Lithium sulfide manufacturing method, lithium sulfide manufacturing raw materials

Abstract

To provide a method of producing lithium sulfide, capable of producing lithium sulfide of high purity at a high yield by a simple process.SOLUTION: The method of producing lithium sulfide is characterized in that lithium sulfide is produced by heating a production raw material comprising a core including a mixture prepared by blending lithium sulfate and a carbon material, and a coating layer consisting of a carbon material coating at least a part of a surface of the core.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing lithium sulfide suitable for use as a sulfide-based solid electrolyte material in, for example, all-solid-state batteries, and to a raw material for producing lithium sulfide. [Background technology]

[0002] Lithium-ion batteries are widely used as power sources in a wide range of vehicles, from electric vehicles (EVs) and hybrid electric vehicles (HEVs) to 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 is dissolved in an organic solvent.

[0003] Such organic electrolytes are flammable and may be damaged by excessive heating or impact. Furthermore, lithium-ion batteries containing organic electrolytes may develop dendrites on the lithium metal surface as they are repeatedly charged and discharged, which can cause internal short circuits between the electrodes and lead to malfunctions. To prevent such internal short circuits, components such as separators are required between the electrodes.

[0004] To improve the safety and durability of conventional lithium-ion batteries that use organic electrolytes, all-solid-state lithium-ion batteries using inorganic solid electrolytes have been proposed. Furthermore, the use of inorganic solid electrolytes can suppress the growth of lithium metal and the resulting internal short-circuiting between electrodes, potentially eliminating components such as separators. Sulfide-based solid electrolytes, one type of inorganic solid electrolyte, have excellent lithium-ion conductivity and are therefore expected to be used primarily in batteries for electric vehicles (EVs). Examples of sulfide-based solid electrolytes currently proposed include the Li2S-P2S5 system, the Li2S-P2S3 system, the Li2S-SiS2 system, the Li2S-Ga2S2 system, and the Li2S-GeS2 system.

[0005] In all of these sulfide-based solid electrolytes, high-purity lithium sulfide (Li2S) is used as a constituent material. 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. Furthermore, Patent Documents 2 and 3 disclose methods for continuously producing lithium sulfide by continuously supplying lithium hydroxide to a rotary kiln through which hydrogen sulfide gas is flowed and allowing it to react with the hydrogen sulfide gas. Patent Document 4 also discloses a method for producing lithium sulfide by mixing lithium sulfate and a carbon material and heating the mixture. This method also discloses that both the lithium sulfate and the carbon material are converted into fine particles to increase the reaction area and thereby reduce the amount of unreacted raw materials. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-151725 [Patent Document 2] Japanese Patent Application Publication No. 2018-035045 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-174787 [Patent Document 4] Japanese Patent Application Laid-Open No. 2013-227180 Summary of the Invention [Problem to be solved by the invention]

[0007] 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, resulting in problems such as a complicated production process and high production costs.

[0008] Furthermore, the inventions disclosed in Patent Documents 2 and 3 require the use of hydrogen sulfide gas, which is toxic to the human body, and have the problem of high equipment costs, such as the need to maintain the airtightness of the reaction apparatus and to treat unreacted hydrogen sulfide gas. Furthermore, the invention disclosed in Patent Document 4 requires a step of processing each of the lithium sulfate and the carbon material into fine particles, which increases the production cost. Furthermore, the finely divided lithium sulfate and the carbon material are likely to scatter inside the reaction vessel when using an inert gas flow or an apparatus that involves the movement of the production raw materials, such as a rotary kiln, which leads to a decrease in the yield of lithium sulfide.

[0009] The present invention has been made in view of the above background, and an object of the present invention is to provide a method for producing lithium sulfide that can produce high-purity lithium sulfide in a high yield through simple steps, and a raw material for producing lithium sulfide. [Means for solving the problem]

[0010] In order to achieve the above object, the present invention is configured as follows. That is, the method for producing lithium sulfide according to the present invention is characterized in that lithium sulfide is produced by heating a raw material for production that includes a core containing a kneaded mixture of lithium sulfate and a carbon material, and a coating layer made of a carbon material that coats at least a part of the surface of the core.

