Sulfide solid electrolyte

Abstract

The present invention relates to a sulfide solid electrolyte comprising a crystal phase and a glass phase, wherein the glass phase includes a phase composed of sulfide-based glass containing Li, P, and S as constituent elements; the crystal phase includes a high-ion conductive crystal phase derived from the sulfide-based glass and an additional crystal phase; the composition of the high-ion conductive crystal phase and the composition of the phase composed of the sulfide-based glass satisfy 30-50 at% of Li, 5-15 at% of P, and 30-60 at% of S; and the additional crystal phase exhibits, in an XRD pattern obtained by powder X-ray diffraction measurement, two or more diffraction peaks different from diffraction peaks indicating the high ion conductive crystal phase within the range in which 2θ = 18.0°-22.5°.

Description

Sulfide solid electrolyte

[0001] The present invention relates to a sulfide solid electrolyte.

[0002] Lithium-ion secondary batteries are widely used in portable electronic devices such as mobile phones and laptop computers, as well as in automobiles. Conventionally, liquid electrolytes have been used in lithium-ion secondary batteries, but concerns about leakage and fire have led to the need for larger casings for safety reasons. Furthermore, improvements have been needed to address the short battery life and narrow operating temperature range.

[0003] In response to this, all-solid-state lithium secondary batteries, which use solid electrolytes as the electrolyte for lithium-ion secondary batteries, are attracting attention because they are expected to offer improved safety, high-speed charging and discharging, and smaller cases.

[0004] Solid electrolytes are broadly classified into sulfide solid electrolytes and oxide solid electrolytes. Sulfide ions constituting sulfide solid electrolytes have higher polarizability than oxide ions constituting oxide solid electrolytes, and exhibit high ionic conductivity. As sulfide solid electrolytes, Li １０ GeP ２ S １２ LGPS type crystals such as Li ６ P.S. ５ Argyrodite-type crystals such as Cl, Li ７ P ３ S １１ LPS crystallized glass and the like are known.

[0005] As a solid electrolyte that is resistant to hydrolysis and has high ionic conductivity, Patent Document 1 discloses a crystallized solid electrolyte that contains lithium, phosphorus, sulfur, and Br as constituent components and has a specific crystal structure.

[0006] Japanese Patent Application Publication No. 2013-201110

[0007] The crystallized solid electrolyte described in Patent Document 1 is crystallized by heat treating sulfide-based glass obtained by mechanical milling. However, the effect of reducing the amount of hydrogen sulfide generated by the crystallized solid electrolyte after the crystallization is limited, and it is desired to achieve higher water resistance.

[0008] Therefore, an object of the present invention is to provide a sulfide solid electrolyte that suppresses the generation of hydrogen sulfide and has excellent water resistance.

[0009] As a result of extensive research, the present inventors have come up with the idea of ​​obtaining a sulfide-based glass phase by heating, melting, and cooling raw materials, by carrying out the heating and cooling under normal pressure conditions and at a very fast cooling rate. They have also found that when a portion of the glass phase obtained in this way is crystallized, in addition to a highly ion-conductive crystalline phase with excellent ionic conductivity, other crystalline phases are precipitated, and that a sulfide solid electrolyte containing these multiple crystalline phases can achieve very good water resistance, which has led to the completion of the present invention.

[0010] That is, the present invention relates to the following [1] to [5]: [1] A sulfide solid electrolyte including a crystalline phase and a glass phase, wherein the glass phase includes a phase made of sulfide-based glass containing Li, P, and S as constituent elements, and the crystalline phase includes a highly ion-conductive crystalline phase derived from the sulfide-based glass and another crystalline phase, wherein the phase made of the sulfide-based glass and the highly ion-conductive crystalline phase have compositions satisfying Li: 30 to 50 at %, P: 5 to 15 at %, and S: 30 to 60 at %, and the other crystalline phase exhibits two or more diffraction peaks in the range of 2θ = 18.0 to 22.5° in an XRD pattern obtained by powder X-ray diffraction measurement that are different from the diffraction peaks indicating the highly ion-conductive crystalline phase. [2] The sulfide solid electrolyte according to [1], wherein the high ion conductive crystalline phase has diffraction peaks at at least 2θ = 20.2 ± 0.5°, 23.8 ± 0.5°, and 29.7 ± 0.5° in an XRD pattern obtained by powder X-ray diffraction measurement. [3] The peak intensity of the diffraction peak at 2θ = 20.2 ± 0.5° of the high ion conductive crystalline phase is I Ａ The peak intensity of the diffraction peak located at the lowest angle among the two or more diffraction peaks of the other crystalline phases shown in the range of 2θ = 18.0 to 22.5° is defined as I α When I α / I Ａ[4] The sulfide solid electrolyte according to any one of [1] to [3], wherein the high ion-conductive crystalline phase further has a diffraction peak at at least one angle selected from the group consisting of 2θ=12.4±0.5°, 14.4±0.5°, and 31.4±0.5° in an XRD pattern obtained by powder X-ray diffraction measurement. [5] The sulfide solid electrolyte according to any one of [1] to [4], wherein the sulfide-based glass phase and the high ion-conductive crystalline phase further contain Ha as a constituent element, wherein the Ha is at least one element selected from the group consisting of F, Cl, Br, and I, and the compositions of the sulfide-based glass phase and the high ion-conductive crystalline phase further satisfy the following composition: Ha: 1 to 12 at%.

[0011] According to the present invention, a sulfide solid electrolyte having excellent water resistance can be obtained, and therefore, when the sulfide solid electrolyte is used in an all-solid-state lithium secondary battery, good battery characteristics can be achieved.

[0012] Fig. 1 is a flow diagram showing a method for producing a sulfide solid electrolyte according to the present embodiment. Fig. 2 is an XRD pattern of the sulfide solid electrolyte obtained in Example 2. Fig. 3 is an XRD pattern of the sulfide solid electrolyte obtained in Example 5.

[0013] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified as desired without departing from the spirit of the present invention. Furthermore, the term "to" indicating a range of values ​​is used to mean that the values ​​before and after it are included as the lower and upper limits.

[0014] The sulfide solid electrolyte according to this embodiment includes a crystalline phase and a glass phase. The glass phase includes a phase made of sulfide-based glass containing Li, P, and S as constituent elements (hereinafter, sometimes simply referred to as a "sulfide-based glass phase"). The crystalline phase includes a highly ion-conductive crystalline phase derived from the sulfide-based glass and another crystalline phase different from the highly ion-conductive crystalline phase.

[0015] The term "high ion-conductive crystalline phase derived from sulfide-based glass" refers to a crystalline phase obtained by crystallizing the sulfide-based glass through heat treatment, and the constituent elements and composition ratios of the phase are considered to be substantially identical. Here, "substantially identical" means that the difference in composition of the high ion-conductive crystalline phase relative to the overall composition, as determined by structural analysis of the XRD pattern, is within ±20% for each component.

[0016] The composition of the sulfide-based glass phase and the highly ion-conductive crystalline phase satisfies Li: 30 to 50 at %, P: 5 to 15 at %, and S: 30 to 60 at %.

[0017] Furthermore, the other crystalline phase exhibits two or more diffraction peaks in the range of 2θ = 18.0 to 22.5° in an XRD pattern obtained by powder X-ray diffraction measurement that are different from the diffraction peak representing the high ion-conductive crystalline phase. Here, the other crystalline phase may consist of only one type of crystalline phase, or may consist of two or more types of crystalline phases. Therefore, the two or more diffraction peaks exhibited in the range of 2θ = 18.0 to 22.5° may be peaks of one type of other crystalline phase, or peaks of two or more types of other crystalline phases.

[0018] <Sulfide-based glass phase and high ion-conductive crystalline phase> The sulfide-based glass phase and high ion-conductive crystalline phase in this embodiment contain Li, P, and S as constituent elements. In addition to these, it is preferable to contain Ha from the viewpoint of lithium ion conductivity. Here, Ha is at least one selected from the group consisting of F, Cl, Br, and I. From the viewpoint of obtaining good oxidation resistance and water resistance, it is more preferable to contain at least one selected from the group consisting of Cl, Br, and I, even more preferable to contain at least one of Cl and Br, and even more preferable to contain Br. It is also even more preferable to contain both Cl and Br.

