Lithium sulfide production method

Abstract

A lithium sulfide production method according to the present invention comprises: a mixing step for preparing a mixed powder containing solid lithium hydroxide, solid sulfur, and a reducing agent containing elemental carbon; and a firing step for firing the mixed powder at a temperature of 600-1200°C in an inert gas or reducing gas atmosphere. During the firing step, a gas accumulation suppression process is performed to suppress the accumulation, in the mixed powder, of gas that is generated during the firing step. It is preferable that the gas accumulation suppression process is performed at 120-1200°C.

Description

Lithium sulfide manufacturing method

[0001] The present invention relates to a method for producing lithium sulfide.

[0002] In recent years, secondary batteries have been attracting attention as an effort to prevent global warming by reducing carbon dioxide emissions. Among these, solid-state batteries are expected to be put into practical use as batteries that combine safety and high energy density. A sulfide solid electrolyte is known as one of the solid electrolytes used in solid-state batteries. A sulfide solid electrolyte is produced, for example, using lithium sulfide as a raw material. Patent Document 1 describes a method for producing lithium sulfide, which includes a calcination step in which a mixture of solid lithium hydroxide and solid sulfur is calcined at 600 to 1200°C in the presence of a reducing agent to obtain lithium sulfide. The document also describes that the method described in this document does not require the use of an organic solvent or a gaseous sulfur source, involves a small number of steps, and produces lithium sulfide with a low content of heterogeneous phases.

[0003] JP 2014-169196 A

[0004] However, as a result of investigations by the present inventors, the production method described in Patent Document 1 did not provide a sufficient yield of lithium sulfide.

[0005] As a result of further investigation, the present inventors have found that in the production method described in Patent Document 1, raw materials and reaction products are likely to boil over from a container such as a sagger during firing, and that this causes a decrease in the yield of lithium sulfide.

[0006] The present invention has been made based on the above-mentioned findings and provides a method for producing lithium sulfide, comprising: a mixing step of preparing a mixed powder containing solid lithium hydroxide, solid sulfur, and a reducing agent containing a carbon element; and a firing step of firing the mixed powder in an inert gas or reducing gas atmosphere, wherein a gas accumulation suppression treatment is performed during the firing step to suppress accumulation of gas generated during the firing step in the mixed powder.

[0007] 1(a) to 1(d) are diagrams illustrating the inconveniences that may occur in the conventional method for producing lithium sulfide using lithium hydroxide as a starting material.

[0008] The present invention will be described below based on preferred embodiments. The method for producing lithium sulfide of the present invention is roughly divided into the following (i) mixing step and (ii) calcination step. (i) Mixing step: A mixed powder containing solid lithium hydroxide, solid sulfur, and a reducing agent containing carbon element is prepared. (ii) Calcination step: The mixed powder obtained in the mixing step is calcined in an inert gas or reducing gas atmosphere. Each step will be described in detail below.

[0009] (i) Mixing Step In this step, lithium hydroxide is used as the starting material for producing lithium sulfide. Lithium sulfate can be used instead of lithium hydroxide as the starting material for producing lithium sulfide. However, because lithium sulfate is more expensive than lithium hydroxide, lithium hydroxide is used as the starting material in this step. The lithium hydroxide used in this step is solid. The term "solid lithium hydroxide" is intended to exclude lithium hydroxide in the form of an aqueous solution. Therefore, lithium hydroxide in a state in which it has absorbed water contained in the air is included in the category of solid lithium hydroxide. It is known that lithium hydroxide exists in anhydrous and hydrated forms, and the lithium hydroxide used in this step may be either anhydrous or hydrated as long as it is solid. There is no particular limitation on the particle size of the lithium hydroxide. Since the present invention produces lithium sulfide by a solid-state reaction, from this perspective, the volume-cumulative particle size D of lithium hydroxide at a cumulative volume of 50% by volume as measured by a laser diffraction / scattering particle size distribution measurement method is used. ５０ is, for example, preferably 1 μm or more, more preferably 10 μm or more, and even more preferably 100 μm or more. ５０ is, for example, preferably 1000 μm or less, more preferably 800 μm or less, even more preferably 600 μm or less, and even more preferably 200 μm or less.

