Lithium sulfide and method for producing sulfide-based solid electrolyte

Abstract

This lithium sulfide is characterized by having a carbon content of 2.0 mass% or less and an oxygen content of 5.0 mass% or less. Impurities contained in the lithium sulfide are preferably one or more impurities selected from among carbon, lithium sulfate, lithium carbonate, and lithium oxide. This method for producing a sulfide-based solid electrolyte is characterized by using said lithium sulfide as a raw 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. 2024-120863, filed on July 26, 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 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, lithium sulfide (Li ２ S) is used.

[0005] As a method for producing 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 particles are obtained from the lithium hydrosulfide. In Patent Document 1, the sulfur oxide content of the produced lithium sulfide particles, SiO ２ The contents of Al and Ca are specified.

[0006] Patent Document 2 discloses a method for obtaining lithium sulfide particles by reacting lithium hydroxide with hydrogen sulfide. In Patent Document 2, the produced lithium sulfide particles have layered cracks on the surface and an average particle diameter d ５０ The thickness is set to 0.1 mm or more and 1.5 mm or less, and the oxygen concentration in the range from the surface to a depth of 5 nm is set to 20.0 atom % or less.

[0007] Patent Document 3 discloses a method for obtaining lithium sulfide powder by reacting lithium hydroxide with hydrogen sulfide. In Patent Document 3, the produced lithium sulfide powder has a solvent content of 0.1% by mass or less, a lithium hydroxide content of 0.3% by mass or less, and a lithium sulfide content of 98.0% by mass or more.

[0008] Furthermore, Patent Document 4 discloses a method for producing lithium sulfide by mixing lithium sulfate and a carbon material and heating the mixture. In this method, it is also disclosed that both the lithium sulfate and the carbon material are made into fine particles to increase the reaction area and thereby reduce the amount of unreacted raw materials.

[0009] Furthermore, Patent Document 5 discloses a method for obtaining lithium sulfide by thermally reducing lithium sulfate with a carbon material in a vacuum atmosphere.

[0010] Japanese Unexamined Patent Publication No. 2006-151725 (A) Japanese Unexamined Patent Application No. 2019-147731 (A) Japanese Unexamined Patent Application No. 2023-116632 (A) Japanese Unexamined Patent Application No. 2013-227180 (A) Japanese Unexamined Patent Application No. 2021-147251 (A)

[0011] However, the method 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 problems such as 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.

[0012] Furthermore, the inventions disclosed in Patent Documents 2 and 3 require 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.

[0013] In contrast to this, in Patent Documents 4 and 5, lithium sulfide is produced by reducing lithium sulfate using a carbon material, so there is no need to use an organic solvent or hydrogen sulfide, and handling and management are relatively easy.

[0014] Recently, there has been a demand for sulfide-based solid electrolytes that are used in high-power batteries and have even better ionic conductivity. Therefore, there is a demand for further improvement in the purity of lithium sulfide, which is a raw material for sulfide-based solid electrolytes. Here, as in Patent Documents 4 and 5, in lithium sulfide produced by reducing lithium sulfate with a carbon material, there is a demand for lithium sulfide containing specific impurities that are restricted in order to improve the ionic conductivity of the sulfide-based solid electrolyte.

[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 that can be used to synthesize a sulfide-based solid electrolyte having excellent ion conductivity, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide.

[0016] In order to solve the above problems, lithium sulfide according to embodiment 1 of the present invention is characterized in that the carbon content is 2.0% by mass or less and the oxygen content is 5.0% by mass or less.

[0017] According to the lithium sulfide of Aspect 1 of the present invention, the carbon content is 2.0 mass % or less and the oxygen content is 5.0 mass % or less, so that it is possible to sufficiently reduce the amount of impurities contained in a sulfide-based solid electrolyte produced using this lithium sulfide as a raw material, and therefore it is possible to improve the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0018] The lithium sulfide of Aspect 2 of the present invention is characterized in that, in the lithium sulfide of Aspect 1 of the present invention, the contained impurities are one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide.

[0019] According to the lithium sulfide of Aspect 2 of the present invention, the impurities contained therein are one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide, and the contents of these impurities are limited so that the carbon content is 2.0% by mass or less and the oxygen content is 5.0% by mass or less. This makes it possible to further sufficiently reduce the amount of impurities contained in a sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0020] 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.

