A method for preparing high-purity lithium sulfide

Abstract

The application relates to a preparation method of high-purity lithium sulfide, which comprises the following steps: (1) mixing lithium sulfate and a carbon source, and obtaining a first product through a first heating reaction under vacuum conditions; (2) obtaining high-purity lithium sulfide through a second heating reaction of the first product in argon-hydrogen mixed gas. The high-purity lithium sulfide is prepared by using a two-step relay reduction method of vacuum carbon thermal pre-reduction and hydrogen deep reduction, compared with the method for preparing lithium sulfide by using carbon thermal reduction or hydrogen reduction alone in the prior art, all solid impurities are converted into gaseous products and discharged in the front end in the preparation method, the product is a battery-grade product after collection, the complicated liquid-solid separation and solvent evaporation steps in the traditional process are fundamentally bypassed, the process flow is greatly simplified, and the operation risk is reduced.

Description

Technical Field

[0001] This application relates to the field of inorganic compound synthesis technology, and in particular to a method for preparing high-purity lithium sulfide. Background Technology

[0002] With the transformation of the global energy structure and the advancement of "dual-carbon" goals, the lithium-ion battery industry, represented by electric vehicles and large-scale energy storage, has experienced explosive growth. The market has placed more stringent demands on battery energy density, cycle life, and safety, prompting cathode material technology to continuously iterate towards higher capacity and lower cost. Against this backdrop, lithium sulfide (Li2S), as a key precursor for preparing high-energy-density lithium-sulfur battery cathode materials and sulfide all-solid-state electrolytes, has become increasingly strategically important. Therefore, developing high-quality, low-cost lithium sulfide preparation processes has become an urgent need in the current new energy materials field.

[0003] Lithium sulfide, as a core precursor in lithium-sulfur batteries, directly determines the final performance of the cathode material due to its purity. Impurities not only trigger side reactions and exacerbate the polysulfide shuttle effect, leading to rapid capacity decay and shortened cycle life, but also accumulate at grain boundaries, hindering ion transport, increasing interfacial impedance, and reducing rate performance. Furthermore, conductive impurities can easily induce micro-short circuits and lithium dendrite growth, posing serious safety hazards, while residual moisture can interfere with material crystallization and dispersion, affecting the preparation process. Therefore, preparing high-purity lithium sulfide is a crucial prerequisite for ensuring high energy density, long cycle life, and safety in batteries.

[0004] Currently, existing lithium sulfide synthesis processes mainly include mechanical ball milling, high-temperature and high-pressure methods, organic solvent methods, and carbothermal reduction methods. However, all of these methods face significant technical bottlenecks in industrial applications. While mechanical ball milling is relatively simple, it has a long reaction cycle, low conversion rate, and often leaves unreacted elemental sulfur or polysulfides in the product, which are difficult to remove completely by physical methods. High-temperature and high-pressure methods can yield high-purity products, but they require extremely high pressure and heat resistance from the reaction equipment and have low single-batch yields, making it difficult to meet the needs of large-scale production. Organic solvent methods typically require liquid ammonia or organolithium reagents, which are not only expensive but also often flammable, explosive, or highly toxic chemicals, posing serious safety hazards and environmental pollution problems. Furthermore, the costs of subsequent solvent recovery and product drying are extremely high.

[0005] The traditional carbothermal reduction method for preparing lithium sulfide (Li2S) has several inherent defects: On the one hand, it is difficult to achieve uniform dispersion at the microscale when solid carbon sources (such as petroleum coke, carbon black, graphite, etc.) and lithium sulfate (Li2SO4) are mechanically mixed, resulting in poor solid-solid interface contact and limited mass transfer between reactants. It requires long-term high-temperature calcination (usually 750-850°C, more than 1-2 hours) to drive the reaction, which is not only energy-intensive and time-consuming, but also prone to producing a large amount of unreacted Li2SO4 residue or excessive carbon residue due to uneven local reaction.

