Lithium sulfide synthesis method, lithium sulfide, and synthesis device

Abstract

The application relates to the technical field of solid-state battery material preparation, and relates to a lithium sulfide synthesis method, lithium sulfide and a synthesis device. The lithium sulfide synthesis method comprises the following steps: providing a reaction system, wherein the reaction system comprises a lithium source, a sulfur source and a composite solvent; the composite solvent comprises an aprotic solvent and a water-carrying agent; under the protection of an inert atmosphere, the reaction system is heated to a reaction temperature, so that the lithium source and the sulfur source react to generate lithium sulfide and by-product water; in the reaction process, the water-carrying agent and the by-product water form azeotropic mixture, and the azeotropic mixture is continuously removed from the reaction system, so as to maintain the low water activity state of the reaction system. Through the composite solvent system and the water-carrying agent and azeotropic water removal mechanism, the water activity in the reaction system can be reduced, the hydrolysis side reaction of lithium sulfide can be effectively inhibited, the selectivity and purity of the target product can be improved, and therefore, the lithium sulfide product with high purity can be obtained.

Description

Technical Field

[0001] This application relates to the field of solid-state battery material preparation technology, and to a method for synthesizing lithium sulfide, lithium sulfide, and synthesis apparatus. Background Technology

[0002] Lithium sulfide is a core raw material for sulfide solid-state batteries, and its purity, particle size, and other parameters directly affect the electrochemical performance of the electrolyte. Currently, the main synthesis routes for lithium sulfide include carbothermal reduction, elemental ball milling, and liquid-phase synthesis. Among these, the liquid-phase method has attracted attention due to its high whiteness and low impurity content. Existing liquid-phase methods typically use lithium hydroxide or similar materials as the lithium source, reacting it with hydrogen sulfide in a solvent.

[0003] However, this reaction process generates water as a byproduct. In existing single-solvent systems, the water generated during the reaction is difficult to remove in time, leading to water accumulation within the system. The presence of water not only limits the reaction kinetics but also easily causes hydrolysis of lithium sulfide, generating impurities such as lithium hydroxide. This results in insufficient purity of the final product, with excessive metal impurities and oxygen content, making it difficult to meet the requirements of high-purity lithium sulfide precursors for high-performance solid-state batteries. Summary of the Invention

[0004] To address the technical problems mentioned in the background section, this invention provides a method for synthesizing lithium sulfide, comprising: A reaction system is provided, comprising a lithium source, a sulfur source, and a composite solvent; the composite solvent comprises an aprotic solvent and a dehydrating agent. Under an inert atmosphere, the reaction system is heated to the reaction temperature, causing the lithium source and the sulfur source to react and generate lithium sulfide and water as a byproduct. During the reaction, the dehydrating agent is used to form an azeotropic mixture with the byproduct water, and the azeotropic mixture is continuously removed from the reaction system to maintain the low water activity state of the reaction system.

[0005] By employing a composite solvent system and utilizing a dehydrating agent and an azeotropic dehydration mechanism, this method can reduce the water activity in the reaction system, effectively suppress the hydrolysis side reaction of lithium sulfide, and improve the selectivity and purity of the target product, thereby obtaining a high-purity lithium sulfide product.

[0006] Optionally, the volume ratio of the aprotic solvent to the dehydrating agent is 1:10 to 10:1; the aprotic solvent includes at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide, or dimethyl sulfoxide; and the dehydrating agent includes at least one of benzene, toluene, xylene, ethylbenzene, pseudotrimethylbenzene, cumene, butylbenzene, n-hexane, n-heptane, cyclohexane, or methyl isobutyl ketone.

[0007] By optimizing the composition and ratio of the composite solvent, the stable operation of the azeotropic system is ensured, the dehydration kinetics are optimized, and the best azeotropic dehydration efficiency and reaction stability are achieved.

[0008] Optionally, before the lithium source reacts with the sulfur source, a pretreatment step is also included: heating the mixture containing the lithium source and the composite solvent to 110-160°C and maintaining the temperature for 1-4 hours to remove free water and some water of crystallization.

[0009] This pretreatment step effectively removes pre-existing moisture from the raw materials and solvents, creating initial conditions for subsequent low water activity reactions and avoiding interference with subsequent synthesis reactions.

[0010] Optionally, the step of heating the reaction system to the reaction temperature to allow the lithium source and the sulfur source to react includes: adding the sulfur source to the pretreated mixture containing the lithium source and the composite solvent, maintaining the reaction temperature at 110–160°C, and reacting for 1–4 hours; and continuing to raise the temperature to 160–200°C and reacting for 1–4 hours.

[0011] By precisely controlling the temperature and time of the main reaction, the reaction kinetics can be precisely controlled, avoiding side reactions caused by excessively high temperatures or incomplete reactions caused by excessively low temperatures, thus ensuring that the reaction is complete and controllable.

[0012] Optionally, the step of continuously removing the azeotropic mixture from the reaction system includes: introducing the gas-phase azeotropic mixture generated during the reaction into a condenser for condensation to obtain condensate and tail gas; and treating the tail gas safely to meet emission standards.

[0013] This step achieves the separation and recovery of solvent and water, while ensuring that exhaust gas emissions meet environmental safety requirements, and safely and efficiently separates and treats the aqueous azeotropic mixture and exhaust gas generated in the reaction.

[0014] Optionally, the reaction process between the lithium source and the sulfur source may include a step of replenishing a dehydrating agent: adding the dehydrating agent to the reaction system dropwise using a dropping device.

[0015] By dynamically replenishing solvent loss, the ratio of aprotic solvent to dehydrating agent is kept stable, ensuring continuous dehydration efficiency and compensating for the dehydrating agent lost due to azeotropic removal during continuous dehydration.

[0016] Optionally, during or after the reaction between the lithium source and the sulfur source, a step of adding a chelating agent is further included; the chelating agent is configured to coordinate with metal impurity ions in the reaction system to form a complex; the addition ratio of the chelating agent to the lithium source is 0.01% to 0.5%.

[0017] Metal impurities are captured through chemical complexation and retained in the liquid phase, facilitating subsequent separation and removal. This process removes residual metal impurity ions from the reaction system and prevents them from contaminating the final product.

