ wherein S represents elemental sulfur and X represents a halogen element.

[17] A method for producing a sulfide solid electrolyte according to

[16] above, further comprising, after precipitating the crystals, performing a post-heat treatment by heating.

[18] A method for producing a sulfide solid electrolyte according to

[17] above, wherein the post-heat treatment is performed at a temperature of 300 to 600°C for 10 minutes to 20 hours in an inert atmosphere at a dew point of -20°C or lower.

[0012] According to the present invention, a sulfide solid electrolyte having good water resistance and good cycle characteristics can be obtained, and therefore a secondary battery incorporating the sulfide solid electrolyte has excellent battery characteristics.

[0013] FIG. 1 is a flow diagram showing one aspect of a method for producing a sulfide solid electrolyte according to this embodiment. FIG. 2 is a flow diagram showing one aspect of a method for producing a sulfide solid electrolyte according to this embodiment. FIG. 3 is an XRD pattern of the sulfide solid electrolyte of Example 1. FIG. 4 is an XRD pattern of the sulfide solid electrolyte of Example 2. FIG. 5 is an XRD pattern of the sulfide solid electrolyte of Example 3. FIG. 6 is an XRD pattern of the sulfide solid electrolyte of Example 4. FIG. 7 is an XRD pattern of the sulfide solid electrolyte of Example 5. FIG. 8 is an XRD pattern of the sulfide solid electrolyte of Example 6. FIG. 9 is an XRD pattern of the sulfide solid electrolyte of Example 7. FIG. 10 is an XRD pattern of the sulfide solid electrolyte of Example 8.

[0014] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified as desired without departing from the gist of the present invention. The term "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits. In this specification, "mass %" and "wt %" and "parts by mass" and "parts by weight" have the same meaning.

[0015] The sulfide solid electrolyte according to this embodiment has a crystalline phase. ａ M ｂ S ｃ X ｄ In this composition formula, R represents an alkali metal element, M represents an element of Groups 2 to 15 of the periodic table that can form a tetracoordinate structure with S, and M is an element of Group 14, M Ａ and M, an element of Group 15 Ｂ wherein S represents a sulfur element and X represents a halogen element. In the above composition formula, a to d, which represent the molar content ratio (composition ratio) of each element, satisfy the following relationship. Here, when two or more elements are contained as elements represented by R, M, and X, a, b, and d, which represent the composition ratios of R, M, and X, respectively, mean the sum of the two or more elements. 3≦a≦7, b=1, 3≦c≦6, and 0.1≦d≦2. In addition to the above, the crystal in this embodiment is composed of M Ａ S ４ Tetrahedral structure and M Ｂ S ４ The crystal has three or more types of tetrahedral structures including a tetrahedral structure. Furthermore, in the crystal, the proportion of Ge in M ​​is 0.05 or less in terms of molar ratio.

[0016] <Crystalline Phase> The composition formula of the crystal contained in the crystalline phase in this embodiment is R ａ M ｂ S ｃ X ｄAs described above, R in the above composition formula represents an alkali metal element. Specifically, it is preferable that at least one selected from the group consisting of Li, Na, and K is contained, more preferably that at least one of Li and Na is contained, and even more preferably that Li is contained. Here, the alkali metal element means a carrier of the solid electrolyte. That is, when the sulfide solid electrolyte according to this embodiment is used in a lithium ion battery, R preferably contains Li. Furthermore, when the sulfide solid electrolyte according to this embodiment is used in a sodium ion battery, R preferably contains Na. Furthermore, when the sulfide solid electrolyte according to this embodiment is used in a potassium ion battery, R preferably contains K.

[0017] The composition formula showing the crystal in this embodiment: R ａ M ｂ S ｃ X ｄ In the formula, when b, which is the composition ratio of M, is used as a reference, i.e., when b=1, the composition ratio a of R, which represents an alkali metal element, is 3 to 7, preferably 5 or more and 7 or less, more preferably 5.2 or more and 6.8 or less, and even more preferably 5.4 or more and 6.6 or less. Here, from the viewpoint of precipitation of the target crystalline phase, the composition ratio a is 3 or more, preferably 5 or more, more preferably 5.2 or more, and even more preferably 5.4 or more. Furthermore, the composition ratio a is set to improve ionic conductivity and hydrogen sulfide (H ２ From the viewpoint of reducing the amount of S) generated, the ratio is 7 or less, preferably 6.8 or less, and more preferably 6.6 or less. When R contains a plurality of alkali metal elements, the composition ratio a means the total ratio of those two or more elements.

[0018] Composition formula in this embodiment: R ａ M ｂ S ｃ X ｄ is the composition formula: R ａ (M ＡｂＡ M ＢｂＢ M ＣｂＣ ) S ｃ X ｄ Here, M Ａ is an element in group 14 of the periodic table, and can form a tetrahedral structure with S.Ｂ is an element in group 15 of the periodic table, and can form a tetrahedral structure with S. Ｃ is an element of Groups 2 to 13 of the periodic table, and is an element that can form a tetracoordinate structure with S. In the above composition formula, element M Ａ The composition ratio of element M is Ｂ The composition ratio of bB and element M Ｃ The composition ratio of bA is bC, and b = bA + bB + bC = 1. Here, M Ａ , M Ｂ , M Ｃ When each of bA, bB, and bC contains two or more elements, the composition ratios bA, bB, and bC each mean the total ratio of those two or more elements.

[0019] In this embodiment, the element M Ａ is an element of the 14th group of the periodic table that can form a tetracoordinate structure with S, and M Ａ S ４ It has a tetrahedral structure. Element M Ａ Specific examples of the element M include Si, Ge, Sn, and Pb. Ａ Among them, from the viewpoint of the stability of the crystal structure and the ease of achieving high ionic conductivity, and from the viewpoint of electrochemical stability among the elements of Group 14, element M is preferably contained. Ａ As the element M, it is preferable that at least one of Si and Sn is contained, and it is more preferable that Si is contained. Ａ When two or more of these elements are contained, it is preferable that they contain Si and Sn.

[0020] Element M Ａ The ratio of element M, i.e., the value of bA, is preferably 0.01 to 0.95, more preferably 0.1 to 0.9, even more preferably 0.2 to 0.8, and even more preferably 0.3 to 0.7. From the viewpoint of improving ionic conductivity, the ratio is preferably 0.01 or more, more preferably 0.1 or more, even more preferably 0.2 or more, and even more preferably 0.3 or more. Ｂ From the viewpoint of mixing a certain amount or more of the above, the ratio is preferably 0.95 or less, more preferably 0.9 or less, even more preferably 0.8 or less, and even more preferably 0.7 or less.

[0021] element M Ａ When Si is contained as element M, the ratio of Si to element M is preferably 0.01 to 0.6, more preferably 0.2 to 0.5, and even more preferably 0.25 to 0.4. From the viewpoint of improving ionic conductivity, the ratio is preferably 0.01 or more, more preferably 0.2 or more, and even more preferably 0.25 or more. From the viewpoint of water resistance, the ratio is preferably 0.6 or less, more preferably 0.5 or less, and even more preferably 0.4 or less.