[0011] According to the present invention, high-purity lithium sulfide is produced without producing solid by-products at room temperature through a simple process of heat-treating a raw material containing lithium sulfate and a carbonaceous material. Furthermore, by using cores containing a mixture of lithium sulfate and a carbonaceous material as the raw material for producing lithium sulfide, scattering of lithium sulfate and the carbonaceous material into a reaction vessel can be prevented even when the raw material is applied to a device involving movement of the raw material, such as a rotary kiln. Furthermore, by using a raw material having a coating layer made of a carbonaceous material that coats at least a portion of the surface of the core, even when the raw material is heated to a reaction temperature at which lithium sulfate softens, lithium sulfate can be prevented from melting and adhering to or remaining in a heating vessel, such as a rotary kiln.

[0012] That is, as for the raw material for production, lithium sulfate is mixed with a carbon material having a higher softening temperature, and the periphery of the mixed material is coated with the carbon material. Therefore, there is no risk of lithium sulfate adhering to the inner wall of a heating vessel such as a rotary kiln and remaining unreacted, thereby reducing the yield of lithium sulfide, or molten lithium sulfate clogging the furnace tube of the rotary kiln, thereby causing equipment malfunction.

[0013] Furthermore, in the method for producing lithium sulfide of the present embodiment, sulfate ions contained in lithium sulfate are reduced by carbon contained in the carbon material at high temperatures to produce lithium sulfide. This eliminates the need to use hydrogen sulfide gas, which is toxic to the human body, as a sulfur source, and eliminates the cost of facilities for treating unreacted gas, etc., making it possible to produce lithium sulfide safely at low cost.

[0014] In the present invention, the heating temperature of the raw materials may be in the range of 700°C or higher and 950°C or lower.

[0015] In the present invention, the amount of carbon material contained in the entire production raw material may be in the range of 1.5 moles or more and 3 moles or less per mole of lithium sulfate contained in the cores.

[0016] In the present invention, the proportion of the carbon material contained in the coating layer with respect to the carbon material contained in the entire production raw material may be in the range of 3 mass % or more and 15 mass % or less.

[0017] In the present invention, the raw materials may be heated using a rotary kiln, and the raw materials may be continuously introduced into the rotary kiln to continuously produce lithium sulfide.

[0018] The raw material for producing lithium sulfide of the present invention is a raw material for producing lithium sulfide used in producing lithium sulfide by reacting lithium sulfate with a carbon material, and is characterized by having a core containing a kneaded mixture of lithium sulfate and a carbon material, and a coating layer made of a carbon material that coats at least a part of the surface of the core. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a method for producing lithium sulfide that can produce high-purity lithium sulfide in high yield through simple steps, and a raw material for producing lithium sulfide. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram showing an example of a raw material for producing lithium sulfide of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] A method for producing lithium sulfide and raw materials for producing lithium sulfide according to one embodiment of the present invention will be described below. 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.

[0022] The method for producing lithium sulfide according to this embodiment is for producing lithium sulfide (LiS), which is a constituent material of a sulfide solid electrolyte used as a solid electrolyte in an all-solid-state battery. Sulfide solid electrolyte materials have high ionic conductivity, are non-flammable, and are highly safe, making them suitable as materials for on-board batteries of electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0023] (Raw material for manufacturing lithium sulfide) FIG. 1 is a schematic diagram showing an example of raw materials for producing lithium sulfide of the present invention. A lithium sulfide production raw material 10 in one embodiment used in the lithium sulfide production method of the present invention has a core 11 containing a mixture obtained by kneading at least lithium sulfate (LiSO) and a carbon material (C), and a coating layer 12 made of a carbon material that coats at least a portion of the surface of the core 11. The manufacturing raw material 10 may be, for example, spherical particles with a particle size of about 0.3 to 3.5 mm.

[0024] Core 11 is made of a kneaded mixture of at least lithium sulfate and a carbon material. The kneaded mixture constituting core 11 may also contain, in addition to lithium sulfate and a carbon material, materials for improving kneadability, such as a solvent and a binder material. The cores 11 may be spherical particles with a particle size of, for example, about 0.5 to 3.0 mm.