[0018] Optionally, the chelating agent includes at least one of ethylenediamine, diethylenetriamine, triethylenetetramine, pyridine, bipyridine, phenanthrene, citric acid, oxalic acid, tartaric acid, acetylacetone, aminotrimethylphosphonic acid, or hydroxyethylidene diphosphonic acid.

[0019] This approach offers a wide range of chelating agents, enhancing the universality of removing different metal impurities, and allowing for the selection of appropriate chelating agents to suit different types of metal impurities.

[0020] Optionally, after the step of adding the chelating agent, the reaction system may further include: a cooling step, cooling the reaction system to 25–80°C; and a reduction step, adding a reducing agent to the reaction system, the reducing agent being configured to react with oxygen impurities in the reaction system; the addition ratio of the reducing agent is 0.1%–10%.

[0021] This method removes oxygen sources, inhibits the formation of oxidation byproducts such as polysulfides, further improves the chemical purity of the product, removes oxygen impurities and oxidizing substances from the system, and prevents lithium sulfide from being oxidized.

[0022] Optionally, the reducing agent includes at least one of hydrazine, hydrazine hydrate, monomethylhydrazine, dimethylhydrazine, carbazide, triethylphosphine, tripropylphosphine, trioctylphosphine, or triphenylphosphine.

[0023] To ensure the reduction reaction proceeds efficiently, eliminate potential oxidizing hazards within the system, and select a highly efficient reducing agent to remove oxygen impurities.

[0024] Optionally, after the reduction step, the process may include: a solid-liquid separation step, in which the reaction product cooled to 25–80°C is filtered to obtain a filter cake; and a washing step, in which the filter cake is washed with a cleaning agent.

[0025] Achieving solid-liquid separation, removing residual solvent and dissolved impurities adhering to the solid surface, and efficiently separating and cleaning high-purity lithium sulfide solid from the reaction solution.

[0026] Optionally, the cleaning agent includes at least one of n-hexane, n-heptane, cyclohexane, toluene, xylene, ethyl acetate, acetone, or tetrahydrofuran.

[0027] By utilizing the difference in solvent polarity, impurities can be effectively washed away while minimizing product loss. A suitable cleaning agent can be selected to remove impurities without dissolving the product.

[0028] Optionally, after the washing step, a drying step is also included, in which the washed solid product is dried under an inert atmosphere to obtain a high-purity lithium sulfide product.

[0029] To obtain a dry lithium sulfide product with low moisture content and high purity, prevent moisture absorption and deterioration during storage, and remove residual solvents and moisture from the solid product.

[0030] Another aspect of the present invention provides a lithium sulfide, which is produced by the lithium sulfide synthesis method described above.

[0031] This lithium sulfide product has high purity and low impurity content, making it directly suitable for the manufacture of high-performance solid-state batteries and meeting the raw material requirements for solid-state battery electrolytes.

[0032] Another aspect of the present invention provides an apparatus for synthesizing lithium sulfide, for preparing lithium sulfide by the lithium sulfide synthesis method described above, comprising: A reaction vessel used to contain the reaction system; Heating components are used to heat the reaction vessel; The dehydration assembly includes a condenser and a water separator. The inlet of the condenser is connected to the gas phase outlet of the reaction vessel, and the outlet of the condenser is connected to the inlet of the water separator, which is configured to remove the separated water.

[0033] This device enables the rational operation of the synthesis method, ensures the precise execution of process parameters and production safety, and provides a dedicated device for the industrial implementation of the aforementioned lithium sulfide synthesis method.

[0034] In summary, this invention solves the hydrolysis and impurity problems caused by the difficulty in removing water, a byproduct, during lithium sulfide synthesis by combining a composite solvent system with a dynamic dehydration mechanism. Furthermore, the deep purification steps involving chelating and reducing agents further reduce the content of metal and oxygen impurities. This technical solution improves the purity and consistency of the final lithium sulfide product while ensuring reaction efficiency, providing a reliable material foundation and process support for the large-scale preparation of high-performance solid-state batteries. Attached Figure Description

[0035] To more clearly illustrate the embodiments of this application, the relevant drawings will be briefly described below. It is understood that the drawings described below are only for illustrating some embodiments of this application, and those skilled in the art can obtain many other technical features and connections not mentioned herein based on these drawings.

[0036] Figure 1 This is a schematic diagram of the lithium sulfide synthesis method of this application; Figure 2 This is a schematic diagram illustrating the implementation steps of one embodiment of the lithium sulfide synthesis method of this application; Figure 3 This is a partial structural schematic diagram of the lithium sulfide synthesis apparatus of this application; Figure 4 This is another structural schematic diagram of the lithium sulfide synthesis apparatus of this application; Figure 5 The image shows the XRD phase characterization of lithium sulfide samples prepared by the lithium sulfide synthesis method of this application. Explanation of reference numerals in the attached figures: 1a. Reaction gas; 1b. Atmospheric gas; 2a. Main solvent; 2b. Dehydrating agent; 2c. Chelating agent; 2d. Impurity remover; 2e. Cleaning agent; 3. Reaction flask; 4. First oil bath; 5. Water separator; 6. Buffer bottle; 7. Absorption bottle; 8. Condenser; 81. Water inlet; 82. Water outlet; 9. Dropper; 10. Second oil bath; 11. Filter scrubber; 12. Collection bottle; 13. Dryer; 14. Vacuum packaging machine. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0038] The technical solutions in the embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0039] In the field of solid-state battery material preparation technology, sulfide solid electrolytes have attracted widespread attention due to their high ionic conductivity. Lithium sulfide, as a key precursor for sulfide solid electrolytes, has parameters such as purity, particle size, and crystal form that directly affect the electrochemical performance of the final electrolyte. Therefore, how to prepare high-purity lithium sulfide has become a core aspect of this technological field.

[0040] In a widely used existing technology, lithium hydroxide and N-methylpyrrolidone are typically used as raw materials, and hydrogen sulfide is introduced to react and prepare lithium sulfide. This method generally involves dehydrating lithium hydroxide under reduced pressure, mixing it with a solvent, and then reacting it with hydrogen sulfide at approximately 130 to 140 degrees Celsius for several hours. The product is then obtained by centrifugation, recrystallization, and drying. The equipment typically includes a reaction flask, a centrifuge, and a vacuum distillation apparatus, and the solvent can be recovered.