[0010] There is no particular limitation on the solid sulfur used in this step, and for example, α-sulfur or β-sulfur can be used. Since the crystal system of sulfur does not affect the properties of the resulting lithium sulfide, sulfur other than these crystal systems may also be used. There is no particular limitation on the particle size of sulfur. As described above, the present invention produces lithium sulfide by a solid-state reaction, and from this perspective, the volume cumulative particle size D of sulfur at 50% cumulative volume as measured by a laser diffraction / scattering particle size distribution measurement method is ５０ is, for example, preferably 1 μm or more, more preferably 10 μm or more, and even more preferably 100 μm or more. ５０ is, for example, preferably 1000 μm or less, more preferably 800 μm or less, even more preferably 600 μm or less, and even more preferably 200 μm or less.

[0011] The reducing agent used in this step is used to promote the conversion of lithium sulfate, which is a by-product in the process of producing lithium sulfide from lithium hydroxide, to lithium sulfide. From this perspective, the reducing agent preferably contains a carbon element. From this perspective, the reducing agent is preferably carbon itself or a reducing organic compound containing carbon.

[0012] Examples of carbon itself used as a reducing agent include various carbon materials such as carbon black, graphite, carbon fiber, and activated carbon. These carbon materials can be used alone or in combination of two or more. These reducing agents have a volume cumulative particle size D at 50% cumulative volume as measured by a laser diffraction / scattering particle size distribution measurement method. ５０ is, for example, preferably 0.1 μm or more, more preferably 1 μm or more, even more preferably 10 μm or more, and even more preferably 90 μm or more. ５０ is preferably, for example, 1000 μm or less, more preferably 300 μm or less, and even more preferably 150 μm or less. ５０ The smaller the value, the greater the contact area when mixed with the raw material, and therefore the more efficiently the reduction can be carried out.

[0013] Examples of reducing organic compounds containing carbon include reducing sugars, polyhydric alcohols, and organic acids. Examples of reducing sugars that can be used include glucose, sucrose, fructose, and lactose. Of these reducing sugars, glucose is preferred because the reduction reaction is easy to control. Examples of polyhydric alcohols that can be used include ethylene glycol, propylene glycol, and glycerin. In particular, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, and polyethylene glycol are preferred. These polyhydric alcohols can be used alone or in combination of two or more. Of these polyols, ethylene glycol is preferred because it has a high proportion of hydroxy groups relative to its molecular weight, resulting in high reducing performance, and is liquid at room temperature, making it easy to handle. Examples of organic acids that can be used include malonic acid, succinic acid, glycolic acid, lactic acid, malic acid, and tartaric acid.

[0014] Among the various reducing agents mentioned above, it is particularly preferable to use at least one selected from carbon black, graphite, carbon fiber, activated carbon, reducing sugars and polyhydric alcohols in view of high reducing ability.

[0015] In this step, the amount of sulfur atoms used per mole of lithium atoms is, for example, preferably 0.5 mol or more, more preferably 1.0 mol or more, and even more preferably 1.5 mol or more. On the other hand, the amount of sulfur atoms used per mole of lithium atoms is, for example, preferably 10 mol or less, more preferably 7.0 mol or less, and even more preferably 4.0 mol or less. When the amount of sulfur atoms used per mole of lithium atoms is within the above range, high-purity lithium sulfide can be obtained. In addition, the production cost can be reduced, the production amount can be improved, and the equipment maintenance load can be further reduced.