[0019] The compositions of the sulfide-based glass phase and the high ion-conductive crystalline phase preferably satisfy the following: Li: 30 to 50 at%, P: 5 to 15 at%, and S: 30 to 60 at%, and Li: 30 to 50 at%, P: 5 to 15 at%, S: 30 to 60 at%, and Ha: 1 to 12 at%. Note that in this specification, at% means atomic %, i.e., atomic %. Furthermore, as mentioned above, the sulfide-based glass phase and the high ion-conductive crystalline phase derived therefrom can be considered to have almost the same constituent elements and composition ratios, but this does not mean that they are strictly the same. It is preferable that the compositions of the sulfide-based glass phase and the high ion-conductive crystalline phase each satisfy the above ranges.

[0020] In the sulfide-based glass phase and the highly ion-conductive crystalline phase of this embodiment, Li is an element responsible for ionic conduction as a solid electrolyte. In the compositions of the sulfide-based glass phase and the highly ion-conductive crystalline phase, the Li content is preferably 30 to 50 at%. 35 to 45 at% is preferred, and 37 to 43 at% is more preferred. From the viewpoint of increasing lithium ion conductivity, the Li content is preferably 30 at% or more, more preferably 35 at% or more, and even more preferably 37 at% or more. Furthermore, from the viewpoint of widening the vitrification range, the Li content is preferably 50 at% or less, more preferably 45 at% or less, and even more preferably 43 at% or less.

[0021] In the sulfide-based glass phase and the high ion-conductive crystalline phase of this embodiment, P is an element that forms a glass network, and among sulfides, the P-S bond is highly resistant to both oxidation and reduction. Therefore, as a solid electrolyte, it has a wide potential window and excellent electrochemical stability. In the composition of the sulfide-based glass phase and the high ion-conductive crystalline phase, the P content is preferably 5 to 15 at%, more preferably 6 to 13 at%, and more preferably 7 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 even more preferably 7 at% or more. Furthermore, from the viewpoint of increasing lithium ion conductivity, the P content is preferably 15 at% or less, more preferably 13 at% or less, and even more preferably 12 at% or less.

[0022] In the sulfide-based glass phase and the high ion-conductive crystalline phase of this embodiment, S is an element that forms P—S bonds with P and is an essential element for forming the glass phase. In the composition of the sulfide-based glass phase and the high ion-conductive crystalline phase, the S content is preferably 30 to 60 at%. 33 to 50 at% is more preferable, and 35 to 47 at% is even more preferable. From the viewpoint of widening the vitrification range, the S content is preferably 30 at% or more, more preferably 33 at% or more, and even more preferably 35 at% or more. Furthermore, 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 even more preferably 47 at% or less.

[0023] In the present embodiment, when the sulfide-based glass phase and the high ion-conductive crystalline phase contain Ha, Ha is an element that contributes to high lithium ion conductivity. In the compositions of the sulfide-based glass phase and the high ion-conductive crystalline phase, the Ha content is preferably 1 to 12 at%; more preferably 3 to 10 at%; and even more preferably 3.5 to 8 at%. Here, from the viewpoint of obtaining high lithium ion conductivity, the Ha content is preferably 1 at% or more, more preferably 3 at% or more, and even more preferably 3.5 at% or more. Furthermore, from the viewpoint of preventing precipitation of lithium halide crystals, the Ha content is preferably 12 at% or less, more preferably 10 at% or less, and even more preferably 8 at% or less. Here, the Ha content refers to the total content of F, Cl, Br, and I.

[0024] Ha is at least one element selected from the group consisting of F, Cl, Br, and I. As described above, from the viewpoint of obtaining good oxidation resistance and water resistance, it is more preferable that at least one of Cl and Br is contained, and it is even more preferable that Br is contained. It is also more preferable that both Cl and Br are contained. For example, of the Ha content, the total content of Cl and Br preferably accounts for 50 at% or more, more preferably 70 at% or more, and may be 100 at%, i.e., it may consist only of Cl and Br. Furthermore, the atomic ratio of the content represented by Cl:Br is preferably 100:0 to 0:100, more preferably 80:20 to 0:100, and even more preferably 60:40 to 0:100.

[0025] The sulfide-based glass phase and the highly ion-conductive crystalline phase in this embodiment may contain elements other than the above-mentioned 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. However, if the other elements are SiO ２ Or B ２ O ３ , P ２ O ５ , Al ２ O ３ In the case of forming oxides such as those mentioned above, it is preferable that such oxides are not contained because they become crystal nuclei. The total content of oxides is preferably 1 mol% or less, more preferably 0.1 mol% or less, and even more preferably 0.05 mol% or less, expressed in mol% based on the oxides, and may be zero, i.e., not contained.

[0026] The sulfide-based glass phase and the highly ion-conductive crystalline phase in this embodiment may contain, among the other elements, Si, B, Ge, Al, Sn, and Sb, which are elements that can occupy the position of P when the sulfide-based glass phase is crystallized.

[0027] When the sulfide-based glass phase and the highly ion-conductive crystalline phase in this embodiment contain Si as a constituent element, Si has the effect of increasing the viscosity of the melt and promoting vitrification. The Si content is preferably 0 to 15 at%, more preferably 0.1 to 10 at%, and even more preferably 1 to 5 at%. Here, from the viewpoint of optimally obtaining the effect of Si, the Si content is preferably 0.1 at% or more, and more preferably 1 at% or more. Furthermore, from the viewpoint of electrochemical stability, the Si content is preferably 15 at% or less, more preferably 10 at% or less, and even more preferably 5 at% or less. However, if Si has a Si—O bond or SiO ２ When Si is contained as an oxide, the Si content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of glass is 3 to 20 mol %, as described above.

[0028] When the sulfide-based glass phase and the highly ion-conductive crystalline phase in this embodiment contain B as a constituent element, B has the effect of increasing the viscosity of the melt and promoting vitrification. The B content is preferably 0 to 15 at %, more preferably 0.1 to 10 at %, and even more preferably 1 to 5 at %. Here, from the viewpoint of optimally obtaining the effect of B, the B content is preferably 0.1 at % or more, and more preferably 1 at % or more. Furthermore, from the viewpoint of lithium ion conductivity, the B content is preferably 15 at % or less, more preferably 10 at % or less, and even more preferably 5 at % or less. However, if B has a B-O bond or B ２ O ３ When B is contained as an oxide, the B content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of the glass is 3 to 20 mol %, as described above.

[0029] When the sulfide-based glass phase and the high ion-conducting crystalline phase in this embodiment contain Ge as a constituent element, Ge has the effect of promoting an improvement in lithium ion conductivity. The Ge content is preferably 0 to 15 at %, more preferably 0.1 to 10 at %, and even more preferably 1 to 5 at %. Here, from the viewpoint of lithium ion conductivity, the Ge content is preferably 0.1 at % or more, and more preferably 1 at % or more. Furthermore, from the viewpoint of ease of glass formation, the Ge content is preferably 15 at % or less, more preferably 10 at % or less, and even more preferably 5 at % or less. However, if Ge has a Ge—O bond or is a GeO ２ When Ge is contained as an oxide, the Ge content is appropriately adjusted so that the total content of the oxides is 1 mol % or less, as described above.

[0030] When the sulfide-based glass phase and the high ion-conductive crystalline phase in this embodiment contain Al as a constituent element, Al has the effect of increasing the viscosity of the melt and promoting vitrification. The Al content is preferably 0 to 15 at %, more preferably 0.1 to 10 at %, and even more preferably 1 to 5 at %. Here, from the viewpoint of optimally obtaining the effect of Al, the Al content is preferably 0.1 at % or more, and more preferably 1 at % or more. Furthermore, from the viewpoint of lithium ion conductivity, the Al content is preferably 15 at % or less, more preferably 10 at % or less, and even more preferably 5 at % or less. However, if Al has an Al—O bond or Al ２ O ３ When Al is contained as an oxide, the Al content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of glass is 3 to 20 mol %, as described above.

[0031] When the sulfide-based glass phase and the highly ion-conductive crystalline phase in this embodiment contain Sn as a constituent element, Sn has the effect of increasing the viscosity of the melt and promoting vitrification. The Sn content is preferably 0 to 15 at%, more preferably 0.1 to 10 at%, and even more preferably 1 to 5 at%. Here, from the viewpoint of optimally obtaining the effects of Sn, the Sn content is preferably 0.1 at% or more, and more preferably 1 at% or more. Furthermore, from the viewpoint of achieving high lithium ion conductivity, the Sn content is preferably 15 at% or less, more preferably 10 at% or less, and even more preferably 5 at% or less. However, if Sn has an Sn—O bond or SnO ２ When Sn is contained as an oxide, the Sn content is appropriately adjusted so that the total content of the oxides is 1 mol % or less, as described above.

