Lithium sulfide and method for manufacturing sulfide solid electrolyte

Abstract

The purpose of the present invention is to provide: lithium sulfide that has sufficiently high purity and is particularly suitable as a feedstock for a sulfide solid electrolyte; and a method, for manufacturing a sulfide solid electrolyte, that uses this lithium sulfide.　The present invention pertains to: lithium sulfide characterized in that the L* value (brightness) as defined in the L*a*b* color space is 85 or more; and a method for manufacturing a sulfide solid electrolyte, the method being characterized in that the aforementioned lithium sulfide is used as a feedstock.

Description

Method for producing lithium sulfide and sulfide solid electrolyte

[0001] This invention relates to lithium sulfide suitable as a constituent material of a sulfide solid electrolyte material for, for example, an all-solid-state battery, and a method for producing a sulfide solid electrolyte using this lithium sulfide. This application claims priority based on Japanese Patent Application No. 2023-203663, filed on December 1, 2023, 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 6000, 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, high-purity lithium sulfide (Li ２ S) is used.

[0005] As a method for producing high-purity lithium sulfide, for example, Patent Document 1 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 the lithium hydrosulfide.

[0006] Furthermore, Patent Document 2 discloses a method for obtaining lithium sulfide by repeating a reaction cycle in which metallic lithium is reacted with sulfur gas or hydrogen sulfide to produce lithium sulfide on the surface of metallic lithium, unreacted metallic lithium is then melted, diffused into and permeated into the lithium sulfide that has already been produced, and the unreacted metallic lithium is then reacted again with sulfur gas or hydrogen sulfide.

[0007] Patent Document 3 proposes a method for producing lithium sulfide by reacting lithium carbonate with hydrogen sulfide. Patent Documents 4 and 5 propose methods for producing lithium sulfide by reacting lithium sulfate with a carbon material.

[0008] Japanese Unexamined Patent Publication No. 2006-151725 (A) Japanese Unexamined Patent Publication No. 09-110404 (A) Japanese Unexamined Patent Application No. 2012-221819 (A) Japanese Unexamined Patent Application No. 2013-227180 (A) Japanese Unexamined Patent Application No. 2021-147251 (A)

[0009] However, the invention disclosed in Patent Document 1 requires the use of an aprotic organic solvent and the organic solvent used must be treated separately, which results in a complicated production process and high production costs. In addition, there is a risk that part of the aprotic organic solvent may remain in the produced lithium sulfide.

[0010] The invention disclosed in Patent Document 2 requires repeated reaction of metallic lithium with sulfur gas or hydrogen sulfide multiple times, resulting in a long production time and low production efficiency. Furthermore, metallic lithium is highly reactive and prone to forming an oxide film on its surface, making it difficult to handle the raw material, for example, by requiring handling in an inert gas atmosphere. Furthermore, if the reaction cycle is insufficient, unreacted materials may remain in the resulting lithium sulfide.

[0011] The invention disclosed in Patent Document 3 requires the use of toxic hydrogen sulfide gas, which has the problem of high equipment costs due to the need to maintain the airtightness of the reaction apparatus, treat unreacted hydrogen sulfide gas, etc. Furthermore, if the reaction is insufficient, there is a risk that unreacted substances will remain in the produced lithium sulfide.

[0012] Here, in Patent Documents 4 and 5, there is no need to use an organic solvent or hydrogen sulfide, and handling and management are relatively easy. However, when lithium sulfate is reacted with a carbon material, unreacted carbon remains in the produced lithium sulfide, and there is a risk that high-purity lithium sulfide cannot be obtained.

[0013] In Patent Document 4, lithium sulfate and carbon powder need to be converted into fine particles to improve reactivity, which increases the number of processing steps and raises concerns about impurities being mixed in during the process of converting the particles into fine particles. Furthermore, depending on the reaction, by-products such as lithium carbonate and lithium oxide may be produced, which raises concerns about a decrease in the purity of the lithium sulfide. In Patent Document 5, the lithium sulfate used as a raw material is specified to promote the reaction and reduce unreacted materials, but the carbon remaining in the lithium sulfide could not be sufficiently reduced.

[0014] As described above, conventional manufacturing methods have not been able to obtain high-purity lithium sulfide. Furthermore, when evaluating trace impurities contained in lithium sulfide, highly accurate analysis is required.

[0015] 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 having a sufficiently high purity and being particularly suitable as a raw material for a sulfide solid electrolyte, and a method for producing a sulfide solid electrolyte using this lithium sulfide.

[0016] As a result of intensive studies conducted by the present inventors to solve the above problems, they have found that impurities (mainly carbon) remaining in lithium sulfide can be evaluated by evaluating the color of lithium sulfide.