[0021] According to the method for producing a sulfide-based solid electrolyte of Aspect 3 of the present invention, the lithium sulfide of 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 sulfide-based solid electrolyte that is high in purity and has excellent ionic conductivity.

[0022] According to the present invention, it is possible to provide lithium sulfide that can be used to synthesize a sulfide-based solid electrolyte having excellent ion conductivity, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide.

[0023] FIG. 1 is a flow chart showing an example of a method for producing lithium sulfide according to an embodiment of the present invention.

[0024] 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.

[0025] The lithium sulfide of this embodiment (lithium sulfide powder, powder containing lithium sulfide as a main component) is used, for example, as a raw material for a sulfide-based solid electrolyte constituting a lithium ion battery. The lithium sulfide of this embodiment has a carbon content of 2.0 mass% or less and an oxygen content of 5.0 mass% or less. Furthermore, in the lithium sulfide of this embodiment, it is preferable that the impurities contained therein are one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide. 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.

[0026] Here, the lithium sulfide of this embodiment is produced by reducing lithium sulfate with a carbon material, as will be described later. ２ SO ４ +2C → Li ２ S + 2CO ２ At this time, impurities mixed into lithium sulfide include unreacted carbon (C) and lithium sulfate (Li ２ SO ４ ), by-product lithium carbonate (Li ２ CO ３ ), lithium oxide (Li ２ In order to control the amount of these impurities mixed in, the carbon content and oxygen content are specified in this embodiment.

[0027] That is, in the lithium sulfide according to this embodiment, by limiting the carbon content to 2.0 mass% or less, the unreacted carbon (C) and the by-product lithium carbonate (Li ２ CO ３In addition, in the lithium sulfide according to this embodiment, by limiting the oxygen content to 5.0 mass % or less, the unreacted lithium sulfate (Li ２ SO ４ ) and by-product lithium carbonate (Li ２ CO ３ ), lithium oxide (Li ２ O) is prevented from being mixed in.

[0028] Here, in the lithium sulfide of this embodiment, the carbon content is more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less. There is no particular restriction on the lower limit of the carbon content, and 0% by mass is most preferable. Furthermore, in the lithium sulfide of this embodiment, the oxygen content is more preferably 2.0% by mass or less, and even more preferably 0.8% by mass or less. There is no particular restriction on the lower limit of the oxygen content, and 0% by mass is most preferable.

[0029] Next, an example of a method for producing lithium sulfide according to this embodiment will be described with reference to FIG. 1.

[0030] (Raw Material Preparation Step S01) First, lithium sulfate and a carbon material are prepared as raw materials. The lithium sulfate may be anhydrous, i.e., without 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 elimination 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.

[0031] 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 ４is 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.

[0032] (Drying 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 until the internal pressure of the vacuum furnace reaches 1×10 Pa or less. Then, an inert gas is introduced to return the pressure to normal pressure, and the vacuum is drawn again to reduce the internal pressure of the vacuum furnace to 1×10 Pa or less. ２ This process is repeated until the pressure is equal to or lower than 100° C. The drying process is then performed by heating the substrate at a temperature of 100° C. to 250° C. for a holding time at the heating temperature of 10 hours to 30 hours.

[0033] (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. Here, the heating temperature in the synthesis step S03 is preferably in the range of 700°C to 950°C. The holding time at the heating temperature is preferably in the range of 500 minutes to 3000 minutes. Furthermore, the average heating rate from room temperature (25°C) to the heating temperature is preferably in the range of 2°C / min to 10°C / min.

[0034] (Cooling step S04) Next, the mixture is naturally cooled to room temperature in the vacuum furnace, and the lithium sulfide produced in an inert gas atmosphere is recovered. Here, the average cooling rate from the heating temperature to room temperature (25°C) is preferably in the range of 1°C / min to 20°C / min.

[0035] 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, and the amount of impurities is sufficiently reduced. Furthermore, since evacuation is performed multiple times in the drying step S02, it is possible to sufficiently reduce the carbon content and oxygen content.

[0036] 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. The lithium sulfide according to this embodiment has a carbon content of 2.0 mass % or less and an oxygen content of 5.0 mass % or less, and the amount of impurities is sufficiently reduced. 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 is produced.