[0032] When the sulfide-based glass phase and the highly ion-conductive crystalline phase in this embodiment contain Sb as a constituent element, Sb has the effect of increasing the viscosity of the melt and promoting vitrification. The Sb content is preferably 0 to 15 at %, more preferably 0.1 to 10 at %, and even more preferably 1 to 5 at %. Here, from the viewpoint of optimally obtaining the effects of Sb, the Sb content is preferably 0.1 at % or more, and more preferably 1 at % or more. Furthermore, from the viewpoint of lithium ion conductivity, the Sb content is preferably 15 at % or less, more preferably 10 at % or less, and even more preferably 5 at % or less. However, if Sb has an Sb—O bond or Sb ２ O ３ When Sb is contained as an oxide, the Sb content is appropriately adjusted so that the total content of oxides is 1 mol % or less, or so that the total content of oxides that become network formers of glass is 3 to 20 mol %, as described above.

[0033] In the compositions of the sulfide-based glass phase and the high ion-conductive crystalline phase in this embodiment, the composition ratio represented by Li / P is preferably 2.5 or more, more preferably 2.5 to 7.0, even more preferably 3.0 to 6.5, and even more preferably 3.2 to 6.2. From the viewpoint of obtaining high lithium ion conductivity, the composition ratio is preferably 2.5 or more, more preferably 3.0 or more, and even more preferably 3.2 or more. Furthermore, from the viewpoint of widening the vitrification range, the composition ratio is preferably 7.0 or less, more preferably 6.5 or less, and even more preferably 6.2 or less. In this specification, the composition ratio refers to the ratio of the content (at %) of each element in the sulfide-based glass phase and the high ion-conductive crystalline phase.

[0034] In the compositions of the sulfide-based glass phase and the highly ion-conductive crystalline phase in this embodiment, the composition ratio represented by S / P is preferably 3.0 to 5.0, more preferably 3.5 to 4.5, and even more preferably 3.8 to 4.2. From the viewpoint of lithium ion conductivity, the composition ratio is preferably 3.0 or more, more preferably 3.5 or more, and even 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 even more preferably 4.2 or less.

[0035] When the sulfide-based glass phase and the high ion-conductive crystalline phase further contain Ha, the composition ratio represented by [(F + Cl + Br) / Ha] is preferably 0.5 or more, more preferably 0.5 to 1.0, even more preferably 0.6 to 1.0, and particularly preferably 0.8 to 1.0. From the viewpoint of oxidation resistance, the composition ratio is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.8 or more. The upper limit of the composition ratio is not particularly limited, and may be 1.0. The composition ratio of 1.0 means that I is not included as Ha. When the sulfide-based glass phase and the high ion-conductive crystalline phase contain I as Ha, from the viewpoint of obtaining the effect of I, the composition ratio may be 0.8 or less, or may be 0.6 or less.

[0036] When the compositions of the sulfide-based glass phase and the high ion-conductive crystalline phase further contain Ha, the composition ratio represented by Ha / P is preferably 0.01 to 1.5, more preferably 0.1 to 1.2, and even more preferably 0.2 to 1.0. From the viewpoint of water resistance, the composition ratio is preferably 0.01 or more, more preferably 0.1 or more, and even more preferably 0.2 or more. Furthermore, from the viewpoint of lithium halide crystal precipitation, the composition ratio is preferably 1.5 or less, more preferably 1.2 or less, and even more preferably 1.0 or less. Here, Ha refers to the sum of F, Cl, Br, and I.

[0037] The constituent elements and their respective contents in the sulfide-based glass phase and the highly ion-conductive crystalline phase are determined by comparing the results of measuring the constituent elements and their respective contents (composition ratios) in the entire sulfide solid electrolyte with the element ratios constituting the crystal structure determined from the XRD pattern. The methods for determining the constituent elements and their respective contents in the entire sulfide solid electrolyte vary depending on the element. For example, P and S can be determined by ICP atomic emission spectroscopy, Li by atomic absorption spectroscopy, and Ha by ion chromatography. Details of each analysis are described in the Examples section below.

[0038] The sulfide-based glass phase in this embodiment is partially crystallized by heat treatment to become the highly ion-conductive crystalline phase in this embodiment. This highly ion-conductive crystalline phase can have a thio-lisicon or LGPS crystalline structure. The highly ion-conductive crystalline phases of the thio-lisicon and LGPS types have high lithium ion conductivity and are useful as solid electrolytes.

[0039] The highly ion-conductive crystalline phase in this embodiment preferably has diffraction peaks at least at 2θ=20.2±0.5°, 23.8±0.5°, and 29.7±0.5° in an XRD pattern obtained by powder XRD measurement.

[0040] The XRD measurement is carried out using CuKα radiation (λ=1.5418 Å) as a radiation source, and the detailed measurement conditions are as described in detail in the Examples section below.

[0041] In the present embodiment, the high ion conductive crystalline phase preferably has, in an XRD pattern obtained by powder X-ray diffraction measurement, at least one additional diffraction peak selected from the group consisting of 2θ = 12.4 ± 0.5 °, 14.4 ± 0.5 °, and 31.4 ± 0.5 ° in addition to 2θ = 20.2 ± 0.5 °, 23.8 ± 0.5 °, and 29.7 ± 0.5 °, preferably at least two additional diffraction peaks, and more preferably at all three. When such diffraction peaks are present, the high ion conductive crystalline phase may be determined to have a thiolisicon-type or LGPS-type crystal structure. These have high lithium ion conductivity.

[0042] The total proportion of the sulfide-based glass phase and the high ion-conductive crystalline phase in the sulfide solid electrolyte according to this embodiment is preferably 50 to 100% by mass, more preferably 70 to 95% by mass, and even more preferably 80 to 90% by mass. From the viewpoint of lithium ion conductivity, the total proportion is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. The total proportion may be 100% by mass, i.e., the sulfide-based glass phase and the high ion-conductive crystalline phase alone. On the other hand, when obtaining the sulfide-based glass phase, both crystalline phases may be obtained, and obtaining only the sulfide-based glass phase and the high ion-conductive crystalline phase requires time and cost. Therefore, from the viewpoint of the manufacturability of the sulfide solid electrolyte, the total proportion is more preferably 95% by mass or less, and even more preferably 90% by mass or less. The proportions of the sulfide-based glass phase and the highly ion-conductive crystalline phase are determined by powdering the sulfide solid electrolyte, performing powder X-ray diffraction (XRD) measurement together with a crystalline powder serving as an internal standard, and then performing Rietveld analysis. The proportion of the sulfide-based glass phase is determined by subtracting the proportion of the crystalline phase from 100% by mass.

[0043] The content of the high ion-conducting crystalline phase in the sulfide solid electrolyte according to this embodiment is preferably 30% by mass or more, more preferably 50% by mass or more, or 30 to 90% by mass, even more preferably 50 to 85% by mass, and even more preferably 60 to 80% by mass. From the viewpoint of lithium ion conductivity, the content is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more. Furthermore, from the viewpoint of optimally obtaining the effects of the sulfide-based glass phase, the content is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less.

[0044] The sulfide solid electrolyte according to this embodiment preferably contains a sulfide-based glass phase in an amount of 1 to 70% by mass, more preferably 5 to 50% by mass, and even more preferably 8 to 40% by mass. From the viewpoint of achieving the softness of the glass phase that facilitates the formation of an interface between the active material and the solid electrolyte, the content is preferably 1% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more. Furthermore, from the viewpoint of optimally obtaining the effects of the highly ion-conductive crystalline phase, the content is preferably 70% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less.

[0045] <Other Crystalline Phases> The sulfide solid electrolyte according to this embodiment includes, in addition to the sulfide-based glass phase and the highly ion-conductive crystalline phase, another crystalline phase. The other crystalline phase exhibits, in an XRD pattern obtained by powder X-ray diffraction measurement, at least two or more diffraction peaks in the range of 2θ = 18.0 to 22.5° that are different from the diffraction peak representing the highly ion-conductive crystalline phase. As described above, the other crystalline phase may consist of only one type of crystalline phase, or may consist of two or more types of crystalline phases.

