Battery-grade lithium sulfide preparation method based on microwave-assisted gas-solid phase reaction and in-situ coating

Abstract

The invention belongs to the technical field of lithium-sulfur battery materials, and particularly relates to a preparation method of battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating. According to the preparation method, high-efficiency and low-consumption preparation of the battery-grade lithium sulfide is realized by adopting a technical path of combining gas-solid phase reaction of a microwave plasma activated sulfur source and a lithium source, adopting a supercritical fluid-assisted in-situ carbon coating process and adopting an alcohol washing-supercritical CO2 drying purification system; meanwhile, the problems of low product purity, high energy consumption, poor stability and the like in the traditional process are solved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-sulfur battery material technology, specifically relating to a method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating. Background Technology

[0002] Battery-grade lithium sulfide (Li₂S), as a highly promising cathode material for lithium-sulfur batteries and a precursor for sulfide solid electrolytes, plays a crucial role in next-generation high-energy-density energy storage systems. Compared to traditional lithium-ion batteries, lithium-sulfur batteries boast a theoretical specific capacity of up to 1675 mAh / g and a theoretical energy density of up to 2600 Wh / kg, making them an important development direction for next-generation high-energy-density batteries. Meanwhile, sulfide solid electrolytes, due to their high ionic conductivity, good mechanical properties, and wide electrochemical window, have become an ideal choice for solid-state lithium batteries, and Li₂S is the core precursor for synthesizing most sulfide solid electrolytes.

[0003] However, current methods for preparing Li2S, such as solvent methods, high-temperature solid-state methods, aqueous methods, and carbothermal reduction methods, all suffer from problems such as insufficient purity, low reaction efficiency, high energy consumption, and poor safety, making it difficult to meet the stringent requirements of battery-grade applications. For example, patent application CN118954546A uses an organic solvent method, employing expensive and dangerous raw materials such as butyllithium and methanethiol, resulting in high costs and safety hazards. Ganfeng Lithium's patent application CN116040587A uses a direct high-temperature solid-phase reaction between metallic lithium and sulfur, which improves purity but requires high temperatures (>600℃), leading to high energy consumption and product sintering. Taihe Technology's patent application CN120622414A uses an aqueous phase method, which, while lower in cost, typically results in product purity ≤99.5%, and carries the risk of hydrolysis leading to even lower purity. Additionally, there are methods using low-cost lithium sulfate as a raw material and carbothermic reduction to prepare Li2S, such as Zhejiang University's patent application CN110137462A, which uses lithium sulfate and a carbon source for high-temperature reduction. However, this method requires high reaction temperatures (>800℃) and presents challenges such as difficulty in controlling carbon impurities in the product and difficulty in purifying and removing impurities after the reaction.

[0004] Microwave-assisted gas-solid phase reaction and in-situ coating technology have great potential in the preparation of battery-grade Li2S materials due to their high efficiency, energy saving, and excellent product performance. Microwave heating technology, based on the principle of dielectric loss, offers advantages such as volumetric heating, rapid temperature rise, and selective heating, significantly improving reaction efficiency and product quality. Simultaneously, in-situ coating technology can construct a protective coating while Li2S particles are forming, effectively enhancing their chemical stability and electrochemical performance. Summary of the Invention

[0005] This invention provides a method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating. This method combines microwave plasma activation of a sulfur source with a lithium source gas-solid phase reaction, employs a supercritical fluid-assisted in-situ carbon coating process, and utilizes an alcohol washing-supercritical CO2 drying purification system. This achieves efficient and low-energy-consumption preparation of battery-grade lithium sulfide, while simultaneously solving the problems of low product purity, high energy consumption, and poor stability in traditional processes. The lithium sulfide prepared by this invention has a purity ≥99.86%, an initial discharge specific capacity of up to 1120 mAh / g, and a capacity retention rate exceeding 70% after 100 cycles.