[0041] However, this existing technology relies on a single aprotic solvent system. Since NMP and water are miscible, the water byproduct generated during the reaction cannot be removed promptly during the reaction itself, but rather depends primarily on a high-temperature, low-pressure dehydration step after the reaction. This post-treatment dehydration method is inefficient and easily leads to water accumulation in the system. The residual water content not only restricts the shift of reaction equilibrium but also inevitably causes hydrolysis of lithium sulfide, generating impurities such as lithium hydroxide, thereby reducing the purity of the product.

[0042] Furthermore, due to the difficulty in removing moisture, the water activity within the reaction system is hard to maintain at a low level. A high water activity environment accelerates the hydrolysis of lithium sulfide, leading to excessive levels of metal and hydroxide impurities in the product. In addition, existing methods typically involve simple recrystallization purification only after the reaction is complete, lacking a dynamic impurity control mechanism during the reaction process. This results in limited removal rates for impurities such as metal ions, making it difficult to meet the stringent purity requirements of high-performance solid-state batteries for precursor materials, such as a total metal impurity content of less than 0.05%.

[0043] Those skilled in the art might consider improving water removal efficiency by increasing the reaction temperature or extending the dehydration time under reduced pressure. However, excessively high temperatures may reduce the conversion rate of the raw materials, while simply extending the dehydration time would significantly increase energy consumption and reduce production efficiency, and would not solve the problem of immediate hydrolysis caused by the real-time generation of water during the reaction. Therefore, there is a pressing technical challenge in the prior art: how to remove byproduct water in real time and efficiently during the reaction to maintain a low water activity environment and suppress side reactions.

[0044] In view of this, the embodiments of the present invention aim to provide a method for synthesizing lithium sulfide in order to solve the above-mentioned technical problems.

[0045] First Implementation Method This embodiment provides a method for synthesizing lithium sulfide, focusing on constructing a composite solvent system capable of coupling reaction and dehydration. This method introduces a dehydrating agent, utilizing its property of forming an azeotropic mixture with the reaction byproduct water, to continuously remove water from the system during the reaction process, thereby maintaining a low water activity state in the reaction environment and effectively inhibiting the hydrolysis of lithium sulfide.

[0046] Please see Figure 1 First, a reaction system is provided, comprising a lithium source, a sulfur source, and a composite solvent. In one specific embodiment, the lithium source may be, for example, anhydrous lithium hydroxide or lithium hydroxide monohydrate; alternatively, other soluble lithium salts such as lithium carbonate or lithium chloride may also be used. The sulfur source may be, for example, hydrogen sulfide gas, or, in another feasible embodiment, a solid sulfur source such as thiourea.

[0047] The composite solvent is key to implementing this scheme and can include an aprotic solvent and a dehydrating agent. In one specific embodiment, the aprotic solvent can be, for example, N-methylpyrrolidone (NMP), which exhibits good chemical stability and dissolution ability of the lithium source at the reaction temperature. To further optimize the solubility, the aprotic solvent may optionally include at least one of dimethylformamide (DMF), dimethylacetamide (DMAc), or dimethyl sulfoxide (DMSO). The dehydrating agent can be, for example, toluene, which is capable of forming a low-boiling-point azeotrope with water. Alternatively, the dehydrating agent may optionally include at least one of benzene, xylene, n-hexane, cyclohexane, or methyl isobutyl ketone (MIBK).

[0048] Regarding the ratio of the composite solvent, in a specific embodiment, the volume ratio of the aprotic solvent to the dehydrating agent can be controlled within the range of 1:10 to 10:1. For example, a ratio of 1:5 can be used when stronger dehydration capability is required; while a ratio of 5:1 can be used when better raw material solubility is required. Preferably, to balance solubility and dehydration efficiency, this volume ratio can be selected as 1:1. This specific ratio design aims to ensure that the solvent system can both fully disperse the reactants and efficiently remove the generated water through azeotropic distillation under heating conditions.

[0049] After the reaction system is constructed, it is heated to the reaction temperature under an inert atmosphere, causing the lithium source and sulfur source to react and generate lithium sulfide and water as a byproduct. In a specific embodiment, the inert atmosphere can be, for example, high-purity nitrogen or argon to prevent oxidation of the raw materials and products. The reaction temperature can be controlled, for example, between 110 and 160 degrees Celsius. Within this temperature range, the dehydrating agent and the water generated in the reaction can form a stable gas-phase azeotropic mixture.

[0050] During the reaction, an azeotropic mixture is formed between the water-carrying agent and the byproduct water. This azeotropic mixture is continuously removed from the reaction system to maintain a low water activity state. This process can be achieved as follows: the gaseous azeotropic mixture generated during the reaction is introduced into a condenser for condensation, yielding condensate and tail gas. The condensate enters a water separator, where, due to the density difference and immiscibility between the water-carrying agent and water, they separate into layers. The aqueous phase is retained or discharged, while the organic phase (water-carrying agent) can optionally be returned to the reaction system, or, in another operating mode, fresh water-carrying agent can be dynamically added to the reaction system via a dripping device.

[0051] To compensate for the loss of dehydrating agent due to azeotropic removal and maintain system stability, a dehydrating agent replenishment step can be included during the reaction of the lithium and sulfur sources. In a specific embodiment, the dehydrating agent can be added to the reaction system at a rate of 0.5 to 5 drops / second using a dropping device. For example, when a decrease in the organic phase refluxed in the water separator is observed, the dropping rate can be adjusted to 2 drops / second. This dynamic replenishment mechanism ensures that the ratio of aprotic solvent to dehydrating agent remains within a preset optimal range throughout the reaction, thereby guaranteeing the continuity of dehydration efficiency.

[0052] To ensure safe emission of the exhaust gas produced during the reaction, it can be introduced into an absorption bottle for safe treatment. In one specific embodiment, the absorption bottle may contain an alkaline solution to neutralize any trace amounts of acidic gas that may be entrained, thus meeting environmental emission standards.