[0046] The crystal structure of the crystalline phase exhibiting two or more diffraction peaks in the range of 2θ=18.0 to 22.5° has not yet been analyzed. However, the diffraction peaks are not lithium phosphate, as can be seen from the Raman spectrum and ３１ This was confirmed by the fact that no peaks of P—O bonds were observed in the P-NMR spectrum. Furthermore, although specific production conditions will be explained in the Examples below, it is unlikely that the lithium phosphate precipitates when heated to 210°C for crystallization. Therefore, it is believed that the crystalline phase exhibiting two or more diffraction peaks in the 2θ=18.0 to 22.5° range is some kind of novel crystalline phase.

[0047] The sulfide solid electrolyte according to this embodiment contains other crystalline phases in addition to the highly ion-conductive crystalline phase, which significantly improves its water resistance. While the reason for this is unclear, it is speculated that the other crystalline phases precipitate near the surfaces of particles made of the highly ion-conductive crystalline phase during heat treatment. It is believed that the other crystalline phases act as a resistance to the reaction between the highly ion-conductive crystalline phase and water, thereby reducing the amount of hydrogen sulfide generated by the highly ion-conductive crystalline phase and improving the water resistance of the sulfide solid electrolyte.

[0048] The other crystalline phase may exhibit two or more diffraction peaks in the range of 2θ=18.0 to 22.5°, and the number of diffraction peaks is preferably 2 to 5, more preferably 2 to 4, and even more preferably 2 to 3.

[0049] The diffraction peaks in the range of 2θ=18.0 to 22.5° are preferably present at one or more positions of, for example, 2θ=18.2±0.5°, 19.2±0.5°, 21.4±0.5°, and 21.8±0.5°, more preferably at two or more positions, and may be present at three or more positions, or may be present at all four positions.

[0050] The diffraction peaks are preferably present at one or more of 2θ=18.2±0.5°, 19.2±0.5°, and 21.8±0.5°, more preferably at two or more of these, and may be present at all three positions.

[0051] The peak intensity of the diffraction peak at 2θ=20.2±0.5° of the high ion-conducting crystalline phase is expressed as I Ａ The peak intensity of the diffraction peak located at the lowest angle among two or more diffraction peaks of other crystalline phases in the range of 2θ = 18.0 to 22.5° is defined as I α When I α / I ＡThe peak intensity ratio represented by the formula (1) is preferably greater than 0.01 and less than 0.6, more preferably 0.05 to 0.5, and even more preferably 0.1 to 0.4. From the viewpoint of suitably improving the water resistance of the sulfide solid electrolyte, the peak intensity ratio is preferably greater than 0.01, more preferably 0.05 or greater, and even more preferably 0.1 or greater. From the viewpoint of lithium ion conductivity, the peak intensity ratio is preferably less than 0.6, more preferably 0.5 or less, and even more preferably 0.4 or less.

[0052] The proportion of the other crystalline phase in the sulfide solid electrolyte is preferably 1 to 40 mass%, more preferably 3 to 30 mass%, and even more preferably 5 to 20 mass%. Here, from the viewpoint of fully exhibiting the properties of the sulfide-based glass phase and the highly ion-conductive crystalline phase, the proportion of the other crystalline phase is preferably 40 mass% or less, more preferably 30 mass% or less, and even more preferably 20 mass% or less. Furthermore, from the viewpoint of water resistance, the proportion of the other crystalline phase is preferably 1 mass% or more, more preferably 3 mass% or more, and even more preferably 5 mass% or more.

[0053] <Other Heterogeneous Phases> The sulfide solid electrolyte according to this embodiment may also contain other heterogeneous phases in addition to the sulfide-based glass phase, the highly ion-conductive crystalline phase, and other crystalline phases. Examples of other heterogeneous phases include crystalline phases other than the other crystalline phases obtained during the production of the sulfide-based glass phase. The proportion of such other heterogeneous phases in the sulfide solid electrolyte is 40% by mass or less, preferably 1 to 40% by mass, more preferably 2 to 35% by mass, and even more preferably 5 to 30% by mass. From the viewpoint of fully utilizing the properties of the sulfide-based glass phase, the highly ion-conductive crystalline phase, and the other crystalline phases, the total proportion of the other heterogeneous phases is 40% by mass or less, preferably 35% by mass or less, and even more preferably 30% by mass or less. Furthermore, the sulfide solid electrolyte may not contain other heterogeneous phases, but from the viewpoint of the manufacturability of the sulfide solid electrolyte, the total proportion of the other heterogeneous phases is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 5% by mass or more.

[0054] <Physical Properties of Sulfide Solid Electrolyte> The sulfide solid electrolyte according to this embodiment, when compacted under 380 MPa, preferably has a lithium ion conductivity of 2.0 mS / cm or more at 25°C. This is due to the high lithium ion conductivity of the sulfide-based glass phase and the high ion-conductive crystalline phase derived therefrom. However, since it is difficult to extract only the sulfide-based glass phase or the high ion-conductive crystalline phase from the sulfide solid electrolyte, the lithium ion conductivity of the sulfide-based glass phase or the high ion-conductive crystalline phase is evaluated based on the lithium ion conductivity of the sulfide solid electrolyte.

[0055] The lithium ion conductivity of the sulfide solid electrolyte according to this embodiment is preferably 2.0 mS / cm or more, and the higher the better. However, this value varies depending on the composition and the proportion of the sulfide glass phase and the highly ion-conductive crystalline phase. For example, when the Br content of the Ha composition is 90 at% or more, the lithium ion conductivity is preferably 2.5 mS / cm or more. Furthermore, when the Cl content of the Ha composition is 90 at% or more, the lithium ion conductivity is preferably 2.0 mS / cm or more. The lithium ion conductivity in this specification is determined by AC impedance measurement using a measurement sample obtained by compacting a sulfide solid electrolyte powder at 380 MPa. Specifically, the AC impedance of the measurement sample is measured at a measurement frequency of 100 Hz to 1 MHz, a measurement voltage of 100 mV, and a measurement temperature of 25°C, and the value obtained from the resulting Nyquist plot is taken as the lithium ion conductivity.

[0056] The sulfide solid electrolyte according to this embodiment was kept in an atmosphere with a dew point of −20° C. for 5 hours. ２ The amount of sulfur generated is preferably 10 mL / g or less, more preferably 1 mL / g or less, and even more preferably 0.1 mL / g or less, and the smaller the better. ２ More specific conditions for measuring the amount of sulfur generated are as follows: First, the sulfide solid electrolyte is passed through a 100 μm mesh to obtain powder with an average particle size of 10 to 20 μm. Then, 10 mg of this powder is mixed with nitrogen (N ２) gas at a flow rate of 0.5 L / min for 5 hours. ２ S) Monitor the amount of generation and calculate the total amount as H ２ The amount of S generated is the above H ２ The amount of S generated is a value that serves as an index of the hydrolysis resistance, i.e., water resistance, of the sulfide solid electrolyte, and can be achieved by the presence of a sulfide-based glass phase or a highly ion-conductive crystalline phase, or by including Ha as a constituent element. In particular, it is preferable that at least one of Cl and Br is included as Ha, and it is more preferable that Br is included. It is also more preferable that both Cl and Br are included.

[0057] The heat resistance of the sulfide solid electrolyte according to this embodiment can be evaluated by the lithium ion conductivity after heat treatment of the sulfide solid electrolyte in a nitrogen atmosphere at 240° C. for 1 hour. Specifically, the lithium ion conductivity after the heat treatment is preferably 0.8 mS / cm or more, and the higher the better, but the value varies depending on the composition and the proportion of the sulfide-based glass phase and the highly ion-conductive crystalline phase, etc.

[0058] If the 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 even more preferably 1.5 mS / cm or less, and the smaller the difference, the more preferable.

[0059] The average particle size (D50) of the sulfide solid electrolyte according to this embodiment is preferably 0.1 to 300 μm, more preferably 0.3 to 50 μm, and even more preferably 0.5 to 5 μm. From the viewpoint of reducing the load of the pulverization process when using the sulfide solid electrolyte in a secondary battery, the average particle size is preferably 300 μm or less, more preferably 50 μm or less, and even more preferably 5 μm or less. Furthermore, from the viewpoint of powder handling, the average particle size is preferably 0.1 μm or more, more preferably 0.3 μm or more, and even more preferably 0.5 μm or more. The average particle size (D50) can be adjusted by subjecting the obtained sulfide solid electrolyte to coarse pulverization. Conventional known methods for coarse pulverization can be used, including, for example, a cutter mill, a planetary ball mill, and a jet mill, and dry pulverization is preferred. The resulting solid electrolyte is then further pulverized to a finer particle size using a wet or dry pulverization method such as a bead mill or a jet mill, and used as a battery material, such as a solid electrolyte for a positive electrode, a negative electrode, or a separator layer.