[0017] The present invention has been made based on the above findings, and the lithium sulfide (lithium sulfide powder) of the first aspect of the present invention is L ＊ a＊ b ＊ L defined in the color space ＊ It is characterized by a value (brightness) of 85 or more.

[0018] According to the lithium sulfide of the first aspect of the present invention, L ＊ a ＊ b ＊ L defined in the color space ＊ Since the value (brightness) is 85 or more, the amount of impurities such as carbon is kept low, the purity is sufficiently high, and it is particularly suitable as a raw material for sulfide solid electrolytes.

[0019] The lithium sulfide of the second aspect of the present invention is the lithium sulfide of the first aspect of the present invention, wherein the L ＊ According to the lithium sulfide of the second aspect of the present invention, the L ＊ a ＊ b ＊ L defined in the color space ＊ Since the value (brightness) is set to 90 or more, the amount of impurities such as carbon is further reduced, and the purity is sufficiently high, making it particularly suitable as a raw material for sulfide solid electrolytes.

[0020] The method for producing a sulfide 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 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 such as carbon in the raw materials is kept low, making it possible to produce a high-purity sulfide solid electrolyte.

[0021] According to the present invention, it is possible to provide lithium sulfide having a sufficiently high purity and being particularly suitable as a raw material for a sulfide solid electrolyte, and a method for producing a sulfide solid electrolyte using this lithium sulfide.

[0022] FIG. 1 is a flow chart showing an example of a method for producing lithium sulfide according to an embodiment of the present invention.

[0023] 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.

[0024] The lithium sulfide according to this embodiment is used, for example, as a raw material for a sulfide solid electrolyte constituting a lithium ion battery. ＊ a ＊ b ＊ L defined in the color space ＊ The value (brightness) is set to 85 or more. ＊ It is preferable that the value (brightness) is 90 or more. ＊ The value (brightness) may be 95 or more. ＊ The value (brightness) may be 100 or less, 98 or less, or 95 or less.

[0025] Here, L ＊ a ＊ b ＊ The color space is a color system that represents the color of an object, and is defined in JIS Z 8781-4. ＊ a ＊ b ＊ In color space, lightness is expressed as L ＊ , chromaticity indicating hue and saturation is a ＊ , b ＊ Chromaticity a ＊ , b ＊ indicates the color direction, and +a ＊ is the red direction, -a ＊ is the green direction, and +b ＊ is yellow direction, -b ＊ indicates blue direction. As these values ​​increase, the color becomes more vivid. ＊ indicates brightness, and L ＊ The larger the value, the whiter (lighter) the color will be, and ＊ The smaller the value of L, the darker the color. ＊By increasing the value (brightness) to 85 or more, the color of the lithium sulfide becomes white, and the content of impurities such as carbon is sufficiently reduced.

[0026] Here, the L of lithium sulfide ＊ a ＊ b ＊ L defined in the color space ＊ The value (brightness) can be measured by a commercially available color difference meter. Usually, when evaluating the carbon content of a substance such as lithium sulfide, a very time-consuming combustion infrared method is performed. However, in the case of lithium sulfide according to the present embodiment, the L value (brightness) can be measured by a color difference meter as described above. ＊ By measuring the value (brightness), it is possible to easily evaluate the carbon content in substances such as lithium sulfide.

[0027] Next, an example of a method for producing lithium sulfide according to this embodiment will be described with reference to FIG. 1.

[0028] (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.

[0029] The mixed powder of lithium sulfate and carbon material was prepared by simply mixing and stirring the lithium sulfate powder and the carbon material powder without granulation. 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.

[0030] (Vacuuming 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 vacuum is drawn stepwise, and the change in internal pressure during each vacuum draw is 1×10 ２ Pa or more 1×10 ３ By drawing the vacuum in stages in this manner, scattering of the raw material powder inside the vacuum furnace is suppressed.

[0031] (Heating step S03) As described above, the internal pressure of the vacuum furnace is set to 1×10 ２ After evacuation to a pressure of 0.2 Pa or less, a heat treatment is performed to reduce lithium sulfate with activated carbon to produce lithium sulfide. Here, the heating temperature in the heating 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 rate of temperature rise to the heating temperature is preferably in the range of 0.5°C / min to 10°C / min.

[0032] (Cooling step S04) Next, the mixture is naturally cooled to room temperature in the vacuum furnace, and the resulting lithium sulfide is recovered. Furthermore, the cooling rate to room temperature is preferably in the range of 1° C. / min to 20° C. / min.

[0033] By the above steps, L ＊ a ＊ b ＊ L defined in the color space ＊ Thus, lithium sulfide having a value (brightness) of 85 or more is produced. That is, the lithium sulfide according to this embodiment is a carbon-reduced lithium sulfide obtained by reducing lithium sulfate with carbon, and although it contains carbon, the amount of impurities such as carbon is sufficiently reduced.