[0022] element M Ａ When Sn is contained as element M, the ratio of Sn to element M is preferably 0.01 to 0.6, more preferably 0.15 to 0.5, and even more preferably 0.2 to 0.4. From the viewpoint of water resistance, the ratio is preferably 0.01 or more, more preferably 0.15 or more, and even more preferably 0.2 or more. From the viewpoint of ionic conductivity, the ratio is preferably 0.6 or less, more preferably 0.5 or less, and even more preferably 0.4 or less.

[0023] In addition, element M Ａ When two or more elements are contained as element M, the ratio of Si to element M may be 0.1 to 0.6, 0.2 to 0.5, or 0.25 to 0.4. Ａ When two or more elements are contained as element M, the ratio of Sn to element M may be 0.1 to 0.6, 0.2 to 0.5, or 0.25 to 0.4.

[0024] Ge is the element M Ａ However, its presence is optional, and the ratio of Ge in M ​​is 0.05 or less. Here, in general, when forming a tetrahedron structure, the ease of forming the tetrahedron structure and the amount of Ge ４＋ Ge has been selected from the viewpoint that it has a moderately large ionic radius, has low interaction with lithium ions, and is easy to increase ionic conductivity. ＡAmong these, it has been found that Ge is a factor in reducing oxidation resistance and reduction resistance. Therefore, from the viewpoint of improving the oxidation resistance and reduction resistance of the sulfide solid electrolyte and obtaining good cycle characteristics when applied to a secondary battery, the above ratio is 0.05 or less, preferably 0.04 or less, and more preferably 0.03 or less. Furthermore, the lower limit of the above ratio is not particularly limited, and it may be 0, i.e., Ge may not be contained.

[0025] In this embodiment, the element M Ｂ is an element in the 15th group of the periodic table that can form a tetracoordinate structure with S, and M Ｂ S ４ It has a tetrahedral structure. Element M Ｂ Specific examples of the element M include P, As, Sb, and Bi. Ｂ It is preferable that the element M contains one or more elements selected from P, As, Sb, and Bi. Among them, from the viewpoint of the stability of the crystal structure and the ease of achieving high ionic conductivity, and from the viewpoint of electrochemical stability among the elements of Group 14, it is preferable to contain at least one of P and Sb, more preferably Sb, and also more preferably P. Furthermore, the element M Ｂ When two or more of these elements are contained, it is preferable that P and Sb are contained.

[0026] Element M Ｂ The ratio of element M, i.e., the value of bB, is preferably 0.05 to 0.99, more preferably 0.1 to 0.9, even more preferably 0.2 to 0.8, and even more preferably 0.3 to 0.7. From the viewpoint of improving ionic conductivity, the ratio is preferably 0.05 or more, more preferably 0.1 or more, even more preferably 0.2 or more, and even more preferably 0.3 or more. Ａ From the viewpoint of mixing a certain amount or more of the above, the ratio is preferably 0.99 or less, more preferably 0.9 or less, even more preferably 0.8 or less, and even more preferably 0.7 or less.

[0027] element M Ｂ When P is contained as element M, the ratio of P to element M is preferably 0 to 0.7, more preferably more than 0 but not more than 0.7, even more preferably 0.01 to 0.5, and even more preferably 0.1 to 0.4. ＢWhen P is contained as P, from the viewpoint of reduction resistance, the ratio is preferably more than 0, more preferably 0.01 or more, and even more preferably 0.1 or more. Also, from the viewpoint of water resistance, the ratio is preferably 0.7 or less, more preferably 0.5 or less, and even more preferably 0.4 or less.

[0028] element M Ｂ When Sb is contained as element M, the ratio of Sb to element M is preferably 0.01 to 0.9, more preferably 0.2 to 0.8, and even more preferably 0.3 to 0.7. From the viewpoint of improving ionic conductivity, the ratio is preferably 0.01 or more, more preferably 0.2 or more, and even more preferably 0.3 or more. Furthermore, from the viewpoint of increasing the ionic conductivity by mixing a larger number of types of elements, the ratio is preferably 0.9 or less, more preferably 0.8 or less, and even more preferably 0.7 or less.

[0029] In addition, element M Ｂ When two or more elements are contained as M, the ratio of P to the element M may be 0 to 0.5, 0.01 to 0.4, or 0.1 to 0.3. Ｂ When two or more elements are contained as element M, the ratio of Sb to element M may be 0.1 to 0.9, 0.2 to 0.8, or 0.3 to 0.7.

[0030] In this embodiment, the element M Ｃ is an element in Groups 2 to 13 of the periodic table that can form a tetracoordinate structure with S, and M Ｃ S ４ It has a tetrahedral structure. Element M Ｃ The presence of element M is optional. Ｃ Specific examples of the element M include Ga, In, Cu, Al, B, Ti, V, Cr, Fe, Zn, Y, Zr, Nb, Ta, W, and La. Ｃ When the element M is contained, it is preferable that the element M contains at least one element selected from the group consisting of Al, B, Ti, Fe, Zn, Y, and Zr, and it is more preferable that the element M contains at least one element selected from the group consisting of Al, B, Zn, and Zr, from the viewpoint of electrochemical stability. Ｃ When two or more of the above are contained, it is also preferable that Al and B are contained.

[0031] Element M Ｃ The ratio of the element M, i.e., the value of bC, is preferably 0 to 0.5, more preferably 0.001 to 0.4, and even more preferably 0.01 to 0.3. Ｃ In the case where the ionic conductivity is improved, the ratio is preferably greater than 0, more preferably 0.001 or greater, and even more preferably 0.01 or greater. In addition, in the case where the ionic conductivity is improved, the ratio is preferably 0.5 or less, more preferably 0.4 or less, and even more preferably 0.3 or less.

[0032] The crystal in this embodiment contains the element M Ａ and element M Ｂ as an essential component, and element M Ｃ is optional. Element M Ａ , element M Ｂ and element M Ｃ The total number of elements is three or more, and the crystal is M Ａ S ４ Tetrahedral structure and M Ｂ S ４ The crystal in this embodiment has three or more types of tetrahedral structures, including a tetrahedral structure. This allows for good water resistance. Although the reason for this is not clear, it is believed that the presence of three or more types of tetrahedral structures makes the material a high-entropy material, resulting in good water resistance. Therefore, the crystal in this embodiment has M Ａ S ４ Tetrahedral structure and M Ｂ S ４ It has three or more types of tetrahedral structures, including a tetrahedral structure, but M Ａ and M Ｂ It is preferable that the elements have a larger atomic number than P.

[0033] Here, the composition formula described in Patent Document 2: Li ６．５ [P ０．２５ Si ０．２５ Ge ０．２５ Sb ０．２５ ]S ５The crystals represented by I are obtained by a solid-state reaction method. Here, the solid-state reaction method is a method in which a raw material mixture containing raw materials is mechanically mixed using media such as mechanical milling, and then heated to cause a solid-state reaction to obtain crystals. In contrast, crystals containing no Ge or a small amount of Ge and M Ａ S ４ Tetrahedral structure and M Ｂ S ４ It is impossible, or even if possible, extremely difficult, to obtain the crystals of this embodiment having three or more types of tetrahedral structures, including a tetrahedral structure, by a solid-state reaction method. Furthermore, it is extremely difficult to achieve the desired types and content ratios of the tetrahedral structures.