[0060] The sulfide solid electrolyte according to this embodiment is suitable as an electrolyte for lithium ion secondary batteries. When used in lithium ion secondary batteries, the sulfide solid electrolyte forms a solid electrolyte layer together with other components such as a binder, as necessary. Conventionally known binders and other components can be used. The content of the sulfide solid electrolyte according to this embodiment is preferably 80 mass % or more, and more preferably 90 mass % or more, of the entire solid electrolyte layer.

[0061] The sulfide solid electrolyte according to this embodiment may be mixed with a positive electrode active material or a negative electrode active material to form a positive electrode layer or a negative electrode layer. The positive electrode active material or a negative electrode active material, current collector, binder, conductive additive, and the like used in the positive electrode layer or the negative electrode layer may be conventionally known materials.

[0062] The lithium ion secondary battery includes a positive electrode layer, a solid electrolyte layer, and a positive electrode layer. It is preferable that at least one layer selected from the group consisting of the positive electrode layer, the solid electrolyte layer, and the positive electrode layer contains the sulfide solid electrolyte according to the present embodiment.

[0063] The lithium ion secondary battery may have any of a variety of shapes, including a coin shape, a sheet shape (film shape), a folded shape, a wound bottomed cylindrical shape, and a button shape, and may be appropriately selected depending on the intended use.

[0064] <<Method for Producing Sulfide Solid Electrolyte>> The method for producing a sulfide solid electrolyte according to this embodiment includes the following steps S1 to S4, in order, as shown in FIG. 1 : Step S1: Mixing raw materials to obtain a raw material mixture containing Li, P, and S; Step S2: Heating the raw material mixture obtained in step S1 to obtain a melt; Step S3: Cooling and solidifying the melt obtained in step S2 to obtain a glass phase; Step S4: Heating the glass phase obtained in step S3 to obtain a highly ion-conductive crystalline phase in which a portion of the glass phase is crystallized, and another crystalline ...

[0065] The heating in step 2 and the cooling and solidification in step 3 are carried out under atmospheric pressure, and the cooling rate during the cooling and solidification is 100°C / sec or more. This results in a sulfide solid electrolyte containing a glass phase, a highly ion-conductive crystalline phase derived therefrom, and other crystalline phases. In this specification, "under atmospheric pressure" means a pressure range of approximately (gauge pressure ±15 kPa).

[0066] Furthermore, in the above step 1, the raw materials are mixed so that the compositions of the resulting sulfide-based glass phase and highly ion-conductive crystalline phase satisfy Li: 30 to 50 at %, P: 5 to 15 at %, and S: 30 to 60 at %, thereby obtaining the sulfide solid electrolyte described in the above section "Sulfide Solid Electrolyte."

[0067] Each step will be explained below.

[0068] <Step 1> In this embodiment, step S1 is step 1 in which raw materials are mixed to obtain a raw material mixture containing Li, P, and S. Specifically, a raw material containing Li, a raw material containing P, and a raw material containing S are mixed to obtain a raw material mixture. Furthermore, when it is desired to obtain a glass phase further containing Ha as a constituent element, a raw material containing Ha is further mixed in addition to the raw material containing Li, the raw material containing P, and the raw material containing S to obtain a raw material mixture containing Li, P, S, and Ha. Here, the Ha element refers to at least one element selected from the group consisting of F, Cl, Br, and I.

[0069] Examples of raw materials containing Li include lithium sulfide (Li ２ S), lithium carbonate (Li ２ CO ３ ), lithium sulfate (Li ２ SO ４ ), lithium oxide (Li ２ Examples of the substance containing Li include lithium compounds such as lithium ion (LiO), lithium hydroxide (LiOH), and metallic lithium. The substance containing Li may be used alone or in combination of two or more.

[0070] From the viewpoint of obtaining a sulfide-based glass phase, it is preferable to use lithium sulfide as the raw material containing Li. Furthermore, when the obtained sulfide solid electrolyte contains a halogen element, lithium halide (LiHa, where Ha is a halogen element) is also preferable as the raw material containing Li. Lithium halide will be described later.

[0071] Examples of raw materials containing the P element include diphosphorus pentasulfide (P ２ S ５ ), diphosphorus trisulfide (P ２ S ３ ) and other phosphorus sulfides, sodium phosphate (Na ３ P.O. ４ The phosphorus-containing substance may be used singly or in combination of two or more.

[0072] The raw material containing the P element is preferably phosphorus sulfide, and more preferably diphosphorus pentasulfide (P), from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide solid electrolyte.２ S ５ ) is more preferred.

[0073] Examples of raw materials containing the S element include lithium sulfide (Li ２ S), diphosphorus trisulfide (P ２ S ３ ), diphosphorus pentasulfide (P ２ S ５ ), other sulfur compounds containing phosphorus, 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 The substance containing the S element may be used alone or in combination of two or more.

[0074] The raw material containing the S element is preferably lithium sulfide or phosphorus sulfide from the viewpoint of preventing the inclusion of elements other than those constituting the target sulfide solid electrolyte. As the phosphorus sulfide, diphosphorus pentasulfide (P ２ S ５ Lithium sulfide is a compound that serves as both a raw material containing Li and a raw material containing S, and phosphorus sulfide is a compound that serves as both a raw material containing S and a raw material containing P.

[0075] Examples of raw materials containing 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. One or more substances containing Ha element may be used, or two or more may be used in combination.

[0076] From the viewpoint of reactivity, the raw material containing the Ha element is preferably a lithium halide, more preferably LiCl, LiBr, or LiI, and even more preferably LiCl or LiBr.

[0077] A raw material mixture may be obtained by further mixing raw materials containing other elements according to the desired composition of the sulfide solid electrolyte. 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 included. These Si, B, Ge, Al, Sn, and Sb are elements that can occupy the P position when the sulfide-based glass phase is crystallized.

[0078] As raw materials containing these other elements, conventionally known raw materials can be used. For example, as a raw material containing Si element, SiO ２ , SiS ２ Among them, from the viewpoint of lithium ion conductivity, SiO ２ These compounds may be used alone or in combination of two or more. Examples of raw materials containing B element include B ２ O ３ , B ２ S ３ Among them, from the viewpoint of water resistance of glass, B ２ O ３ These compounds may be used alone or in combination of two or more. The raw material containing Ge element is 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. Examples of raw materials containing Al element include Al ２ S ３ , Al ２ O ３ , AlCl ３ Among them, Al is the most suitable from the viewpoint of lithium ion conductivity and water resistance. ２ S ３ , AlCl ３ is preferred, and Al２ S ３ These compounds may be used alone or in combination of two or more. Examples of raw materials containing Sn include SnS, 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 These compounds may be used alone or in combination of two or more. Examples of raw materials containing Sb include Sb ２ S ３ , Sb ２ O ３ , Sb ２ O ５ , SbCl ３ , SbCl ５ Among them, from the viewpoint of lithium ion conductivity and water resistance, Sb ２ S ３ , SbCl ３ is preferred, and Sb ２ S ３ These compounds may be used alone or in combination of two or more.

[0079] These raw materials are blended appropriately according to the composition of the sulfide-based glass phase and the resulting high ion-conductive crystalline phase. Specifically, to obtain the sulfide solid electrolyte described in the above "Sulfide Solid Electrolyte," the raw materials are mixed so that the resulting glass phase and the resulting high ion-conductive crystalline phase have a composition of Li: 30 to 50 at%; P: 5 to 15 at%; and S: 30 to 60 at%. Furthermore, the glass phase and the high ion-conductive crystalline phase preferably further contain Ha as a constituent element, and in addition to the above composition, the Ha content is preferably 1 to 12 at%. The blending of the raw materials also determines the crystal structure of the resulting high ion-conductive crystalline phase.

[0080] In addition to the above, preferred embodiments of the resulting glass phase and the highly ion-conductive crystalline phase derived therefrom are the same as preferred embodiments of the "sulfide-based glass phase and the highly ion-conductive crystalline phase" in the "sulfide solid electrolyte" above.