[0016] The amount of the reducing agent used, when converted into carbon atoms and expressed as a molar ratio per mole of lithium atoms, is preferably 0.05 or more, more preferably 0.1 or more, and even more preferably 0.2 or more. By setting the amount of the reducing agent in this manner, it is possible to suppress the remaining of lithium sulfate. Furthermore, in this step, when converted into carbon atoms and expressed as a molar ratio per mole of lithium atoms, it is preferably 0.5 or less, more preferably 0.45 or less, and even more preferably 0.4 or less. By setting the amount of the reducing agent in this manner, it is possible to suppress the remaining of the reducing agent itself. As a result, by setting the amount of the reducing agent per mole of lithium atoms within the above range, high-purity lithium sulfide can be obtained.

[0017] In this process, there is no particular limitation on the means for mixing solid lithium hydroxide, solid sulfur, and a reducing agent containing carbon element. These may be mixed by dry mixing or wet mixing. For dry mixing, devices such as a high-speed mixer, super mixer, turbosphere mixer, Henschel mixer, Nauta mixer, ribbon blender, and V-type mixer can be used. These uniform mixing operations are not limited to the mechanical means exemplified above. Before or after mixing, particle size adjustment may be performed by pulverization using a jet mill or the like, as necessary. For wet mixing, devices such as a ball mill, disper mill, homogenizer, vibration mill, sand grind mill, and attritor can be used. There is no particular limitation on the type of liquid medium used in wet mixing, as long as it does not affect the three components. By the above operations, a mixed powder containing the three components can be obtained. In some cases, other components may be added to the three components to prepare the mixed powder.

[0018] (ii) Calcination Step The mixed powder obtained in the mixing step is calcined in this step. Calcination can be carried out by, for example, filling the mixed powder into a sagger and heating it in a calcination furnace. In this specification, "calcination" refers to the heating period from the start of heating the mixed powder at room temperature until the target lithium sulfide is produced. During calcination, the mixture does not need to be heated all the time; heating may be interrupted midway through the calcination, the mixture may be cooled to room temperature, and then heating may be resumed to produce the target lithium sulfide. Such an operation is also included in the "calcination" referred to in this specification.

[0019] The calcination can be carried out under an inert gas or reducing gas atmosphere. Examples of inert gases used for the calcination include nitrogen gas and rare gases such as helium, argon, and neon. Examples of reducing gases used for the calcination include hydrogen gas, hydrogen gas diluted with an inert gas, carbon monoxide gas, and hydrogen sulfide gas.

[0020] The maximum temperature reached in the firing is, for example, preferably 1200°C or lower, more preferably 1100°C or lower, and even more preferably 1000°C or lower. By keeping the maximum temperature reached at or below this level, it is possible to suppress the generation of phases other than lithium sulfide. On the other hand, the maximum temperature reached in the firing is, for example, preferably 600°C or higher, more preferably 700°C or higher, and even more preferably 800°C or higher. By keeping the maximum temperature at or above this level, it is possible to reliably generate lithium sulfide. The time for which the maximum temperature is maintained can be appropriately adjusted so as to reliably generate lithium sulfide from the raw material lithium hydroxide.

[0021] The present inventors believe that the reaction in this process is different between the low-temperature firing region and the high-temperature firing region, and consists of the following two stages: Low-temperature firing region: 8LiOH + 4S → 3Li ２ S+Li ２ SO ４ +4H ２ O (1) High temperature firing region Li ２ SO ４ +2C → Li ２ S + 2CO ２(2) Therefore, the total reaction is as follows: 8LiOH + 4S + 2C → 4Li ２ S+4H ２ O + 2CO ２ (3) As a result of the inventors' investigations, it was found that the low-temperature firing range in which the reaction in reaction formula (1) occurs most efficiently is up to about 500°C or less, and the high-temperature firing range in which the reaction in reaction formula (2) occurs is about 500°C or more.