[0034] In the method for producing a sulfide solid electrolyte according to this embodiment, the lithium sulfide according to this embodiment is used as a raw material. Since the lithium sulfide according to this embodiment has a sufficiently reduced amount of impurities such as carbon as described above, the amount of impurities is also reduced in a sulfide solid electrolyte produced using this as a raw material, and a sulfide solid electrolyte with excellent properties is produced.

[0035] According to the lithium sulfide of this embodiment having the above-described configuration, L ＊ a ＊ b ＊ L defined in the color space ＊ Since the value (brightness) is 85 or more, the amount of impurities such as carbon is kept low, the purity is sufficiently high, and it is particularly suitable as a raw material for sulfide solid electrolytes.

[0036] Furthermore, in the lithium sulfide according to this embodiment, L ＊ a ＊ b ＊ L defined in the color space ＊ When the value (brightness) is 90 or more, the amount of impurities such as carbon is further reduced, the purity is sufficiently high, and the material is particularly suitable as a raw material for sulfide solid electrolytes.

[0037] In the method for producing a sulfide solid electrolyte according to the present embodiment, the lithium sulfide according to the present embodiment is used as a raw material, and therefore the amount of impurities such as carbon in the raw material is kept low, making it possible to produce a high-purity sulfide solid electrolyte.

[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.

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

[0040] (Invention Examples 1 to 7, Comparative Example 1) Lithium sulfate powder (average particle size 20 μm) and activated carbon powder (average particle size 8 μm) were prepared and weighed out so as 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 internal pressure is increased to 1×10 Pa. ３ The vacuum was drawn so that the pressure was decreased stepwise by Pa. Then, the mixture was held at 780°C for 12 hours and then naturally cooled, thereby producing lithium sulfide of Invention Examples 1 to 7 and Comparative Example 1.

[0041] (Comparative Example 2) A lithium sulfate powder (average particle size 20 μm) and an activated carbon powder (average particle size 8 μm) were prepared, weighed out so as to have the molar ratio shown in Table 1, mixed, and granulated to obtain mixed particles. The mixed particles were 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 then held at 780° C. for 12 hours and then naturally cooled to produce lithium sulfide of Comparative Example 2.

[0042] The L of lithium sulfide of Examples 1 to 7 of the present invention and Comparative Examples 1 and 2 obtained as described above ＊ The value (brightness) and carbon content were evaluated as follows.

[0043] (L ＊ The obtained lithium sulfide was crushed in an agate mortar, and the crushed lithium sulfide was flattened on a glass plate in a glove box. The lithium sulfide was measured using a portable color difference meter TES-3250 (measurement range: L ＊ = 5 to 100, measurement conditions: CIE 2° standard field of view, light source: white LED, minimum measurement interval: 2 seconds) ＊ The values ​​were measured.

[0044] (Carbon Content) 0.1 g of the obtained lithium sulfide was weighed out, and the O content was measured using a CS analyzer CSLS-600 manufactured by LECO. ２ Complete combustion at 1400°C in an atmosphere ２ The carbon content was determined by measuring the gas.

[0045]

[0046] In Comparative Example 1, L ＊ The value was 61, and the carbon content was as high as 2.1 mass%. This is presumably because the ratio of activated carbon mixed as a raw material was high, and a large amount of unreacted carbon remained. ＊ The value was 70, and the carbon content was as high as 0.4 mass%. This is presumably because the mixed particles of lithium sulfate and activated carbon were heat-treated, and some of the activated carbon did not react sufficiently, leaving unreacted carbon.

[0047] In contrast, in Examples 1 to 7 of the present invention, L ＊ The carbon content was sufficiently reduced, with the value of 85 or more and the carbon content being 0.31 mass% or less. ＊ The carbon content was 0.24 mass% or less, which was even lower. ＊ It is confirmed that the larger the value, the lower the carbon content and the reduced amount of impurities.

[0048] As described above, it has been confirmed that the present invention can provide lithium sulfide of sufficiently high purity that is particularly suitable as a raw material for a sulfide solid electrolyte, and a method for producing a sulfide solid electrolyte using this lithium sulfide.

[0049] According to the present invention, it is possible to provide lithium sulfide that is sufficiently pure and particularly suitable as a raw material for a sulfide solid electrolyte, and a method for producing a sulfide solid electrolyte using this lithium sulfide.

Claims

1. L ＊ a ＊ b ＊ L defined in color space ＊ Lithium sulfide having a brightness value of 85 or more.

2. Said L ＊ 2. The lithium sulfide according to claim 1, characterized in that the value (brightness) is 90 or more.

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

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