[0053] By employing the aforementioned composite solvent system and dynamic dehydration mechanism, this embodiment significantly reduces the water activity within the reaction system. The ingenious aspect of this embodiment lies in achieving a simultaneous reaction-dehydration coupling mode. This mode overcomes the technical bias that water generation during the reaction leads to a shift in the hydrolysis equilibrium, suppressing side reactions from both thermodynamic and kinetic perspectives. Specifically, continuous water removal shifts the reaction equilibrium towards lithium sulfide formation, improving the selectivity of the target product; simultaneously, the low water activity environment prevents lithium sulfide hydrolysis, thereby significantly improving the purity of the final product, ensuring that the total metal impurity content can be stably controlled below 0.05%, meeting the stringent requirements for high-performance solid-state battery precursors.

[0054] Second Implementation Method Based on the reaction system constructed in the first embodiment, this embodiment further provides a full-process process control scheme, and integrates a deep impurity control strategy on this basis. The scheme focuses on the timeline operation control from raw material pretreatment to final product drying. Through precise temperature and time management and the synergistic introduction of chemical additives, metal impurity ions and oxygen impurities are targeted and removed at the molecular level, thereby breaking through the limits of simple physical separation and realizing the preparation of ultra-high purity lithium sulfide.

[0055] First, a pretreatment step may be included before the reaction between the lithium source and the sulfur source. In one specific embodiment, a mixture containing the lithium source and the composite solvent is heated to 110 to 160 degrees Celsius and held at that temperature for 1 to 4 hours to remove free water and some of the water of crystallization. For example, the mixture may be heated to 130 degrees Celsius and held at that temperature for 2 hours. As another feasible option, the heating temperature may also be optionally controlled at 110 degrees Celsius and held at that temperature for 4 hours to accommodate raw material systems with high heat sensitivity. Preferably, to prevent the raw materials from being oxidized during the pretreatment process, this step may be carried out under an inert atmosphere, such as high-purity nitrogen or argon.

[0056] By implementing the aforementioned pretreatment steps, pre-existing moisture in the raw materials and solvents can be effectively removed, preventing interference with subsequent synthesis reactions. The ingenious aspect of this approach lies in the fact that lithium sulfide synthesis is extremely sensitive to water activity. If trace amounts of free water or water of crystallization in the raw materials are not removed before the reaction, they will rapidly consume the sulfur source or lead to product hydrolysis in the early stages of the reaction. High-temperature isothermal treatment not only evaporates the free water but also promotes the detachment of some water of crystallization from the crystal lattice, creating initial conditions for subsequent low-water-activity reactions. This reduces the load on the dehydration components during the reaction and improves the overall dehydration efficiency.

[0057] In this embodiment, the step of heating the reaction system to the reaction temperature to allow the lithium source and sulfur source to react may include: adding the sulfur source to a pretreated mixture containing the lithium source and a composite solvent, maintaining the reaction temperature at 110 to 160 degrees Celsius, reacting for 1 to 4 hours, and then further heating to 160 to 200 degrees Celsius and reacting again for 1 to 4 hours. In one specific embodiment, in the first stage, the reaction temperature may be controlled, for example, at 140 degrees Celsius, and the reaction time is 3 hours. In another application scenario, to accelerate the reaction rate, the reaction temperature may be optionally adjusted to 160 degrees Celsius, and the reaction time is correspondingly shortened to 1 hour. After the first stage reaction, the temperature is raised again to above 160 degrees Celsius, for example, to 200 degrees Celsius, and reacted for approximately 1 to 4 hours, which is beneficial for obtaining lithium sulfide products. The sulfur source may be added by continuously introducing hydrogen sulfide gas, or, in another feasible approach, by adding a solid sulfur source in batches.

[0058] By precisely controlling the reaction temperature and time, precise control of reaction kinetics can be achieved, avoiding side reactions caused by excessively high temperatures or incomplete reactions caused by excessively low temperatures. Specifically, the temperature range of 110 to 200 degrees Celsius is the optimal reaction window, verified by numerous experiments. Within this range, the azeotropic dehydration efficiency of the composite solvent is the highest, while the reactivity of the lithium and sulfur sources is moderate. If the temperature is below 110 degrees Celsius, the azeotropic effect weakens, and the water removal efficiency is low; if the temperature is above 200 degrees Celsius, the solubility of the reaction gas in the reaction system decreases, resulting in a very low conversion rate. Therefore, the selection of this parameter range balances the contradiction between reaction rate and raw material utilization.

[0059] To further improve product purity and break through the limits of physical separation, a chelating agent can be added during or after the reaction between the lithium source and the sulfur source.

[0060] In one specific embodiment, the chelating agent can be configured to coordinate with metal impurity ions in the reaction system to form a complex. The addition ratio of the chelating agent to the lithium source can be controlled within the mass ratio range of 0.01% to 0.5%. For example, when the amount of lithium source is 100g, the amount of chelating agent added can be, for example, 0.1g (i.e., 0.1%). As another feasible option, if the metal impurity content in the raw material is high, this ratio can be optionally adjusted to 0.5%; if the purity of the raw material is high, this ratio can also be optionally reduced to 0.01%.

[0061] Regarding the specific type of chelating agent, in one particular embodiment, the chelating agent can be, for example, aminotrimethylphosphonic acid (ATMP) or hydroxyethylidene diphosphonic acid (HEDP), these phosphonic acid chelating agents have a very strong complexing ability for transition metal ions. Alternatively, amine compounds such as ethylenediamine, diethylenetriamine, and triethylenetetramine, or organic acid compounds such as citric acid, oxalic acid, and tartaric acid, or even nitrogen-containing heterocyclic compounds such as pyridine, bipyridine, and phenanthroline can be used. Preferably, to adapt to aprotic solvent environments, acetylacetone or triethylenetetramine, which have good solubility in organic solvents, can be selected.

[0062] By introducing a chelating agent, this embodiment can remove residual metal impurity ions from the reaction system. Traditional filtration and washing mainly remove unreacted solid particles or large-sized impurities, but have limited effectiveness in removing trace metal ions (such as Fe³⁺, Ni²⁺, Cu²⁺, etc.) dissolved in the solvent. The chelating agent alters the partitioning behavior of these metal ions in the solid-liquid phases by forming stable, high-molecular-weight complexes with them, making them more likely to remain in the liquid phase, thus enabling effective removal in subsequent solid-liquid separation steps. This embodiment reduces the content of transition metal impurities in the product to below 30 ppm.