[0025] For example, either anhydrous or monohydrate with a purity of 99.9% or more can be used as lithium sulfate constituting the core 11. It is preferable to select finely divided lithium sulfate in order to improve kneadability.

[0026] The carbon material that forms the core 11 by kneading with lithium sulfate and the carbon material that forms the coating layer 12 that covers the surface of the kneaded product can be, for example, activated carbon, carbon black, etc. In this embodiment, activated carbon powder is used as the carbon material.

[0027] The carbon material constituting the core 11 and the carbon material constituting the coating layer 12 can also be carbon materials with different properties. For example, the carbon material constituting the coating layer 12 can be a carbon material with a larger particle size than the carbon material constituting the core 11, or activated carbon can be used as the carbon material constituting the core 11 and carbon black can be used as the carbon material constituting the coating layer 12.

[0028] When a solvent is added to the kneaded material that constitutes the core 11, for example, pure water or an organic solvent can be used as the solvent. In this embodiment, pure water is used as the solvent. A synthetic resin can be used as the binder when adding a binder to the kneaded material that constitutes the core 11. In this embodiment, polyvinyl alcohol (PVA) is used as the binder.

[0029] The coating layer 12 is made of a carbon material and is formed to a thickness in the range of, for example, 0.5 μm or more and 30 μm or less. Such coating layer 12 needs to cover at least a portion of the surface of the core 11, and preferably covers the entire surface of the core 11. The coverage rate of the coating layer 12 with respect to the core 11 may be, for example, in the range of 50% or more and 100% or less.

[0030] As a method for producing the raw material for producing lithium sulfide having the above-mentioned configuration, for example, a mixed powder of lithium sulfate powder and activated carbon powder is kneaded with pure water (solvent) and PVA (binder), formed into granules, and then dried to obtain cores 11 (granulated powder). Note that a solvent and a binder may be added during kneading, if necessary.

[0031] As a specific example, lithium sulfate and activated carbon are mixed in a molar ratio of 1.0:2.0, kneaded with water and PVA in a kneader, and then extrusion-granulated in a granulator and dried to form cores (granulated powder) 11 with a particle size of about 0.5 to 3 mm. If a solvent is added, the solvent is removed during the extrusion-granulation process and by drying after granulation.

[0032] Then, by applying a carbon material to the surface of the obtained granulated powder, it is possible to obtain raw material 10 for producing lithium sulfide of this embodiment, in which the surface of cores 11 containing a kneaded mixture of lithium sulfate and carbon material is coated with coating layer 12 made of carbon material. In order to uniformly apply and coat the surface of cores 11 with the carbon material, for example, cores 11 and the carbon material are placed in a container and mixed with a strength that does not cause the cores to crumble.

[0033] The amount of carbon material contained in the entire raw material 10 for producing lithium sulfide (the total amount of carbon material in the core 11 and the coating layer 12) may be in the range of 1.5 mol or more and 3 mol or less per mol of lithium sulfate contained in the core 11. The proportion of the carbon material contained in the coating layer 12 among the carbon material contained in the entire raw material for producing lithium sulfide may be in the range of 3 mass % or more and 15 mass % or less.

[0034] (Method of producing lithium sulfide) A method for producing lithium sulfide according to one embodiment of the present invention uses the above-described raw material for producing lithium sulfide, that is, a raw material for producing lithium sulfide, which has a core containing a mixture obtained by kneading lithium sulfate and a carbon material, and a coating layer made of a carbon material that coats at least a portion of the surface of the core.

[0035] In this embodiment, the above-mentioned raw materials are heated using a heating device, for example, a rotary kiln (rotary furnace), to generate lithium sulfide. By heating the raw materials, the lithium sulfate contained in the cores and the carbon material contained in the cores and coating layer undergo a reaction shown in the following formula 1, and lithium sulfide is generated. Li2SO4+2C→Li2S+2CO2···(1)

[0036] When lithium sulfide is produced according to this embodiment, the above-described raw materials for producing lithium sulfide are continuously supplied into the rotary kiln, and the inside of the heating device is kept in an inert gas atmosphere or a vacuum atmosphere to prevent an oxidation reaction. For example, argon gas is used as the inert gas to be flowed. The inside of the rotary kiln is then heated to a predetermined reaction temperature. This allows lithium sulfide to be produced continuously and efficiently.

