Lithium sulfide and method for manufacturing sulfide solid electrolyte

Abstract

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.SOLUTION: A lithium sulfide according to the present invention has an Lvalue (brightness) of 85 or more as defined in the L*a*b* color space. Preferably, the Lvalue is 90 or more. A method for manufacturing a sulfide solid electrolyte according to the present invention employs the lithium sulfide as a feedstock.SELECTED DRAWING: None

Description

Technical Field

[0001] This invention relates to lithium sulfide suitable as a constituent material of a sulfide solid electrolyte material for, for example, all-solid-state batteries, and a method for producing a sulfide solid electrolyte using this lithium sulfide.

Background Art

[0002] Lithium-ion batteries are widely used as power sources, from vehicles such as EVs (electric vehicles) and HEVs (hybrid electric vehicles) to electronic devices such as mobile phones and notebook computers. Conventional lithium-ion batteries use an organic electrolyte solution in which a lithium salt such as lithium hexafluorophosphate (LiPF 6 ) is dissolved in an organic solvent.

[0003] Such organic electrolyte solutions are flammable and may be damaged by excessive temperature rise or impact. In addition, in a lithium-ion battery using metallic lithium as the negative electrode, dendrite-like metallic lithium may grow on the surface of the negative electrode during charging, which may cause an internal short circuit between the electrodes and lead to problems.

[0004] In order to improve the safety and durability of conventional lithium-ion batteries using such organic electrolyte solutions, 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 systems, Li 2 S-P 2 S 3 systems, Li 2 S-SiS 2 systems, Li 2 S-Ga 2 S 2 systems, Li 2 S-GeS 2 systems, etc. In any of these sulfide-based solid electrolytes, high-purity lithium sulfide (Li 2 S) is used as a constituent material.

[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 this lithium hydrosulfide.

[0006] Further, Patent Document 2 discloses a method in which lithium metal is reacted with sulfur gas or hydrogen sulfide to form lithium sulfide on the surface of the lithium metal, then the unreacted lithium metal is melted and diffused and penetrated into the already formed lithium sulfide, and then the reaction cycle of reacting the unreacted lithium metal with sulfur gas or hydrogen sulfide again is repeated to obtain lithium sulfide.

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

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0009] Incidentally, in Patent Document 1, in the invention disclosed in Patent Document 1, it is necessary to use an aprotic organic solvent, and since the treatment of the used organic solvent is separately required, there is a problem that the manufacturing process is complicated and the manufacturing cost is high. In addition, there is a possibility that a part of the aprotic organic solvent remains in the generated lithium sulfide.

[0010] In the invention disclosed in Patent Document 2, it is necessary to react metallic lithium with sulfur gas or hydrogen sulfide a plurality of times repeatedly, and there is a problem that the manufacturing time becomes long and the manufacturing efficiency is low. In addition, since metallic lithium has high reactivity and an oxide film is likely to be generated on the surface, it is difficult to handle raw materials such as being required to handle in an inert gas atmosphere. Further, when the reaction cycle is insufficient, there is a possibility that unreacted substances remain in the generated lithium sulfide.

[0011] In the invention disclosed in Patent Document 3, it is necessary to use toxic hydrogen sulfide gas, and there is a problem that the equipment cost becomes high, such as management of the airtightness of the reaction apparatus and treatment of unreacted hydrogen sulfide gas. In addition, when the reaction is insufficient, there is a possibility that unreacted substances remain in the generated lithium sulfide.

[0012] Here, in Patent Documents 4 and 5, it is not necessary to use an organic solvent or hydrogen sulfide, and handling and management are relatively easy. However, when lithium sulfate and a carbon material are reacted, unreacted carbon may remain in the produced lithium sulfide, and there is a possibility that high-purity lithium sulfide cannot be obtained.

[0013] In addition, in Patent Document 4, in order to improve the reactivity, it is necessary to make lithium sulfate and carbon powder into fine particles, the number of treatment steps increases, and there is a concern that impurities are mixed in the process of making them into fine particles. Further, depending on the reaction, by-products such as lithium carbonate and lithium oxide are generated, and there is a concern that the purity of lithium sulfide decreases. In Patent Document 5, by defining lithium sulfate as a raw material, the reaction is promoted and the reduction of unreacted substances is attempted, but it has not been possible to sufficiently reduce the carbon remaining in lithium sulfide.

[0014] As described above, with the conventional manufacturing method, it has not been possible to obtain lithium sulfide with high purity. In addition, when evaluating trace impurities contained in lithium sulfide, it has been necessary to perform highly accurate analysis.

[0015] This invention has been made in view of the above-described circumstances, and an object thereof 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 manufacturing a sulfide solid electrolyte using this lithium sulfide.

Means for Solving the Problems

[0016] As a result of intensive studies by the inventors to solve the above problems, it has been found that by evaluating the color tone of lithium sulfide, it is possible to evaluate impurities (mainly carbon) remaining in lithium sulfide.