[0063] Furthermore, after the chelating agent addition step, a cooling step and a reduction step may also be included. In a specific embodiment, the reaction system is first cooled to 25 to 80 degrees Celsius. For example, it can be naturally cooled to 60 degrees Celsius. Subsequently, a reducing agent is added to the reaction system, the reducing agent being configured to undergo a reduction reaction with oxygen impurities in the reaction system. The proportion of the reducing agent added can be controlled within the range of 0.1% to 10% by mass. For example, 0.5% hydrazine hydrate can be added as a reducing agent.

[0064] As another feasible option, the reducing agent may optionally include hydrazine compounds such as hydrazine, monomethylhydrazine, dimethylhydrazine, and carbazide, or phosphine compounds such as triethylphosphine, tripropylphosphine, trioctylphosphine, and triphenylphosphine. Preferably, considering reactivity and safety, hydrazine hydrate or triphenylphosphine are commonly used choices.

[0065] By adding a reducing agent, this embodiment can remove oxygen impurities and oxidizing substances from the system, preventing lithium sulfide from being oxidized. The technical principle is that lithium sulfide easily comes into contact with trace amounts of oxygen or generates oxidation byproducts such as polysulfides during preparation. These impurities can severely affect the electrochemical performance of the solid electrolyte. The reducing agent can preferentially react with dissolved oxygen, peroxides, or polysulfides in the system, converting them into harmless products (such as nitrogen gas, oxide precipitates, etc.), thereby protecting lithium sulfide from oxidation. This approach, combined with the aforementioned step of adding a chelating agent, constructs a comprehensive impurity defense system, ensuring that the chemical purity of the final product reaches over 99.9%.

[0066] After completing the above chemical reaction and temperature control, a solid-liquid separation step and a washing step may be included to obtain a solid product. In one specific embodiment, the reaction mixture containing the chelate complex and the reduction product is cooled to 25 to 80 degrees Celsius and then filtered to obtain a filter cake. Subsequently, the filter cake is washed with a cleaning agent.

[0067] For example, n-hexane can be used as a cleaning agent, and washing can be performed three times. Alternatively, the cleaning agent may optionally include at least one of n-heptane, cyclohexane, toluene, xylene, ethyl acetate, acetone, or tetrahydrofuran. Preferably, to obtain the best cleaning effect and reduce product loss, a mixed solvent washing method can be used, for example, first washing with a small amount of acetone to remove polar impurities, and then washing with n-hexane to remove organic residues; the number of washing cycles can be controlled between three and six.

[0068] Solid-liquid separation and washing can achieve solid-liquid separation, removing residual solvents, chelating agent complexes, and reduction byproducts adhering to the solid surface. The technical principle lies in utilizing the compatibility differences between the cleaning agent and the composite solvent and impurities to displace the residual reaction liquid from the lithium sulfide solid surface. Multiple washing operations can significantly reduce the amount of residual impurities in the filter cake, especially removing unreacted lithium ion sources, metal complexes, or byproduct salts, thereby further improving the chemical purity of the product.

[0069] Following the washing step, a drying step may also be included. In one specific embodiment, the washed solid product is dried under an inert atmosphere to obtain a high-purity lithium sulfide product. For example, it can be vacuum dried at 80 degrees Celsius for 12 hours under nitrogen protection. As another feasible option, the drying temperature can also be optionally adjusted to 60 degrees Celsius, and the drying time extended to 24 hours to avoid the influence of high temperature on the crystal form of the product. The inert atmosphere can be, for example, an argon atmosphere filling a glove box, or in another operating mode, a flowing nitrogen purging the drying chamber.

[0070] Drying under an inert atmosphere allows for the production of low-moisture, high-purity dried lithium sulfide products, preventing moisture absorption and deterioration during storage. The beneficial effect lies in the fact that lithium sulfide is highly hygroscopic, rapidly absorbing moisture and hydrolyzing when exposed to air. Drying in an inert atmosphere eliminates the contact path between the product and airborne water and oxygen, ensuring that the final product's moisture content remains at a low level, such as below 50 ppm, thus meeting the raw material standards for the synthesis of high-performance solid-state battery electrolytes.

[0071] This embodiment also includes a step of treating the tail gas after the reaction. Since the reaction system will produce irritating gases such as hydrogen sulfide during the reaction, the method of this application sets up tail gas treatment equipment such as an absorption bottle at the gas phase outlet of the reaction equipment. The tail gas treatment equipment is equipped with a tail gas absorbent, such as an alkaline solution or other strong oxidant, to absorb the tail gas generated by the reaction and make the tail gas meet the safety standards before being discharged.

[0072] Specifically, the exhaust gas absorbent can be selected from one or more of sodium hydroxide, sodium hypochlorite, sodium thiosulfate, calcium hydroxide, calcium hypochlorite, potassium hydroxide, and hydrogen peroxide; this embodiment is not limited to a single one.

[0073] In summary, this implementation method integrates precise thermal process control with a thorough chemical impurity removal strategy, forming a complete closed-loop process. From pretreatment dehydration to reaction temperature control, then to chelation for metal removal and reduction for oxygen removal, and finally washing and drying, each step is interconnected, jointly ensuring the high purity and high stability of the final product.

[0074] Third Implementation Method Based on the reaction system architecture provided in the first embodiment and the process control flow provided in the second embodiment, this embodiment, combined with a specific device configuration, further provides a specific execution example of a lithium sulfide synthesis method. This embodiment details the complete operational steps from raw material input to finished product packaging, and coordinates the corresponding equipment components to achieve industrial-scale implementation of the process and product consistency.

[0075] Please see Figure 2 The specific implementation example of the lithium sulfide synthesis method of this application includes the following steps: Step S1: Construct a composite solvent system.

