A method for preparing lithium sulfide by one-step continuous dehydration and sulfidation of lithium hydroxide monohydrate

Abstract

This invention belongs to the field of battery materials technology, specifically relating to a one-step continuous dehydration and sulfidation method for preparing lithium sulfide from lithium hydroxide monohydrate. This invention uses LiOH·H₂O as the lithium source for preparing lithium sulfide, significantly reducing costs. The dehydration and sulfidation steps are completed continuously and uninterruptedly within the same reaction vessel, using the same batch of materials, through program control. There are no intermediate material transfer, separation, or intermediate processing steps, avoiding exposure to air for moisture absorption, oxidation, and the introduction of impurities, thus ensuring high product purity.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a method for preparing lithium sulfide by one-step continuous dehydration and sulfidation of lithium hydroxide monohydrate. Background Technology

[0002] As the global energy structure shifts towards a green and low-carbon model, electric vehicles and large-scale energy storage systems are placing unprecedented demands on electrochemical energy storage technologies. Traditional liquid lithium-ion batteries face severe challenges in terms of safety and driving range due to the inherent flammability of organic electrolytes and energy density limitations. Against this backdrop, solid-state batteries are considered the ultimate solution for next-generation power batteries. They use non-flammable solid electrolytes instead of liquid electrolytes, potentially solving safety issues fundamentally and achieving a leap in energy density thanks to their higher voltage window and compatibility with high-capacity lithium metal anodes.

[0003] Among numerous solid-state electrolyte technologies, sulfide solid-state electrolytes have become one of the most promising mainstream directions for industrialization due to their extremely high lithium-ion conductivity (some materials can rival or even surpass liquid electrolytes) and excellent mechanical ductility. Lithium sulfide (Li2S) is an indispensable cornerstone material in sulfide solid-state battery systems, both as a core electrolyte material and as a key precursor for constructing high-capacity sulfide cathodes. Its purity, cost, and physicochemical properties directly determine the performance, reliability, and commercial prospects of the final solid-state battery. Therefore, developing an advanced process for the large-scale, low-cost, and high-purity preparation of lithium sulfide has become one of the key core technologies for overcoming the bottlenecks in solid-state battery materials and promoting its industrialization.

[0004] Currently, the mainstream industrial methods for preparing lithium sulfide include liquid-phase methods, carbothermal reduction methods, and combustion-explosion methods. However, these existing methods have significant shortcomings in meeting the stringent requirements of solid-state batteries for precursor materials (low cost, high purity, and low oxygen content).

[0005] (1) Liquid phase method: Anhydrous lithium hydroxide (LiOH) powder is suspended in an organic solvent such as N-methylpyrrolidone (NMP), and H2S is introduced at low temperature to generate lithium hydrogen sulfide (LiHS). After filtration, washing and drying, an intermediate is obtained, and finally Li2S is obtained by high-temperature pyrolysis under an inert atmosphere. The disadvantages of this method are: the raw material cost is high, and the price of anhydrous LiOH, which is higher than that of lithium hydroxide monohydrate (LiOH·H2O), is contrary to the industry's demand for cost reduction; the process is complex and easy to introduce pollution. The multi-step operation and the use of organic solvents lead to low efficiency and may leave residual impurities that damage the electrochemical performance of the material; the purity of the product is difficult to guarantee. The complex production process increases the risk of contamination by oxygen and moisture, resulting in increased impurity content, which seriously affects the ionic conductivity and cycle life of the final solid-state battery module.

[0006] (2) Carbothermic reduction method: Lithium sulfate (Li2SO4) is used as the lithium and sulfur source, and carbon (such as graphite, pyrolysis carbon of sucrose, etc.) is used as the reducing agent. The reaction is carried out under an inert atmosphere at 800-1000℃. Lithium sulfate is reduced to lithium sulfide by carbon, and carbon dioxide is generated at the same time. The disadvantages of this method are: the process is extremely complex, involving high-temperature solid-phase reaction and subsequent wet washing to remove by-product salts, and the process is lengthy; the product purity is low, and it is very easy to introduce impurities such as carbon and sodium, which require complex purification, resulting in a low pass rate as a battery-grade material.

[0007] (3) Combustion and Explosion Method: This method involves mixing metallic lithium and elemental sulfur in a stoichiometric ratio in a strictly anhydrous and oxygen-free glove box environment, and then heating the mixture in a sealed container to initiate a violent chemical reaction. The raw material cost is extremely high, as metallic lithium is expensive.