[0034] In contrast to this, in this embodiment, a melting method is adopted in which a mixture of raw materials is heated and melted, and then cooled and solidified, as described below, to obtain the desired crystals, and the crystal structure and performance as a sulfide solid electrolyte can be controlled. The reason for this is believed to be that by undergoing the steps of heating and melting the raw material mixture and then cooling, the constituent components become uniform in a molten state, and by cooling it, a crystal structure in which the constituent components are uniformly dispersed can be obtained. Note that the manufacturing method according to this embodiment is not to be interpreted as being limited to the above-mentioned mechanism of action. In particular, with regard to the uniformity of the constituent components, it is particularly important to consider the effect of the PS ４ Tetrahedron, multiple MS ４ Although the tetrahedrons tend to be non-uniform, the uniformity of this tetrahedron structure can be determined by Rietveld structural analysis of the XRD pattern. In particular, when the crystal structure has an F-43m space group, it can be easily determined by the presence or absence of a diffraction pattern of the (111) plane on the low-angle side of the (200) plane. That is, when a diffraction pattern of the (111) plane is obtained, PS ４ Tetrahedrons and various MS ４ It can be said that the tetrahedrons are not uniformly dispersed.

[0035] In the crystal of this embodiment, the element M Ａ , element M Ｂ and element M Ｃ The total number of elements is three or more, and the element M Ａ and element M Ｂis essential, but M in the above composition formula is an element M from the viewpoint of improving ionic conductivity. Ａ and element M Ｂ Preferably, the element M is composed of three or more elements in total, including two or more of the elements Ｂ It is more preferable that M in the composition formula is composed of four or more elements in total. Ａ and element M Ｂ It is also preferable that both of contain two or more elements.

[0036] In the composition formula of the crystal of this embodiment, M may be composed of a total of three or more elements, but from the viewpoint of improving ionic conductivity, it is preferably composed of a total of four or more elements, more preferably five or more. There is no particular upper limit on the number of elements, but in a practical configuration, eight or less elements are preferred, and seven or less elements are more preferred.

[0037] In this embodiment, an index of the crystallinity is an ionic radius parameter expressed by the following formula using the ionic radii of the elements constituting the element M: Ionic radius parameter=(b １ ×r １ ) + (b ２ ×r ２ ) + ... + (b ｎ ×r ｎ )

[0038] Here, M in the composition formula is M １ ~M ｎ When it is assumed that the element is composed of n kinds of elements (n is a natural number), b １ ~b ｎ are respectively, M １ ~M ｎ That is, (b １ +b ２ +...+b ｎ ) = 1. Also, r in the above formula １ ~r ｎ are respectively, M １ ~M ｎ represents the ionic radius (Å) of the tetracoordinate valence of each element in the crystal.

[0039] It has been found that the larger the value of the ionic radius parameter, the larger the crystal lattice, and as a result, the higher the ionic conductivity tends to be.

[0040] The value of the ionic radius parameter is preferably 0.47 to 0.55, more preferably 0.49 to 0.54, and even more preferably 0.495 to 0.53. From the viewpoint of ionic conductivity, the value is preferably 0.47 or more, more preferably 0.49 or more, and even more preferably 0.495 or more. From the viewpoint of ease of crystal structure formation, the value is preferably 0.55 or less, more preferably 0.54 or less, and even more preferably 0.53 or less.

[0041] The ionic radius (crystal radius) of each element constituting element M used in the ionic radius parameter indicates the crystal radius, and is the ionic radius of the tetravalent coordination of each element in a crystal, among the ionic radii described in the following Reference 1. Reference 1: R. D. Shannon et al., Acta Cryst. A32 (1976) 751-767.

[0042] Typical examples of the ionic radii of specific elements are as follows. The following values ​​are the ionic radii of the tetracoordinated state, which is the highest valence state in the crystal: P: 0.31 Å, Si: 0.40 Å, Ge: 0.53 Å, Sb: 0.63 Å, Sn: 0.69 Å. Of the above examples, Sb ５＋ The data for the tetracoordinate structure of Sb is not described in the above-mentioned Reference 1. Therefore, the value of Sb: 0.63 Å is a value inferred from the tetracoordinate structures of surrounding elements. In this way, when a value is not available, it may be inferred from the values ​​of surrounding elements.

[0043] In this embodiment, the composition ratio c of S, which is a sulfur element, is 3 or more and 6 or less, preferably 4 or more and 5.3 or less, and more preferably 14.2 or more and 5.1 or less. Here, in the crystal of this embodiment, S forms a tetrahedral structure with the element M, and also forms a tetrahedral structure with S. ２－From the viewpoint of forming an adequate tetrahedral structure with the element M, the composition ratio c is 3 or more, preferably 4 or more, and more preferably 4.2 or more. From the viewpoint of reducing the proportion of isolated sulfur and realizing good water resistance, the composition ratio c is 6 or less, preferably 5.3 or less, and more preferably 5.1 or less.

[0044] The crystal in this embodiment contains a halogen element represented by X as a constituent element. Examples of halogen elements include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Among these, from the viewpoint of improving ionic conductivity, it is preferable to contain at least one selected from the group consisting of chlorine, bromine, and iodine, more preferably at least one of bromine and iodine, and even more preferably iodine. Furthermore, from the viewpoint of improving ionic conductivity, it is also preferable to contain two or more halogen elements. In this case, it is preferable to contain at least two selected from the group consisting of chlorine, bromine, and iodine, and more preferably bromine and iodine.

[0045] In this embodiment, the composition ratio d of X, which is a halogen element contained in the crystal, is 0.1 or more and 2 or less, preferably 0.5 or more and 1.8 or less, more preferably 0.7 or more and 1.7 or less, and even more preferably 0.9 or more and 1.6 or less. Here, from the viewpoint of improving ionic conductivity, the ratio is 0.1 or more, preferably 0.5 or more, more preferably 0.7 or more, and even more preferably 0.9 or more. Furthermore, from the viewpoint of precipitation of the target crystalline phase, the ratio is 2 or less, preferably 1.8 or less, more preferably 1.7 or less, and even more preferably 1.6 or less.

[0046] The crystal in this embodiment may further contain other elements as constituent elements in addition to the above-mentioned R, M, S, and X. Examples of other elements include N, C, and O. These elements include CN, CO, and the like. ｎ , NO ｎ , S.O. ｎ The crystal may be present in the form of the above.

[0047] In order to improve ionic conductivity, the crystal in this embodiment preferably has a crystal structure of the F-43m space group, and more preferably has a cubic crystal structure of the F-43m space group. Furthermore, it is more preferable that the crystal has an argyrodite-type crystal structure, and even more preferable that the crystal has a cubic argyrodite-type crystal structure. The argyrodite-type crystal structure in this specification refers to a crystal structure having the composition formula Ag ８ GeS ６ This is the crystalline structure possessed by a group of compounds derived from minerals represented by the formula: When the crystal in this embodiment is a cubic crystal of the F-43m space group, the lattice constant is preferably 10.25 to 10.45 Å, and more preferably 10.30 to 10.42 Å. That is, the lattice constant is preferably 10.25 Å or more, more preferably 10.30 Å or more, and is preferably 10.45 Å or less, and more preferably 10.42 Å or less. The lattice constant of the crystal can be determined by Rietveld analysis of an XRD pattern or from representative peak positions. When determining the lattice constant from representative peak positions, it is desirable to calculate an average value using two or more peaks.

[0048] The sulfide solid electrolyte according to this embodiment may consist of only the crystalline phase made of the above crystals, or may contain other crystalline phases or an amorphous phase.

