Lithium sulfide and method for producing sulfide-based solid electrolyte

Abstract

To provide lithium sulfide having sufficiently high purity and capable of efficiently synthesizing a sulfide-based solid electrolyte, and a method for producing a sulfide-based solid electrolyte using the lithium sulfide.SOLUTION: The lithium sulfide of the present invention has a 50% volume-based diameter within a range of 0.1 μm to 600 μm, and a full width at half maximum of a peak at 2θ=27° ±0.03° measured by X-ray diffraction within a range of 0.10° to 0.50°. The method for producing a sulfide-based solid electrolyte of the present invention uses the above-described lithium sulfide as a raw material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to lithium sulfide suitable as a constituent material of sulfide-based solid electrolyte materials used in, for example, all-solid-state batteries, and to a method for producing a sulfide-based solid electrolyte using this lithium sulfide. [Background technology]

[0002] Lithium-ion batteries are widely used as power sources in vehicles such as EVs (electric vehicles) and HEVs (hybrid electric vehicles), as well as electronic devices such as mobile phones and laptops. Conventional lithium-ion batteries use an organic electrolyte solution, in which a lithium salt such as lithium hexafluorophosphate (LiPF6) is dissolved in an organic solvent.

[0003] These organic electrolytes are flammable and can be damaged by excessive heating or impact. In addition, in lithium-ion batteries that use metallic lithium in the negative electrode, dendrites of metallic lithium grow on the surface of the negative electrode during charging, which can cause internal short circuits between the electrodes and lead to malfunctions.

[0004] To improve the safety and durability of conventional lithium-ion batteries that use organic electrolytes, all-solid-state lithium-ion batteries using sulfide-based solid electrolytes have been proposed. Examples of sulfide-based solid electrolytes currently proposed include Li2S-P2S5, Li2S-P2S3, Li2S-SiS2, Li2S-Ga2S2, and Li2S-GeS2. In all of these sulfide-based solid electrolytes, lithium sulfide (Li2S) is used as a constituent material.

[0005] Here, as a method for producing lithium sulfide, for example, Patent Document 1 proposes a method in which lithium hydroxide and hydrogen sulfide are reacted in an aprotic organic solvent to produce lithium hydrosulfide, and then the reaction solution is dehydrosulfided to produce lithium sulfide. Patent Document 2 proposes a method in which an aqueous solution containing lithium hydroxide is subjected to microfiltration to obtain purified lithium hydroxide, this purified lithium hydroxide is reacted with hydrogen sulfide in an aprotic solvent while distilling off the water produced, and lithium sulfide is obtained, and this lithium sulfide is washed and dried with an organic solvent.

[0006] Patent Document 3 discloses a method in which lithium hydroxide is reacted with hydrogen sulfide in an aprotic organic solvent to produce lithium hydrosulfide, and lithium sulfide is obtained from this lithium hydrosulfide. Furthermore, Patent Document 4 proposes a method for producing lithium sulfide by reacting lithium sulfate with a carbon material. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 07-330312 [Patent Document 2] WO04 / 106232 publication [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-151725 [Patent Document 4] Patent Publication No. 2021-147251 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the methods disclosed in Patent Documents 1 to 3 require the use of an aprotic organic solvent and the organic solvent used must be treated separately, which results in problems such as complicated production processes and high production costs. In addition, there is a risk that part of the aprotic organic solvent may remain in the produced lithium sulfide.

[0009] Furthermore, in the methods disclosed in Patent Documents 1 to 3, the lithium sulfide obtained has a regular hexahedral shape and a large particle size. When this lithium sulfide is used to produce a sulfide-based solid electrolyte, the contact area between the lithium sulfide and other raw materials is small, so that it takes a long time to synthesize the sulfide-based solid electrolyte, and it is not possible to efficiently produce the sulfide-based solid electrolyte.

[0010] Furthermore, in Patent Document 4, lithium sulfate and a carbon material are granulated to form relatively large granulated powder having a diameter of approximately 0.05 mm and a length of 1 mm, and these are reacted to produce lithium sulfide. This makes it possible to suppress oxidative decomposition due to moisture and oxygen in the air and to produce highly pure lithium sulfide.