[0008] In summary, existing lithium sulfide preparation technologies have two main interrelated drawbacks when adapting to the stringent requirements of the solid-state battery industry for precursor materials: 1. High raw material costs, which do not align with the industry's cost reduction trend: Existing mainstream methods (such as the liquid phase method and the combustion explosion method) all rely on high-purity anhydrous lithium hydroxide or metallic lithium as starting materials. The price of these raw materials is much higher than their hydrated form or other basic lithium salts, resulting in high initial material costs for lithium sulfide, which contradicts the cost control requirements necessary for the large-scale commercialization of solid-state batteries.

[0009] 2. Complex and lengthy process flow with pollution risks: Existing methods generally involve multiple discontinuous physical or chemical steps. For example, liquid-phase methods involve multiple separation and conversion units such as dispersion-reaction-filtration-washing-drying-pyrolysis; carbothermal reduction methods require high-temperature calcination and subsequent wet purification. These complex processes not only lead to low production efficiency and increased equipment investment, but also introduce risks of solvent residue, impurity introduction, and material oxidation due to exposure to air during multiple operations, directly affecting the purity and consistency of the final product. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a method for preparing lithium sulfide by one-step continuous dehydration and sulfidation of lithium hydroxide monohydrate.

[0011] A method for preparing lithium sulfide by one-step continuous dehydration and sulfidation of lithium hydroxide monohydrate includes the following steps: Dehydration pretreatment: Lithium hydroxide monohydrate is placed in a reactor, purged with inert gas, and heated to the first temperature range. Inert gas is continuously introduced for constant temperature dehydration treatment. Water vapor is discharged from the gas outlet at the top of the reactor to obtain anhydrous lithium hydroxide. Sulfidation reaction: The gas source of the reactor is switched from inert gas to hydrogen sulfide gas, and the reaction system is heated to the second temperature range to carry out gas-solid reaction; Post-processing: After the reaction is complete, the gas source of the reactor is switched back to inert gas for purging, and then cooled to room temperature under inert atmosphere protection to obtain lithium sulfide powder. The first temperature range is 105℃~155℃, and the second temperature range is 250℃~400℃.

[0012] This invention uses lithium hydroxide monohydrate as the lithium source for preparing lithium sulfide, which significantly reduces costs compared to the anhydrous LiOH or metallic lithium used in the prior art. The dehydration and sulfidation steps are carried out continuously and uninterruptedly in the same reaction vessel for the same batch of materials through program control, without any material transfer, separation or intermediate processing steps. This avoids the material from being exposed to air to absorb moisture, oxidize and introduce impurities, thus ensuring the high purity of the product.

[0013] Preferably, in the dehydration pretreatment, the heating rate is 2℃ / min to 10℃ / min, the inert gas flow rate is 100mL / min to 2000mL / min, the purging time is 10min to 30min, and the isothermal dehydration treatment time is 60min to 300min.

[0014] Preferably, in the sulfidation reaction, the heating rate is 5℃ / min to 15℃ / min, the flow rate of hydrogen sulfide gas is 100mL / min to 2000mL / min, and the gas-solid reaction time is 6h to 22h.

[0015] Preferably, the reactor is equipped with a stirring mechanism.

[0016] Preferably, the gas outlet pipe and valves of the reactor are insulated.

[0017] Preferably, the air outlet is connected to a TDLAS analyzer.

[0018] Preferably, the inert gas includes either argon or nitrogen.

[0019] Preferably, the lithium hydroxide monohydrate is spread evenly inside the reactor.

[0020] Preferably, the lithium hydroxide monohydrate is in powder or granular form.

