Lithium sulfide and sulfide solid electrolyte production method

Abstract

This lithium sulfide is characterized in that the 50% diameter on a volume basis is in the range of 0.1-600 μm, inclusive, and the half-value width of a peak at 2θ= 27°±0.03° as measured using X-ray diffraction is in the range of 0.10-0.50°, inclusive. This sulfide solid electrolyte production method is characterized by using said lithium sulfide as a starting material.

Description

Method for producing lithium sulfide and sulfide-based solid electrolyte

[0001] This 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. This application claims priority based on Japanese Patent Application No. 2024-070606, filed on April 24, 2024, the contents of which are incorporated herein by reference.

[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 contain lithium hexafluorophosphate (LiPF) in an organic solvent as an electrolyte. ６ An organic electrolyte solution containing a lithium salt such as CI, CI, or CI, is used.

[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] In order to improve the safety and durability of conventional lithium ion batteries using such organic electrolytes, all-solid-state lithium ion batteries using sulfide-based solid electrolytes have been proposed. Examples of currently proposed sulfide-based solid electrolytes include Li ２ S-P ２ S ５ system, Li ２ S-P ２ S ３ system, Li ２ S-SiS ２ system, Li ２ S-Ga ２ S ２ system, Li ２ S-GeS ２ In any of these sulfide-based solid electrolytes, lithium sulfide (Li ２ S) is used.

[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 microfiltrated to obtain purified lithium hydroxide, and this purified lithium hydroxide and hydrogen sulfide are reacted in an aprotic solvent while distilling off the produced water to obtain lithium sulfide, and this lithium sulfide is washed and dried with an organic solvent.

[0006] Patent Document 3 discloses a method of producing lithium sulfide from lithium hydrosulfide by reacting lithium hydroxide with hydrogen sulfide in an aprotic organic solvent, and Patent Document 4 proposes a method of producing lithium sulfide by reacting lithium sulfate with a carbon material.

[0007] Japanese Patent Application Publication No. 07-330312 (A) WO04 / 106232 Publication (A) Japanese Patent Application Publication No. 2006-151725 (A) Japanese Patent Application Publication No. 2021-147251 (A)

[0008] However, the methods disclosed in Patent Documents 1 to 3 require the use of an aprotic organic solvent and additional treatment of the used organic solvent, which results in problems such as complicated production steps 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 about 0.05 mm and a length of 1 mm, and these granulated powders 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 lithium sulfide with high purity.

[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.

[0013] In order to solve the above problems, lithium sulfide according to aspect 1 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-value 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.

[0014] According to the lithium sulfide of Aspect 1 of the present invention, the 50% volumetric diameter is within the range of 0.1 μm to 600 μm, which ensures a sufficient contact area between the lithium sulfide and other raw materials when producing a sulfide-based solid electrolyte, thereby enabling efficient synthesis of the sulfide-based solid electrolyte. Furthermore, the half-width of the peak at 2θ = 27° ± 0.03° measured by X-ray diffraction is within the range of 0.10° to 0.50°, which sufficiently reduces the amount of impurities other than lithium sulfide. Therefore, by using this lithium sulfide as a raw material, it becomes possible to produce a sulfide-based solid electrolyte with stable properties.

[0015] The lithium sulfide of Aspect 2 of the present invention is characterized in that the lithium sulfide of Aspect 1 of the present invention has a lithium oxide content of 0.15 mass% or less as measured by X-ray diffraction. Since the lithium sulfide of Aspect 2 of the present invention has a lithium oxide content of 0.15 mass% or less as measured by X-ray diffraction, 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 having excellent conductivity.

[0016] The method for producing a sulfide-based solid electrolyte according to Aspect 3 of the present invention is characterized by using the lithium sulfide according to Aspect 1 or Aspect 2 of the present invention. According to the method for producing a sulfide-based solid electrolyte according to Aspect 3 of the present invention, the lithium sulfide according to Aspect 1 or Aspect 2 of the present invention is used, and therefore the amount of impurities is kept low, making it possible to produce a high-purity sulfide-based solid electrolyte. Furthermore, the contact area between the lithium sulfide and other raw materials is ensured, making it possible to efficiently synthesize the sulfide-based solid electrolyte.

[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.

[0018] FIG. 1 is a flow chart showing an example of a method for producing lithium sulfide according to an embodiment of the present 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 (lithium sulfide powder, powder containing lithium sulfide as a main component) according to this embodiment is used, for example, as a raw material for sulfide-based solid electrolytes constituting lithium-ion batteries. The lithium sulfide according to this embodiment has a volume-based 50% diameter in the range of 0.1 μm to 600 μm, and a half-width of a peak at 2θ = 27° ± 0.03° measured by X-ray diffraction in the range of 0.10° to 0.50°. Furthermore, the lithium sulfide according to this embodiment preferably has a lithium oxide content of 0.15 mass% or less as measured by X-ray diffraction. Although not particularly limited, the lithium sulfide content in the lithium sulfide powder is preferably 99 mass% or more, more preferably 99.5 mass% or more, and may be 99.9 mass% or more.

[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 this embodiment will be explained.