[0049] The content of the crystals in the sulfide solid electrolyte according to this embodiment is preferably 50 to 100% by mass, more preferably 60 to 100% by mass, and even more preferably 70 to 90% by mass. From the viewpoint of improving ionic conductivity, the content is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The content of the crystals may be 100% by mass, i.e., the sulfide solid electrolyte according to this embodiment may consist solely of the crystals. On the other hand, from the viewpoint of formability, the content is preferably less than 100% by mass, and more preferably 90% by mass or less.

[0050] When the crystals in this embodiment have a crystal structure of the F-43m space group, the content of the crystals in the sulfide solid electrolyte is the same as above, i.e., preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, and may be 100% by mass, less than 100% by mass, or 90% by mass or less.

[0051] <Sulfide Solid Electrolyte> The secondary particle diameter of the sulfide solid electrolyte according to this embodiment is preferably small from the viewpoint of obtaining good ionic conductivity when used in a secondary battery. Specifically, the secondary particle diameter is preferably 10 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less. The lower limit of the secondary particle diameter is not particularly limited, but is usually 0.1 μm or more. The secondary particle diameter can be measured using a microtrack device.

[0052] The lithium ion conductivity of the sulfide solid electrolyte according to this embodiment at 25° C. is 0.1×10 －３ S / cm or more is preferable, and 0.5×10 －３ S / cm or more is more preferable, and 1×10 －３ The upper limit of the lithium ion conductivity is not particularly limited, but it is usually 1×10 －１ The lithium ion conductivity is 0.05 S / cm or less. The lithium ion conductivity can be measured by an AC impedance method. Specifically, the lithium ion conductivity is a value measured using an AC impedance measuring device (for example, a potentiostat / galvanostat VSP manufactured by Bio-Logic Sciences Instruments) under the following conditions: a measurement frequency of 100 Hz to 1 MHz, a measurement voltage of 100 mV, and a measurement temperature of 25°C.

[0053] The sulfide solid electrolyte according to this embodiment can be identified by analyzing the elemental composition using various methods such as crystal structure analysis by X-ray diffraction (XRD) measurement, ICP emission spectrometry measurement, atomic absorption spectrometry measurement, and ion chromatography measurement. For example, M and S can be measured by ICP emission spectrometry measurement, R can be measured by atomic absorption spectrometry measurement, and X can be measured by ion chromatography measurement.

[0054] The sulfide solid electrolyte according to this embodiment is suitable for use in electrode mixtures and solid electrolyte layers used in secondary batteries, and is particularly suitable for all-solid-state secondary batteries, and more suitable for all-solid-state lithium-ion secondary batteries. That is, the electrode mixture is used in secondary batteries and contains the sulfide solid electrolyte and an active material. The solid electrolyte layer is used in secondary batteries and contains the sulfide solid electrolyte.

[0055] The electrode mixture, the solid electrolyte layer, and the all-solid-state lithium ion secondary battery may further contain another solid electrolyte. The other solid electrolyte is not particularly limited, and examples thereof include a conventionally known solid electrolyte having an argyrodite-type crystal structure, Li ３ P.S. ４ Examples of suitable solid electrolytes include solid electrolytes having an LGPS type crystal structure, and Li-PS-Ha (Ha is a halogen element) type crystallized glass type solid electrolytes.

[0056] The active material contained in the electrode mixture may be a conventionally known material. For example, the positive electrode active material is not particularly limited as long as it can reversibly absorb and release alkali metal ions, intercalate and deintercalate alkali metal ions, or dope and dedope counter anions of the alkali metal ions. Here, the alkali metal ion is preferably lithium ion. Specific examples of the positive electrode active material include lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel manganate, composite metal oxides, and polyanion olivine-type positive electrodes.

[0057] The negative electrode active material is not particularly limited as long as it can reversibly absorb and release alkali metal ions, detach and insert (intercalate) alkali metal ions, or reversibly dope and dedope counter anions of the alkali metal ions. Here, the alkali metal ions are preferably lithium ions. Specific examples of the negative electrode active material include 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, and lithium titanate.

[0058] The solid electrolyte layer may contain the sulfide solid electrolyte according to the present embodiment, but may also contain other additives such as a binder. Conventionally known binders can be used, including butadiene rubber, acrylate butadiene rubber, styrene butadiene rubber, polyvinylidene fluoride, and polytetrafluoroethylene. The binder content in the solid electrolyte layer may also be within a conventionally known range.

[0059] The all-solid-state secondary battery is not particularly limited as long as it includes a positive electrode and a negative electrode in addition to the sulfide solid electrolyte according to this embodiment. The positive electrode and the negative electrode may be an electrode mixture containing the sulfide solid electrolyte according to this embodiment. The positive electrode active material may be the same as the positive electrode active material described in the electrode mixture, and the positive electrode may further include a positive electrode current collector, a binder, a conductive additive, and the like, as necessary. The positive electrode current collector may be made of aluminum, an alloy thereof, a thin metal plate such as stainless steel, or the like.

[0060] The negative electrode active material can be the same as the negative electrode active material described in the electrode mixture, and the negative electrode may further contain, as necessary, a negative electrode current collector, a binder, a conductive additive, etc. The negative electrode current collector can be a thin metal plate such as copper or aluminum.

[0061] <<Method for Producing Sulfide Solid Electrolyte>> The method for producing the sulfide solid electrolyte according to this embodiment is not particularly limited as long as it can produce the sulfide solid electrolyte described above in <<Sulfide Solid Electrolyte>>. Preferred aspects of the resulting sulfide solid electrolyte are also the same as the preferred aspects described above in <<Sulfide Solid Electrolyte>>.

[0062] As shown in FIG. 1 , an example of a method for producing a sulfide solid electrolyte according to this embodiment includes the following steps in order: Step S1: A step of mixing raw materials containing elements R, M, S, and X to obtain a raw material mixture; Step S2: A step of obtaining a melt of the raw material mixture obtained in Step S1 by heat treatment; Step S3: A step of cooling the melt obtained in Step S2 to precipitate crystals; Among the elements in Step S1, R represents an alkali metal element, M represents an element that can form a tetracoordinate structure with S among elements of Groups 2 to 15 of the periodic table, and M is an element of Group 14, MＡ and M, an element of Group 15 Ｂ wherein S represents a sulfur element and X represents a halogen element.

[0063] 2, the manufacturing method according to this embodiment may further include the following step S4 after the step S3: Step S4: A step of performing a post-heat treatment by heating after the crystals are precipitated in step S3.

[0064] Each step will be explained in order.

[0065] <Step S1: Mixing> Step S1 in this embodiment is a step of mixing raw materials containing elements R, M, S, and X to obtain a raw material mixture. Among the elements in the above step, R represents an alkali metal element, M represents an element that can form a tetra-coordinate structure with S among elements of Groups 2 to 15 of the periodic table, and M is an element of Group 14, M Ａ and M, an element of Group 15 Ｂ wherein S represents a sulfur element and X represents a halogen element.

[0066] Examples of the alkali metal element (R) include lithium element (Li), sodium element (Na), and potassium element (K). When the obtained sulfide solid electrolyte is applied to a lithium ion secondary battery, the raw material preferably contains a substance such as lithium element (Li) as the alkali metal element (R).