[0037] The lithium sulfide of this embodiment configured as described above has a carbon content of 2.0 mass % or less and an oxygen content of 5.0 mass % or less, which makes it possible to sufficiently reduce the amount of impurities contained in a sulfide-based solid electrolyte produced using this lithium sulfide as a raw material, thereby improving the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0038] In the lithium sulfide of the present embodiment, when the impurities contained therein are one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide, the contents of these impurities contained when lithium sulfide is produced by reducing lithium sulfate with a carbon material are limited so that the carbon content is 2.0% by mass or less and the oxygen content is 5.0% by mass or less, and it becomes possible to further sufficiently reduce the amount of impurities contained in a sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0039] According to the method for producing a sulfide-based solid electrolyte of the present embodiment, the amount of impurities is kept low because lithium sulfide of the present embodiment is used, and therefore a sulfide-based solid electrolyte with high purity and excellent ionic conductivity can be produced.

[0040] Although one embodiment of the present invention has been described above, the present invention is not limited thereto and can be appropriately modified within the scope of the technical concept of the invention. In this embodiment, lithium sulfate monohydrate is used as the lithium sulfate, but the present invention is not limited thereto and an anhydrous lithium sulfate having no water of crystallization may also be used.

[0041] A confirmation experiment conducted to confirm the effectiveness of the present invention will be described.

[0042] Lithium sulfate monohydrate powder (average particle size 20 μm) and activated carbon powder (average particle size 8 μm) were prepared and mixed in a molar ratio of C / Li ２ SO ４ The lithium sulfate monohydrate powder and activated carbon were weighed so that the mass ratio was 2 or more and 4 or less. The weighed lithium sulfate monohydrate powder and activated carbon were placed in a sagger, and an alumina lid was placed on the sagger. As a drying step, the sagger was placed in a vacuum furnace, and the internal pressure of the vacuum furnace was adjusted to 1×10 ２ The vacuum was drawn until the internal pressure of the vacuum furnace reached 1×10 Pa or less. After that, an inert gas was introduced to return the pressure to normal pressure, and the vacuum was drawn again to reduce the internal pressure of the vacuum furnace to 1×10 ２ This procedure was repeated. The number of repetitions is shown in Table 1. The internal pressure of the vacuum furnace was set to 1×10 ２ The mixture was dried by heating under the conditions shown in Table 1 at a pressure of 100 Pa or less. Then, as a synthesis step, a heat treatment was carried out under the conditions shown in Table 1, followed by natural cooling, to obtain lithium sulfide. From the results of XRD analysis of the obtained lithium sulfide and the raw materials used, it is believed that the lithium sulfide contains one or more impurities selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide.

[0043] The carbon content of the lithium sulfide obtained as described above was measured by a combustion infrared spectrometry. The oxygen content of the lithium sulfide was also measured by a fusion infrared spectrometry. Appropriate measurement conditions were set, and the measurement results when the analysis subject was completely combusted and melted are shown in Table 1.

[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 SP-300 manufactured by Bio-Logic Science Instruments 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 oxygen content of lithium sulfide was 5.41% by mass, and the sulfide-based solid electrolyte produced using this lithium sulfide had an ionic conductivity of 1.45 S / cm. In Comparative Example 2, the carbon content of lithium sulfide was 2.85% by mass, and the sulfide-based solid electrolyte produced using this lithium sulfide had an ionic conductivity of 1.68 S / cm. In Comparative Example 3, the carbon content of lithium sulfide was 3.34% by mass and the oxygen content was 5.85% by mass, and the sulfide-based solid electrolyte produced using this lithium sulfide had an ionic conductivity of 1.02 S / cm. In all Comparative Examples, the XRD analysis results of the obtained lithium sulfide and the raw materials used suggest that one or more impurities selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide were contained.

[0048] In contrast, in Examples 1 to 6 of the present invention, the carbon content of the lithium sulfide was 2.0% by mass or less and the oxygen content was 5.0% by mass or less, and the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 2.68 S / cm or more, which was higher than that of Comparative Examples 1 to 3.

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

[0050] It is possible to provide lithium sulfide that can be used to synthesize a sulfide-based solid electrolyte having excellent ion conductivity, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide.

Claims

1. Lithium sulfide having a carbon content of 2.0% by mass or less and an oxygen content of 5.0% by mass or less.

2. The lithium sulfide according to claim 1, characterized in that the impurities contained therein are one or more selected from the group consisting of carbon, lithium sulfate, lithium carbonate, and lithium oxide.

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.

Images