Sulfide solid electrolyte and method for producing sulfide solid electrolyte

Abstract

Provided is a sulfide solid electrolyte that is characterized by having an LGPS type crystal structure belonging to the space group P42 / nmc, and is characterized in that the half value width of a peak of 2θ=29.58°±1.0° is 0.1 or less in an X-ray diffraction measurement using CuKα rays.

Description

Sulfide solid electrolyte and method for producing sulfide solid electrolyte

[0001] The present invention relates to a sulfide solid electrolyte suitable for use in, for example, all-solid-state batteries, and a method for producing the sulfide solid electrolyte. This application claims priority based on Japanese Patent Application No. 2023-201708, filed on November 29, 2023, the contents of which are incorporated herein by reference.

[0002] In recent years, sulfide-based solid electrolytes have attracted attention as electrolytes for lithium-ion secondary batteries because they have high ionic conductivity and are safer than liquid electrolytes. A widely used method for producing sulfide solid electrolytes involves first mechanically milling a mixture of raw materials to vitrify or amorphousize them, followed by heat treatment to synthesize the sulfide solid electrolyte through a solid-state reaction.

[0003] For example, Patent Document 1 describes a process for producing sulfide glass and glass ceramics, which are types of sulfide-based solid electrolytes, in which a mixture of metallic lithium, elemental sulfur, and elemental phosphorus is vitrified by mechanical milling and then heat-treated. Patent Documents 2 and 3 also describe a process for producing a sulfide-based solid electrolyte with an LGPS crystal structure in which a mixture of various sulfides is amorphized by mechanical milling, followed by heat treatment and crystallization by a solid-phase reaction. That is, a method in which a mixture of electrolyte raw materials is subjected to mechanical milling to diffuse and mix the contained elements and homogenize the chemical composition of the entire mixture before heat treatment has become a common method for producing a solid electrolyte with sufficient ionic conductivity.

[0004] Furthermore, Patent Document 4 describes a method for producing a crystalline sulfide-based solid electrolyte, in which elemental sulfur or a sulfur compound is mixed with a solid electrolyte raw material, and the mixture is heat-treated.

[0005] Japanese Patent Publication No. 2003-208919 (A) Japanese Patent No. 5527673 (B) Japanese Patent No. 5888609 (B) Japanese Patent Publication No. 2023-048303 (A)

[0006] However, in all-solid-state batteries and the like that use a sulfide solid electrolyte, if the ionic conductivity of the sulfide solid electrolyte is low, the resistance increases. The sulfide solid electrolytes produced by the production methods described in Patent Documents 1 to 4 had insufficient ionic conductivity and could not be used as sulfide solid electrolytes that constitute high-power batteries.

[0007] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a sulfide solid electrolyte that is excellent in ionic conductivity and is particularly suitable for high-power solid-state batteries, and a method for producing the sulfide solid electrolyte.

[0008] In order to solve the above problems, the present inventors have conducted extensive research and have found that the ionic conductivity can be significantly improved by increasing the crystallinity of the sulfide solid electrolyte.

[0009] The present invention has been made based on the above-mentioned findings, and the sulfide solid electrolyte of aspect 1 of the present invention is characterized in that it has an LGPS-type crystal structure belonging to the space group P42 / nmc, and in X-ray diffraction measurement using CuKα rays, the half-width of the peak at 2θ = 29.58° ± 1.0° is 0.1 or less.

[0010] The sulfide solid electrolyte of aspect 1 of the present invention has an LGPS-type crystal structure belonging to the space group P42 / nmc, and in X-ray diffraction measurement using CuKα radiation, the half-width of the peak at 2θ = 29.58° ± 1.0° is 0.1 or less, and therefore has sufficiently high crystallinity and excellent ionic conductivity, making it possible to configure a high-power solid-state battery.

[0011] A method for producing a sulfide solid electrolyte according to Aspect 2 of the present invention is a method for producing a sulfide solid electrolyte, comprising: a raw material mixing step of mixing raw materials containing elements constituting the sulfide solid electrolyte to obtain a mixed raw material; and a production step of heat-treating the mixed raw material to produce the sulfide solid electrolyte, wherein the raw material mixing step uses elemental sulfur as a raw material, and when the mixed raw material is heated to 120°C, a volume ratio of the elemental sulfur to the entire mixed raw material is 20% or more, and the production step uses a heating temperature of T°C, a holding time of h hours, and T×h is 1000 or more.