[0006] On the other hand, since carbothermic reduction reactions require high temperatures to proceed effectively, traditional resistance heating methods have slow heating rates (typically ≤10°C / min). The reaction system remains at high temperatures for extended periods, easily leading to excessive Li2S grain growth, lithium volatilization loss, and the formation of impurities such as Li2O and elemental sulfur through side reactions. The final product often requires multiple purification steps (such as water washing, acid washing, and recrystallization) to obtain battery-grade or electrolyte-grade Li2S, resulting in a lengthy process and low yield. Furthermore, traditional carbon sources often leave residual ash and metallic impurities after the reaction, further increasing the purification burden.

[0007] Therefore, developing a simple and low-cost method for preparing high-purity lithium sulfide has become an urgent problem to be solved. Summary of the Invention

[0008] To address the aforementioned technical problems, this application aims to provide a method for preparing high-purity lithium sulfide. The method described in this application requires no post-processing purification, is simple in process, and has low cost. The high-purity lithium sulfide obtained has a purity greater than 99.5%, thus solving the problem of carbon residue in the carbothermic reduction method.

[0009] To achieve this objective, the following technical solution is adopted in this application:

[0010] In a first aspect, this application provides a method for preparing high-purity lithium sulfide, the method comprising the following steps:

[0011] (1) A mixture of lithium sulfate and a carbon source is subjected to a first heating reaction under vacuum conditions to obtain the first product;

[0012] (2) The first product is subjected to a second heating reaction in an argon-hydrogen mixture to obtain high-purity lithium sulfide.

[0013] The method for preparing high-purity lithium sulfide provided in this application involves, on the one hand, the carbothermic pre-reduction of lithium sulfate under vacuum conditions, with the reaction formula: Li2SO4+2C→Li2S+2CO2. Since the reaction produces gaseous products, according to the LeChatelier principle, the vacuum condition can drive the reaction equilibrium to the direction of lithium sulfide formation, thereby reducing the starting temperature of the carbothermic reduction reaction and achieving significant energy-saving effect. At the same time, the lower reaction temperature effectively inhibits the excessive growth of lithium sulfide crystals, resulting in finer and more uniform product particle size.

[0014] On the other hand, this application also utilizes the hydrogen reduction method. The strong reducing property of H2 not only completely converts the residual lithium sulfate in the first step into lithium sulfide, but also converts the trace amounts of free carbon that may remain in the first step into gaseous CH4 or CO and discharges them with the tail gas (the reaction formula is: C + 2H2 → CH4, or C + H2O → CO + H2), thereby achieving "zero carbon residue" in the product. The only byproducts in the entire reaction process are water vapor and trace amounts of carbon oxides, both of which are gaseous. After the reaction, no post-processing purification steps are required (such as ethanol extraction, filtration, evaporation, secondary calcination, etc.). High-purity lithium sulfide powder can be obtained by directly collecting it under an inert atmosphere.

[0015] The preparation method provided in this application achieves a technological breakthrough in the carbothermic reduction method, which eliminates the need for post-processing purification. Through a two-step relay design, all solid impurities are converted into gaseous products at the front end and discharged. The collected products are battery-grade products, which fundamentally bypasses the cumbersome liquid-solid separation and solvent evaporation steps in traditional processes, greatly simplifying the process and reducing operational risks.

[0016] The following are preferred technical solutions of this application, but are not intended to limit the technical solutions provided in this application. Through the following preferred technical solutions, the technical objectives and beneficial effects of this application can be better achieved.

[0017] In some embodiments, the molar ratio of lithium sulfate to the carbon source is 1:(1.50-2.0), for example, it can be 1:1.50, 1:1.55, 1:1.60, 1:1.65, 1:1.70, 1:1.75, 1:1.80, 1:1.85, 1:1.90, 1:1.95 or 1:2.0, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] This application further controls the molar ratio of lithium sulfate and carbon source to 1:(1.50-2.0). The amount of carbon source added in this application is controlled between 80% and 100% of the theoretical amount (lower than the theoretical amount of traditional processes), thus avoiding the problem of excessive carbon residue from the source. The remaining unreacted lithium sulfate is treated by hydrogen reduction in the second step. A clear division of labor and relay cooperation are formed between the two steps. If the amount of carbon source added is too large, it will not only cause carbon residue, but also increase the amount of hydrogen used in step (2) and the processing time in order to reduce the amount of carbon residue. If the amount of carbon source added is too small, it will not be able to effectively reduce lithium sulfate, resulting in low purity of lithium sulfide in the product.