[0076] like Figure 3 As shown, in a specific embodiment, the main solvent 2a (i.e., the aprotic solvent) and the dehydrating agent 2b are mixed and added to the reaction flask 3. The volume ratio of the main solvent 2a to the dehydrating agent 2b can be from 1:10 to 10:1. For example, N-methylpyrrolidone can be selected as the main solvent 2a and toluene as the dehydrating agent 2b. The composite solvent system has good dispersibility and can achieve azeotropic removal of water under heating conditions. During operation, the solvent can be introduced into the reaction flask 3 by metering pump or manual measurement and stirred evenly under an inert atmosphere.

[0077] Step S2: Raw material addition and pretreatment.

[0078] Please see Figure 3 The lithium source is added to the composite solvent system, and heated to 110-160 degrees Celsius in an oil bath 4 under an inert atmosphere. The temperature is maintained for 1-4 hours to remove free water and some of the water of crystallization from the system. In one specific embodiment, the inert atmosphere can be achieved by introducing an atmospheric gas 1b, which can be, for example, high-purity nitrogen or argon. The first oil bath 4 provides a uniform heat source to ensure stable material temperature within the reaction flask 3. The purpose of this step is to create an initial environment with low water activity for subsequent reactions, preventing moisture in the raw materials from interfering with the formation of lithium sulfide.

[0079] Step S3: Introduce hydrogen sulfide to react and remove the byproduct water.

[0080] A predetermined volume of reaction gas, such as hydrogen sulfide, is introduced into the pretreated system, and then the addition of hydrogen sulfide is stopped. In the first stage, the reaction system is maintained at 110 to 160 degrees Celsius for 1 to 4 hours. Then, in the second stage, the temperature is further increased to 160 to 200 degrees Celsius for another 1 to 4 hours to obtain lithium sulfide product. In one specific embodiment, reaction gas 1a is introduced through a pipeline below the liquid phase of reaction bottle 3 to increase the gas-liquid contact area. The water-carrying agent 2b is condensed by condenser 8 and recovered in water separator 5. The cooling water inlet 81 and outlet 82 of condenser 8 are connected to an external circulating water system to ensure condensation efficiency. Irritating gases such as hydrogen sulfide are safely discharged after being absorbed by alkaline solution or other strong oxidants in absorption bottle 7 after passing through buffer bottle 6.

[0081] During the reaction, the dehydrating agent 2b forms an azeotropic mixture with water, continuously removing the generated water from the reaction system to maintain a low water activity state. The dehydrating agent 2b can be replenished via the dropper 9 at a rate of 0.5 to 5 drops per second.

[0082] Chelating agent 2c is added during or after the reaction. The ratio of chelating agent 2c to lithium source can be 0.01% to 0.5%. This step enables the reaction and dehydration to proceed simultaneously, and the exhaust gas treatment system ensures the safety of the operation.

[0083] Step S4: Impurity removal and filtration washing.

[0084] like Figure 4 As shown, reaction flask 3 is transferred to a second oil bath 10 and cooled to 25 to 80 degrees Celsius. Reducing agent 2d is added, with the addition ratio ranging from 0.1% to 10%. After thorough stirring, the mixture is transferred to a filter washer 11 for solid-liquid separation of the reaction product. The product is then washed multiple times using cleaning agent 2e, for example, 3 to 6 times, to remove residual impurities. The filtrate can be piped into a collection bottle 12 for centralized treatment.

[0085] In one specific embodiment, the filter scrubber 11 may be equipped with a microporous filter plate or filter cloth to achieve efficient solid-liquid separation. This step, through a combination of chemical reduction and physical washing, deeply removes oxygen impurities and residual solvents from the system.

[0086] Step S5: Drying and Packaging.

[0087] The resulting solid product is dried under a protective atmosphere 1b to obtain a high-purity lithium sulfide product. In one specific embodiment, the solid product is transferred to a dryer 13 for heated drying. The dried product is then sealed in a glove box using a vacuum packaging machine 14. The glove box maintains a low-moisture, low-oxygen environment, for example, a water content of less than 1 ppm and an oxygen content of less than 1 ppm. The vacuum packaging machine 14 seals the product in a barrier bag to prevent moisture absorption and deterioration during storage. This step ensures the physical stability of the final product, facilitating transportation and subsequent use.

[0088] Through the orderly execution of the above five steps, this implementation method transforms abstract process parameters into specific equipment operation actions. This step-by-step method tightly couples the principles of chemical reaction with the functions of engineering equipment. For example, the combination of the water separator 5 and the condenser 8 enables continuous water removal, while the combination of the glove box and the vacuum packaging machine 14 enables anhydrous packaging of the product. This fully enclosed and automated operation minimizes the risk of contamination from human intervention, ensures consistency in product quality between batches, and provides a reproducible engineering model for the large-scale preparation of high-purity lithium sulfide.

[0089] Fourth Implementation Method Based on the synthesis methods provided in the first to third embodiments described above, this embodiment provides a lithium sulfide product obtained by the above methods. Due to its high purity and low impurity content, this product is particularly suitable as a precursor material for sulfide solid electrolytes and for use in the manufacture of high-performance solid-state batteries.

[0090] Please see Figure 5 In one specific embodiment, the purity of the lithium sulfide product can reach, for example, 99.9% or higher. As another feasible approach, by optimizing the impurity control steps in the aforementioned synthesis process, the purity of the product can optionally be increased to 99.95% or even 99.99%. This high purity characteristic is achieved through the synergistic effect of dehydration in the aforementioned composite solvent system and deep impurity removal by chemical additives.

[0091] Regarding the impurity content in the product, in one specific embodiment, the total metal impurity content of the product, detected by inductively coupled plasma mass spectrometry (ICP-MS), can be controlled, for example, below 0.05%. Further, the transition metal impurity content can be, for example, below 30 ppm. Transition metal impurities can include elements such as iron, nickel, and copper. As another feasible option, the alkali metal impurity content can be, for example, below 400 ppm. Alkali metal impurities can include elements such as sodium, potassium, calcium, magnesium, and aluminum. In addition, the trace metal impurity content can be, for example, below 30 ppm. These trace metal impurities can include elements such as chromium, titanium, zinc, and vanadium. These specific impurity limits are preferred ranges set based on the stringent requirements for the purity of raw materials in solid-state battery electrolytes.