[0022] The inventors further investigated Reaction Formula (1) and Reaction Formula (2) and found that, as shown in Figure 1, in the low-temperature firing region where Reaction Formula (1) occurs, lithium hydroxide reacts with sulfur to produce lithium sulfide (mp = 938°C) and lithium sulfate (mp = 859°C), which are substances with high melting points. It is believed that these substances are produced in the form of a film on the mixed powder containing molten sulfur (mp = 122°C) (see Figure 1(b)). On the other hand, in the high-temperature firing region where Reaction Formula (2) occurs, sulfur gas (bp = 445°C) and CO are produced in the space partitioned by the film containing lithium sulfide and lithium sulfate. ２ The gas accumulates, and the internal pressure of the space increases (see FIG. 1(c)). When the internal pressure of the space increases and the film can no longer withstand the pressure, the film breaks, causing the raw material containing molten sulfur to scatter or boil over (see FIG. 1(d)).

[0023] As described above, when lithium sulfide is produced from lithium hydroxide, there is a problem that the raw material may fly off or boil over during the process. As a result, the yield of the target lithium sulfide is reduced. Therefore, in the present production method, sulfur gas and CO generated during the calcination step are removed. ２ A gas accumulation prevention process is carried out during the firing process to prevent gases from accumulating in the mixed powder, thereby preventing the scattering and boiling over of raw materials.

[0024] As an example of the gas accumulation suppression treatment, in the (A) firing step, when a film of lithium sulfide and / or lithium sulfate is formed on the upper surface of the mixed powder containing molten sulfur in the low-temperature firing region shown in FIG. 1(b), at least a part of the film is destroyed, and sulfur gas and CO are released from the mixed powder covered by the film.２ It is advantageous to remove gases such as fumes, etc. By destroying this coating, it is possible to effectively prevent the raw material from scattering or boiling over as shown in Figure 1(d).

[0025] When the operation (A) is performed, the film may be subjected to a pulverization or crushing treatment in order to destroy at least a part of the film. For example, an apparatus for pulverizing or crushing the film may be installed in the reaction system, and the film may be destroyed by the apparatus.

[0026] As another example of the gas accumulation suppression treatment, it is also advantageous to suppress the formation of a film of lithium sulfide and / or lithium sulfate on the upper surface of the mixed powder during the (B) firing step. This operation prevents the formation of the film. Therefore, this operation also effectively suppresses the scattering or boiling over of the raw materials as shown in FIG. 1(d).

[0027] When performing the operation (B), it is preferable to agitate the mixed powder before the film is formed to prevent the film from being formed. For this purpose, the mixed powder can be vibrated continuously or discontinuously. The mixed powder can also be agitated by rotating a stirring blade, and the formation of the film can be suppressed by blowing air.

[0028] The operations (A) and (B) are preferably carried out at, for example, 120°C or higher and 1200°C or lower. In particular, they are preferably carried out in a low-temperature firing region, which is the temperature region in which the coating is formed. From this viewpoint, the operations (A) and (B) are preferably carried out at 120°C or higher, which is the low-temperature firing region, more preferably at 150°C or higher, and even more preferably at 180°C or higher. Furthermore, the operations (A) and (B) are preferably carried out at 500°C or lower, which is the low-temperature firing region, more preferably at 450°C or lower, and even more preferably at 420°C or lower. The operations (A) or (B) may be carried out not only in these temperature regions but also throughout the entire firing step (i.e., from the start of temperature rise to the completion of lithium sulfide production). From the viewpoint of economy, it is preferable to carry out the operations (A) or (B) in the low-temperature firing region and not to carry out the operations (A) or (B) in the high-temperature firing region.

[0029] As an alternative method, in the case of the above-mentioned operation (A), if a film is formed in the low-temperature firing zone, the reaction system can be cooled to room temperature, the film can be destroyed, and then firing can be performed again (i.e., firing via the low-temperature firing zone and the high-temperature firing zone). Performing operation (A) in this manner has the advantage that the lithium sulfate and the reducing agent in the film can be uniformly mixed again, accelerating the reaction. Furthermore, as a result of the progress of reaction formula (1), the entire product is cooled and returns to a solid state in a state where the amounts of sulfur and lithium hydroxide, which are substances with low melting points, are reduced. This also has the advantage that the fired product is less likely to become liquid when heated again, and a film is less likely to form in the first place.