[0019] This application fundamentally solves the long-standing carbon residue problem in carbothermic reduction by combining three methods: underfeeding of carbon source, vacuum decarbonization, and hydrogen decarbonization. Underfeeding of carbon source (80%-100% of theoretical amount) avoids the introduction of excessive carbon, vacuum conditions improve the carbon reduction efficiency (achieving more complete conversion with the same amount of carbon), and the hydrogen atmosphere converts trace residual carbon into gaseous carbides. The synergistic effect of these three methods results in higher purification efficiency and higher purity of lithium sulfide.

[0020] In some embodiments, the molar ratio of the lithium sulfate to the carbon source is 1:(1.60-1.80), for example, it can be 1:1.60, 1:1.65, 1:1.70, 1:1.75 or 1:8.0, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] In some embodiments, the carbon source includes any one or a combination of at least two of graphite powder, activated carbon, carbon black, amorphous carbon, glucose, or sucrose. Typical but non-limiting combinations include a combination of graphite powder and carbon black, a combination of amorphous carbon and glucose, a combination of glucose and sucrose, and a combination of graphite powder, glucose, and sucrose.

[0022] In some embodiments, the temperature of the first heating reaction is 700°C-800°C, for example, it can be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C or 800°C, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0023] This application further controls the temperature of the first heating reaction to 700℃-800℃. The temperature of the first heating reaction affects the reduction effect of lithium sulfate. If the temperature of the first heating reaction is too high, byproducts such as lithium oxide or lithium carbonate will be generated, reducing product quality and making cost control difficult. If the temperature of the first heating reaction is too low, the main reaction will not be able to proceed effectively, resulting in an extremely low conversion rate of lithium sulfate.

[0024] In some embodiments, the first heating reaction time is 2h-6h, for example, it can be 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] In some embodiments, the temperature of the second heating reaction is 700°C-800°C, for example, it can be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C or 800°C, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0026] This application further controls the temperature of the second heating reaction to 700℃-800℃. The temperature of the second heating reaction affects the purity of the final lithium sulfide product. If the temperature of the second heating reaction is too high, the reaction will be violent, requiring high control over temperature uniformity and gas distribution, resulting in unstable product quality. If the temperature of the second heating reaction is too low, a longer reaction time is required, which is not conducive to continuous industrial production.

[0027] This application integrates the temperature reduction effect of vacuum carbothermal reduction with the deep purification effect of hydrogen reduction. The former uses vacuum pumping to reduce the partial pressure of the product gas to lower the reduction reaction temperature, while the latter uses the gaseous diffusion characteristics and strong reducing properties of H2 to completely eliminate unreacted substances and carbon residues. The two functions are complementary and the temperature windows are compatible, making it possible to achieve one-step continuous operation (changing atmosphere in the same furnace).

[0028] In some embodiments, the second heating reaction time is 2h-6h, for example, it can be 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] In some embodiments, the vacuum level of the vacuum condition is 10. -1 atm-10 -3 atm, for example, can be 10 -1 atm, 10 -2 atm or 10 -3 The value is atm, but not limited to the listed values; other unlisted values ​​within the range also apply.