[0021] During the dehydration process of lithium hydroxide monohydrate, the water vapor generated during dehydration condenses and flows back in the cold areas of the system (such as the outlet pipe and flange connection), causing material agglomeration and entrapment, which severely hinders the subsequent sulfidation reaction. Water vapor diffuses slowly inside the reaction vessel, especially in statically packed material beds, easily forming localized high-humidity areas, resulting in uneven dehydration and affecting the subsequent sulfidation reaction. This invention precisely places the gas outlet at the top of the reactor. On one hand, the water vapor generated by the reaction fills the entire container and spontaneously flows upward, forming an upward airflow. On the other hand, the introduced gas also first fills the entire container. Since the outlet is at the top, the unreacted gas will flow out from the top along with the water vapor. Therefore, this invention places the outlet at the top of the reactor to maintain a stable upward airflow. Utilizing the principle of natural convection of hot gas, it drives the high-temperature humid gas to be discharged rapidly upward, minimizing stagnation and condensation in cold areas such as side walls and flanges. Active heating and insulation are applied to key parts prone to "cold spots," such as exhaust pipes and valves, to ensure that their wall temperature is always higher than the dew point temperature of the process gas under the current conditions, thereby eliminating any possibility of condensate formation. A high-precision online dew point meter is equipped at the system exhaust port to monitor the absolute water vapor content in the exhaust gas in real time and continuously, ensuring that the water vapor concentration in the system is stably lower than the preset strict standard (e.g., ≤10 ppm) before switching to hydrogen sulfide gas.

[0022] During the sulfidation stage, a temperature gradient within the reactor can lead to over-sulfidation of the material in high-temperature regions (potentially generating polysulfides and other byproducts), while the reaction in low-temperature regions is insufficient, severely impacting the purity and uniformity of the final product. This invention introduces a mechanical stirring device into the reactor. During the dehydration stage, this device breaks up the static accumulation of the material, forcing internal moisture to diffuse outwards and preventing localized clumping; during the sulfidation stage, it ensures sufficient and uniform contact between the material and the reacting gases, promoting complete reaction.

[0023] Because the product lithium sulfide is highly hygroscopic and deteriorates easily, and reacts with oxygen at high temperatures to form lithium polysulfides, traditional open-top sampling analysis during production is not feasible to determine the completeness of the reaction. Otherwise, oxygen and moisture would be introduced, disrupting the inert atmosphere and contaminating the product. This invention employs a multi-channel TDLAS analyzer at the exhaust port during the sulfidation stage to simultaneously, in real-time, and continuously monitor the exhaust gas, calculating the hydrogen sulfide gas consumption rate and conversion rate in real time. When the consumption rate approaches zero, or the inlet and outlet concentrations are nearly identical, it indicates that the reaction is essentially complete.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention uses LiOH·H2O as the lithium source for preparing lithium sulfide, which is different from the existing technology (using anhydrous LiOH or metallic lithium), significantly reducing costs. It proposes a core process of "in-situ continuous dehydration and sulfidation one-step method", which completes the dehydration and sulfidation steps continuously and uninterruptedly in the same reaction vessel and with the same batch of materials through program control. There are no intermediate material transfer, separation or intermediate treatment steps, which avoids the absorption of moisture and oxidation (generating Li2O) and the introduction of impurities when exposed to air, thus ensuring the high purity of the product.

[0025] This technical solution can be implemented in common reaction equipment with programmed temperature control, closed-loop operation, and controllable atmosphere switching capabilities, such as tubular furnaces, fluidized bed reactors, or stirred tank reactors. This demonstrates the process's excellent scalability and industrialization potential. Detailed Implementation

[0026] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods.

[0027] The technical solution of this invention is designed as follows: using inexpensive and readily available lithium hydroxide monohydrate as the sole lithium source, in a closed reaction system, through precise programmed temperature control, the two key chemical reaction processes of water of crystallization removal and hydrogen sulfide gas-phase sulfidation are continuously completed within the same reaction vessel, ultimately directly yielding lithium sulfide product. The steps are as follows:

[0028] Step 1: System preparation and raw material loading 1. Raw material loading: Lithium hydroxide monohydrate is directly added to a reactor equipped with a stirring device and a gas outlet at the top. The material layer must be kept loose to ensure uniform heat transfer and gas diffusion during the subsequent reaction. In addition, key parts such as exhaust pipes and valves that are prone to forming "cold spots" are heat-traced and insulated.

[0029] 2. System sealing and purification: Close the reactor, open the valve for inert gas (such as high-purity argon or nitrogen), and introduce gas into the system at a flow rate of 100-2000 ml per minute for 10-30 minutes to completely remove air and water vapor from the reactor and pipelines, and establish an inert environment free of water and oxygen.

[0030] Step 2: Dehydration by temperature control 1. Start heating: Start the program temperature control system and start low-speed stirring to raise the temperature of the reaction zone from room temperature to the dehydration target temperature (T1) at a heating rate of 2-10℃ per minute.