[0006] The method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating includes the following steps: (1) Microwave plasma activation of sulfur: Sulfur powder is introduced into a microwave plasma reactor and activated into active sulfur species under an inert atmosphere using microwave plasma to obtain activated sulfur powder. (2) Fluidized bed sulfidation reaction: Battery-grade lithium hydroxide is placed in a fluidized bed reactor and an inert gas is introduced. The temperature is maintained at 300-400℃ for 30-60 min to dehydrate the battery-grade lithium hydroxide and generate porous LiOH. The activated sulfur powder is then introduced into the fluidized bed reactor in the form of gas flow and reacted completely at a certain temperature to obtain crude Li2S. (3) Supercritical fluid-assisted carbon coating: The crude Li2S and the carbon source are initially mixed in an inert atmosphere at a certain mass ratio. The mixture is then transferred to a supercritical CO2 reactor. Liquid CO2 is slowly injected into the reactor, and the temperature and pressure are increased to make the CO2 reach the supercritical state and react for a period of time. After the pressure is slowly released, the material is then subjected to carbonization heat treatment in an inert atmosphere to form a uniform carbon coating layer on the carbon source, thus obtaining carbon-coated Li2S solid. (4) Purification and supercritical fluid drying: The carbon-coated Li2S solid was washed and filtered with organic solvent to obtain wet material. The wet material was placed in a supercritical reactor and liquid CO2 was introduced. The temperature and pressure were controlled to make the CO2 reach the supercritical state and circulate to remove the residual organic solvent. The organic solvent was further recovered and recycled through a condenser on one side. Finally, the pressure was slowly released to obtain dry and loose carbon-coated battery-grade Li2S powder.

[0007] Furthermore, in step (1), the conditions for obtaining the activated sulfur powder are: microwave frequency of 2.45 GHz, power of 5-10 kW (preferably 5 kW), plasma temperature of 200-400 ℃ (preferably 350 ℃); and inert atmosphere of argon or nitrogen.

[0008] Furthermore, in step (2), the preferred condition for dehydration to generate porous LiOH is to maintain the temperature at 380°C for 30 min.

[0009] Furthermore, in step (1): the average particle size of the sulfur powder is 5-30 μm, preferably 20 μm.

[0010] Furthermore, the reaction conditions in step (2) are: temperature of 380-480 ℃ and time of 30-60 min (preferably temperature of 450 ℃ and time of 45 min).

[0011] Furthermore, in step (3), the carbon source is pitch.

[0012] Furthermore, the mass ratio of crude Li2S to carbon source is 100:(5-10) (preferably 100:8).

[0013] Furthermore, the heating and pressurization conditions described in step (3) are: pressure of 15-25 MPa and temperature of 100-150℃ (preferably temperature of 120℃ and pressure of 20 MPa).

[0014] Furthermore, the conditions for carbonization heat treatment in step (3) are: carbonization heat treatment at 500-700 °C for 0.5-3 h (preferably carbonization heat treatment at 600 °C for 1 h).

[0015] Furthermore, in step (4): the organic solvent is anhydrous ethanol, and the number of washing times is 2-4 times (preferably 3 times); the control of a certain temperature and pressure is: temperature is 35-45 ℃, pressure is 10-14 MPa (preferably temperature is 40 ℃, pressure is 12 MPa).

[0016] Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) Microwave plasma activation technology is adopted to selectively excite sulfur molecules through microwave energy, thereby reducing the activation energy of the reaction and lowering the reaction temperature from the traditional >600℃ to below 480℃, resulting in a significant reduction in energy consumption. In addition, gas-solid phase reaction and fluidized bed design are carried out to utilize the efficient contact between the obtained porous LiOH and gaseous sulfur species, shortening the reaction time to within 1 h and eliminating solvent pollution.

[0017] (2) Using supercritical fluid-assisted coating method, the high diffusivity and low viscosity of supercritical CO2 enable the carbon source to uniformly coat Li2S particles. Under the optimal carbon coating conditions, the coating layer is dense and uniform, which can improve its air stability, increase the specific surface area, and has excellent electrochemical performance. It not only improves the first discharge specific capacity, but also has a high capacity retention rate after 100 cycles.