[0030] In some embodiments, the volume fraction of hydrogen in the argon-hydrogen mixture is 5 vol.% to 75 vol.%, for example, it can be 5 vol.%, 10 vol.%, 15 vol.%, 20 vol.%, 25 vol.%, 30 vol.%, 35 vol.%, 40 vol.%, 45 vol.%, 50 vol.%, 55 vol.%, 60 vol.%, 65 vol.%, 70 vol.%, or 75 vol.%, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0031] In some embodiments, the carbon content in the high-purity lithium sulfide is ≤0.05wt.%, for example, it can be 0.05wt.%, 0.04wt.%, 0.03wt.%, 0.02wt.%, 0.01wt.%, 0.005wt.% or 0.001wt.%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] The high-purity lithium sulfide obtained in this application has a carbon content of less than 0.05 wt% and a purity of ≥99.5%, and can be directly used for the synthesis of sulfide solid electrolytes (such as Li6PS5Cl) and the preparation of cathode materials for lithium-sulfur batteries.

[0033] As a preferred embodiment of the preparation method described in this application, the preparation method includes the following steps:

[0034] (1) Lithium sulfate and carbon source are mixed in a molar ratio of 1:(1.60-1.90) under a vacuum of 10 -1 atm-10 -3 Under atm conditions, the first heating reaction was carried out at 700℃-800℃ for 2h-6h to obtain the first product;

[0035] (2) The first product is subjected to a second heating reaction at 700℃-800℃ in an argon-hydrogen mixture with a hydrogen volume fraction of 5 vol.%-75 vol.% for 2 h-6 h to obtain high-purity lithium sulfide with a carbon content ≤0.05 wt.%.

[0036] The numerical range described in this application includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, this application will not exhaustively list the specific point values ​​included in the range.

[0037] Compared with the prior art, this application has at least the following beneficial effects:

[0038] This application utilizes a two-step relay reduction method of vacuum carbothermal pre-reduction and deep hydrogen reduction to prepare high-purity lithium sulfide. Compared with existing technologies that use carbothermal reduction or hydrogen reduction alone to prepare lithium sulfide, the preparation method of this application converts all solid impurities into gaseous products at the front end and discharges them. The collected products are battery-grade products, which fundamentally bypasses the cumbersome liquid-solid separation and solvent evaporation steps in traditional processes, greatly simplifying the process and reducing operational risks. Detailed Implementation

[0039] The technical solution of this application will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely to help understand this application and should not be regarded as specific limitations on this application.

[0040] The scope of this application can be defined by lower and upper limits. The selected lower and upper limits define the boundaries of a specific range. The range defined in this way can be defined by the inclusion or exclusion of endpoints. Any endpoint can be independently selected for inclusion or exclusion, and all lower and upper limits can be arbitrarily combined to form new ranges. That is, any lower limit can be combined with any upper limit to form a valid range. For example, if the ranges of 60~120 and 80~110 are listed for specific parameters, it should be understood that the ranges of 60~110 and 80~120 also fall within the scope of this application. In addition, if the minimum range values ​​1 and 2 are listed, and the maximum range values ​​3, 4 and 5 are also listed, then all ranges of 1~3, 1~4, 1~5, 2~3, 2~4 and 2~5 fall within the scope of this application. In this application, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0~5" means that all real numbers between 0 and 5 have been fully listed in this document, and "0~5" is only a shortened representation of this set of numerical combinations. When a parameter is expressed as an integer ≥2, it is equivalent to listing positive integers that meet the requirements, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. When a parameter is expressed as an integer selected from "2~10", it is equivalent to listing any integer among 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0041] In this application, "at least two combinations" refers to a quantity greater than or equal to 2 unless otherwise specified. For example, "any one or at least two combinations" means any one of the listed items, or a combination of at least two of the listed items formed in a manner that does not conflict and enables the implementation of this application. In this application, unless otherwise specified, the features or solutions corresponding to "and / or" cover any one of two or more related listed items, as well as any and all combinations of the related listed items. The arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a set consisting of A, B, and combinations of A and B, where "containing A and / or B" can be understood, depending on the context of the statement, as containing A, containing B, or simultaneously containing both A and B. In this application, "optional" means that the corresponding feature, component, step or solution is not necessary, that is, it is selected from either "with" or "without". If there are multiple "optional" limitations in a technical solution, unless otherwise specified and there is no technical conflict or mutual constraint, each "optional" limitation is independent and does not affect the others.