[0012] According to the method for producing a sulfide solid electrolyte of Aspect 2 of the present invention, elemental sulfur is used as a raw material, and when the mixed raw material is heated to 120°C, the volume ratio of the elemental sulfur to the entire mixed raw material is set to 20% or more. In the production step, the heating temperature is set to T°C, the holding time is set to h hours, and T×h is set to 1000 or more. Therefore, a sulfide solid electrolyte having high crystallinity and excellent ionic conductivity can be produced.

[0013] According to the present invention, it is possible to provide a sulfide solid electrolyte that has excellent ionic conductivity and is particularly suitable for high-power solid-state batteries, and a method for producing this sulfide solid electrolyte.

[0014] 1 is a flow chart showing an example of a method for producing a sulfide solid electrolyte according to an embodiment of the present invention. 2 is a result of XRD measurement of the sulfide solid electrolyte according to Example 1-2 of the present invention.

[0015] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the following embodiments are specifically described to provide a better understanding of the gist of the invention, and do not limit the present invention unless otherwise specified.

[0016] The sulfide solid electrolyte according to this embodiment is used, for example, as a solid electrolyte constituting an all-solid-state battery. Sulfide solid electrolytes have relatively high ionic conductivity, are non-flammable, and are highly safe, and are therefore applied to electric vehicles and the like. Here, in order to construct a high-power solid-state battery, a sulfide solid electrolyte with even better ionic conductivity is required.

[0017] Therefore, the sulfide solid electrolyte according to this embodiment has an LGPS-type crystal structure belonging to the space group P42 / nmc, and in X-ray diffraction measurement using CuKα radiation, the half-width of the peak at 2θ = 29.58° ± 1.0° is 0.1 or less. Although not particularly limited, the half-width of the peak at 2θ = 29.58° ± 1.0° may be 0.096 or less, or 0.094 or less. Furthermore, although not particularly limited, the half-width of the peak at 2θ = 29.58° ± 1.0° may be 0.001 or more, 0.010 or more, or 0.050 or more. That is, in the sulfide solid electrolyte according to this embodiment, the maximum peak (peak at 2θ = 29.58° ± 1.0°) in the X-ray diffraction pattern of the sulfide solid electrolyte having an LGPS-type crystal structure has a high intensity and a small half-width. Therefore, the sulfide solid electrolyte according to this embodiment is sufficiently crystallized.

[0018] As described above, the sulfide solid electrolyte of this embodiment has an LGPS-type crystal structure belonging to the space group P42 / nmc. When measured by X-ray diffraction measurement using CuKα rays, peaks of the following formulas (A) to (F) are detected as diffraction peaks, and as for the peak of formula (G), either no peak is detected, or when the diffraction intensity of the peak of formula (F) is IA and the diffraction intensity of the peak of formula (G) is IB, the peak of formula (G) is detected at a diffraction intensity IB such that the peak intensity ratio of IB to IA is less than 50%. 2θ=17.38°±1.0°...(A) 2θ=20.18°±1.0°...(B) 2θ=20.44°±1.0°...(C) 2θ=23.96°±1.0°...(D) 2θ=26.96°±1.0°...(E) 2θ=29.58°±1.0°...(F) 2θ=27.33°±1.0°...(G)

[0019] Next, the method for producing a sulfide solid electrolyte according to this embodiment will be described with reference to the flow diagram of Fig. 1. As shown in Fig. 1, this embodiment includes a raw material mixing step S01 and a production step S02.

[0020] (Raw Material Mixing Step S01) First, raw materials containing each element constituting the sulfide solid electrolyte are mixed to obtain a raw material mixture. Here, elemental sulfur is used as the raw material. This elemental sulfur is mixed so that the volume ratio of elemental sulfur to the entire raw material mixture when the raw material mixture is heated to 120°C is 20% or more. Although not particularly limited, the volume ratio of elemental sulfur to the entire raw material mixture when the raw material mixture is heated to 120°C may be 30% or more, or may be 40% or more. Furthermore, although not particularly limited, the volume ratio of elemental sulfur to the entire raw material mixture when the raw material mixture is heated to 120°C may be 80% or less, 60% or less, or 40% or less. In order to utilize the solid-liquid reaction between each raw material and liquid sulfur to generate a sulfide solid electrolyte, excess elemental sulfur exceeding the stoichiometric composition of the sulfide solid electrolyte is added to the raw material mixture. Furthermore, in order to promote elemental diffusion and chemical reaction in the subsequent production step S02, it is preferable that each raw material and excess elemental sulfur be in the form of powder or granules (powder-like powder, particulate granules, or an aggregate of powder and granules), and among these, powder is more preferable.