[0067] As the raw material containing an alkali metal element (R), an appropriate combination of substances containing an alkali metal element, such as an alkali metal element alone or a compound containing an alkali metal element, can be used. When the alkali metal element (R) is lithium element (Li), an appropriate combination of substances containing Li, such as Li alone or a compound containing Li, can be used as the lithium element.

[0068] Examples of raw materials containing lithium element (Li) include lithium sulfide (Li ２ S), lithium iodide (LiI), lithium carbonate (Li ２ CO ３ ), lithium sulfate (Li２ SO ４ ), lithium oxide (Li ２ Examples of the raw material containing lithium element (Li) include lithium compounds such as lithium ion (LiO), lithium hydroxide (LiOH), and metallic lithium. One type of raw material containing lithium element (Li) may be used, or two or more types may be used in combination.

[0069] As the raw material containing lithium element (Li), lithium sulfide is preferably used from the viewpoint of obtaining a sulfide material. In addition, when the obtained sulfide solid electrolyte contains a halogen element, it is also preferable to use lithium halide (LiX, where X is a halogen element) as the raw material containing lithium element (Li). Lithium halide will be described later.

[0070] Among the elements of Groups 2 to 15 of the periodic table, those that can form a tetracoordinate structure with S and M, an element of Group 14 Ａ and M, an element of Group 15 Ｂ As the raw material containing a total of three or more elements (M), conventionally known raw materials can be used. For example, the raw material containing the element (M) can be an appropriate combination of a simple substance consisting of the element (M), an oxide of the element (M), a sulfide of the element (M), and a halide of the element (M). Specific examples of elements are shown below.

[0071] When the element M contains phosphorus (P), the raw material containing phosphorus (P) may be an appropriate combination of substances containing P, such as simple P or a compound containing P. The raw material containing phosphorus (P) may be used alone or in combination of two or more.

[0072] As a raw material containing phosphorus (P), diphosphorus pentasulfide (P ２ S ５ ), diphosphorus trisulfide (P ２ S ３ ) and other phosphorus sulfides, sodium phosphate (Na ３ P.O. ４ ) and elemental phosphorus.

[0073] When the element M contains tin (Sn), examples of raw materials containing tin (Sn) include Sn (single Sn), SnS, and SnS. ２, SnO, SnO ２ , SnCl ２ Among them, from the viewpoint of lithium ion conductivity, SnS ２ , SnCl ２ is preferred, and SnS ２ The raw material containing tin (Sn) may be used alone or in combination of two or more.

[0074] When the element M contains antimony (Sb), examples of raw materials containing antimony (Sb) include Sb (single Sb), Sb ２ S ３ , Sb ２ S ５ The raw material containing antimony (Sb) may be used alone or in combination of two or more.

[0075] When the element M contains germanium (Ge), examples of raw materials containing Ge include GeO ２ , GeS, GeS ２ , GeCl ２ Among them, from the viewpoint of lithium ion conductivity, GeS ２ , GeCl ２ is preferred, and GeS ２ These compounds may be used alone or in combination of two or more.

[0076] When the element M contains indium (In), examples of raw materials containing In include In ２ O ３ , In ２ S ３ , InCl ３ Among them, from the viewpoint of lithium ion conductivity, In ２ S ３ , InCl ３ is preferred, In ２ S ３ These compounds may be used alone or in combination of two or more.

[0077] When the element M contains copper (Cu), the raw material containing Cu may be, for example, Cu ２O, CuO, Cu ２ S, CuS, CuCl ２ Among them, from the viewpoint of lithium ion conductivity, CuS, CuCl ２ These compounds may be used alone or in combination of two or more.

[0078] When the element M contains silicon (Si), examples of raw materials containing Si include Si (simple Si), SiO ２ , SiS ２ Among them, from the viewpoint of lithium ion conductivity, SiO ２ The raw material containing silicon element (Si) may be used alone or in combination of two or more.

[0079] When the element M contains aluminum (Al), examples of raw materials containing Al include Al ２ S ３ , Al ２ O ３ , AlCl ３ Among them, from the viewpoint of lithium ion conductivity, Al ２ S ３ , AlCl ３ is preferred, and Al ２ S ３ These compounds may be used alone or in combination of two or more.

[0080] As the raw material containing sulfur element (S), raw materials containing S such as simple S or compounds containing S can be used in appropriate combination. The raw material containing sulfur element (S) may be used alone or in combination of two or more. ２ S is a compound that serves as both a raw material containing sulfur element (S) and a raw material containing lithium element (Li) as described above. ２ S ５ is a compound that serves as both a raw material containing sulfur element (S) and the above-mentioned raw material containing phosphorus element (P). Furthermore, SnS is a compound that serves as both a raw material containing sulfur element (S) and the above-mentioned raw material containing Sn. ２ S ３is a compound that serves as both a raw material containing elemental sulfur (S) and the above-mentioned raw material containing Sb.

[0081] Examples of raw materials containing sulfur element (S) include phosphorus pentasulfide (P ２ S ５ ), diphosphorus trisulfide (P ２ S ３ ), other sulfur compounds containing phosphorus, elemental sulfur, and compounds containing sulfur. ２ S, CS ２ , iron sulfide (FeS, Fe ２ S ３ , FeS ２ , Fe １－ｘ S, etc.), bismuth sulfide (Bi ２ S ３ ), copper sulfide (CuS, Cu ２ S, Cu １－ｘ S, etc.

[0082] Examples of raw materials containing a halogen element (X) include lithium halides (LiHa) 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. Among these, lithium halides are preferred from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide solid electrolyte. These compounds may be used alone or in combination of two or more. Furthermore, the halogen may be HX or X ２ It may be introduced as a gas, and when X is Br or I, Br ２ Ya I ２ It may be introduced as

[0083] The raw materials can be mixed, for example, by mixing in a mortar, mixing using media such as a planetary ball mill, or medialess mixing such as a pin mill, a powder mixer, or airflow mixing. The raw materials may be made amorphous by mixing before heating.

[0084] <Step S2: Heat Melting> Step S2 in this embodiment is a process of obtaining a melt of the raw material mixture obtained in step S1 by heat treatment.

[0085] The specific method for heating and melting the raw material mixture is not particularly limited, and the raw materials are placed in a heat-resistant container and heated in a heating furnace. The raw material mixture may be sealed in a heat-resistant container. Alternatively, the melting may be carried out in an atmosphere containing elemental sulfur. Examples of the atmosphere containing elemental sulfur include a mixed gas atmosphere of a gas containing elemental sulfur, such as sulfur gas, hydrogen sulfide gas, or sulfur dioxide gas, and an inert gas.

[0086] The heat-resistant container may be a heat-resistant container made of carbon, a heat-resistant container containing an oxide such as quartz, quartz glass, borosilicate glass, aluminosilicate glass, alumina, zirconia, or mullite, a heat-resistant container containing a nitride such as silicon nitride or boron nitride, or a heat-resistant container containing a carbide such as silicon carbide, etc. Furthermore, these heat-resistant containers may be formed in bulk from the above-mentioned materials, or may be containers on which a layer of carbon, oxide, nitride, carbide, or the like is formed, such as a carbon-coated quartz tube.

[0087] As the furnace body in the heating furnace used for heating and melting, a conventionally known furnace having a heating section can be used as appropriate, and the material and size of the furnace body can also be selected arbitrarily.