[0011] However, since a relatively large granulated powder is used, the size of the obtained lithium sulfide is large. When this lithium sulfide is used to produce a sulfide-based solid electrolyte, the contact area between the lithium sulfide and other raw materials is small, so that it takes a long time to synthesize the sulfide-based solid electrolyte, and it is not possible to efficiently produce a sulfide-based solid electrolyte.

[0012] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide lithium sulfide that has a sufficiently high purity and that can be used to efficiently synthesize a sulfide-based solid electrolyte, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide. [Means for solving the problem]

[0013] In order to solve the above problems, lithium sulfide according to a first aspect of the present invention is characterized in that its volume-based 50% diameter is in the range of 0.1 μm or more and 600 μm or less, and its half-width of the peak at 2θ=27°±0.03° as measured by X-ray diffraction is in the range of 0.10° or more and 0.50° or less.

[0014] According to the lithium sulfide of the first aspect of the present invention, the volume-based 50% diameter is set to be within the range of 0.1 μm or more and 600 μm or less. Therefore, when producing a sulfide-based solid electrolyte, the contact area between the lithium sulfide and other raw materials is ensured, and the sulfide-based solid electrolyte can be efficiently synthesized. Furthermore, the half-width of the peak at 2θ=27°±0.03° measured by X-ray diffraction is within the range of 0.10° or more and 0.50° or less, which means that the amount of impurities other than lithium sulfide is sufficiently reduced. By using this lithium sulfide as a raw material, it is possible to produce a sulfide-based solid electrolyte with stable properties.

[0015] The lithium sulfide of the second aspect of the present invention is the lithium sulfide of the first aspect of the present invention, characterized in that the lithium oxide content measured by X-ray diffraction is 0.15 mass % or less. According to the lithium sulfide of the second aspect of the present invention, the lithium oxide content measured by X-ray diffraction is 0.15 mass% or less, and therefore the content of lithium oxide as an impurity is kept low. By using this lithium sulfide as a raw material, it is possible to produce a sulfide-based solid electrolyte having excellent conductivity.

[0016] The method for producing a sulfide-based solid electrolyte according to the third aspect of the present invention is characterized by using the lithium sulfide according to the first or second aspect of the present invention. According to the method for producing a sulfide-based solid electrolyte of Aspect 3 of the present invention, the amount of impurities is kept low and a high-purity sulfide-based solid electrolyte can be produced because the lithium sulfide of Aspect 1 or Aspect 2 of the present invention is used. Furthermore, the contact area between the lithium sulfide and other raw materials is ensured, allowing the sulfide-based solid electrolyte to be synthesized efficiently. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide lithium sulfide that has a sufficiently high purity and that can be used to efficiently synthesize a sulfide-based solid electrolyte, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a flow chart showing an example of a method for producing lithium sulfide according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] 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.

[0020] The lithium sulfide of this embodiment is used, for example, as a raw material for a sulfide-based solid electrolyte that constitutes a lithium ion battery. The lithium sulfide according to this embodiment has a volume-based 50% diameter in the range of 0.1 μm or more and 600 μm or less, and a half-value width of a peak at 2θ=27°±0.03° measured by X-ray diffraction in the range of 0.10° or more and 0.50° or less. Furthermore, in the lithium sulfide according to this embodiment, the content of lithium oxide measured by X-ray diffraction is preferably 0.15 mass % or less.

[0021] Here, the reasons for specifying the volume-based 50% diameter, lithium oxide, half-width of the peak at 2θ=27°±0.03° measured by X-ray diffraction, and lithium oxide content measured by X-ray diffraction as described above for lithium sulfide according to the present embodiment will be explained.