[0042] In this application, technical features or solutions described using open-ended terms such as "comprising" or "including" do not exclude additional non-conflicting elements beyond those listed unless otherwise specified. They are considered to disclose both closed-ended features or solutions consisting solely of the listed elements and open-ended features or solutions that may include additional non-conflicting elements beyond the listed elements. For example, if A includes a1, a2, and a3, unless otherwise specified, this means that A can consist only of a1, a2, and a3, or it can include other non-conflicting elements based on a1, a2, and a3. This corresponds to the disclosure of technical solutions such as "A consists of a1, a2, and a3," "A is selected from a1, a2, and a3," and "A not only includes a1, a2, and a3, but may also include other non-conflicting elements." All embodiments and optional embodiments of this application, unless otherwise specified and without technical conflict, can be combined to form new technical solutions, and such combinations fall within the scope of this application. The term "embodiment" as used in this application means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art can explicitly and implicitly understand that the embodiments described in this application can be combined with other embodiments that do not conflict with existing technologies. The ordinal numbers "first," "second," "third," and "fourth," etc., used in this application in the expressions "first aspect," "second aspect," "third aspect," and "fourth aspect," are for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly specifying the importance or quantity of the indicated technical features. They serve only as a non-exhaustive enumeration and do not constitute a closed limitation on quantity.

[0043] In this application, the order in which the steps are written in the methods described in the various embodiments does not imply a strict execution order. The actual execution order of each step should be determined according to its function and possible internal logic. Unless otherwise specified, all steps in this application can be executed in the order they are written, or in any order without technical conflict. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) executed sequentially, or it may include steps (b) and (a) executed sequentially. If the method also includes step (c), then step (c) can be added to the method in any order without conflict, including but not limited to the execution order of steps (a), (b), and (c), steps (a), (c), and (b), steps (c), (a), and (b), etc.

[0044] Unless otherwise specified, all reagents and consumables used in the following examples and comparative examples were purchased from conventional reagent manufacturers in the art; unless otherwise specified, the experimental methods and techniques used were conventional methods and techniques in the art.

[0045] Example 1

[0046] This embodiment provides a method for preparing high-purity lithium sulfide, the method comprising the following steps:

[0047] (1) Lithium sulfate and graphite powder were mixed at a molar ratio of 1:1.5 to a carbon source, and the mixture was subjected to a vacuum of 10. -2 Under atm conditions, the first heating reaction was carried out at 750℃ for 4 hours to obtain the first product;

[0048] (2) The first product was heated at 750°C for 4 hours in an argon-hydrogen mixture with a hydrogen volume fraction of 10 vol.% to obtain high-purity lithium sulfide.

[0049] Example 2

[0050] This embodiment provides a method for preparing high-purity lithium sulfide, the method comprising the following steps:

[0051] (1) Lithium sulfate and amorphous carbon were mixed at a molar ratio of 1:1.6 to a carbon source and subjected to a vacuum of 10. - 1 Under atm conditions, the first heating reaction was carried out at 700℃ for 6 hours to obtain the first product;

[0052] (2) The first product was heated at 700°C for 6 hours in an argon-hydrogen mixture with a hydrogen volume fraction of 30 vol.% to obtain high-purity lithium sulfide.

[0053] Example 3

[0054] This embodiment provides a method for preparing high-purity lithium sulfide, the method comprising the following steps:

[0055] (1) Lithium sulfate and glucose were mixed at a molar ratio of 1:1.9 to a carbon source, and the mixture was subjected to a vacuum of 10. -3 Under atm conditions, the first heating reaction was carried out at 800℃ for 2 hours to obtain the first product;

[0056] (2) The first product was heated at 800°C for 2 hours in an argon-hydrogen mixture with a hydrogen volume fraction of 30 vol.% to obtain high-purity lithium sulfide.

[0057] Example 4

[0058] This embodiment provides a method for preparing high-purity lithium sulfide. The only difference from Example 1 is that the molar ratio of lithium sulfate and carbon source in step (1) is 1:1, while the other steps remain unchanged.