[0037] The reaction temperature (heating temperature) may be set in the range of 700°C or higher and 950°C or lower, preferably 750°C or higher and 850°C or lower. By setting the reaction temperature at 700°C or higher, the reaction rate can be kept within a practical range, thereby increasing productivity. Furthermore, by setting the reaction temperature at 950°C or lower, melting of the produced lithium sulfide can be prevented. In particular, by setting the reaction temperature at 850°C or lower, it is possible to prevent lithium sulfate contained in the nuclei of the production raw material from melting and flowing out before the reaction.

[0038] The retention time at the reaction temperature, i.e., the time required for the raw materials to be brought to the temperature range described above and completely reacted (in this embodiment, the time required for the raw materials to pass through the heated section of the rotary kiln), is preferably 1 hour or more, more preferably 3 hours or more. The retention time at the reaction temperature is preferably 48 hours or less, more preferably 24 hours or less.

[0039] The rotation speed of the rotary kiln should be set to 1 rpm or more and 10 rpm or less. The flow rate of argon gas flowing inside the rotary kiln is, for example, 0.5 L / min. -1 Over 3.0Lmin -1 It may be set in the following range.

[0040] Through the above steps, high-purity lithium sulfide can be produced. According to the method for producing lithium sulfide of the present embodiment, high-purity lithium sulfide is produced by a simple process of heat-treating a production raw material containing lithium sulfate and a carbon material, without producing any solid by-products at room temperature.

[0041] Furthermore, by using, as the raw material for producing the above-mentioned lithium sulfide, a raw material for production that has a core containing a kneaded mixture of lithium sulfate and a carbon material and a coating layer made of a carbon material that coats at least a part of the surface of the core, even when this raw material for production is heated to a reaction temperature at which lithium sulfate softens, it is possible to prevent lithium sulfate from melting and adhering to the inner wall of a heating vessel such as a rotary kiln.

[0042] That is, as for the raw material for production, lithium sulfate is mixed with a carbon material having a higher softening temperature, and the periphery of the mixed material is coated with the carbon material. Therefore, there is no risk of lithium sulfate adhering to the inner wall of a heating vessel such as a rotary kiln and remaining unreacted, thereby reducing the yield of lithium sulfide, or molten lithium sulfate clogging the furnace tube of the rotary kiln, thereby causing equipment malfunction.

[0043] Furthermore, in the method for producing lithium sulfide of the present embodiment, sulfate ions contained in lithium sulfate are reduced by carbon contained in the carbon material at high temperatures to produce lithium sulfide. This eliminates the need to use hydrogen sulfide gas, which is toxic to the human body, as a sulfur source, and eliminates the cost of facilities for treating unreacted gas, etc., making it possible to produce lithium sulfide safely at low cost.

[0044] Although one embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This embodiment can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. This embodiment and its modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Example]

[0045] The method for producing lithium sulfide according to this embodiment was verified below. In the verification, lithium sulfate, activated carbon (carbon material), and water (solvent) were kneaded using a kneader (SPG-25T, manufactured by Dalton Co., Ltd.) to obtain a kneaded mixture. Furthermore, PVA, which is used as a binder, was dissolved in water to a predetermined concentration and added to the kneader. This kneaded mixture was then extrusion-granulated using a granulator to produce granulated powder (cores) with particle sizes of approximately 0.5 mm to 3 mm. In the following Examples 1 to 6, carbon powder was applied to the particle surfaces of this granulated powder to form a coating layer, and the granulated powder was then thoroughly dried to remove the solvent and water of crystallization, thereby obtaining the production raw material (sample) for Examples 1 to 6. In Comparative Example 1, the above-mentioned granulated powder (cores) was used as the production raw material (sample) as is.