[0031] 2. Dehydration Reaction: T1 is strictly controlled within the range of 105℃ to 155℃, with an optimal range of 115-125℃. At this temperature, continuous stirring is maintained to break up the static accumulation of materials, enhance mass transfer, and prevent agglomeration. Simultaneously, inert gas is continuously introduced as a carrier gas for 60-300 minutes. A high-precision online dew point meter is installed at the system exhaust port to continuously monitor the water vapor content in the exhaust gas in real time. This ensures that the water vapor concentration in the system is consistently below a stringent preset standard (e.g., 10 ppm) before switching to hydrogen sulfide gas. The reaction occurring during this process is: LiOH·H2O(s) → LiOH(s) + H2O(g)↑. The water of crystallization is completely removed and carried out of the system by the gas flow, and the raw material is converted in situ into highly active porous anhydrous lithium hydroxide solid.

[0032] Step 3: Switching gases and increasing temperature for vulcanization 1. Gas Switching: After the dehydration stage is completed, while maintaining the inert gas flow and the current temperature (T1), the gas source is switched from inert gas to hydrogen sulfide gas through the gas switching valve. This operation does not interrupt the reaction process, and the materials do not move in any way.

[0033] 2. Secondary heating: After the gas switching is completed, the temperature control program continues to run, which can increase the stirring speed to enhance mixing, and raise the temperature of the reaction zone from T1 to the target temperature of the sulfurization reaction (T2) at a heating rate of 5-15℃ per minute.

[0034] 3. Sulfidation reaction: Temperature (T2) is strictly controlled within the range of 250℃ to 400℃, with the optimal range being 300-400℃. At this temperature, stirring is maintained, and the hydrogen sulfide gas flow rate is adjusted to 100-2000 ml per minute to ensure sufficient contact between the hydrogen sulfide gas and the high-temperature anhydrous lithium hydroxide, facilitating the gas-solid phase reaction. The core reaction is: 2LiOH(s) + H2S(g) → Li2S(s) + 2H2O(g)↑. The water vapor produced as a byproduct of the reaction is promptly carried away by the gas flow.

[0035] Step 4: Isothermal vulcanization and reaction completion 1. Insulation reaction: Under temperature T2, a hydrogen sulfide atmosphere is maintained for an isothermal sulfidation reaction, typically lasting 6–22 hours. A multi-channel TDLAS analyzer is installed at the outlet to monitor the lithium sulfide concentration in the exhaust gas in real time to ensure complete reaction.

[0036] 2. Tail gas treatment: The tail gas (containing unreacted H2S and water vapor) flowing out during the reaction process is directly introduced into the tail gas absorption device for safe treatment.

[0037] Step 5: System Cooling and Product Collection 1. Inert protection cooling: After the reaction is completed, the inlet gas is switched back to inert gas. Under the protection of the inert atmosphere, the heating system is turned off, and the reactor is allowed to cool naturally or be cooled to room temperature by program control. Then the stirring is turned off.

[0038] 2. Product Collection: After the system cools down, open the reactor under an inert gas atmosphere and directly remove the reaction product to obtain white, high-purity lithium sulfide powder. The product does not require any intermediate or post-processing steps such as washing, filtering, drying, or pulverizing.

[0039] Example 1: 1. Weigh 100.0 g of lithium hydroxide monohydrate (LiOH·H2O) powder and place it into a vertical reactor equipped with a mechanical stirrer. An outlet is installed at the top of the reactor, connected to high-purity argon and hydrogen sulfide gas lines. The outlet pipe is equipped with a high-precision online dew point meter, and the exhaust pipe and valves are insulated and heated throughout the process using fiberglass insulation sleeves. The exhaust gas is ultimately introduced into an alkaline absorption bottle.

[0040] 2. Seal the reaction system and start stirring (30 rpm). Purge with argon gas at a flow rate of 300 mL / min for 30 minutes to ensure that the system is in an inert environment free of water and oxygen.

[0041] 3. Start the temperature ramp-up program, raising the temperature of the reaction zone to 120°C at a rate of 5°C / min. Once reached, maintain this temperature and continue to purge argon gas as a carrier gas at a flow rate of 100 mL / min for 120 minutes. Monitor the dew point using an online dew point meter. Once the dew point of the discharged gas is consistently below -60°C (approximately corresponding to a water vapor concentration below 10 ppm), it indicates that the water of crystallization has been completely removed.