[0092] By strictly controlling the content of metal impurities, the lithium sulfide product provided in this embodiment can significantly reduce the risk of side reactions in the subsequent electrolyte synthesis process. The underlying ingenuity lies in the fact that transition metal impurities often catalyze the decomposition of sulfide electrolytes or cause abnormally high electronic conductivity, thereby triggering internal short circuits in the battery. Controlling such impurities at the ppm level can block these failure pathways at the source, improving the reliability of the final battery assembly.

[0093] By maintaining an extremely low moisture content, this embodiment effectively prevents the hydrolysis of lithium sulfide during storage and transportation. The underlying principle is that lithium sulfide readily reacts with water to form hydrogen sulfide and lithium hydroxide, which not only reduces product purity but also poses safety hazards. The low moisture content ensures the chemical stability of the product when used as a precursor, avoiding fluctuations in electrolyte performance due to raw material deterioration.

[0094] Regarding the application of the product, in one specific embodiment, lithium sulfide can be used as a raw material to prepare sulfide solid electrolytes. The sulfide solid electrolyte can be, for example, lithium thiophosphate (LPSC), lithium thiogermanium phosphate (LGPS), or derivatives thereof. Alternatively, the lithium sulfide can also be optionally used to prepare other sulfur-containing lithium-ion battery cathode materials. Preferably, the lithium sulfide is particularly suitable for high-energy-density solid-state battery systems sensitive to impurities, such as in the electrolyte synthesis stage for drone power batteries or AI data center energy storage batteries.

[0095] By providing high-purity lithium sulfide, this embodiment can improve the ionic conductivity of the final solid-state electrolyte. The beneficial effect lies in the fact that the ionic conductivity of an electrolyte material is closely related to its lattice integrity and impurity concentration. Electrolytes synthesized using high-purity lithium sulfide exhibit lower grain boundary resistance and more unobstructed lithium-ion transport channels, thereby enabling the battery to achieve higher rate performance and longer cycle life. This directly meets the current demands of solid-state battery technology for increasingly demanding performance indicators of key raw materials.

[0096] Fifth Implementation Method Based on the methods and products described in the first to fourth embodiments, this embodiment provides a dedicated apparatus for implementing the above-described lithium sulfide synthesis method. This apparatus aims to achieve closed-loop operation of the synthesis process, ensuring precise execution of process parameters and production safety, and completing the reaction and post-processing in a low-water, low-oxygen environment.

[0097] like Figure 3 and Figure 4As shown, in one specific embodiment, the lithium sulfide synthesis apparatus may include a reaction vessel, a heating assembly, and a dehydration assembly. The reaction vessel is used to contain the reaction system. In one specific embodiment, the reaction vessel includes the aforementioned reaction flask 3, which may be, for example, a three-necked flask or a high-pressure reactor. Its material may optionally include borosilicate glass, stainless steel, duplex stainless steel, high-nickel alloy, titanium, zirconium, tantalum, or Hastelloy, to meet the reaction requirements of different corrosive environments. The reaction vessel is typically equipped with a stirring port, gas inlet / outlet, and temperature monitoring port to ensure uniform material mixing and real-time status monitoring.

[0098] The heating assembly is configured to heat the reaction vessel. In one specific embodiment, the heating assembly may be, for example, the first oil bath 4 and the second oil bath 10 mentioned above. Figure 3 As shown, the reaction flask 3 can be placed in the first oil bath 4, where heat is transferred through a heating medium. Alternatively, the heating assembly may optionally include an electric heating mantle or an infrared heating lamp, the heating power of which can be adjusted according to the volume of the reaction system, for example, controlled between 500 watts and 2000 watts. Preferably, the heating assembly is equipped with a temperature controller to achieve precise constant temperature control of 110 to 160 degrees Celsius, with temperature fluctuations controlled, for example, within ±1 degree Celsius.

[0099] like Figure 3 As shown, the dehydration assembly may include the aforementioned condenser 8 and water separator 5. In a specific embodiment, the inlet of the condenser 8 is connected to the gas phase outlet of the reaction flask 3. The condenser 8 may be, for example, a straight condenser tube or a spherical condenser tube, and its cooling medium may optionally include circulating water or an ethylene glycol solution. The outlet of the condenser 8 is connected to the inlet of the water separator 5. The water separator 5 is used to remove the separated water. For example, the water separator 5 may be based on the principle of density difference, allowing the condensed organic phase to flow back to the reaction flask 3, while the aqueous phase accumulates at the bottom and is discharged through a valve. This configuration enables continuous separation of the reaction-generated water without interrupting the reaction process.

[0100] Furthermore, the device may also include a gas handling system. In one specific embodiment, the gas handling system may include a reaction gas inlet and an ambient gas inlet. A reaction gas 1a, such as hydrogen sulfide, may be introduced through a conduit to the bottom of the reaction vessel 3 to increase the gas-liquid contact area. An ambient gas 1b, such as nitrogen or argon, may be introduced through another conduit to maintain an inert atmosphere within the system.

[0101] The apparatus may also include an exhaust gas treatment unit. For example, the exhaust gas treatment unit includes the aforementioned buffer bottle 6 and absorption bottle 7. The exhaust gas is buffered by the buffer bottle 6 and then introduced into the absorption bottle 7. The absorption bottle 7 may contain an alkaline solution to neutralize unreacted acidic gases, achieving safe emission standards. The presence of the buffer bottle 6 prevents the absorbent from being drawn back into the reaction system, thus providing a safety buffer.

[0102] After the reaction is complete, the device can also integrate a filtration and washing unit. In one specific embodiment, such as... Figure 4 As shown, the filter washing unit can be the aforementioned filter washer 11. The filter washer 11 can be connected to the reaction flask 3 via a pipeline, or it can serve as an independent unit to receive transferred materials. The filter washer 11 can be equipped with a microporous filter plate or filter cloth to achieve efficient solid-liquid separation. The filtrate can be piped into a collection bottle 12 for centralized treatment. The cleaning agent 2e can be added to the filter washer 11 via a spray device or pipeline to wash the filter cake multiple times.