[0059] Example 5

[0060] This embodiment provides a method for preparing high-purity lithium sulfide. The only difference from Example 1 is that the molar ratio of lithium sulfate to carbon source in step (1) is 1:2.5, while the other steps remain unchanged.

[0061] Example 6

[0062] This embodiment provides a method for preparing high-purity lithium sulfide. The only difference from Example 1 is that in step (2), the temperature of the first heating reaction is changed from 700°C to 600°C, while the other steps remain unchanged.

[0063] Example 7

[0064] This embodiment provides a method for preparing high-purity lithium sulfide. The only difference from Example 1 is that the temperature of the second heating reaction in step (2) is changed from 700°C to 900°C, while the other steps remain unchanged.

[0065] Comparative Example 1

[0066] This comparative example provides a method for preparing high-purity lithium sulfide, which differs from Example 1 only in that the reaction atmosphere in step (1) is a nitrogen atmosphere.

[0067] Comparative Example 2

[0068] This comparative example provides a method for preparing high-purity lithium sulfide. The only difference from Example 1 is that the molar ratio of lithium sulfate to carbon source in step (1) is 1:3, and step (2) is not performed, i.e., carbothermic reduction is performed in the case of excess carbon.

[0069] Comparative Example 3

[0070] This comparative example provides a method for preparing high-purity lithium sulfide. The only difference from Example 1 is that step (1) is omitted, and in step (2), the "first product" is replaced with lithium sulfate raw material, that is, only hydrogen reduction is performed.

[0071] test:

[0072] The high-purity lithium sulfide prepared in the examples and comparative examples was tested using the following methods, and the test results are shown in Table 1 below.

[0073] (1) Carbon content (wt%): The carbon content in lithium sulfide was determined using high-frequency combustion-infrared absorption method.

[0074] (2) Lithium sulfide purity (%): The content of impurity ions was detected by inductively coupled plasma atomic emission spectrometry and converted into the purity of lithium sulfide.

[0075] Table 1

[0076]

[0077] The test results show that:

[0078] (1) As can be seen from Examples 1-3, this application uses a two-step relay reduction method of vacuum carbothermal pre-reduction and hydrogen deep fine reduction to prepare high-purity lithium sulfide. Compared with the existing technology of using carbothermal reduction or hydrogen reduction alone to prepare lithium sulfide, the preparation method of this application converts all solid impurities into gaseous products at the front end and discharges them. After the products are collected, they are battery-grade products. This fundamentally bypasses the cumbersome liquid-solid separation and solvent evaporation steps in the traditional process, greatly simplifies the process flow and reduces the operational risk.

[0079] (2) By comparing Example 1 with Examples 4-5, it can be seen that this application further controls the molar ratio of the lithium sulfate and the carbon source to 1:(1.50-2.0). The amount of carbon source added in this application is controlled between 80% and 100% of the theoretical amount (lower than the theoretical amount of the traditional process), thus avoiding the problem of excessive carbon residue from the source. The remaining unreacted lithium sulfate is treated by hydrogen reduction in the second step. A clear division of labor and relay cooperation are formed between the two steps. If the amount of carbon source added is too large, it will not only cause carbon residue, but also increase the amount of hydrogen used in step (2) and the processing time in order to reduce the amount of carbon residue. If the amount of carbon source added is too small, it will not be able to effectively reduce lithium sulfate.

[0080] (3) By comparing Example 1 with Example 6-7, it can be seen that the temperature of the first heating reaction in this application is 700℃-800℃ and the temperature of the second heating reaction is 700℃-800℃. If the temperature of the first heating reaction is too low, the main reaction will not be able to proceed effectively, resulting in a very low conversion rate of lithium sulfate. If the temperature of the second heating reaction is too high, the reaction will be violent, and the control requirements for temperature uniformity and gas distribution are high, resulting in unstable product quality.