[0042] 4. After dehydration, while maintaining 120℃ and an argon atmosphere, switch the inlet gas to hydrogen sulfide gas at a flow rate of 100 mL / min. Increase the stirring speed to 60 rpm, and then raise the reaction zone temperature to 350℃ at a rate of 5℃ / min. Maintain the reaction at 350℃ under a hydrogen sulfide atmosphere for 8 hours. Connect the outlet to a TDLAS analyzer to monitor the exhaust gas continuously in real time, and calculate the hydrogen sulfide gas consumption rate and conversion rate. When the consumption rate approaches zero, or the inlet and outlet concentrations are nearly identical, the reaction is considered essentially complete.

[0043] 5. After the reaction is complete, switch the inlet gas back to argon (300 mL / min), stop heating, and allow the reactor to cool naturally to room temperature under argon protection. Then stop stirring. Remove the product from the glove box to obtain a white powder with a purity greater than 99.9%.

[0044] Example 2: 1. Accurately weigh 100.00 grams of LiOH·H2O.

[0045] 2. Using the same vertical reactor system as in Example 1, add the raw materials into the reactor. Seal the reaction system and turn on the mechanical pump to evacuate the system to a low vacuum of 10. -1 After Pa, high-purity argon gas is introduced to a slightly positive pressure. This displacement operation is repeated three times to accelerate the removal of air from the container and ensure that the container is an anhydrous and oxygen-free environment. Finally, argon gas is continuously introduced at a stable flow rate of 300 mL / min under normal pressure.

[0046] 3. Start the heating and stirring program (30 rpm), setting the heating rate to 3℃ / min and the target temperature to 120℃. Once the temperature reaches 120℃, maintain this temperature and continuously introduce argon gas (flow rate 300 mL / min) as the carrier gas. Set the dehydration time to 120 minutes. During this period, monitor the dew point using a high-precision online dew point meter to confirm that the exhaust dew point meets the standard.

[0047] 4. After the dehydration stage is completed, without changing the temperature, switch the inlet gas from argon to hydrogen sulfide gas through the three-way valve, setting the flow rate to 300 mL / min. After switching the gas, increase the stirring speed to 50 rpm and immediately start the second stage of temperature program: raise the temperature of the reaction zone from 120℃ to the target sulfidation temperature of 300℃ at a rate of 5℃ / min.

[0048] 5. Once the temperature reaches and stabilizes at 300℃, start timing. At this temperature, maintain a hydrogen sulfide gas flow rate of 300 mL / min for isothermal vulcanization. During the reaction, monitor the progress using a multi-channel TDLAS analyzer. End this stage when the monitoring data indicates the reaction is complete. The total vulcanization reaction time in this example is approximately 8 hours.

[0049] 6. After the reaction is complete, shut off the hydrogen sulfide supply and simultaneously switch the three-way valve back to the argon gas path, purging at a flow rate of 300 mL / min. Initiate the cooling program, cooling the reaction zone to below 50°C at a rate of 2°C / min, and then stop stirring. Collect all solid products in a glove box filled with high-purity argon gas; the purity was found to be greater than 99.9%.

[0050] Example 3: Verifying the adaptability of the process to different physical states 1. Weigh 100.0 grams of columnar lithium hydroxide monohydrate granules (approximately 2 mm in diameter and 3-5 mm in length) obtained through extrusion granulation and sieving. Granular raw materials can effectively reduce system pressure drop and reduce dust entrainment.

[0051] 2. Spread the granular raw material evenly in the feed pan of a vertical reactor equipped with a stirring paddle, and purge the system as in Example 1.

[0052] 3. Start low-speed stirring (20 rpm) and heat to 125℃ at a rate of 5℃ / min. To overcome the resistance to water vapor diffusion inside the particles, increase the argon flow rate to 400 mL / min and dehydrate for 120 minutes. Use an online dew point meter to ensure that the core water of crystallization in the particles is completely removed.

[0053] 4. After dehydration, the inlet gas is switched to hydrogen sulfide gas at a flow rate of 300 mL / min, and the stirring speed is increased to 50 rpm to ensure effective gas penetration into the granular bed. The temperature is then increased to 350℃ at a rate of 10℃ / min, and the hydrogen sulfide concentration in the exhaust gas is monitored in real time using a multi-channel TDLAS analyzer to determine the endpoint. Complete reaction at 350℃ takes approximately 10 hours.

[0054] 5. Cool to room temperature under argon protection, stop stirring, and remove the product. The particle morphology remains intact, but the texture becomes brittle and fragile, with a purity greater than 99.9%.