[0017] This invention has been made based on the above findings. The lithium sulfide of Aspect 1 of this invention is L * a * b * It is characterized in that the L* value (lightness) defined in the color space is 85 or more.

[0018] According to the lithium sulfide of Aspect 1 of this invention, L * a * b * Since the L * value (lightness) defined in the color space is 85 or more, the amount of impurities such as carbon is suppressed to be small, and the purity is sufficiently high and it is particularly suitable as a raw material for a sulfide solid electrolyte.

[0019] The lithium sulfide of Aspect 2 of this invention is characterized in that in the lithium sulfide of Aspect 1 of this invention, the L* value (lightness) is 90 or more. According to the lithium sulfide of Embodiment 2 of the present invention, L * a * b * Since the L* value (lightness) defined in the color space is 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 Embodiment 3 of the present invention is characterized by using the lithium sulfide of Embodiment 1 or Embodiment 2 of the present invention. According to the method for producing a sulfide solid electrolyte according to Embodiment 3 of the present invention, since the lithium sulfide of Embodiment 1 or Embodiment 2 of the present invention is used, the amount of impurities such as carbon in the raw material is reduced, and a high-purity sulfide solid electrolyte can be produced.

Advantages of the Invention

[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 sulfide solid electrolytes, and a method for producing a sulfide solid electrolyte using this lithium sulfide.

Brief Description of the Drawings

[0022]

Figure 1

Modes for Carrying Out the Invention

[0023] Hereinafter, embodiments of the present invention will be described with reference to the attached drawings. Note that each of the embodiments shown below is specifically described to better understand the gist of the invention, and does 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. And the lithium sulfide according to this embodiment is L * a * b* The L value (lightness) defined in the color space * is 85 or more. Note that the L * value (lightness) is preferably 90 or more.

[0025] Here, the L * a * b * color space is a color system representing the color of an object and is defined by JIS Z 8781-4. The L * a * b * In the color space, the lightness is represented by L * , and the chromaticity indicating the hue and saturation is represented by a * ,b * . The chromaticity a * ,b * indicates the direction of the color. +a * is the red direction, -a * is the green direction, and +b * is the yellow direction, -b * is the blue direction. As these numerical values increase, the color becomes more vivid. The lightness L * indicates brightness. When the value of L * is large, the color tone becomes white (pale), and when the value of L * is small, the color tone becomes black (dark). That is, by making the L * value (lightness) 85 or more and large, the color tone of lithium sulfide has become white, indicating that the content of impurities such as carbon is sufficiently suppressed.

[0026] Here, the L * a * b * value (lightness) defined in the color space of lithium sulfide * can be measured by a commercially available color difference meter. Note that usually, when evaluating the carbon content contained in substances such as lithium sulfide, it is necessary to perform a very time-consuming combustion infrared method. However, in the lithium sulfide according to this embodiment, as described above, the L *By measuring the value (brightness), it becomes possible to simply evaluate the carbon content contained in substances such as lithium sulfide.

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

[0028] (Raw material preparation step S01) First, prepare lithium sulfate and a carbon material as raw materials. Lithium sulfate may be anhydrous without crystal water or monohydrate. If it is lithium sulfate monohydrate, due to the volume change during heating and temperature rise, fine cracks due to the detachment of crystal water will occur on the surface of lithium sulfate, increasing the surface area and enhancing the reactivity. In the present embodiment, it is preferable to use lithium sulfate monohydrate in which the weight loss during heating up to 120 °C is in the range of 5% or more and 25% or less. In addition, as the carbon material serving as a reducing agent, for example, activated carbon, carbon black, etc. can be used. In the present embodiment, it is preferable to use activated carbon as the carbon material.

[0029] Regarding the mixed powder of lithium sulfate and the carbon material, the powder of lithium sulfate and the powder of the carbon material were simply mixed and stirred without granulation. Here, the average particle size (d50) of lithium sulfate is preferably in the range of 10 μm or more and 100 μm or less. Also, the average particle size (d50) of the carbon material is preferably in the range of 1 μm or more and 10 μm or less.

[0030] (Vacuum evacuation step S02) Put the prepared lithium sulfate and carbon material into the furnace of the vacuum furnace, and evacuate the internal pressure of the vacuum furnace until it becomes 1×10 2 Pa or less. The vacuum evacuation is carried out step by step, and the change in the internal pressure in one vacuum evacuation is set in the range of 1×10 2 Pa or more and 1×10 3 Pa or less. By evacuating step by step in this way, the scattering of the raw material powder in the vacuum furnace is suppressed.

[0031] (Heating Step S03) As described above, after evacuating the inside of the vacuum furnace until the internal pressure becomes 1×10 2 Pa or less, 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 or higher and 900°C or lower. Also, the holding time at the heating temperature is preferably in the range of 500 minutes or longer and 3000 minutes or shorter. Furthermore, the heating rate to the heating temperature is preferably in the range of 0.5°C / min or higher and 10°C / min or lower.