[0018] (3) The purification system of alcohol washing-supercritical CO2 drying can not only improve the purity of the product and ensure no solvent residue, but also obtain high-performance carbon-coated battery-grade lithium sulfide material with original nanostructure, no agglomeration and high specific surface area by completely avoiding the destructive effect of capillary force. Moreover, the ethanol and CO2 used in the purification process can be recycled and reused, making the process green and environmentally friendly. Detailed Implementation

[0019] In this invention, the pitch-based carbon source is a carbon precursor obtained by processing coal tar pitch (coal tar distillation residue). The specific processing steps are as follows: 1) Grinding: Ball mill or grind until the particle size D50 is 5μm.

[0020] 2) Pretreatment and pre-oxidation: Treat at 200℃ for 3 hours in an inert atmosphere, and then pre-oxidize in air at 300℃ for 5 hours.

[0021] 3) Carbonization and activation: Carbonize at 500℃ in an inert atmosphere for 4 hours, and then activate with CO2 at 1000℃ for 8 hours.

[0022] For the above treatment, medium-temperature coal tar pitch is selected, and its key indicators are: softening point 75-95℃ and quinoline insoluble matter (QI) content QI<0.1%.

[0023] The typical characteristic parameters of the pitch-based carbon material obtained by this method are: specific surface area of ​​1300 m². 2 / g, total pore volume is 1.2 cm³ 3 / g, pore size distribution is micropores or mesopores (25 nm).

[0024] In this invention, acetylene black was purchased from Aladdin.

[0025] In this invention, only asphalt can be used as a carbon precursor to achieve in-situ coating, while acetylene black cannot, for the following reasons: Due to its unique physical properties, coal tar pitch is flowable and has good plasticity after heating or treatment. After pretreatment and carbonization, it can form abundant pores and a continuous coating layer with a high specific surface area. Therefore, it can combine with active substances to achieve in-situ coating, forming a core-shell structure with a strong bond. Acetylene black, on the other hand, is a solid powder, a chain-like nanoparticle, and has no flowability and a low specific surface area. Therefore, it can only be physically mixed, and the contact is insufficient, making it easy to separate and unable to achieve in-situ coating.

[0026] The sulfur (sublimed sulfur) used in the following examples is of analytical grade.

[0027] Example 1 A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating includes the following steps: (1) Sulfur powder with an average particle size of 20 μm was introduced into a microwave plasma reactor at a rate of 500 g / h. The microwave frequency was 2.45 GHz and the power was 8 kW. The plasma temperature was maintained at 350℃ under an Ar atmosphere. The sulfur powder was activated into active sulfur species by microwave plasma to obtain activated sulfur powder.

[0028] (2) 500 g of battery-grade lithium hydroxide (lithium hydroxide monohydrate, LiOH content ≥ 56.5%, the same below) was placed in a fluidized bed reactor and Ar gas was introduced. The reactor was heated to 380°C at a heating rate of 10 °C / min and held for 30 min to dehydrate it and generate porous LiOH. Under the Ar atmosphere, the activated sulfur powder (Li:S molar ratio of 2:1, the same below) was introduced into the fluidized bed reactor in the form of gas flow. The temperature was raised to 450°C and reacted at this temperature for 45 min to obtain crude Li2S.

[0029] (3) The crude Li2S and the pitch-based carbon source were initially mixed in an Ar atmosphere at a mass ratio of 100:8 to obtain a mixture. The mixture was transferred to a supercritical CO2 reactor, and liquid CO2 was injected into the reactor at a rate of 1.5 MPa / min. Then, the temperature was raised to 120°C and the pressure was increased to 20 MPa at a rate of 3 °C / min to make it reach the supercritical state (temperature above 31.1°C, pressure above 7.38 MPa). After the reaction was completed, the pressure was slowly released at a rate of 0.2 MPa / min. The material was then subjected to carbonization heat treatment at 600°C for 1 hour in an Ar atmosphere to convert the pitch-based carbon source into a uniform carbon coating layer, thereby obtaining carbon-coated Li2S solid.