[0081] (4) As can be seen from Example 1 and Comparative Example 1, this application performs carbothermic pre-reduction of lithium sulfate under vacuum conditions. Since the reaction generates gaseous products, according to the LeChatelier principle, the vacuum condition can drive the reaction equilibrium to shift towards the generation of lithium sulfide, thereby reducing the starting temperature of the carbothermic reduction reaction and achieving significant energy-saving effect. At the same time, the lower reaction temperature effectively inhibits the excessive growth of lithium sulfide crystals, and the product particle size is finer and more uniform.

[0082] (5) As can be seen from Example 1 and Comparative Examples 2-3, this application fundamentally solves the long-standing problem of carbon residue in the carbothermic reduction method by combining three methods: insufficient carbon source feeding, vacuum carbon reduction, and hydrogen carbon removal. Among them, insufficient carbon source feeding (80%-100% of the theoretical amount) avoids the introduction of excessive carbon, vacuum conditions can improve the carbon reduction efficiency (achieving more complete conversion under the same amount of carbon), and hydrogen atmosphere converts trace residual carbon into gaseous carbides. The three factors work together to make the purification efficiency of lithium sulfide higher and the purity of the obtained lithium sulfide higher.

[0083] In summary, this application utilizes a two-step relay reduction method of vacuum carbothermal pre-reduction and deep hydrogen reduction to prepare high-purity lithium sulfide. Compared with existing technologies that use carbothermal reduction or hydrogen reduction alone to prepare lithium sulfide, the preparation method of this application converts all solid impurities into gaseous products at the front end and discharges them. The collected products are battery-grade products, fundamentally bypassing the cumbersome liquid-solid separation and solvent evaporation steps in traditional processes, greatly simplifying the process flow and reducing operational risks.

[0084] The applicant declares that the above description is only a specific implementation of this application, but the protection scope of this application is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application fall within the protection and disclosure scope of this application.

Claims

1. A method for preparing high-purity lithium sulfide, characterized in that, The preparation method includes the following steps: (1) A mixture of lithium sulfate and a carbon source is subjected to a first heating reaction under vacuum conditions to obtain the first product; (2) The first product is subjected to a second heating reaction in an argon-hydrogen mixture to obtain high-purity lithium sulfide.

2. The preparation method according to claim 1, characterized in that, The molar ratio of lithium sulfate to the carbon source is 1:(1.50-2.0).

3. The preparation method according to claim 1 or 2, characterized in that, The molar ratio of lithium sulfate to the carbon source is 1:(1.60-1.80).

4. The preparation method according to any one of claims 1-3, characterized in that, The carbon source includes any one or a combination of at least two of the following: graphite powder, activated carbon, carbon black, amorphous carbon, glucose, or sucrose.

5. The preparation method according to any one of claims 1-4, characterized in that, The temperature of the first heating reaction is 700℃-800℃; And / or, the duration of the first heating reaction is 2-6 hours.

6. The preparation method according to any one of claims 1-5, characterized in that, The temperature of the second heating reaction is 700℃-800℃; And / or, the second heating reaction takes 2-6 hours.

7. The preparation method according to any one of claims 1-6, characterized in that, The vacuum level of the vacuum condition is 10. -1 atm-10 -3 atm.

8. The preparation method according to any one of claims 1-7, characterized in that, The volume fraction of hydrogen in the argon-hydrogen mixture is 5 vol.% to 75 vol.%.

9. The preparation method according to any one of claims 1-8, characterized in that, The carbon content in the high-purity lithium sulfide is ≤0.05 wt.%.

10. The preparation method according to any one of claims 1-9, characterized in that, The preparation method includes the following steps: (1) Lithium sulfate and carbon source are mixed in a molar ratio of 1:(1.60-1.90) under a vacuum of 10 -1 atm-10 -3 Under atm conditions, the first heating reaction was carried out at 700℃-800℃ for 2h-6h to obtain the first product; (2) The first product is subjected to a second heating reaction at 700℃-800℃ in an argon-hydrogen mixture with a hydrogen volume fraction of 5 vol.%-75 vol.% for 2 h-6 h to obtain high-purity lithium sulfide with a carbon content ≤0.05 wt.%.

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