[0022] (Volume-based 50% diameter) In the lithium sulfide of this embodiment, if the volume-based 50% 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 volume-based 50% diameter of the lithium sulfide is too large or small, it will be difficult to handle and will be more likely to contain impurities when producing a 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 volume-based 50% diameter of lithium sulfide is preferably 0.1 μm or more, and more preferably 5 μm or more. On the other hand, the upper limit of the volume-based 50% diameter of lithium sulfide is preferably 30 μm or less, and more preferably 18 μm or less.

[0024] (Half-width 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 properties of the sulfide-based solid electrolyte synthesized using lithium sulfide as a raw material may be degraded. 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° to 0.50°. Note that 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, and more preferably 0.105° or more. Meanwhile, 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, and 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 as an impurity, it is possible to improve the conductivity of a 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. 1.

[0027] (Raw Material Preparation Step S01) First, lithium sulfate and a carbon material are prepared as raw materials. The lithium sulfate may be anhydrous, having no 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 the 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%. In addition, the carbon material used as the reducing agent may be, for example, activated carbon or carbon black. In this embodiment, it is preferable to use activated carbon as the carbon material.

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

[0029] (Dehydration step S02) The prepared lithium sulfate and carbon material are placed in a vacuum furnace, and the internal pressure of the vacuum furnace is increased to 1×10 ２ The mixture is then heated at a temperature of 200° C. to 300° C. for a holding time of 300 minutes to 1200 hours, for dehydration.

[0030] (Synthesis Step S03) After the dehydration treatment, a heating treatment is performed 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 in the range of 600°C to 900°C. The holding time at the heating temperature is preferably in the range of 500 minutes to 3000 minutes. Furthermore, the heating rate to the heating temperature is preferably in the range of 0.5°C / min to 10°C / min.

[0031] (Cooling step 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 in the range of 1° C. / min to 20° C. / min.

[0032] (Pulverization step S05) Next, the obtained lithium sulfide particles are pulverized, and particle size control is performed so that the volume-based 50% diameter is in the range of 0.1 μm to 600 μm. There are no particular limitations on the pulverization method, but in this embodiment, the 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, the lithium sulfide according to this embodiment is used as a raw material. As described above, the lithium sulfide according to this embodiment has a sufficiently reduced amount of impurities and a controlled particle size, and 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] The lithium sulfide of this embodiment configured as described above has a volume-based 50% diameter in the range of 0.1 μm to 600 μm, which ensures a sufficient contact area between the lithium sulfide and other raw materials when producing a sulfide-based solid electrolyte, allowing for efficient synthesis of the sulfide-based solid electrolyte. Furthermore, the half-width of the peak at 2θ = 27° ± 0.03° measured by X-ray diffraction is in the range of 0.10° to 0.50°, which sufficiently reduces the amount of impurities other than lithium sulfide. Using this lithium sulfide as a raw material makes it possible to produce a sulfide-based solid electrolyte with stable properties.

[0036] In the lithium sulfide of the present 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 becomes 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 thereto and can be modified as appropriate within the scope of the technical concept of the invention. In this embodiment, a ball mill is used in the pulverization step S05, but the present invention is not limited thereto and any existing pulverization means may be appropriately selected and applied.

[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 to achieve the molar ratios shown in Table 1. The weighed lithium sulfate powder and activated carbon were mixed in a mortar to obtain a mixed powder. This 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 adjusted to 1×10 ２ The mixture was evacuated to a pressure of 100 Pa, and then held at 250°C for 5 hours for dehydration. Then, a heat treatment was carried out under the conditions shown in Table 1, followed by natural cooling to obtain lithium sulfide particles. The lithium sulfide particles were then pulverized using a ball mill. 250 g of lithium sulfide particles and 5 mm diameter ZrO ２790 g of balls were charged, and pulverization was carried out for 24 hours. In this manner, lithium sulfide of Inventive Example 1-9 was obtained. Also, a commercially available lithium sulfide product was prepared as Comparative Example 1-4.

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

[0043] (Lithium Oxide Content and Half-Width of 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 conditions of CuKα, 2° / min, and 0.01°. The lithium oxide content was determined from the XRD obtained above as the intensity ratio of lithium oxide to the intensity of lithium sulfide. Furthermore, 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 Li １０．３５ Sn ０．２７ Si １．０８ P １．６５ S １２ A sulfide-based solid electrolyte consisting of lithium sulfide (LiS) was prepared in a glove box. ２ Sulfide-based solid electrolytes were obtained by weighing out tin (S), tin (Sn), silicon (Si), phosphorus (P), and sulfur (S) in a non-stoichiometric ratio and mixing them in a mortar. The mixture was then fired in an alumina crucible at the temperature shown in Table 1 for 6 hours.

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

[0046]

[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 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 The viscosity was 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 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 The viscosity was 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.

[0050] 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.

Claims

1. Lithium sulfide having a volume-based 50% diameter in the range of 0.1 μm to 600 μm, and a half-width of the peak at 2θ = 27° ± 0.03° measured by X-ray diffraction in the range of 0.10° to 0.50°.

2. The lithium sulfide according to claim 1, characterized in that 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, characterized in that the lithium sulfide according to claim 1 or 2 is used as a raw material.

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