[0055] Example 4: Scaled-up Implementation 1. Accurately weigh 5.00 kg of industrial-grade lithium hydroxide monohydrate powder.

[0056] 2. The raw materials were fed into a 50L pilot-scale reactor equipped with a stirrer and jacketed heating system using a vacuum feeding system. The reactor body was closed, and low-speed stirring (20 rpm) was started. The reactor was evacuated and purged with nitrogen three times, and finally a slight positive pressure (0.02 MPa) was maintained, with nitrogen continuously introduced (flow rate 2 L / min). The gas outlet pipe and valves of the reactor were insulated with fiberglass insulation sleeves, and a high-precision online dew point meter was connected.

[0057] 3. Start the heating system and raise the material temperature to 120℃ at an average heating rate of 5℃ / min, then maintain this temperature. Increase the stirring speed to 35 rpm and continue dehydration under these conditions for 240 minutes, recovering the dehydrated process water through a condenser. Monitor the process with an online dew point meter to ensure thorough dehydration.

[0058] 4. After dehydration is complete, close the nitrogen inlet valve. Introduce hydrogen sulfide gas into the reactor and control the system pressure to slowly rise to 0.15 MPa and maintain it stable through the back pressure valve.

[0059] 5. Under the conditions of maintaining stirring and the aforementioned pressure, the material temperature was increased from 120℃ to 350℃ at an average heating rate of 5℃ / min. At 350℃ and 0.15 MPa pressure, hydrogen sulfide was continuously introduced at a flow rate of 2 L / min to maintain pressure stability. The reaction process was monitored using a multi-channel TDLAS analyzer, and the hydrogen sulfide introduction rate was adjusted according to consumption to maintain pressure stability. The isothermal sulfidation reaction was carried out for 20 hours.

[0060] 6. After the reaction is complete, cut off the hydrogen sulfide supply and switch to nitrogen purging. Release the pressure inside the reactor to atmospheric pressure and continue purging for 30 minutes. Then, start the cooling system to reduce the material temperature to below 60°C and stop stirring.

[0061] 7. Under nitrogen protection, the product is transferred to a sealed container protected by inert gas through the discharge valve at the bottom of the reactor. The purity is tested to be greater than 99.9%.

[0062] It should be noted that when numerical ranges are mentioned in the claims of this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. To avoid redundancy, the present invention describes preferred embodiments.

[0063] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0064] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing lithium sulfide by one-step continuous dehydration and sulfidation of lithium hydroxide monohydrate, characterized in that, Includes the following steps: Dehydration pretreatment: Lithium hydroxide monohydrate is placed in a reactor, purged with inert gas, and heated to the first temperature range. Inert gas is continuously introduced for constant temperature dehydration treatment. Water vapor is discharged from the gas outlet at the top of the reactor to obtain anhydrous lithium hydroxide. Sulfidation reaction: The gas source of the reactor is switched from inert gas to hydrogen sulfide gas, and the reaction system is heated to the second temperature range to carry out gas-solid reaction; Post-processing: After the reaction is complete, the gas source of the reactor is switched back to inert gas for purging, and then cooled to room temperature under inert atmosphere protection to obtain lithium sulfide powder. The first temperature range is 105℃~155℃, and the second temperature range is 250℃~400℃.

2. The preparation method according to claim 1, characterized in that, In the dehydration pretreatment, the heating rate is 2℃ / min to 10℃ / min, the inert gas flow rate is 100mL / min to 2000mL / min, the purging time is 10min to 30min, and the isothermal dehydration treatment time is 60min to 300min.

3. The preparation method according to claim 1, characterized in that, In the sulfidation reaction, the heating rate is 5℃ / min to 15℃ / min, the flow rate of hydrogen sulfide gas is 100mL / min to 2000mL / min, and the gas-solid reaction time is 6h to 22h.

4. The preparation method according to claim 1, characterized in that, The reactor is equipped with a stirring mechanism.

5. The preparation method according to claim 1, characterized in that, The gas outlet pipes and valves of the reactor are insulated.

6. The preparation method according to claim 1, characterized in that, The air outlet is connected to a TDLAS analyzer.

7. The preparation method according to claim 1, characterized in that, The inert gas includes either argon or nitrogen.

8. The preparation method according to claim 1, characterized in that, The lithium hydroxide monohydrate is spread evenly inside the reactor.

9. The preparation method according to claim 1, characterized in that, The lithium hydroxide monohydrate is in powder or granular form.