[0081] It is preferable that the particle size of each raw material be small from the viewpoint of shortening the holding time during heating and melting in the subsequent step 2. Furthermore, if the particle size of the raw material is too large, it may affect the homogeneity of the glass phase or sulfide solid electrolyte, and from this viewpoint as well, it is preferable that the particle size of the raw material be small to a certain extent. However, the production method according to this embodiment has excellent composition controllability. Therefore, even if raw materials with particle sizes that may reduce homogeneity in conventional production methods are used, the production method according to this embodiment can produce a more homogeneous glass phase or sulfide solid electrolyte.

[0082] From the above viewpoint, specifically, the particle size of each raw material is preferably 1 mm or less, more preferably 500 μm or less, even more preferably 250 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less. Smaller particle sizes are preferable, but a practical lower limit is about 0.1 μm, and the particle size may be 1 μm or more, or may be 5 μm or more. Furthermore, as described above, according to the manufacturing method of this embodiment, a homogeneous glass phase or sulfide solid electrolyte is easily obtained even when raw materials with relatively large particle sizes are used. In consideration of this, for example, from the viewpoint of reducing manufacturing costs, the particle size of each raw material may be 10 μm or more, 100 μm or more, or 250 μm or more.

[0083] From these, the particle size of each raw material is preferably 0.1 μm to 1 mm, more preferably 1 to 500 μm, even more preferably 5 to 250 μm, even more preferably 5 to 100 μm, and particularly preferably 5 to 50 μm. Furthermore, from the viewpoint of production costs, the particle size of each raw material is preferably 10 μm to 1 mm, more preferably 100 μm to 1 mm, and even more preferably 250 to 500 μm. In this specification, the particle size of each raw material refers to the average particle size (D50) expressed as the median diameter obtained from a volume-based particle size distribution chart obtained by measuring the particle size distribution using a laser diffraction particle size distribution analyzer MT3300EXII manufactured by Microtrac.

[0084] 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.

[0085] <Step 2> Step S2 in this embodiment is step 2 in which the raw material mixture obtained in step 1 is heated to obtain a melt.

[0086] The specific method for heating and melting the raw material mixture is not particularly limited, as long as the heating is performed under atmospheric pressure. In the production method according to this embodiment, the heating for melting the raw material mixture is performed under atmospheric pressure, thereby reducing the compositional difference between the composition of the raw material mixture and the composition of the resulting sulfide-based glass phase and the highly ion-conductive crystalline phase derived therefrom, or the composition of the sulfide solid electrolyte. Furthermore, by performing the heating under a gas atmosphere containing the element S, the compositional difference can be further suppressed. Note that in the production method according to this embodiment, the heating and melting are performed in an atmosphere-controlled environment under atmospheric pressure, and do not need to be performed in a sealed tube. However, this does not in any way exclude the use of a sealed tube.

[0087] Here, the difference between the sealed tube environment and the controlled atmosphere environment under normal pressure conditions is that in the sealed tube environment, the pressure inside the container is a vacuum, i.e., less than (gauge pressure -15 kPa), whereas in the controlled atmosphere environment under normal pressure conditions, the pressure inside the container during heating and melting is within the range of (gauge pressure ±15 kPa).

[0088] That is, in the manufacturing method according to the present embodiment, the heating and melting in step 2 is performed under normal pressure conditions (gauge pressure ±15 kPa). At this time, the composition ratio of the resulting glass phase or highly ion-conductive crystalline phase expressed as [P / Li] relative to the composition ratio of the raw material mixture expressed as [P / Li] is preferably 95 to 105%, for example.

[0089] The pressure during the heat melting may be normal pressure (gauge pressure ±15 kPa), preferably (gauge pressure ±10 kPa), more preferably (gauge pressure ±5 kPa), and even more preferably (gauge pressure ±2 kPa). Furthermore, the composition ratio of the resulting glass phase or highly ion-conductive crystalline phase expressed as [P / Li] relative to the composition ratio expressed as [P / Li] of the raw material mixture is preferably 95 to 105%, more preferably 97 to 103%, even more preferably 99 to 101%, with the closer to 100% being more preferable. Here, the composition ratio is preferably 95% or more, more preferably 97% or more, even more preferably 99% or more, and is preferably 105% or less, more preferably 103% or less, and even more preferably 101% or less.

[0090] With respect to the composition ratio [P / Li] of the raw material mixture relative to the composition ratio [P / Li] of the resulting glass phase or highly ion-conductive crystalline phase, Li is an element that is difficult to volatilize, and P is an element that is easily volatilized. Therefore, comparing the composition ratio [P / Li] of the raw material mixture with that of the glass phase or highly ion-conductive crystalline phase is appropriate as an index of the compositional deviation between the composition of the raw material mixture and the composition of the resulting sulfide-based glass phase or highly ion-conductive crystalline phase or the composition of the sulfide solid electrolyte.

[0091] Examples of heat-resistant containers for containing the raw material mixture include heat-resistant containers made of carbon, heat-resistant containers containing oxides such as quartz, quartz glass, borosilicate glass, aluminosilicate glass, alumina, zirconia, and mullite, heat-resistant containers containing nitrides such as silicon nitride and boron nitride, and heat-resistant containers containing carbides such as silicon carbide. 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.

[0092] The melting temperature when the raw material mixture is heated varies depending on the raw materials used and the composition of the raw material mixture, but is, for example, preferably 600 to 950°C, more preferably 630 to 850°C, and even more preferably 650 to 750°C. Here, from the viewpoint of reaction rate, the 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 composition deviation due to volatilization of components, the melting temperature is preferably 950°C or lower, more preferably 850°C or lower, and even more preferably 750°C or lower.

[0093] The heat-melting time varies depending on the scale, but is preferably 10 minutes to 10 hours, more preferably 30 minutes to 9.5 hours, even more preferably 45 minutes to 9 hours, and particularly preferably 1 to 9 hours. From the viewpoint of smoothly progressing the reaction, the heat-melting time is preferably 10 minutes or more, more preferably 30 minutes or more, even more preferably 45 minutes or more, and particularly preferably 1 hour or more. From the viewpoint of productivity, the heat-melting time is preferably 10 hours or less, more preferably 9.5 hours or less, and even more preferably 9 hours or less.

[0094] The atmosphere in the container during heat melting is not particularly limited. For example, it may be a nitrogen atmosphere, an argon atmosphere, or a gas atmosphere containing an S element. Among these, a gas atmosphere containing an S element is preferred from the viewpoint of more suitably suppressing composition deviation.

[0095] When the gas atmosphere contains S element, examples of the S element source include sulfur gas, hydrogen sulfide gas, sulfur dioxide gas, etc., and sulfur gas is preferred from the viewpoint of reactivity with the raw materials. Alternatively, the gas containing S element may be introduced by adding elemental sulfur powder and vaporizing the elemental sulfur powder by heating during melting. The elemental sulfur powder may be added together with the raw materials when mixing the raw materials to obtain a raw material mixture, or may be added separately after obtaining the raw material mixture. However, when the elemental sulfur powder is added together with the raw materials when obtaining the raw material mixture, the mass of the obtained raw material mixture does not include the mass of the elemental sulfur powder. The gas containing S element may be introduced by adding the above-mentioned sulfur gas, hydrogen sulfide gas, sulfur dioxide gas, etc., and the gas containing S element resulting from the addition of elemental sulfur powder.

[0096] The gas containing S element is preferably used as a mixed gas with, for example, an inert gas, which is the S element source gas. The mixing ratio with the inert gas is arbitrary and is not particularly limited as long as the cumulative amount of S element introduced relative to the mass of the raw material mixture can be set to a desired value.

[0097] Examples of the inert gas include nitrogen gas, argon gas, and helium gas, and these may be used alone or in combination of two or more.

[0098] The dew point during heat melting is preferably −20° C. or lower, and although there is no particular lower limit, it is usually about −80° C. The oxygen concentration is preferably 1000 ppm by volume or lower.

[0099] In step 2, whether the molten mixture of raw materials is completely dissolved can be confirmed by the absence of peaks derived from crystals in high-temperature X-ray diffraction measurement. It can also be confirmed by heating the molten mixture at a predetermined temperature and tilting the molten mixture to check whether it has fluidity.

[0100] <Step 3> Step S3 in this embodiment is step 3 in which the melt obtained in step 2 is cooled and solidified to obtain a glass phase.

[0101] Cooling and solidification are carried out under atmospheric pressure. As mentioned above, "under atmospheric pressure" means an atmosphere under a pressure of about (gauge pressure ±15 kPa). The pressure during cooling and solidification may be under atmospheric pressure conditions of (gauge pressure ±15 kPa), preferably (gauge pressure ±10 kPa), more preferably (gauge pressure ±5 kPa), and even more preferably (gauge pressure ±2 kPa).