[0030] When the operation (A) or (B) is carried out in the low-temperature firing zone, the temperature in the reaction system can be maintained at the above-mentioned temperature for 1 hour or more, 2 hours or more, or 3 hours or more, and the temperature in the reaction system can be maintained at the above-mentioned temperature for 10 hours or less, 9 hours or less, or 8 hours or less.

[0031] The lithium sulfide obtained by the above-mentioned operations can be subjected to a post-treatment process of pulverization and sieving to obtain a powder having an appropriate particle size distribution.

[0032] The lithium sulfide obtained by this production method is useful, for example, as a raw material for a sulfide solid electrolyte of a lithium ion battery. ２ S ５ ) or other sulfides by mechanical milling reaction to obtain, for example, Li ７ P ３ S １１ and LiPS ４Alternatively, a crystalline solid electrolyte, for example, a solid electrolyte having a crystal phase with an argyrodite crystal structure, can be synthesized by firing a mixture of lithium sulfide, diphosphorus pentasulfide, and a lithium halide such as lithium chloride (LiCl) and / or lithium bromide (LiBr) under an inert gas atmosphere or a hydrogen sulfide atmosphere. The substance to be reacted with lithium sulfide to synthesize the solid electrolyte is not particularly limited. For example, in addition to the above-mentioned diphosphorus pentasulfide, silicon sulfide (SiS ２ ), germanium sulfide (GeS ２ ) etc.

[0033] With respect to the above-described embodiments, the present invention further discloses the following method for producing lithium sulfide. [1] A method for producing lithium sulfide, comprising: a mixing step of preparing a mixed powder containing solid lithium hydroxide, solid sulfur, and a reducing agent containing elemental carbon; and a calcination step of calcining the mixed powder in an inert gas or reducing gas atmosphere, wherein a gas accumulation suppression treatment is performed during the calcination step to suppress accumulation of gas generated during the calcination step in the mixed powder. [2] The gas accumulation suppression treatment includes: A: destroying at least a portion of a coating of lithium sulfide and / or lithium sulfate formed on an upper surface of the mixed powder during the calcination step, thereby removing the gas from within the mixed powder coated with the coating, or B: suppressing formation of a coating of lithium sulfide and / or lithium sulfate on the upper surface of the mixed powder during the calcination step. [3] The method for producing lithium sulfide according to [1] or [2], wherein the gas accumulation suppression treatment includes: a: subjecting a coating of lithium sulfide and / or lithium sulfate formed on the upper surface of the mixed powder during the firing step to a pulverization or crushing treatment to destroy at least a part of the coating, or b: stirring the mixed powder during the firing step to suppress the formation of a coating of lithium sulfide and / or lithium sulfate during the firing step. [4] The method for producing lithium sulfide according to any one of [1] to [3], wherein the gas accumulation suppression treatment is performed at a temperature of 120°C or higher and 1200°C or lower. [5] The method for producing lithium sulfide according to any one of [1] to [4], wherein the reducing agent containing a carbon element is at least one selected from carbon black, graphite, carbon fiber, activated carbon, reducing sugar, and polyhydric alcohol. [6] The method for producing lithium sulfide according to any one of [1] to [5], wherein the reducing agent containing carbon element is used so that the ratio of carbon atoms to lithium atoms is 0.05 or more and 0.5 or less in terms of molar ratio in atomic terms.

[0034] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass."