[0030] (4) Purification of the carbon-coated Li₂S solid: After washing three times with anhydrous ethanol and filtering, a wet material was obtained. The wet material was placed in a supercritical reactor, and CO₂ was introduced. The temperature was controlled at 40°C and the pressure at 12 MPa, so that the CO₂ reached a supercritical state and circulated for 4 h to remove the organic solvent ethanol. The ethanol was further recovered and recycled through a condenser on one side. Finally, the pressure was slowly released at a rate of 0.2 MPa / min to obtain dry, loose carbon-coated battery-grade lithium sulfide. The purity of the prepared battery-grade Li₂S was tested, and its electrochemical performance was tested in a standard coin cell as a positive electrode material. The results are shown in Table 1.

[0031] Example 2 A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating includes the following steps: (1) Sulfur powder with an average particle size of 20 μm was introduced into a microwave plasma reactor at a rate of 500 g / h. The microwave frequency was 2.45 GHz and the power was 8 kW. The plasma temperature was maintained at 320℃ in an Ar atmosphere. The sulfur powder was activated into active sulfur species by microwave plasma to obtain activated sulfur powder.

[0032] (2) 500 g of battery-grade lithium hydroxide was placed in a fluidized bed reactor and Ar gas was introduced. The temperature was increased to 380°C at a rate of 10 °C / min and held for 30 min to dehydrate it and generate porous LiOH. Under the Ar atmosphere, the activated sulfur powder was then introduced into the fluidized bed reactor in the form of a gas stream. The temperature was raised to 450°C and reacted at this temperature for 45 min to obtain crude Li2S.

[0033] (3) The crude Li2S and the pitch-based carbon source were initially mixed in an Ar atmosphere at a mass ratio of 100:8 to obtain a mixture. The mixture was transferred to a supercritical CO2 reactor, and liquid CO2 was injected into the reactor at a rate of 1.5 MPa / min. Then, the temperature was raised to 120°C and the pressure was increased to 20 MPa at a rate of 3 °C / min to make it reach the supercritical state (temperature above 31.1°C, pressure above 7.38 MPa). After the reaction was completed, the pressure was slowly released at a rate of 0.2 MPa / min. The material was then subjected to carbonization heat treatment at 500°C for 1 hour in an Ar atmosphere to convert the pitch-based carbon source into a uniform carbon coating layer, thereby obtaining carbon-coated Li2S solid.

[0034] (4) Purification of the carbon-coated Li₂S solid: After washing three times with anhydrous ethanol and filtering, a wet material was obtained. The wet material was placed in a supercritical reactor, and CO₂ was introduced. The temperature was controlled at 45°C and the pressure at 12 MPa, so that the CO₂ reached a supercritical state and circulated for 4 h to remove the organic solvent ethanol. The ethanol could be further recovered and recycled through a condenser on one side. Finally, the pressure was slowly released at a rate of 0.2 MPa / min to obtain dry, loose carbon-coated battery-grade lithium sulfide. The purity of the prepared battery-grade Li₂S was tested, and its electrochemical performance was tested in a standard coin cell as a positive electrode material. The results are shown in Table 1.

[0035] Example 3 A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating includes the following steps: (1) Sulfur powder with an average particle size of 20 μm was introduced into a microwave plasma reactor at a rate of 500 g / h. The microwave frequency was 2.45 GHz and the power was 8 kW. The plasma temperature was maintained at 350℃ in an Ar atmosphere. The sulfur powder was activated into active sulfur species by microwave plasma to obtain activated sulfur powder.

[0036] (2) 500 g of battery-grade lithium hydroxide was placed in a fluidized bed reactor and Ar gas was introduced. The temperature was increased to 390°C at a rate of 10 °C / min and held for 30 min to dehydrate it and generate porous LiOH. Under the Ar atmosphere, the activated sulfur powder was then introduced into the fluidized bed reactor in the form of a gas stream. The temperature was raised to 450°C and reacted at this temperature for 45 min to obtain crude Li2S.