[0102] The cooling rate during solidification is 100°C / sec or higher, preferably 100 to 100,000°C / sec, more preferably 500 to 50,000°C / sec, and even more preferably 1,000 to 10,000°C / sec. Here, by using an ultra-rapid cooling rate of 100°C / sec or higher, which is faster than conventional methods, the proportion of a sulfide-based glass phase containing Li, P, and S as constituent elements can be increased to, for example, 60 mass% or more. This sulfide-based glass phase can achieve higher lithium ion conductivity than conventional sulfide-based glasses, thereby also increasing the lithium ion conductivity of the sulfide solid electrolyte. Furthermore, by performing the cooling and solidification under atmospheric pressure conditions, the compositional deviation between the composition of the raw material mixture and the composition of the resulting glass phase and the highly ion-conductive crystalline phase derived therefrom, or the composition of the sulfide solid electrolyte, can be reduced.

[0103] The cooling rate is 100°C / sec or more, but from the viewpoint of ease of glass formation, it is preferably 500°C / sec or more, more preferably 1000°C / sec or more. Furthermore, the upper limit of the cooling rate is not particularly limited, but from the viewpoint of equipment capacity, it is preferably 100,000°C / sec or less, more preferably 50,000°C / sec or less, and even more preferably 10,000°C / sec or less. The above-mentioned ultra-rapid cooling can be achieved, for example, by using a rapid-cooling twin roll.

[0104] This sulfide-based glass phase can achieve higher lithium ion conductivity than conventional sulfide-based glasses. Furthermore, by performing the cooling and solidification under atmospheric pressure, the compositional difference between the raw material mixture and the resulting sulfide-based glass phase and the resulting highly ion-conductive crystalline phase or sulfide solid electrolyte can be reduced.

[0105] <Step 4> Step S4 according to this embodiment is step 4 in which the glass phase obtained in step 3 is heated to obtain a highly ion-conductive crystalline phase in which part of the glass phase is crystallized, and other crystalline phases.

[0106] The heating temperature in step 4, i.e., the temperature at which crystallization is performed to obtain the high ion conductivity crystalline phase and other crystalline phases, can be determined based on the crystallization temperature at which the glass phase becomes the high ion conductivity crystalline phase. Specifically, from the viewpoint of effectively promoting the crystallization, a temperature of (crystallization temperature - 15°C) or higher is preferred, and it may be (crystallization temperature - 10°C) or higher, or a temperature higher than the crystallization temperature, or even (crystallization temperature + 1°C) or higher. Furthermore, it has been found that heating at high temperatures precipitates other heterogeneous phases, which are crystals with low ionic conductivity, next to the high ion conductivity crystalline phase. Therefore, from the viewpoint of preventing the precipitation of crystals with low ionic conductivity, the crystallization temperature is preferably lower than the temperature at which crystals with low ionic conductivity precipitate, more preferably lower than (crystallization temperature + 20°C), and even more preferably lower than (crystallization temperature + 15°C). That is, the crystallization temperature is preferably higher than (crystallization temperature - 15°C), and more preferably higher than (crystallization temperature - 15°C) and lower than the temperature at which crystals with low ionic conductivity precipitate. Alternatively, the temperature may be (crystallization temperature - 10°C) or more and (crystallization temperature + 20°C) or less, or may be (crystallization temperature + 20°C) or more and (crystallization temperature + 1°C) or more and (crystallization temperature + 15°C) or less. Within the above ranges, a highly ion-conductive crystalline phase is obtained from the glass phase, and other crystalline phases are also obtained.

[0107] The crystallization temperature in this specification refers to the peak top temperature of the exothermic peak observed when differential scanning calorimetry (DSC) is performed on the sulfide-based glass phase, which can be said to be a precursor of the sulfide solid electrolyte obtained in step 3, and the temperature is increased at a heating rate of 5°C / min.

[0108] The heating time during crystallization is, for example, 1 minute to 6 hours, preferably 1 minute to 3 hours, more preferably 5 minutes to 2 hours, and even more preferably 10 minutes to 100 minutes. From the viewpoint of stable quality, the heating time is preferably 1 minute or more, more preferably 5 minutes or more, and even more preferably 10 minutes or more. From the viewpoint of productivity, the heating time is preferably 6 hours or less, more preferably 3 hours or less, even more preferably 2 hours or less, and even more preferably 100 minutes or less. The content ratio of the high ion-conducting crystalline phase can be adjusted by adjusting the heating temperature and heating time during crystallization.

[0109] The heating atmosphere during crystallization may be a nitrogen atmosphere, an argon atmosphere, a dry air atmosphere, etc. Among these, a nitrogen atmosphere or an argon atmosphere is preferred from the viewpoint of not impairing ionic conductivity.

[0110] The dew point during crystallization is preferably −30° C. or lower. The oxygen concentration during crystallization may be a dry air environment, but from the viewpoint of safety, it is preferably 5% by volume or lower.

[0111] After heating, the sulfide solid electrolyte according to this embodiment can be obtained by cooling to room temperature at a rate of, for example, 1 to 10,000° C. / min.

[0112] The sulfide solid electrolyte obtained above may be used as it is for at least one selected from the group consisting of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer, or may be used after being subjected to pulverization, drying, or the like depending on the application.

[0113] 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 4 are working examples, and Examples 5 and 6 are comparative examples.

[0114] <Test Examples> <Example 1: Melt-quenching method> Li was melted under a dry nitrogen gas atmosphere. ３．４ P １．０ S ４．０ Br ０．３４Lithium sulfide powder (Sigma, purity 99.98%), diphosphorus pentasulfide powder (Sigma, purity 99%), and lithium bromide powder (Sigma, purity 99.995%) were weighed and mixed in a mortar to obtain a raw material mixture (Step 1). The obtained raw material mixture was placed in a carbon container under a nitrogen atmosphere containing sulfur powder (Sigma, purity 99.998%) as an S element source and a dew point of −50° C. or less, and then placed in an electric furnace inside a glove box. The pressure was increased to +1 kPa and the temperature was increased to 750° C. for 1 hour to obtain a completely dissolved molten material (Step 2). Next, the tip of the container was heated to melt the melt, and the molten material was poured onto a twin roll and cooled to room temperature at a cooling rate of 3000°C / s to obtain a glass phase (step 3). The pressure at this time was (gauge pressure + 1 kPa), and the cooling rate was adjusted by the melt outflow rate, the roll gap of the twin rolls, and the rotation speed. The glass phase obtained above was heated in a nitrogen atmosphere at 210°C for 10 minutes to crystallize a portion of the glass phase (step 4), and then cooled to room temperature at a cooling rate of 300°C / min to obtain a sulfide solid electrolyte containing a glass phase, a highly ion-conductive crystalline phase, and other crystalline phases. DSC measurement of the glass phase obtained above was performed at a heating rate of 5°C / min, and the temperature at the top of the exothermic peak, i.e., the crystallization temperature, was confirmed to be 223°C.

[0115] Examples 2 and 3: Melt-quenching method A sulfide solid electrolyte containing a glass phase, a highly ion-conductive crystalline phase, and other crystalline phases was obtained in the same manner as in Example 1, except that the cooling rate in step 3 was 8,000°C / sec (Example 2) or 10,000°C / sec (Example 3). The crystallization temperature of the glass phase obtained in step 3 was confirmed to be 217°C (Example 2) or 215°C (Example 3).

[0116] Example 4: Melt quenching method: Composition ratio is Li ３．４ P １．０ S ４．０ Cl ０．１ Br ０．２A sulfide solid electrolyte containing a glass phase, a highly ion-conducting crystalline phase, and other crystalline phases was obtained in the same manner as in Example 1, except that lithium sulfide powder (manufactured by Sigma, purity 99.98%), diphosphorus pentasulfide powder (manufactured by Sigma, purity 99%), lithium bromide powder (manufactured by Sigma, purity 99.995%), and lithium chloride powder (manufactured by Sigma, purity 99.995%) were weighed to obtain a raw material mixture so as to have a composition ratio of (Li: 39.1 at%, P 11.5 at%, S: 46.0 at%, Cl: 1.1 at%, Br: 2.3 at%). The crystallization temperature of the glass phase obtained in step 3 was confirmed to be 219°C.