[0088] The heat-melting temperature is not particularly limited as long as the raw material mixture is melted, but is preferably 600°C or higher, more preferably 600 to 1000°C, even more preferably 630 to 950°C, even more preferably 650°C or higher but less than 900°C, and particularly preferably 650°C or higher but less than 850°C. Here, from the viewpoint of homogenizing the melt in a short time, the heat-melting temperature is preferably 600°C or higher, more preferably 630°C or higher, and even more preferably 650°C or higher. Furthermore, from the viewpoint of suppressing deterioration or decomposition of components in the melt, the heat-melting temperature is preferably 1000°C or lower, more preferably 950°C or lower, even more preferably less than 900°C, and particularly preferably less than 850°C. The heat-melting temperature is the temperature of the melt produced in the furnace body and can be adjusted by the heating unit provided in the furnace body.

[0089] The heat-melting time is not particularly limited as long as the raw material mixture is melted, but is, for example, preferably 10 minutes to 10 hours, more preferably 30 minutes to 9.5 hours, even more preferably 45 minutes to 9 hours, and particularly preferably 1 to 9 hours. From the viewpoint of smoothly progressing the reaction, the heat-melting time is preferably 10 minutes or more, more preferably 30 minutes or more, even more preferably 45 minutes or more, and particularly preferably 1 hour or more. Furthermore, from the viewpoint of suppressing deterioration or decomposition of components in the melt due to heating, the heat-melting time is preferably 10 hours or less, more preferably 9.5 hours or less, and even more preferably 9 hours or less.

[0090] The heat melting is preferably performed in an inert atmosphere, such as a nitrogen gas atmosphere, an argon gas atmosphere, or a helium gas atmosphere. Alternatively, the heat melting may be performed in a gas atmosphere containing elemental sulfur.

[0091] The dew point during heat melting is preferably −20° C. or lower in order to prevent side reactions between the melt and water vapor, oxygen, etc. The lower limit of the dew point is not particularly limited, but is usually about −80° C. The oxygen concentration is preferably 1000 ppm by volume or lower.

[0092] The pressure during heating and melting is not particularly limited as long as the raw material mixture is melted, but for example, normal pressure or slight pressure is preferred, and normal pressure is more preferred.

[0093] Heat melting is more preferably carried out in a temperature range of 600 to 1000°C for 10 minutes to 10 hours, and in addition to the above, it is more preferably carried out under one or more of the following conditions: in an inert atmosphere, under a dew point of -20°C or less, and under normal pressure or slight pressure, even more preferably under two or more of the following conditions, and particularly preferably under all three of the following conditions.

[0094] In step S2, whether the raw material mixture is completely melted can be confirmed by the absence of peaks derived from crystals in high-temperature X-ray diffraction measurement.

[0095] In step S2, a compound that will become a crystal nucleus may be contained in the melt in order to facilitate the precipitation of crystals in the subsequent step S3. The method for containing the compound that will become a crystal nucleus in the melt is not particularly limited, but examples thereof include a method of adding the compound that will become a crystal nucleus to raw materials, and a method of directly adding the compound that will become a crystal nucleus to the melt that has been heated and melted.

[0096] Examples of compounds that can serve as crystal nuclei include oxides, oxynitrides, nitrides, carbides, other chalcogen compounds, and halides. Compounds that can serve as crystal nuclei are preferably compounds that have a certain degree of compatibility with the melt. Note that compounds that are completely incompatible with the melt cannot serve as crystal nuclei.

[0097] <Step S3: Cooling> Step S3 in this embodiment is a step of cooling the melt obtained in step S2 to precipitate crystals. ａ M ｂ S ｃ X ｄ The crystals obtained in step S3 may be used as the sulfide solid electrolyte, or the crystals that have been subjected to a post-heat treatment in the subsequent step S4 may be used as the sulfide solid electrolyte.

[0098] In step S3, the molten material obtained in step S2 is discharged at any time from a discharge port provided in the furnace body, and the process proceeds to a step of cooling and solidifying. A known method can be used to cool the molten material, and there is no particular limitation. For example, from the viewpoint of increasing the cooling rate, cooling using a twin roller, which is generally considered to have the fastest quenching rate, is preferred.

[0099] The cooling rate is preferably 0.1 to 10,000°C / sec, more preferably 0.5 to 5,000°C / sec, and even more preferably 1 to 1,000°C / sec. From the viewpoint of improving compositional homogeneity and suppressing quality variations, the cooling rate is preferably 0.1°C / sec or more, more preferably 0.5°C / sec or more, and even more preferably 1°C / sec or more. The upper limit of the cooling rate is not particularly limited, but taking into account the cooling rate of a twin roller, which is generally said to have the fastest quenching rate, the upper limit is 1,000,000°C / sec or less. From the viewpoint of practical production, the cooling rate is more preferably 10,000°C / sec or less, more preferably 5,000°C / sec or less, and even more preferably 1,000°C / sec or less.

[0100] On the other hand, a sulfide solid electrolyte having a crystalline phase may be obtained by slow cooling. Alternatively, a sulfide solid electrolyte having both a crystalline phase and an amorphous phase may be obtained. When slowly cooling, the cooling rate is preferably 0.01 to 500°C / sec, more preferably 0.05 to 450°C / sec. Alternatively, it may be 0.01 to 10°C / sec, or 0.05 to 5°C / sec. Here, the cooling rate is preferably 0.01°C / sec or more, more preferably 0.05°C / sec or more, and is preferably 500°C / sec or less, more preferably 450°C / sec or less. Alternatively, the cooling rate may be 10°C / sec or less, or 5°C / sec or less. The cooling rate may be adjusted appropriately depending on the crystallization conditions.

[0101] The atmosphere during cooling is preferably a low-moisture, inert atmosphere, similar to the heating and melting in step S2. Cooling and solidification is preferably carried out under atmospheric pressure. "Atmospheric pressure" here means that the pressure is not controlled during cooling. Specifically, the pressure is about 0.8 to 1.2 atm.

[0102] <Step S4: Post-heat treatment> Step S4 in this embodiment is an optional step of performing post-heat treatment by heating again after the crystals are precipitated in step S3.

[0103] The post-heat treatment in step S4 promotes crystallization if the solid obtained in step S3 contains an amorphous phase. The post-heat treatment may also rearrange ions within the crystalline structure to increase lithium ion conductivity.

[0104] That is, the post-heat treatment refers to at least one of a heat treatment for crystallizing the obtained solid and a heat treatment for rearranging ions in the crystal structure.

[0105] The temperature in the post-heat treatment is preferably 300 to 600° C., more preferably 350 to 600° C., even more preferably 355 to 550° C., and even more preferably 360 to 500° C. Here, from the viewpoint of shortening the heat treatment time, the temperature in the post-heat treatment is preferably 300° C. or higher, more preferably 350° C. or higher, even more preferably 355° C. or higher, and even more preferably 360° C. or higher. Furthermore, from the viewpoint of preventing sintering of particles, the temperature in the post-heat treatment is preferably 600° C. or lower, more preferably 550° C. or lower, and even more preferably 500° C. or lower.