[0037] (3) The crude Li2S and the pitch-based carbon source were initially mixed in an Ar atmosphere at a mass ratio of 100:8 to obtain a mixture. The mixture was transferred to a supercritical CO2 reactor, and liquid CO2 was injected into the reactor at a rate of 1.5 MPa / min. Then, the temperature was raised to 120°C and the pressure was increased to 20 MPa at a rate of 3 °C / min to make it reach the supercritical state (temperature above 31.1°C, pressure above 7.38 MPa). After the reaction was completed, the pressure was slowly released at a rate of 0.2 MPa / min. The material was then subjected to carbonization heat treatment at 700°C for 1 hour in an Ar atmosphere to convert the pitch-based carbon source into a uniform carbon coating layer, thereby obtaining carbon-coated Li2S solid.

[0038] (4) Purification of the carbon-coated Li₂S solid: After washing three times with anhydrous ethanol and filtering, a wet material was obtained. The wet material was placed in a supercritical reactor, and CO₂ was introduced. The temperature was controlled at 40°C and the pressure at 12 MPa, so that the CO₂ reached a supercritical state and circulated for 4 h to remove the organic solvent ethanol. The ethanol could be further recovered and recycled through a condenser on one side. Finally, the pressure was slowly released at a rate of 0.2 MPa / min to obtain dry, loose carbon-coated battery-grade lithium sulfide. The purity of the prepared battery-grade Li₂S was tested, and its electrochemical performance was tested in a standard coin cell as a positive electrode material. The results are shown in Table 1.

[0039] Comparative Example 1 A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating differs from Example 1 in that it omits the step of microwave plasma activation of sulfur in step (1) and replaces it with direct high-temperature gasification, including the following steps: (1) Under an Ar atmosphere, sulfur powder with an average particle size of 20 μm is heated to 450 °C to completely vaporize it, and the sulfur vapor is maintained at the temperature for later use.

[0040] (2) 500 g of battery-grade lithium hydroxide was placed in a fluidized bed reactor and Ar gas was introduced. The temperature was increased to 380°C at a rate of 10 °C / min and held for 30 min to dehydrate it and generate porous LiOH. Under the Ar atmosphere, the sulfur vapor was introduced into the fluidized bed reactor in the form of a gas stream, the temperature was raised to 450°C, and the reaction was carried out at this temperature for 45 min to obtain crude Li2S.

[0041] (3) The crude Li2S and the pitch-based carbon source were initially mixed in an Ar atmosphere at a mass ratio of 100:8 to obtain a mixture. The mixture was transferred to a supercritical CO2 reactor, and liquid CO2 was injected into the reactor at a rate of 1.5 MPa / min. Then, the temperature was raised to 120°C and the pressure was increased to 20 MPa at a rate of 3 °C / min to make it reach the supercritical state (temperature above 31.1°C, pressure above 7.38 MPa). After the reaction was completed, the pressure was slowly released at a rate of 0.2 MPa / min. The material was then subjected to carbonization heat treatment at 600°C for 1 hour in an Ar atmosphere to convert the pitch-based carbon source into a uniform carbon coating layer, thereby obtaining carbon-coated Li2S solid.

[0042] (4) Purification of the carbon-coated Li2S solid: After washing three times with anhydrous ethanol and filtering, a wet material was obtained. The wet material was placed in a supercritical reactor, and CO2 was introduced. The temperature was controlled at 40°C and the pressure at 12 MPa, so that the CO2 reached a supercritical state and circulated for 4 h to remove the organic solvent ethanol. The ethanol could be further recovered and recycled through a condenser on one side. Finally, the pressure was slowly released at a rate of 0.2 MPa / min to obtain a dry and loose control sample. The purity of the prepared control sample was tested, and it was used as a positive electrode material to perform electrochemical performance tests in a standard coin cell. The results are shown in Table 1.

[0043] Comparative Example 2 A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating differs from Example 1 in that mechanical dry mixing is used instead of supercritical CO2-assisted mixing in step (3), including the following steps: (1) Sulfur powder with an average particle size of 20 μm was introduced into a microwave plasma reactor at a rate of 500 g / h. The microwave frequency was 2.45 GHz and the power was 8 kW. The plasma temperature was maintained at 350℃ under an Ar atmosphere. The sulfur powder was activated into active sulfur species by microwave plasma to obtain activated sulfur powder.