[0117] Example 5: Mechanical Milling Method Step 1 was performed in the same manner as in Example 1 to obtain a raw material mixture. The resulting raw material mixture was then further mixed using a planetary ball mill (Fritsch, P-7) to obtain a glass phase (Steps 2'+3'). Mixing using the planetary ball mill involved mechanical milling for 20 hours at 400 rpm under a nitrogen atmosphere using zirconia balls with a particle size of 4 mm and a zirconia pot. The dew point during operation was maintained at -70°C or below. The resulting glass phase was heated in a nitrogen atmosphere at 210°C for 10 minutes to crystallize a portion of the glass phase (Step 4), followed by cooling to room temperature at a cooling rate of 300°C / min to obtain a sulfide solid electrolyte containing a glass phase and a highly ion-conductive crystalline phase. The crystallization temperature of the glass phase was 207°C.

[0118] Example 6: Mechanical Milling Method A raw material mixture was obtained by carrying out step 1 in the same manner as in Example 4. Then, steps 2'+3' and 4 were carried out in the same manner as in Example 5 to obtain a sulfide solid electrolyte containing a glass phase and a highly ion-conductive crystalline phase. The crystallization temperature of the glass phase was 210°C.

[0119] <Evaluation: Glass Phase> In each example, the glass phase obtained in step 3 or step 2'+3' was weighed in a glove box and dissolved in an alkaline aqueous solution, and composition analysis was performed for each element. Specifically, P and S were analyzed by ICP atomic emission spectroscopy (apparatus: Hitachi High-Tech Science Corporation, model number PS3520UVDDII). Li was analyzed by atomic absorption spectroscopy (apparatus: Hitachi High-Tech Corporation, model number ZA3300; when measuring Li, CsCl was added so that the solution concentration became 0.1%). Br and Cl were analyzed by ion chromatography (apparatus: Thermo Fisher Scientific, model number ICS-2100 (column: AS11HC), H ２ O ２ The glass phase composition was confirmed by adding a small amount of HCl and diluting with ultrapure water.

[0120] Evaluation: Sulfide Solid Electrolyte Composition Ratio and Content Ratio The sulfide solid electrolytes obtained in Examples 1 to 6 were crushed in a mortar and passed through a 100 μm sieve to obtain a powder of solid electrolyte precursor having an average particle diameter D50 of approximately 20 μm. The resulting powder was used as a sample. An X-ray diffraction (XRD) apparatus (Rigaku Corporation, SmartLab) was used to measure the crystallinity of the samples in a non-exposed environment, using a CuKα ray source (λ = 1.5418 Å), a tube voltage of 45 kV, a tube current of 200 mA, a scan angle of 10 to 100°, a scan rate of 5° / min, and a step count of 0.01° / step. Silicon powder was added as an internal standard during XRD measurement, and the crystallinity (wt%) was determined by Rietveld analysis.

[0121] In the obtained XRD pattern, the value of 2θ at which a diffraction peak was observed was determined. The peak intensity of the diffraction peak at 2θ = 20.2 ± 0.5° was determined as I Ａ The peak intensity of the diffraction peak located at the lowest angle among the diffraction peaks that are shown in the range of 2θ = 18.0 to 22.5° and are different from the diffraction peaks that indicate the high ion conductive crystalline phase is defined as I α and use them to calculate I Ａ / I αThe peak intensity ratios represented by the formula (I) were also determined. Here, the composition ratio of the sulfide-based glass phase can be determined to be the same as the composition ratio of the high ion-conductive crystalline phase, but just to be sure, it was compared with the composition ratio determined by the evaluation of the glass phase. As a result, it was confirmed that the composition ratio of the glass phase before step 3 was within a range of ±20% for all components relative to the composition ratio of the high ion-conductive crystalline phase. The XRD patterns of the sulfide solid electrolytes obtained in Examples 2 and 5 are shown in Figures 2 and 3, respectively.

[0122] <Water Resistance> The obtained sulfide solid electrolyte was crushed in a mortar and then passed through a 100 μm sieve to obtain a powder with an average particle size (D50) of about 20 μm, which was used as a sample. 10 mg of the sample was placed in a humidified N ２ The gas was allowed to flow and exposed for 5 hours, and the hydrogen sulfide concentration in the gas was monitored to determine the total amount of hydrogen sulfide generated. ２ The gas was passed through at 0.5 L / min. In this way, the total amount of hydrogen sulfide generated was detected to evaluate the hydrolysis resistance as an evaluation of water resistance. A hydrogen sulfide concentration meter (Model 3000RS, manufactured by Techne Measurement Co., Ltd.) was used as the detector tube. The results are shown in the "Water Resistance H" column in Table 1. ２ The amount of sulfur generated (mL / g) is shown in the table below.

[0123] <Lithium ion conductivity> The sulfide solid electrolyte obtained in each example was pulverized in a mortar and then passed through a 100 μm sieve to obtain a powder with an average particle size (D50) of approximately 20 μm, which was used as a sample. The above sample was compressed under a pressure of 380 MPa to obtain a measurement sample, and measured using an AC impedance measurement 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 calculated from the obtained Nyquist plot. The results are shown in Table 1 under "σ" Ｌｉ＋ (mS / cm)".

[0124]

[0125] From the above results, it was found that the sulfide solid electrolytes of Examples 1 to 3 were able to achieve high water resistance, despite having the same composition of the sulfide-based glass phase and the highly ion-conductive crystalline phase as the sulfide solid electrolyte of Example 5. Similarly, the sulfide solid electrolyte of Example 4 was able to achieve higher water resistance than the sulfide solid electrolyte of Example 6. This is thought to be because, in producing the sulfide solid electrolytes of Examples 1 to 4, a melt-quenching method employing specific conditions was used to obtain a glass phase, and then the glass phase was crystallized by heating, thereby precipitating another crystalline phase that exhibits two or more diffraction peaks in the range of 2θ = 18.0 to 22.5° that are different from the diffraction peak indicating the highly ion-conductive crystalline phase, thereby suppressing reaction with water. Furthermore, from the results of the water resistance of the sulfide solid electrolytes of Examples 1 to 3, the peak intensity ratio I α / I Ａ The higher the content ratio of the other crystalline phases, the more remarkable the water resistance effect.

[0126] 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. 2023-168391) filed on September 28, 2023, the contents of which are incorporated herein by reference.

Claims

1. A sulfide solid electrolyte comprising a crystalline phase and a glass phase, wherein the glass phase comprises a phase made of a sulfide-based glass containing Li, P, and S as constituent elements, the crystalline phase comprises a highly ion-conductive crystalline phase derived from the sulfide-based glass, and another crystalline phase, wherein the phase made of the sulfide-based glass and the highly ion-conductive crystalline phase have compositions satisfying Li: 30-50 at%, P: 5-15 at%, and S: 30-60 at%, and the other crystalline phase exhibits two or more diffraction peaks in the range of 2θ = 18.0 to 22.5° that are different from the diffraction peaks exhibiting the highly ion-conductive crystalline phase in an XRD pattern obtained by powder X-ray diffraction measurement.

2. The sulfide solid electrolyte according to claim 1, wherein the highly ion-conductive crystalline phase has diffraction peaks at least at 2θ = 20.2 ± 0.5°, 23.8 ± 0.5°, and 29.7 ± 0.5° in an XRD pattern obtained by powder X-ray diffraction measurement.

3. The peak intensity of the diffraction peak at 2θ=20.2±0.5° of the highly ion-conductive crystalline phase is expressed as I Ａ The peak intensity of the diffraction peak located at the lowest angle side among the two or more diffraction peaks shown in the range of 2θ = 18.0 to 22.5 ° of the other crystal phase is I α When I α / I Ａ The sulfide solid electrolyte according to claim 2, wherein the peak intensity ratio represented by is greater than 0.01 and less than 0.

6.

4. The sulfide solid electrolyte according to claim 1 or 2, wherein the highly ion-conductive crystalline phase further has a diffraction peak at at least one selected from the group consisting of 2θ = 12.4 ± 0.5 °, 14.4 ± 0.5 °, and 31.4 ± 0.5 ° in an XRD pattern obtained by powder X-ray diffraction measurement.

5. The sulfide solid electrolyte according to claim 1 or 2, wherein the sulfide-based glass phase and the highly ion-conductive crystalline phase further contain Ha as a constituent element, the Ha being at least one element selected from the group consisting of F, Cl, Br, and I, and the compositions of the sulfide-based glass phase and the highly ion-conductive crystalline phase further satisfy Ha: 1 to 12 at %.