[0022] (50% diameter by volume) In the lithium sulfide of this embodiment, if the volumetric median diameter is too large, the contact area between the lithium sulfide and other raw materials will be small when producing a sulfide-based solid electrolyte, and it will take a long time to synthesize the sulfide-based solid electrolyte. On the other hand, if the volumetric median diameter of the lithium sulfide is too small, it will be difficult to handle and will be more likely to contain impurities when producing the sulfide-based solid electrolyte, which may reduce the purity of the synthesized sulfide-based solid electrolyte and result in poor properties.

[0023] Therefore, in this embodiment, the volume-based 50% diameter of lithium sulfide is set to be in the range of 0.1 μm or more and 600 μm or less. The lower limit of the 50% volumetric diameter of lithium sulfide is preferably 0.1 μm or more, and more preferably 5 μm or more, while the upper limit of the 50% volumetric diameter of lithium sulfide is preferably 30 μm or less, and more preferably 18 μm or less.

[0024] (FWHM of the peak at 2θ=27°±0.03° measured by X-ray diffraction) In the lithium sulfide of this embodiment, if the amount of impurities is large, the characteristics of the sulfide-based solid electrolyte synthesized using lithium sulfide as a raw material may be deteriorated. Therefore, in the lithium sulfide of this embodiment, the half width of the peak at 2θ=27°±0.03° measured by X-ray diffraction is set to be in the range of 0.10° or more and 0.50° or less. The lower limit of the half-width of the peak at 2θ=27°±0.03° measured by X-ray diffraction is preferably 0.100° or more, more preferably 0.105° or more, while the upper limit of the half-width of the peak at 2θ=27°±0.03° measured by X-ray diffraction is preferably 0.200° or less, more preferably 0.130° or less.

[0025] (Lithium oxide content measured by X-ray diffraction) In the lithium sulfide of this embodiment, by suppressing the content of lithium oxide, which is an impurity, it is possible to improve the conductivity of the sulfide-based solid electrolyte synthesized using this lithium sulfide. Therefore, in this embodiment, it is preferable to limit the lithium oxide content measured by X-ray diffraction to 0.15 mass % or less. The upper limit of the lithium oxide content measured by X-ray diffraction is more preferably 0.15 mass% or less, and even more preferably 0.07 mass% or less. On the other hand, there is no particular restriction on the lower limit of the lithium oxide content measured by X-ray diffraction, and it is most preferably 0 mass%.

[0026] Next, an example of a method for producing lithium sulfide according to this embodiment will be described with reference to FIG.

[0027] (Raw material preparation process S01) First, lithium sulfate and a carbon material are prepared as raw materials. Lithium sulfate may be anhydrous, free of water of crystallization, or may be monohydrate. In the case of lithium sulfate monohydrate, a volume change during heating causes fine cracks to form on the surface of lithium sulfate due to the removal of water of crystallization, increasing the surface area and enhancing reactivity. In this embodiment, it is preferable to use lithium sulfate monohydrate whose weight loss during heating up to 120°C is in the range of 5% to 25%. The carbon material that serves as the reducing agent may be, for example, activated carbon, carbon black, etc. In this embodiment, it is preferable to use activated carbon as the carbon material.

[0028] The mixed powder of lithium sulfate and carbon material was simply mixed and stirred without granulation. Here, the mixing ratio (molar ratio) of lithium sulfate and the carbon material, C / Li2SO4, is preferably in the range of 2 or more and 4 or less. The average particle size (d50) of the lithium sulfate is preferably in the range of 10 μm to 100 μm, and the average particle size (d50) of the carbon material is preferably in the range of 1 μm to 10 μm.

[0029] (Dehydration process S02) The prepared lithium sulfate and carbon material were placed in a vacuum furnace, and the internal pressure of the vacuum furnace was increased to 1×10 2 The vacuum is drawn until the pressure becomes less than 1 Pa. The mixture is then heated to a temperature of 200° C. or higher and 300° C. or lower for a holding time at the heating temperature of 300 minutes or higher and 1200 hours or lower, for dehydration.