[0044] (2) 500 g of battery-grade lithium hydroxide was placed in a fluidized bed reactor and Ar gas was introduced. The temperature was increased to 380°C at a rate of 10°C / min and held for 30 min to dehydrate it and generate porous LiOH. Under the Ar atmosphere, the activated sulfur powder was introduced into the fluidized bed reactor in the form of a gas stream. The temperature was raised to 450°C and reacted at this temperature for 45 min to obtain crude Li2S.

[0045] (3) The crude Li2S and the pitch-based carbon source were mechanically dry-mixed in a ball mill at a mass ratio of 100:8 in an Ar atmosphere at a speed of 350 rpm for 3 hours. Then, the material was subjected to carbonization heat treatment at 600°C in an Ar atmosphere for 1 hour to perform carbon coating treatment on the Li2S solid, and carbon-coated Li2S solid was obtained.

[0046] (4) Purification of the carbon-coated Li₂S solid: After washing three times with anhydrous ethanol and filtering, a wet material was obtained. The wet material was placed in a supercritical reactor, and CO₂ was introduced. The temperature was controlled at 40°C and the pressure at 12 MPa, so that the CO₂ reached a supercritical state and circulated for 4 h to remove the organic solvent ethanol. The ethanol could be further recovered and recycled through a condenser on one side. Finally, the pressure was slowly released at a rate of 0.2 MPa / min to obtain a dry and loose control sample. The purity of the prepared control sample was tested, and it was used as a positive electrode material to perform electrochemical performance tests in a standard coin cell. The results are shown in Table 1.

[0047] Comparative Example 3 A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating differs from Example 1 in that conventional vacuum drying is used instead of supercritical CO2 drying in step (4), and includes the following steps: (1) Sulfur powder with an average particle size of 20 μm was introduced into a microwave plasma reactor at a rate of 500 g / h. The microwave frequency was 2.45 GHz and the power was 8 kW. The plasma temperature was maintained at 350℃ in an Ar atmosphere. The sulfur powder was activated into active sulfur species by microwave plasma to obtain activated sulfur powder.

[0048] (2) 500 g of battery-grade lithium hydroxide was placed in a fluidized bed reactor and Ar gas was introduced. The temperature was increased to 380°C at a rate of 10 °C / min and held for 30 min to dehydrate it and generate porous LiOH. Under the Ar atmosphere, the activated sulfur powder was then introduced into the fluidized bed reactor in the form of a gas stream. The temperature was raised to 450°C and reacted at this temperature for 45 min to obtain crude Li2S.

[0049] (3) The crude Li2S and the pitch-based carbon source were initially mixed in an Ar atmosphere at a mass ratio of 100:8 to obtain a mixture. The mixture was transferred to a supercritical CO2 reactor, and liquid CO2 was injected into the reactor at a rate of 1.5 MPa / min. Then, the temperature was raised to 120°C and the pressure was increased to 20 MPa at a rate of 3 °C / min to make it reach the supercritical state (temperature above 31.1°C, pressure above 7.38 MPa). After the reaction was completed, the pressure was slowly released at a rate of 0.2 MPa / min. The material was then subjected to carbonization heat treatment at 600°C for 1 hour in an Ar atmosphere to convert the pitch-based carbon source into a uniform carbon coating layer, thereby obtaining carbon-coated Li2S solid.

[0050] (4) Purification of the carbon-coated Li2S solid: After washing three times with anhydrous ethanol and filtering, a wet material was obtained. The wet material was then placed in a vacuum drying oven and dried for 4 hours at 80°C. Finally, a dry and loose control sample was obtained. The purity of the prepared control sample was tested, and it was used as the positive electrode material to perform electrochemical performance tests in a standard coin cell. The results are shown in Table 1.