[0046] Example 1: The reaction temperature was set to 780° C., and the production raw materials were prepared so that the ratio of the carbon material in the coating layer to the total amount of carbon material was 3 mass %. Example 2: The reaction temperature was set to 780° C., and the production raw materials were prepared so that the ratio of the carbon material in the coating layer to the total amount of carbon material was 5 mass %. Example 3: The reaction temperature was set to 780° C., and the production raw materials were prepared so that the ratio of the carbon material in the coating layer to the total amount of carbon material was 10 mass %. Example 4: The reaction temperature was set to 780° C., and the production raw materials were prepared so that the ratio of the carbon material of the coating layer to the total amount of carbon material was 15 mass %. Example 5: The reaction temperature was set to 700° C., and the production raw materials were prepared so that the ratio of the carbon material in the coating layer to the total amount of carbon material was 5 mass %. Example 6: The reaction temperature was set to 850° C., and the production raw materials were prepared so that the ratio of the carbon material in the coating layer to the total amount of carbon material was 5 mass %. Comparative Example 1: The reaction temperature was set at 780° C., and raw materials (cores only) without a coating layer were prepared.

[0047] Each sample powder was then heat-treated in a rotary kiln (manufactured by Nikkato Corporation) having an alumina furnace core tube (reaction vessel) while flowing argon gas inside the furnace core tube for a time sufficient to allow all of the charged manufacturing raw materials to react, thereby producing lithium sulfide. The mass of the recovered lithium sulfide was then measured.

[0048] The yield was calculated as the ratio of the mass of lithium sulfide of each sample to the maximum mass of lithium sulfide that could be produced from the stoichiometric amount. In addition, the furnace tube of the rotary kiln after the reaction was observed to check for the presence or absence of attached residues or blockages in the furnace tube. These results are shown in Table 1.

[0049] [Table 1]

[0050] According to the results shown in Table 1, in Examples 1 to 6, which used a production raw material having a core containing a kneaded mixture of lithium sulfate and a carbon material and a coating layer made of a carbon material that coated at least a portion of the surface of the core, the amount of deposits on the furnace tube was small, and it was confirmed that lithium sulfide could be obtained in a high yield of 80% to 85%. It was also confirmed that lithium sulfide could be obtained continuously and efficiently without clogging the furnace tube with deposits.

[0051] On the other hand, in Comparative Example 1, which used raw materials not coated with a carbon material, a large amount of deposits formed on the furnace tube, and the yield of lithium sulfide was only about 50%. In addition, the deposits caused clogging of the furnace tube, making it difficult to continuously produce lithium sulfide due to maintenance. Therefore, it was confirmed that the present invention makes it possible to continuously produce high-purity lithium sulfide in high yield through a simple process.

Claims

1. 1. A method for producing lithium sulfide, comprising heating a raw material for production, the raw material comprising: a core containing a mixture of lithium sulfate and a carbon material; and a coating layer made of a carbon material that coats at least a portion of the surface of the core.

2. 2. The method for producing lithium sulfide according to claim 1, wherein the heating temperature of the production raw materials is in the range of 700°C or higher and 950°C or lower.

3. 3. The method for producing lithium sulfide according to claim 1, wherein the amount of the carbon material contained in the entire production raw materials is in the range of 1.5 mol or more and 3 mol or less per 1 mol of lithium sulfate contained in the cores.

4. 4. The method for producing lithium sulfide according to claim 1, wherein a ratio of the carbon material contained in the coating layer to the carbon material contained in the entire production raw materials is in a range of 3 mass% or more and 15 mass% or less.

5. 5. The method for producing lithium sulfide according to claim 1, wherein the raw materials are heated using a rotary kiln, and the raw materials are continuously introduced to produce lithium sulfide continuously.

6. A raw material for producing lithium sulfide used in producing lithium sulfide by reacting lithium sulfate with a carbon material, A raw material for producing lithium sulfide, comprising: a core containing a mixture of lithium sulfate and a carbon material; and a coating layer made of a carbon material that coats at least a portion of the surface of the core.

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