[0035] [Example 1] Lithium hydroxide (D ５０ = 14.2 μm), sulfur (D ５０ = 41.8 μm) and activated carbon (D ５０ (=130.6 μm) was used. The amount of sulfur used was 2.0 moles of sulfur atoms per mole of lithium atoms. The amount of activated carbon used, calculated as carbon atoms, was 0.40 moles of carbon atoms per mole of lithium atoms. The raw materials were dry-mixed to obtain a mixed powder. The obtained mixed powder was filled into a sagger and placed in a firing furnace. The inner dimensions of the sagger were 110 mm wide, 52 mm deep, and 50 mm high. The mixed powder was filled into the sagger to a thickness of 38.4 mm. The firing furnace was heated from room temperature to 400°C and maintained at that temperature for 3 hours for low-temperature firing. The firing atmosphere was nitrogen. Next, heating of the firing furnace was stopped and the furnace was cooled to room temperature. The sagger was removed from the furnace and the mixed powder was stirred to destroy the coating formed on the top surface of the mixed powder. Thereafter, the sagger was placed back in the firing furnace, and the temperature inside the furnace was raised to 900°C, and high-temperature firing was carried out by maintaining that temperature for 6 hours. The mixed powder was not stirred during the high-temperature firing. In this manner, the target lithium sulfide was obtained. The yield of the obtained lithium sulfide was 94.8%. No scattering or boiling over of the mixed powder was observed during the firing.

[0036] Example 2 Lithium hydroxide, sulfur, and carbon black were used as raw materials. The amount of sulfur used was 2.0 moles of sulfur atoms per mole of lithium atoms. The amount of carbon black used, calculated as carbon atoms, was 0.4 moles of carbon atoms per mole of lithium atoms. Except for this, lithium sulfide was obtained in the same manner as in Example 1. No scattering or overflow of the mixed powder was observed during firing. The yield of lithium sulfide obtained was 96.9%.

[0037] Comparative Example 1 Lithium hydroxide, sulfur, and sucrose were used as raw materials. The amount of sulfur used was 2.0 moles per mole of lithium. The amount of sucrose used, calculated as carbon, was 0.4 moles per mole of lithium. The raw materials were dry-mixed to obtain a mixed powder. The obtained mixed powder was filled into a sagger and placed in a firing furnace. The firing furnace was heated from room temperature to 900°C, and high-temperature firing was performed by maintaining that temperature for 6 hours. The firing atmosphere was nitrogen. As a result, boiling over of the mixed powder was observed during firing. The yield of lithium sulfide obtained was 69.8%.

[0038] As described above in detail, according to the present invention, lithium sulfide can be produced in high yield.

Claims

1. A mixing step of preparing a mixed powder containing solid lithium hydroxide, solid sulfur, and a reducing agent containing a carbon element; and a firing step of firing the mixed powder in an inert gas or reducing gas atmosphere, wherein during the firing step, a gas accumulation suppression treatment is performed to suppress the accumulation of the gas generated during the firing step into the mixed powder. A method for producing lithium sulfide.

2. As the gas accumulation suppression treatment, A: At least a part of the film of lithium sulfide and / or lithium sulfate formed on the upper surface side of the mixed powder during the firing step is destroyed to remove the gas from the mixed powder covered by the film, or B: During the firing step, formation of a film of lithium sulfide and / or lithium sulfate on the upper surface side of the mixed powder is suppressed. The method for producing lithium sulfide according to claim 1.

3. As the gas accumulation suppression treatment, a: A pulverization treatment or a disintegration treatment is performed on the film of lithium sulfide and / or lithium sulfate formed on the upper surface side of the mixed powder during the firing step to destroy at least a part of the film, or b: The mixed powder during the firing step is stirred to suppress the formation of a film of lithium sulfide and / or lithium sulfate during the firing step. The method for producing lithium sulfide according to claim 1 or 2.

4. The method for producing lithium sulfide according to claim 1 or 2, wherein the gas accumulation suppression treatment is performed at 120°C or higher and 1200°C or lower.

5. The method for producing lithium sulfide according to claim 1 or 2, wherein the reducing agent containing a carbon element is at least one selected from carbon black, graphite, carbon fiber, activated carbon, reducing sugar, and polyhydric alcohol.

6. The method for producing lithium sulfide according to claim 1 or 2, wherein the reducing agent containing a carbon element is used such that the ratio of carbon atoms to lithium atoms is 0.05 or more and 0.5 or less in terms of molar ratio in atomic conversion.

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