[0051] Comparative Example 4 A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating differs from Example 1 in that acetylene black is used instead of pitch-based carbon source in step (3), and includes the following steps: (1) Sulfur powder with an average particle size of 20 μm was introduced into a microwave plasma reactor at a rate of 500 g / h. The microwave frequency was 2.45 GHz and the power was 8 kW. The plasma temperature was maintained at 350℃ under an Ar atmosphere. The sulfur powder was activated into active sulfur species by microwave plasma to obtain activated sulfur powder.

[0052] (2) 500 g of battery-grade lithium hydroxide was placed in a fluidized bed reactor and Ar gas was introduced. The temperature was increased to 380°C at a rate of 10°C / min and held for 30 min to dehydrate it and generate porous LiOH. Under the Ar atmosphere, the activated sulfur powder was introduced into the fluidized bed reactor in the form of a gas stream. The temperature was raised to 450°C and reacted at this temperature for 45 min to obtain crude Li2S.

[0053] (3) The crude Li2S and acetylene black carbon source were initially mixed in an Ar atmosphere at a mass ratio of 100:8 to obtain a mixture. The mixture was transferred to a supercritical CO2 reactor, and liquid CO2 was injected into the reactor at a rate of 1.5 MPa / min. Then, the temperature was raised to 120°C and the pressure was increased to 20 MPa at a rate of 3 °C / min to make it reach the supercritical state (temperature above 31.1°C, pressure above 7.38 MPa). After the reaction was completed, the pressure was slowly released at a rate of 0.2 MPa / min. The material was then subjected to carbonization heat treatment at 600°C for 1 hour in an Ar atmosphere to convert the acetylene black carbon source into a uniform carbon coating layer, thus obtaining carbon-coated Li2S solid.

[0054] (4) Purification of the carbon-coated Li₂S solid: After washing three times with anhydrous ethanol and filtering, a wet material was obtained. The wet material was placed in a supercritical reactor, and CO₂ was introduced. The temperature was controlled at 40°C and the pressure at 12 MPa, so that the CO₂ reached a supercritical state and circulated for 4 h to remove the organic solvent ethanol. The ethanol could be further recovered and recycled through a condenser on one side. Finally, the pressure was slowly released at a rate of 0.2 MPa / min to obtain a dry and loose control sample. The purity of the prepared control sample was tested, and it was used as a positive electrode material to perform electrochemical performance tests in a standard coin cell. The results are shown in Table 1.

[0055] Table 1. Comparison of sample purity and assembled battery performance in Examples 1-3 and Comparative Examples 1-4 Note: The product purity in the table refers to the purity of Li2S, excluding the carbon coating layer.

[0056] Note: The data detected in the examples of this invention were tested using ICP, IC and other testing instruments. The initial discharge specific capacity and the capacity retention rate after 100 cycles were tested by the Blue Battery Testing System CT3002A.

[0057] As shown in Table 1, compared with Example 1, Comparative Example 1 showed significantly higher purity (99.96%) and initial capacity (1120 mAh / g) than Comparative Example 1 (99.50%, 980 mAh / g). Comparative Example 1 did not use microwave plasma activation, resulting in low sulfur vapor reactivity and incomplete reaction with LiOH. Residual LiOH or Li₂O impurities reduced product purity and electrochemical performance. Compared with Example 1, Comparative Example 2 lacked supercritical CO₂-assisted mixing, relying solely on mechanical mixing. This resulted in uneven dispersion of the pitch-based carbon source, severe agglomeration of the carbonized product, and a reduced specific surface area to 8.5 m². 2The concentration of carbon in Example 3 was significantly reduced, severely impacting ion transport and leading to a substantial decrease in capacity and cycle performance. Compared to Example 1, Comparative Example 3 showed similar purity and initial capacity, but Example 1 exhibited a much larger specific surface area and superior cycle performance. This was due to the vacuum drying process, which caused nanoparticles to agglomerate due to the surface tension of water, resulting in a loss of more than half of the specific surface area. This lengthened the ion transport path and reduced the number of active sites during charge and discharge, exacerbating capacity decay. Compared to Example 1, Comparative Example 4, although possessing higher purity (99.90%), only achieved physical mixing using acetylene black carbon, without any coating effect. The poor conductive contact between carbon and lithium sulfide resulted in poor stability in air, as well as poor capacity and cycle performance in battery applications.