[0030] (Synthesis step S03) After dehydration, the material is heated in a vacuum furnace and lithium sulfate is reduced with activated carbon to produce lithium sulfide particles. Here, the heating temperature in the synthesis step S03 is preferably within a range of 600° C. to 900° C. The holding time at the heating temperature is preferably within a range of 500 minutes to 3000 minutes. Furthermore, the rate of temperature rise up to the heating temperature is preferably within the range of 0.5° C. / min to 10° C. / min.

[0031] (Cooling process S04) Next, the mixture is naturally cooled to room temperature in the vacuum furnace, and the resulting lithium sulfide particles are collected. Furthermore, the cooling rate to room temperature is preferably within the range of 1° C. / min to 20° C. / min.

[0032] (Crushing process S05) Next, the obtained lithium sulfide particles are pulverized, and the particle size is controlled so that the volume-based 50% diameter is in the range of 0.1 μm or more and 600 μm or less. There are no particular limitations on the pulverization method, but in this embodiment, lithium sulfide particles are pulverized using a ball mill and then sieved.

[0033] The lithium sulfide of this embodiment is produced by the steps described above. The lithium sulfide of this embodiment is carbon-reduced lithium sulfide obtained by reducing lithium sulfate with carbon, in which the amount of impurities is sufficiently reduced and the particle size is controlled by pulverization.

[0034] In the method for producing a sulfide-based solid electrolyte according to this embodiment, lithium sulfide according to this embodiment is used as a raw material. As described above, the lithium sulfide of the present embodiment has a sufficiently reduced amount of impurities and a controlled particle size. Therefore, the amount of impurities is also reduced in a sulfide-based solid electrolyte produced using this as a raw material, and a sulfide-based solid electrolyte with excellent properties can be efficiently produced.

[0035] According to the lithium sulfide of the present embodiment configured as described above, the volume-based 50% diameter is within the range of 0.1 μm or more and 600 μm or less. Therefore, when producing a sulfide-based solid electrolyte, the contact area between the lithium sulfide and other raw materials is ensured, and the sulfide-based solid electrolyte can be efficiently synthesized. Furthermore, the half-width of the peak at 2θ=27°±0.03° measured by X-ray diffraction is within the range of 0.10° or more and 0.50° or less, which means that the amount of impurities other than lithium sulfide is sufficiently reduced. By using this lithium sulfide as a raw material, it is possible to produce a sulfide-based solid electrolyte with stable properties.

[0036] In the lithium sulfide of this embodiment, when the lithium oxide content measured by X-ray diffraction is 0.15 mass% or less, the content of lithium oxide as an impurity is kept low, and by using this lithium sulfide as a raw material, it is possible to produce a sulfide-based solid electrolyte with excellent conductivity.

[0037] According to the method for producing a sulfide-based solid electrolyte of this embodiment, the lithium sulfide of this embodiment is used as a raw material, so the amount of impurities is kept low, and a high-purity sulfide-based solid electrolyte can be produced. Furthermore, because the particle size of the lithium sulfide is controlled, the contact area between the lithium sulfide and other raw materials is ensured, and the sulfide-based solid electrolyte can be efficiently synthesized.

[0038] 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. In this embodiment, a ball mill is used in the pulverization step S05, but the present invention is not limited to this, and any existing pulverization means may be appropriately selected and applied. [Example]

[0039] A confirmation experiment conducted to confirm the effectiveness of the present invention will be described.

[0040] Lithium sulfate powder (average particle size 20 μm) and activated carbon powder (average particle size 8 μm) were prepared and weighed out to achieve the molar ratio shown in Table 1. The weighed lithium sulfate powder and activated carbon were mixed in a mortar to obtain a mixed powder. The mixed powder was transferred to an alumina crucible and placed in a vacuum furnace equipped with a glove box. The internal pressure of the vacuum furnace was increased to 1×10 2 The mixture was evacuated to a pressure of 20 Pa, and then held at 250°C for 5 hours to carry out a dehydration treatment. Then, the mixture was subjected to a heat treatment under the conditions shown in Table 1, and then allowed to cool naturally to obtain lithium sulfide particles. Next, the lithium sulfide particles were pulverized using a ball mill. 250 g of lithium sulfide particles and 790 g of ZrO2 balls with a diameter of 5 mm were placed in a 500 mL container, and pulverization was carried out for 24 hours. In this manner, lithium sulfide of Inventive Example 1-9 was obtained. Furthermore, as Comparative Example 1-4, a commercially available purchased lithium sulfide was prepared.