[0021] Elemental sulfur refers to sulfur that does not contain elements other than sulfur, except for inevitable impurities. Unless otherwise specified, in other descriptions of this embodiment, each raw material may contain inevitable impurities. Furthermore, the elemental sulfur to be added may be any of sulfur allotropes, such as α sulfur (orthorhombic sulfur), β sulfur (monoclinic sulfur), γ sulfur (monoclinic sulfur), and rubber sulfur, or may contain multiple allotropes.

[0022] The mixing method in the raw material mixing step S01 is not particularly limited as long as it can uniformly mix the raw materials. Examples of various existing methods include a general mixer, blender, ball mill, bead mill, vibration mill, and V-type mixer. Furthermore, when the raw materials 11 contain sulfides, the mixing process in the raw material mixing step S01 is preferably carried out in a gas atmosphere that does not react with the raw materials. Therefore, it is preferably carried out in an inert atmosphere such as nitrogen, argon, or other rare gases. Additionally, it is preferable that the atmospheric gas used does not contain moisture or oxygen gas. In particular, the moisture content in the atmospheric gas is preferably 500 ppm or less, more preferably 100 ppm or less, and even more preferably 10 ppm or less. By keeping the moisture content in the atmospheric gas within this range, oxidation due to moisture is suppressed, enabling the production of a high-quality sulfide solid electrolyte.

[0023] (Production Step S02) In the production step S02, the mixed raw materials placed in a firing container such as a crucible or a sagger are heated to produce a sulfide solid electrolyte. As for the material of the firing container, it is preferable that the inner wall of the container is made of a material that is not easily reactive with the melt, such as elemental sulfur or sulfides, in other words, a material that is not easily corroded by sulfurization, such as alumina, zirconia, carbon, or silicon carbide.

[0024] In the production step S02, the heating temperature is set to T°C, the holding time is set to h hours, and T×h is set to 1000 or more. Note that T×h is preferably 2000 or more, and more preferably 3000 or more. There is no particular upper limit to T×h, but from the viewpoint of productivity and economy, it is preferably set to 20000 or less, and more preferably 15000 or less.

[0025] Furthermore, the heating temperature in the generation step S02 is preferably 400°C or higher, more preferably 450°C or higher, and even more preferably 500°C or higher. On the other hand, the heating temperature is preferably 1000°C or lower, more preferably 650°C or lower. Furthermore, it is preferable that the holding time at the heating temperature is appropriately set so as to satisfy the above-mentioned T×h≧1000. Although not particularly limited, the holding time at the heating temperature may be 1 hour or longer, 1.5 hours or longer, or even 5 hours or longer. Furthermore, although not particularly limited, the holding time at the heating temperature may be 50 hours or shorter, 44 hours or shorter, or 40 hours or shorter.

[0026] Furthermore, the heat treatment in the production step S02 is preferably carried out in a gas atmosphere that does not react with the mixed raw materials, the intermediate product, or the sulfide solid electrolyte. Therefore, it is preferably carried out in an inert atmosphere such as nitrogen, argon, or other rare gases. Additionally, it is preferable that the atmospheric gas used does not contain moisture or oxygen gas. In particular, the moisture content in the atmospheric gas is preferably 500 ppm or less, more preferably 100 ppm or less, and even more preferably 10 ppm or less. By keeping the moisture content in the atmospheric gas within this range, reaction with moisture is suppressed, enabling the production of a high-quality sulfide solid electrolyte.

[0027] The sulfide solid electrolyte of this embodiment configured as described above has an LGPS-type crystal structure belonging to the space group P42 / nmc, and in X-ray diffraction measurement using CuKα radiation, the half-width of the peak at 2θ = 29.58° ± 1.0° is 0.1 or less, which indicates sufficiently high crystallinity and excellent ionic conductivity. Therefore, it is possible to configure a high-power solid-state battery.