[0103] Furthermore, the apparatus may also include a drying and packaging unit. In one specific embodiment, the drying and packaging unit includes a dryer 13 and a vacuum packaging machine 14. The dryer 13 may be, for example, a vacuum drying oven or a fluidized bed dryer, and its interior may be circulated with an inert gas for protection. The vacuum packaging machine 14 may be located inside a glove box. The glove box provides a low-water, low-oxygen operating environment, for example, a water content of less than 1 ppm and an oxygen content of less than 1 ppm. The product completes the entire process from drying to packaging within the glove box, avoiding contact with outside air.

[0104] Through the integration of the aforementioned hardware, the synthesis apparatus provided in this embodiment enables automated and closed-loop operation of the synthesis method. This apparatus connects the reaction, dehydration, impurity removal, drying, and packaging processes into a coherent system via hardware connections, reducing the risk of material exposure to air during transfer. In particular, the direct connection between the dehydration component and the reaction vessel allows for the hardware-based implementation of the dynamic dehydration mechanism, ensuring a low water activity environment throughout the reaction process. Furthermore, the integration of the glove box and vacuum packaging machine 14 solves the technical challenge of lithium sulfide products' susceptibility to moisture absorption and deterioration, ensuring the storage stability of the final product. This apparatus configuration is not only suitable for small-scale laboratory preparations but can also be extended to industrial production scenarios by scaling up the reaction vessel size and heating power, demonstrating excellent engineering scalability.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for synthesizing lithium sulfide, characterized in that, include: A reaction system is provided, the reaction system comprising a lithium source, a sulfur source, and a composite solvent; the composite solvent comprises an aprotic solvent and a dehydrating agent; Under an inert atmosphere, the reaction system is heated to the reaction temperature, causing the lithium source and the sulfur source to react and generate lithium sulfide and water as a byproduct. During the reaction, the dehydrating agent forms an azeotropic mixture with the byproduct water, and the azeotropic mixture is continuously removed from the reaction system to maintain the low water activity state of the reaction system.

2. The method for synthesizing lithium sulfide according to claim 1, characterized in that, The volume ratio of the aprotic solvent to the dehydrating agent is 1:10 to 10:1; The aprotic solvent includes at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide, or dimethyl sulfoxide; The dehydrating agent includes at least one of benzene, toluene, xylene, ethylbenzene, pseudotrimethylbenzene, cumene, butylbenzene, n-hexane, n-heptane, cyclohexane, or methyl isobutyl ketone.

3. The method for synthesizing lithium sulfide according to claim 1, characterized in that, Before the lithium source reacts with the sulfur source, a pretreatment step is included: heating the mixture containing the lithium source and the composite solvent to 110-160°C and maintaining the temperature for 1-4 hours to remove free water and some water of crystallization.

4. The method for synthesizing lithium sulfide according to claim 3, characterized in that, The step of heating the reaction system to the reaction temperature to allow the lithium source and the sulfur source to react includes: The sulfur source is added to the pretreated mixture containing the lithium source and the composite solvent, and the reaction temperature is maintained at 110-160°C for 1-4 hours; the temperature is then increased to 160-200°C and the reaction is continued for 1-4 hours.

5. The method for synthesizing lithium sulfide according to claim 1, characterized in that, The step of continuously removing the azeotropic mixture from the reaction system includes: The gas-phase azeotropic mixture generated during the reaction is introduced into a condenser for condensation to obtain condensate and tail gas. The exhaust gas is treated safely to meet emission standards.

6. The method for synthesizing lithium sulfide according to claim 5, characterized in that, The reaction process between the lithium source and the sulfur source also includes a step of replenishing the dehydrating agent: the dehydrating agent is added dropwise to the reaction system using a dripping device.

7. The method for synthesizing lithium sulfide according to any one of claims 1 to 6, characterized in that, The reaction between the lithium source and the sulfur source may include a step of adding a chelating agent during or after the reaction. The chelating agent is configured to coordinate with metal impurity ions in the reaction system to form a complex; the addition ratio of the chelating agent to the lithium source is 0.01% to 0.5%.

8. The method for synthesizing lithium sulfide according to claim 7, characterized in that, The chelating agent includes at least one of ethylenediamine, diethylenetriamine, triethylenetetramine, pyridine, bipyridine, phenanthrene, citric acid, oxalic acid, tartaric acid, acetylacetone, aminotrimethylphosphonic acid, or hydroxyethylidene diphosphonic acid.

9. The method for synthesizing lithium sulfide according to claim 7, characterized in that, Following the step of adding the chelating agent, the method further includes: The cooling step involves cooling the reaction system to 25–80°C. The reduction step involves adding a reducing agent to the reaction system, the reducing agent being configured to react with oxygen impurities in the reaction system; the proportion of the reducing agent added is 0.01% to 10%.

10. The method for synthesizing lithium sulfide according to claim 9, characterized in that, The reducing agent includes at least one of hydrazine, hydrazine hydrate, monomethylhydrazine, dimethylhydrazine, carbazide, triethylphosphine, tripropylphosphine, trioctylphosphine, or triphenylphosphine.

11. The method for synthesizing lithium sulfide according to claim 9, characterized in that, Following the restoration step, the method further includes: In the solid-liquid separation step, the reaction product, cooled to 25–80°C, is filtered to obtain a filter cake; Washing step: Wash the filter cake with a cleaning agent.

12. The method for synthesizing lithium sulfide according to claim 11, characterized in that, The cleaning agent includes at least one of n-hexane, n-heptane, cyclohexane, toluene, xylene, ethyl acetate, acetone, or tetrahydrofuran.

13. The method for synthesizing lithium sulfide according to claim 11, characterized in that, Following the washing step, the process also includes: The drying step involves drying the washed solid product under an inert atmosphere to obtain a high-purity lithium sulfide product.

14. A lithium sulfide, characterized in that, It is prepared by the lithium sulfide synthesis method as described in any one of claims 1 to 13.

15. A lithium sulfide synthesis apparatus, characterized in that, A method for preparing lithium sulfide by the lithium sulfide synthesis method according to any one of claims 1 to 13, comprising: A reaction vessel for containing the reaction system; A heating assembly for heating the reaction vessel; A dehydration assembly includes a condenser and a water separator, wherein the inlet of the condenser is connected to the gas phase outlet of the reaction vessel, the outlet of the condenser is connected to the inlet of the water separator, and the water separator is configured to remove the separated water.

Images