[0032] (Cooling Step S04) Next, it is naturally cooled to room temperature inside the vacuum furnace, and the produced lithium sulfide is recovered. Furthermore, the cooling rate to room temperature is preferably in the range of 1°C / min or higher and 20°C / min or lower.

[0033] Through the above steps, lithium sulfide with an L ※ a ※ b ※ value (brightness) defined in the color space of 85 or higher is produced. That is, the lithium sulfide according to this embodiment is carbon-reduced lithium sulfide obtained by reducing lithium sulfate with carbon and contains carbon, but the amount of impurities such as carbon is sufficiently reduced. ※ In the method for manufacturing 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 in the sulfide solid electrolyte produced using this as a raw material is also reduced, and a sulfide solid electrolyte with excellent characteristics is produced.

[0034] According to the lithium sulfide of this embodiment configured as described above, in the L

[0035] a * a * b * color space, a lithium sulfide with an L *Since the value (brightness) is 85 or more, the amount of impurities such as carbon is suppressed to be small, and the purity is sufficiently high, which is particularly suitable as a raw material for the sulfide solid electrolyte.

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

[0037] In the method for producing a sulfide solid electrolyte according to the present embodiment, since the lithium sulfide according to the present embodiment is used as a raw material, the amount of impurities such as carbon in the raw material is suppressed to be small, and a high-purity sulfide solid electrolyte can be produced.

[0038] As described above, one embodiment of the present invention has been described. However, the present invention is not limited thereto, and can be appropriately changed without departing from the technical idea of the invention.

Examples

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

[0040] (Examples 1 to 7 of the present invention, Comparative Example 1) Powder of lithium sulfate (average particle size 20 μm) and powder of activated carbon (average particle size 8 μm) were prepared and weighed so as to have the molar ratios shown in Table 1. The weighed powder of lithium sulfate and activated carbon were mixed in a mortar to obtain a mixed powder. This mixed powder was transferred to an alumina crucible and loaded into the furnace of a vacuum furnace equipped with a glove box. The vacuum was drawn so that the internal pressure of the vacuum furnace decreased step by step to 1×10 2 Pa, and then to 1×10 3 Pa. After holding at 780 °C for 12 hours and then naturally cooling, lithium sulfides of Examples 1 to 7 of the present invention and Comparative Example 1 were produced.

[0041] (Comparative Example 2) Powder of lithium sulfate (average particle size: 20 μm) and powder of activated carbon (average particle size: 8 μm) were prepared, weighed so as to have the molar ratios shown in Table 1, mixed and granulated to obtain mixed granules. These mixed granules were transferred to an alumina crucible and loaded into the furnace of a vacuum furnace with a glove box. The inside pressure of the vacuum furnace was reduced to 1×10 2 Pa in one step. Then, after holding at 780 °C for 12 hours and allowing natural cooling, lithium sulfide of Comparative Example 2 was produced.

[0042] Regarding the L * value (brightness) and carbon content of lithium sulfide of Examples 1 to 7 and Comparative Examples 1 and 2 obtained as described above, evaluation was carried out as follows.

[0043] (L * value) The obtained lithium sulfide was pulverized in an agate mortar, and the pulverized lithium sulfide was flattened on a glass plate in a glove box. Using a portable color difference meter TES-3250 manufactured by Nippon Denshoku Industries Co., Ltd. (measurement range: L * = 5 to 100, measurement conditions: CIE2° standard viewing field, light source: white LED, minimum interval of measurement interval: 2 seconds), the L * value of the color coordinates was measured.

[0044] (Carbon content) 0.1 g of the obtained lithium sulfide was weighed, and using a CS analyzer CSLS-600 manufactured by LECO, it was completely combusted at 1400 °C in an O 2 atmosphere, and the generated CO 2 gas was measured to determine the carbon content.

[0045]

Table 1

[0046] In Comparative Example 1, L *The value was 61, and the carbon content was as high as 2.1 mass%. It is presumed that this was because the mixing ratio of the activated carbon mixed as a raw material was high and a large amount of unreacted carbon remained. In Comparative Example 2, L * The value was 70, and the carbon content was as high as 0.4 mass%. It is presumed that this was because, after heat-treating the mixed grains of lithium sulfate and activated carbon, some of the activated carbon did not react sufficiently and unreacted carbon remained.

[0047] In contrast, in Invention Examples 1 to 7, L * The value was 85 or more, the carbon content was 0.31 mass% or less, and the carbon content was sufficiently reduced. Also, in Invention Examples 1 to 6, L * The value was 90 or more, and the carbon content was 0.24 mass% or less, and the carbon content was even lower. Comparing Invention Examples 1 to 7, L * It was confirmed that the larger the value, the lower the carbon content and the lower the amount of impurities.

[0048] As described above, according to the present invention, it was confirmed that it is possible to provide lithium sulfide having a sufficiently high purity 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 the color space ＊ Lithium sulfide characterized in that the value (brightness) is 85 or more.

2. The above-mentioned L ＊ 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, characterized by using lithium sulfide according to Claim 1 or Claim 2 as a raw material.

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