[0028] Furthermore, according to the method for producing a sulfide solid electrolyte of the present embodiment, elemental sulfur is used as a raw material, and when the mixed raw material is heated to 120°C, the volume ratio of the elemental sulfur to the entire mixed raw material is set to 20% or more. In the production step, the heating temperature is set to T°C, the holding time is set to h hours, and T×h is set to 1000 or more. Therefore, a sulfide solid electrolyte having high crystallinity and excellent ionic conductivity can be produced.

[0029] Although one embodiment of the present invention has been described above, the present invention is not limited to this and can be modified as appropriate within the scope of the technical idea of ​​the invention.

[0030] A confirmation experiment conducted to confirm the effectiveness of the present invention will be described.

[0031] (Invention Examples 1-1 and 1-2) As raw materials, Li ２ S, Ge, P, and S were prepared, weighed to a predetermined ratio, and mixed in a mortar. Here, the volume ratio of elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%. The mixed raw material was placed in an alumina firing container and loaded into a firing furnace. Then, firing was performed under conditions of an Ar atmosphere, a heating temperature of T°C, a holding time of h hours, and T×h = 3000. As a result, a sulfide solid electrolyte made of LGPS material was produced.

[0032] (Invention Examples 2-1 and 2-2) As raw materials, Li ２ S, Ge, P, and S were prepared, weighed to a predetermined ratio, and mixed in a mortar. Here, the volume ratio of elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%. The mixed raw material was placed in an alumina firing container and loaded into a firing furnace. Then, firing was performed under conditions of an Ar atmosphere, a heating temperature of T°C, a holding time of h hours, and T×h = 1000. As a result, a sulfide solid electrolyte made of LGPS material was produced.

[0033] (Comparative Example 1) As a raw material, Li ２S, Ge, P, and S were prepared, weighed to a predetermined ratio, and mixed in a mortar. Here, the volume ratio of elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%. The mixed raw material was placed in an alumina firing container and loaded into a firing furnace. Then, firing was performed under conditions of an Ar atmosphere, a heating temperature of T°C, a holding time of h hours, and T×h = 500. As a result, a sulfide solid electrolyte made of LGPS material was produced.

[0034] (Invention Examples 3-1 and 3-2) As raw materials, Li ２ S, Si, Sn, P, and S were prepared, weighed to a predetermined ratio, and mixed in a mortar. Here, elemental sulfur was set so that the volume ratio of elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%. The mixed raw material was placed in an alumina firing container and loaded into a firing furnace. Then, firing was performed under conditions of an Ar atmosphere, a heating temperature of T°C, a holding time of h hours, and T x h = 3000. As a result, a sulfide solid electrolyte made of an LSSPS material was produced.

[0035] (Invention Examples 4-1 and 4-2) As raw materials, Li ２ S, Si, Sn, P, and S were prepared, weighed to a predetermined ratio, and mixed in a mortar. Here, elemental sulfur was set so that the volume ratio of elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%. The mixed raw material was placed in an alumina firing container and loaded into a firing furnace. Then, firing was performed under conditions of an Ar atmosphere, a heating temperature of T°C, a holding time of h hours, and T x h = 1000. As a result, a sulfide solid electrolyte made of an LSSPS material was produced.

[0036] (Comparative Example 2) As a raw material, Li ２S, Si, Sn, P, and S were prepared, weighed to a predetermined ratio, and mixed in a mortar. Here, elemental sulfur was set so that the volume ratio of elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%. The mixed raw material was placed in an alumina firing container and loaded into a firing furnace. Then, firing was performed under conditions of an Ar atmosphere, a heating temperature of T°C, a holding time of h hours, and T x h = 500. As a result, a sulfide solid electrolyte made of an LSSPS material was produced.

[0037] The obtained sulfide solid electrolyte was subjected to X-ray diffraction measurement (XRD measurement) using CuKα radiation to measure the half-width of the peak at 2θ = 29.58° ± 1.0°. Furthermore, the obtained sulfide solid electrolyte was measured for ionic conductivity. The XRD measurement method and the method for evaluating ionic conductivity are described below.

[0038] <XRD Measurement> X-ray diffraction measurement (XRD measurement) was carried out using CuKα radiation. For the XRD measurement, a Bruker XRD device "D8 ADVANCE" was used, and θ-2θ measurement was performed in the range of 10°≦2θ≦55° under the conditions of a step width of 0.01° and an integration time of 1.2 seconds / step, and the half-width of the peak at 2θ = 29.58° ± 1.0° was evaluated. The measurement sample was prepared in a glove box in an argon atmosphere, and the sulfide solid electrolyte was pulverized in an agate mortar and sealed in a sealable measurement cell, and powder X-ray diffraction measurement was carried out while maintaining a state where it was not exposed to the atmosphere.