[0106] The post-heat treatment time is preferably 10 minutes to 20 hours, more preferably 10 minutes to 10 hours, even more preferably 30 minutes to 9.5 hours, even more preferably 45 minutes to 9 hours, and particularly preferably 1 to 9 hours. From the viewpoint of production stability, the heat treatment time is preferably 10 minutes or more, more preferably 30 minutes or more, even more preferably 45 minutes or more, and particularly preferably 1 hour or more. From the viewpoint of production cost, the post-heat treatment time is preferably 20 hours or less, more preferably 10 hours or less, even more preferably 9.5 hours or less, and even more preferably 9 hours or less.

[0107] The atmosphere for the post-heat treatment is preferably an inert atmosphere. Examples of the inert atmosphere include a nitrogen gas atmosphere, an argon gas atmosphere, and a helium gas atmosphere. The dew point during the post-heat treatment is preferably −20° C. or lower, and although there is no particular lower limit, it is usually about −80° C. The oxygen concentration is preferably 1000 ppm by volume or lower.

[0108] The post-heat treatment is more preferably carried out, for example, in a temperature range of 300 to 600°C for 10 minutes to 20 hours, and in addition to the above, it is further more preferably carried out under at least one of an inert atmosphere and a condition with a dew point of -20°C or lower, and particularly preferably under both conditions.

[0109] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these. Examples 1 to 8, 12, and 13 are working examples, and Examples 9 to 11 are comparative examples.

[0110] Examples 1 to 11 Raw materials containing each element were weighed and mixed in a mortar in a dry nitrogen gas atmosphere so as to have the composition ratios shown in Table 1 to obtain a raw material mixture. The raw materials containing each element were lithium sulfide powder (manufactured by Sigma, purity 99.98%) as a raw material containing Li and S, diphosphorus pentasulfide powder (manufactured by Sigma, purity 99%) as a raw material containing P and S, and SnS as a raw material containing Sn. ２ Sb powder (manufactured by Mitsuwa Chemical Co., Ltd., purity 99.5%) was used as a raw material containing Sb. ２ S ３ SiS powder (manufactured by Mitsuwa Chemical Co., Ltd., purity 99.98%) was used as a raw material containing Si. ２ Powder (Mitsuwa Chemical Co., Ltd., purity 99%) was used as a Ge-containing raw material. ２ Lithium bromide powder (manufactured by Sigma, purity 99.995%) was used as a raw material containing Br and Li, lithium iodide powder (manufactured by Tokyo Chemical Industry Co., Ltd., purity 99.9%, low moisture grade) was used as a raw material containing I and Li, and lithium chloride powder (manufactured by Sigma, purity 99%) was used as a raw material containing Cl and Li. ２ S ３ When powder was used, a sulfur raw material (manufactured by Sigma, purity 99.98%) was also used to adjust the composition.

[0111] The raw material mixture obtained above was placed in a quartz tube and vacuum sealed. It was then heated in an electric furnace at 700 to 800°C for 1 hour to obtain a completely dissolved molten material. It was then cooled to room temperature at a rate of 1 to 5°C / second, and crystals were precipitated to obtain a sulfide solid electrolyte.

[0112] Table 1 shows the composition formula: ａ M ｂ S ｃ X ｄ When the element M is expressed as ４ / MS ４ ), M.S. ４ The type of tetrahedral structure (tetrahedral species) and the value of the ionic radius parameter are also shown. The values ​​used for the ionic radius parameter are the tetracoordinate ionic radii of the valence that each element is thought to take in the solid electrolyte crystal structure, and are as follows: P: 0.31 Å, Si: 0.40 Å, Ge: 0.53 Å, Sb: 0.63 Å, Sn: 0.69 Å.

[0113] Example 12 and Example 13 The crystals obtained in Example 7 were further subjected to post-heat treatment in a dry nitrogen gas atmosphere at 400°C for 1 hour (Example 12) or at 450°C for 1 hour (Example 13), and then cooled again to room temperature at a rate of 1°C / second, thereby obtaining a sulfide solid electrolyte.

[0114] Evaluation X-ray Diffraction Measurement (XRD) For the sulfide solid electrolytes of Examples 1 to 13, the sulfide solid electrolytes were pulverized in a mortar and passed through a 100 μm sieve to obtain powders with a D50 of approximately 10 to 20 μm. Here, D50 refers to the volume-based median diameter. XRD measurements were performed on the obtained powders using an X-ray diffractometer (Rigaku Corporation, SmartLab) in an environment not exposed to the atmosphere 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 rate: 5° / min, number of steps: 0.01° / step. Silicon was mixed as an internal standard to a concentration of 10% by mass, and measurements were performed.

[0115] As an example, XRD patterns of the sulfide solid electrolytes of Examples 1 to 8 are shown in Figures 3 to 10. From the XRD patterns, it was confirmed that crystals of the same composition as those shown in Table 1 were obtained in all of Examples 1 to 11. Furthermore, it was confirmed that Examples 12 and 13 had the same composition as those in Example 7. Furthermore, although the composition remained unchanged, it was suggested that the crystal structure had changed from that of Example 7 due to post-heat treatment. The peak at 2θ = 28.5° in Figures 3 to 10 is a Si internal standard. The broad peak observed at 2θ = 16 to 17° is a peak derived from the window material of the XRD holder not exposed to air. All of the crystals obtained above were cubic argyrodite-type crystals with a crystal structure in the F-43m space group.

[0116] Furthermore, each MS possessed by the crystals in the sulfide solid electrolytes in Examples 1 to 13 ４ The tetrahedral structure was confirmed by Rietveld analysis of the XRD pattern. Furthermore, the absence of a diffraction pattern (diffraction peak) of the (111) plane on the low-angle side of the (200) plane in the XRD pattern confirmed that the tetrahedral structures were uniformly distributed. ４ The types and numbers of tetrahedral structures are shown in Table 1 under "Tetrahedral type."

[0117] <Lithium Ion Conductivity> The sulfide solid electrolytes of Examples 1 to 13 were pulverized in a mortar and passed through a 100 μm sieve to obtain powders with a D50 of approximately 10 to 20 μm. Here, D50 refers to the volume-based median diameter. The resulting powders were used as samples to measure lithium ion conductivity using an AC impedance measuring device (VSP potentiostat / galvanostat, manufactured by Bio-Logic Sciences Instruments). The results are shown in the "Ion Conductivity [mS / cm]" column in Table 1. 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.

[0118] <Water Resistance: Lithium Ion Conductivity Retention Rate> The sulfide solid electrolytes of Examples 1 to 11 were crushed in a mortar and passed through a 100 μm sieve to obtain powders with a D50 of approximately 10 to 20 μm. 150 mg of the resulting powder was weighed out as a sample, and nitrogen gas with a dew point of −20°C was passed through at a flow rate of 0.5 L / min for 1 hour. The sample was then vacuum heated at 80°C for 1 hour, and the lithium ion conductivity was measured under the same conditions as those described in the <Lithium Ion Conductivity> section above. The results are shown in Table 1 under "Conductivity (after exposure) [mS / cm]" in the "Water Resistance" section. The lithium ion conductivity values ​​obtained in this test were calculated as the lithium ion conductivity retention rate, with the value obtained in the <Lithium Ion Conductivity> section set to 100% (reference). The results are shown in Table 1 under "Conductivity (Retention Rate) [%]" in the "Water Resistance" section.