[0058] A comparison of Examples 1-3 shows that, through adjustment and optimization of experimental conditions, the lithium sulfide obtained by the method in Example 1 has the best purity and application performance. In Examples 2 and 3, the effects of low-temperature carbonization (500°C) are significant: incomplete carbonization and poor coating quality lead to reduced conductivity, cycle performance, and stability in batteries. Conversely, the effects of excessively high-temperature carbonization (700°C) include: excessive graphitization and the risk of interfacial side reactions (consuming active material and increasing interfacial impedance), reduced air stability of the sample, and decreased capacity and cycle performance in batteries.

Claims

1. A method for preparing battery-grade lithium sulfide based on microwave-assisted gas-solid phase reaction and in-situ coating, comprising the following steps: (1) microwave plasma activation of sulfur: passing sulfur powder into a microwave plasma reactor, using microwave plasma to activate the sulfur powder into active sulfur species under an inert atmosphere, to obtain activated sulfur powder; (2) fluidized bed sulfuration reaction: placing battery-grade lithium hydroxide in a fluidized bed reactor and passing inert gas, keeping at 300-400℃ for 30-60 min to make the battery-grade lithium hydroxide dehydrate to form porous LiOH; then introducing the activated sulfur powder into the fluidized bed reactor in the form of gas flow, reacting completely at a certain temperature to obtain Li2S crude product; (3) supercritical fluid-assisted carbon coating: preliminarily mixing the Li2S crude product with a carbon source in a certain mass ratio under an inert atmosphere, transferring the mixture to a supercritical CO2 reactor, slowly injecting liquid CO2 into the reactor, increasing temperature and pressure, making CO2 reach the supercritical state and reacting for a period of time, then slowly releasing the pressure, and then performing carbonization heat treatment of the material under an inert atmosphere to make the carbon source form a uniform carbon coating layer, to obtain carbon-coated Li2S solid; (4) purification and supercritical fluid drying: washing and filtering the carbon-coated Li2S solid using an organic solvent to obtain wet material, placing the wet material in a supercritical reactor, passing in liquid CO2, controlling a certain temperature and pressure, making CO2 reach the supercritical state and circulating to take away residual organic solvent, and further recycling the organic solvent through a condensing device on one side; finally slowly releasing the pressure to obtain dry and loose carbon-coated battery-grade Li2S powder.

2. The method of claim 1, wherein the lithium sulfide is prepared by the steps of: In step (1), the conditions for obtaining the activated sulfur powder are: microwave frequency of 2.45 GHz, power of 5-10 kW, and plasma temperature of 200-400℃; the inert atmosphere is argon or nitrogen. ​ 3. The method for preparing battery-grade lithium sulfide according to claim 1, characterized in that, In step (1), the average particle size of the sulfur powder is 5-30 μm.

4. The method for preparing battery-grade lithium sulfide according to claim 1, characterized in that, In step (2), the conditions for preparing the Li2S crude product are: temperature of 380-480℃ and time of 30-60 min.

5. The method for preparing battery-grade lithium sulfide according to claim 1, characterized in that, In step (3), the carbon source is pitch.

6. The method for preparing battery-grade lithium sulfide according to claim 1, characterized in that, The mass ratio of the Li2S crude product to the carbon source is 100: (5-10).

7. The method for preparing battery-grade lithium sulfide according to claim 1, characterized in that, In step (3), the temperature and pressure increase are: pressure of 15-25 MPa and temperature of 100-150℃.

8. The method of claim 1, wherein the lithium sulfide is prepared by the process comprising: In step (3), the carbonization heat treatment conditions are: carbonization heat treatment at 500-700℃ for 0.5-3 h. ​ 9. The method for preparing battery-grade lithium sulfide according to claim 1, characterized in that, In step (4), the organic solvent is anhydrous ethanol, and the washing is performed 2-4 times; the certain temperature and pressure are: temperature of 35-45℃ and pressure of 10-14 MPa.

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