[0039] <Ionic Conductivity> The obtained solid electrolyte was taken out from a glove box in an argon atmosphere and then crushed in an agate mortar. 0.3 g of the resulting solid electrolyte was weighed out and filled into a stainless steel ionic conductivity measurement cell (cylindrical, inner diameter 17 mm). The ionic conductivity (mS / cm) was then measured by an AC impedance method using a measuring device "Potentio / Galvanostat SP-300" manufactured by Biologic, Inc., under the conditions of a measurement temperature of 25°C, a measurement frequency of 1 Hz to 1 MHz, and an applied pressure of 360 MPa to the measurement cell.

[0040]

[0041] Inventive Examples 1-1, 1-2, 2-1, and 2-2 and Comparative Example 1 are LGPS materials. In Inventive Examples 1-1, 1-2, 2-1, and 2-2, the volume ratio of the elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40% or more, the product T×h of the heating temperature T°C and the holding time h hours was 1000 or more, and the half-width of the peak at 2θ = 29.58° ± 1.0° was 0.096 or less. On the other hand, in Comparative Example 1, the volume ratio of the elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%, the product T×h of the heating temperature T°C and the holding time h hours was 500, and the half-width of the peak at 2θ = 29.58° ± 1.0° was 0.105. It can be seen that Inventive Examples 1-1, 1-2, 2-1, and 2-2 have higher crystallinity than Comparative Example 1. Inventive Examples 1-1, 1-2, 2-1, and 2-2, the ionic conductivity was 9.6 mS / cm or more, while the ionic conductivity was 8.0 mS / cm in Comparative Example 1. Inventive Examples 1 and 2 had superior ionic conductivity to Comparative Example 1.

[0042] Inventive Examples 3-1, 3-2, 4-1, and 4-2 and Comparative Example 2 are LSSPS materials. In Inventive Examples 3-1, 3-2, 4-1, and 4-2, the volume ratio of the elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40% or more, the product T×h of the heating temperature T°C and the holding time h hours was 1000 or more, and the half-width of the peak at 2θ = 29.58° ± 1.0° was 0.096 or less. On the other hand, in Comparative Example 2, the volume ratio of the elemental sulfur to the entire mixed raw material when the mixed raw material was heated to 120°C was 40%, the product T×h of the heating temperature T°C and the holding time h hours was 500, and the half-width of the peak at 2θ = 29.58° ± 1.0° was 0.103. It can be seen that Inventive Examples 3-1, 3-2, 4-1, and 4-2 have higher crystallinity than Comparative Example 2. Inventive Examples 3-1, 3-2, 4-1, and 4-2, the ionic conductivity was 7.6 mS / cm or more, while the ionic conductivity was 6.1 mS / cm in Comparative Example 2. Inventive Examples 3-1, 3-2, 4-1, and 4-2 were superior in ionic conductivity to Comparative Example 2.

[0043] As a result of the above confirmatory experiments, it was confirmed that the present invention can provide a sulfide solid electrolyte having excellent ionic conductivity and particularly suitable for high-power solid-state batteries, and a method for producing this sulfide solid electrolyte.

[0044] According to the present invention, it is possible to provide a sulfide solid electrolyte that has excellent ionic conductivity and is particularly suitable for high-power solid-state batteries, and a method for producing this sulfide solid electrolyte.

Claims

1. A sulfide solid electrolyte having an LGPS type crystal structure belonging to the space group P42 / nmc, and characterized in that in X-ray diffraction measurement using CuKα radiation, the half-width of the peak at 2θ=29.58°±1.0° is 0.1 or less.

2. A method for producing a sulfide solid electrolyte, comprising: a raw material mixing step of mixing raw materials containing each element constituting the sulfide solid electrolyte to obtain a mixed raw material; and a production step of heat-treating the mixed raw material to produce the sulfide solid electrolyte, wherein in the raw material mixing step, elemental sulfur is used as a raw material, and when the mixed raw material is heated to 120°C, the volume ratio of the elemental sulfur to the entire mixed raw material is 20% or more, and in the production step, the heating temperature is T°C, the holding time is h hours, and T×h is 1000 or more.

Images