[0119] <Battery Characteristics> The sulfide solid electrolytes of Examples 1 to 11 were crushed in a mortar and passed through a 45 μm sieve to obtain powders with a D50 of about 5 μm. LiNi coated with lithium niobate having an average particle size of 7 μm was used as the positive electrode active material. １／３ Co １／３ Mn １／３ O ２ Using particles (cathode active material), 34 parts by mass of the sulfide-based solid electrolyte powder prepared above, 60 parts by mass of the cathode active material, and 6 parts by mass of a conductive additive (acetylene black, manufactured by Denka Co., Ltd., HS100) were mixed to prepare a cathode composite. 80 mg of the sulfide-based solid electrolyte powder prepared above was placed in a plastic cylinder with a diameter of 10 mm and pressure-molded to form a solid electrolyte layer. Next, 6 mg of the cathode composite prepared above was placed in the same cylinder and pressure-molded again to form a cathode layer. Furthermore, indium foil and lithium foil were placed on the opposite side of the cathode composite to form a cathode layer. In this way, an all-solid-state lithium ion secondary battery (half cell) was prepared, and a charge / discharge test was performed at a confining pressure of 10 kN.

[0120] The obtained half cell was subjected to a voltage test at 25°C, with a voltage of 4.3 to 2.5 V vs. Li ＋ Charge and discharge were repeated 10 times within the range of 0.05 C for both charging and discharging. The rate was 0.05 C for both charging and discharging. The ratio of the discharge capacity at the 10th cycle to the discharge capacity at the 1st cycle (discharge capacity retention rate [%]) was calculated and evaluated according to the following criteria: ○: Discharge capacity retention rate was 90% or more, very good △: Discharge capacity retention rate was 70% or more and less than 90%, good ×: Discharge capacity retention rate was less than 70%, poor The results are shown in "Battery Characteristics" in Table 1.

[0121]

[0122] From the above results, the sulfide solid electrolyte according to this embodiment has excellent water resistance and good battery characteristics. In particular, when the results of Examples 1 to 8 are compared with the results of Examples 9 and 11, it is clear that the crystals are MS. ４ By including three or more types of tetrahedral structures, very good water resistance was achieved. On the other hand, from the results of Examples 8 and 10, it was found that the crystals were MS ４Even when three or more types of tetrahedral structures are included, the cycle characteristics deteriorate when a certain amount of Ge is included. ４ This is thought to be because the inclusion of a certain amount of tetrahedral structures reduces the oxidation resistance and reduction resistance.

[0123] Furthermore, the lithium ion conductivity at 25° C. of Example 7 was 3.2 mS / cm, whereas the lithium ion conductivity of Examples 12 and 13 was 6.5 mS / cm and 7.2 mS / cm, respectively. This indicates that the lithium ion conductivity can be increased by further performing post-heat treatment by heating after the precipitation of crystals.

[0124] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-206287) filed on December 6, 2023, the contents of which are incorporated herein by reference.

Claims

1. A sulfide solid electrolyte having a crystalline phase, wherein the crystalline phase has a composition formula: R ａ M ｂ S ｃ X ｄ In the composition formula, R represents an alkali metal element, M represents an element that can form a tetracoordination structure with S among elements of Groups 2 to 15 of the periodic table, and M is an element of Group 14 of the periodic table. Ａ and M, an element of group 15 Ｂ wherein S represents a sulfur element, and X represents a halogen element, and in the composition formula, 3≦a≦7, b=1, 3≦c≦6, and 0.1≦d≦2 are satisfied, and the crystal is Ａ S ４ Tetrahedral structure and M Ｂ S ４ A sulfide solid electrolyte having three or more types of tetrahedral structures including a tetrahedral structure, wherein in the crystal, a ratio of Ge to M is 0.05 or less in terms of molar ratio.

2. The sulfide solid electrolyte according to claim 1, wherein R in the composition formula contains at least one element selected from the group consisting of Li, Na and K.

3. M included in M ​​in the composition formula Ａ As the element, one or more elements selected from Si, Ge, Sn and Pb are included, and M included in M ​​in the composition formula Ｂ The sulfide solid electrolyte according to claim 1, comprising one or more selected from P, As, Sb, and Bi.

4. M included in M ​​in the above composition formula Ｂ The sulfide solid electrolyte according to claim 1 , comprising Sb as a sulfide.

5. The sulfide solid electrolyte according to claim 2, wherein in the crystal, the proportion of Sb in M ​​is 0.01 to 0.9 in terms of molar ratio.

6. M included in M ​​in the above composition formula Ａ The sulfide solid electrolyte according to claim 1 , comprising Si as a sulfide.

7. The sulfide solid electrolyte according to claim 4, wherein in the crystal, the proportion of Si in M ​​is 0.01 to 0.6 in terms of molar ratio.

8. M included in M ​​in the above composition formula Ｂ The sulfide solid electrolyte according to claim 1 , comprising P as a sulfide.

9. The sulfide solid electrolyte according to claim 8, wherein in the crystal, a ratio of P to M is greater than 0 and not greater than 0.7 in terms of molar ratio.

10. In the above composition formula, M is an element of Group 14. Ａ and M, an element of Group 15 Ｂ The sulfide solid electrolyte according to claim 1, which is composed of a total of three or more elements including at least two of the above.

11. The sulfide solid electrolyte according to claim 1, wherein M in the composition formula is composed of a total of four or more elements.

12. The sulfide solid electrolyte according to claim 1, wherein the crystal has a crystal structure of the F-43m space group.

13. The sulfide solid electrolyte according to claim 12, wherein the content of crystals having a crystal structure of the F-43m space group in the sulfide solid electrolyte is 50 mass % or more.

14. The sulfide solid electrolyte according to claim 1, wherein the crystal has an ionic radius parameter, expressed by the following formula using the ionic radius of an element constituting M, of 0.47 to 0.

55. Ionic radius parameter=(b １ ×r １ ) + (b ２ ×r ２ ) + ... + (b ｎ ×r ｎ In the above composition formula, M is １ ~M ｎ In the formula, b １ ~b ｎ Each of the M １ ~M ｎ represents the composition ratio of each element, (b １ +b ２ + ... + b ｎ )=1, and r １ ~r ｎ Each of the M １ ~M ｎ represents the ionic radius (Å) of the tetracoordinate valence of each element in the crystal.

15. The lithium ion conductivity at 25°C is 0.1 x 10 －３ The sulfide solid electrolyte according to claim 1, wherein the sulfide solid electrolyte has a specific surface area of ​​1.5 S / cm or more.

16. A method for producing a sulfide solid electrolyte according to any one of claims 1 to 15, comprising the steps of: mixing raw materials containing elements R, M, S, and X to obtain a raw material mixture; obtaining a melt of the raw material mixture by a heat treatment; and cooling the melt to precipitate crystals, wherein, among the elements, R represents an alkali metal element, M represents an element that can form a tetracoordination structure with S among elements of Groups 2 to 15 of the periodic table, and M is an element of Group 14. Ａ and M, an element of group 15 Ｂ wherein S represents a sulfur element and X represents a halogen element.

17. The method for producing a sulfide solid electrolyte according to claim 16, further comprising, after precipitating the crystals, carrying out a post-heat treatment by heating.

18. The method for producing a sulfide solid electrolyte according to claim 17, wherein the post-heat treatment is carried out at a temperature of 300 to 600° C. for 10 minutes to 20 hours in an inert atmosphere with a dew point of −20° C. or lower.

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