[0041] The volume-based 50% diameter of the lithium sulfide, the lithium oxide content measured by X-ray diffraction, and the half-value width of the peak at 2θ=27°±0.03° measured by X-ray diffraction were evaluated as follows.

[0042] (50% diameter by volume) The particle size-controlled lithium sulfide was added to 3 mL of isopropyl alcohol (IPA) and dispersed using a TAITEC ultrasonic homogenizer CP-80R. Then, using a Horiba laser diffraction / scattering particle size analyzer LP-90, the lithium sulfide dispersed in isopropyl alcohol (IPA) was added dropwise while flowing acetonitrile (ACN) as the dispersion solvent. The volume-based 50% diameter was measured under a refractive index of 1.99.

[0043] (Lithium oxide content and half-width of the peak at 2θ=27°±0.03°) Lithium sulfide was pulverized in an agate mortar, and then subjected to powder X-ray diffraction measurement using a Smartlab manufactured by Rigaku Corporation under the conditions of CuKα, 2° / min, and 0.01°. The content of lithium oxide was determined as the intensity ratio of lithium oxide to the intensity of lithium sulfide by the XRD obtained above. Further, the lithium oxide content and the half-width of the peak at 2θ=27°±0.03° were determined using Fullprof.

[0044] Next, the obtained lithium sulfide was used to 10.35 Sn 0.27 Si 1.08 P 1.65 S 12 A sulfide-based solid electrolyte consisting of (LSSPS) was fabricated. In a glove box, lithium sulfide (LiS), tin (Sn), silicon (Si), phosphorus (P), and sulfur (S) were weighed in a non-stoichiometric ratio and mixed in a mortar. The mixture was then fired in an alumina crucible at the temperature shown in Table 1 for 6 hours to obtain a sulfide-based solid electrolyte.

[0045] The obtained sulfide-based solid electrolyte was pulverized and 0.2 g was placed in a SUS conductivity measurement cell, and then AC impedance measurements were carried out in the range of 1 Hz to 7 MHz using a Solartron SP-300 at room temperature (25°C) and with a pressure of 360 MPa applied. The evaluation results are shown in Table 1.

[0046] [Table 1]

[0047] In Comparative Example 1, the volume-based 50% diameter was 1000 μm, and the half-value width of the peak at 2θ=27°±0.03° measured by X-ray diffraction was 0.55°. The conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 5.0×10 -6 S / cm. In Comparative Example 2, the volume-based 50% diameter was 0.04 μm, and the conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 3.2 × 10 -4 S / cm.

[0048] In Comparative Example 3, the volume-based 50% diameter was 1150 μm, and the half-value width of the peak at 2θ=27°±0.03° measured by X-ray diffraction was 0.60°. The conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 1.7×10 -6 S / cm. In Comparative Example 4, the half-width of the peak at 2θ=27°±0.03° measured by X-ray diffraction was 0.05°, and the conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 4.3×10 -3 S / cm.

[0049] As described above, it has been confirmed that the present invention can provide lithium sulfide that has a sufficiently high purity and that can be used to efficiently synthesize a sulfide-based solid electrolyte, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide.

Claims

1. The volume-based 50% diameter is in the range of 0.1 μm or more and 600 μm or less, 1. Lithium sulfide, characterized in that the half-width of a peak at 2θ=27°±0.03° as measured by X-ray diffraction is in the range of 0.10° or more and 0.50° or less.

2. 2. The lithium sulfide according to claim 1, wherein the lithium oxide content measured by X-ray diffraction is 0.15 mass% or less.

3. A method for producing a sulfide-based solid electrolyte, comprising using the lithium sulfide according to claim 1 or 2 as a raw material.

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