Composite lithium supplement agent and preparation method and application thereof

Abstract

The invention provides a composite lithium supplement agent and a preparation method and application thereof, and belongs to the field of lithium ion batteries, the composite lithium supplement agent comprises Li5FeO4 particles and a coating layer at least partially coating the surfaces of the Li5FeO4 particles, and the coating layer comprises a LiF interface layer and a fluorine-doped carbon layer which are sequentially arranged outwards along the surfaces of the Li5FeO4 particles; wherein the total thickness of the coating layer is 5-50 nm; the total content of the fluorine element in the Li5FeO4 lithium supplement agent coated with the fluorinated composite carbon layer is 0.5 to 5.0 at.%. The composite coating layer composed of the inner LiF interface layer and the outer fluorine-doped carbon layer is constructed on the surface of the Li5FeO4 particle in situ, so that the air stability and the processing performance of the material are remarkably improved, an efficient electronic conductive network is formed, the intrinsic conductivity of the material is improved, and meanwhile, the material has a good application prospect. The technical problems of high residual alkali, low conductivity and poor air stability of the Li5FeO4 lithium supplement agent are solved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a composite lithium replenishing agent, its preparation method, and its application. Background Technology

[0002] With the ever-increasing demand for energy storage, the development of lithium-ion batteries has received increasing attention. Li5FeO4, as a high-capacity lithium replenisher, can effectively compensate for the irreversible capacity loss during the first charge-discharge process of lithium-ion batteries, improving the overall energy density of the battery. However, unmodified Li5FeO4 materials have significant technical drawbacks: firstly, their surface has a high content of residual alkali (such as Li2O and Li2CO3), which not only leads to processing difficulties but also causes slurry gelation, affecting battery manufacturing processes; secondly, their intrinsic electronic conductivity is extremely low, limiting the full realization of their electrochemical activity; and thirdly, they are chemically reactive, extremely sensitive to moisture and carbon dioxide in the air, have poor stability, and require harsh storage and processing conditions. These shortcomings severely restrict their large-scale commercial application.

[0003] Surface coating is an effective means to improve the electrochemical performance and stability of electrode materials. Common coating materials include metal oxides (such as Al2O3 and ZrO2), phosphates (such as AlPO4), and carbon materials. Among them, carbon coating has a significant advantage in improving electronic conductivity. However, traditional carbon coatings (such as those using glucose or pitch as carbon sources) have poor compatibility with the strongly alkaline Li5FeO4 surface, making it difficult to form a stable and robust interface layer. This results in limited effectiveness in preventing the outward migration of residual alkali and the inward penetration of external moisture, leading to unsatisfactory long-term air stability and storage performance of the modified material. Summary of the Invention

[0004] In view of the technical problems existing in the background art, this application provides a composite lithium replenishing agent and its preparation method and application, aiming to solve the technical problem that the poor compatibility between the traditional carbon coating layer and the Li5FeO4 surface leads to uneven coating and weak interfacial bonding, which seriously affects the long-term storage stability and electrochemical performance of the lithium replenishing agent.

[0005] In a first aspect, embodiments of this application provide a composite lithium replenishing agent, comprising Li5FeO4 particles and a coating layer at least partially coated on the surface of the Li5FeO4 particles, the coating layer comprising a LiF interface layer and a fluorine-doped carbon layer sequentially disposed outward along the surface of the Li5FeO4 particles; The total thickness of the coating layer is 5-50 nm; the total fluorine content in the composite lithium supplement is 0.5-5.0 at.%.

[0006] In the technical solution of this application embodiment, a composite coating layer consisting of an inner LiF interface layer and an outer fluorine-doped carbon layer is constructed in situ on the surface of Li5FeO4 particles. The inner LiF layer chemically passivates the surface and permanently fixes residual alkali, thereby improving the material's air stability and processing performance. The outer fluorine-doped carbon layer not only constructs a dense physical barrier but also forms a highly efficient electronic conductive network, significantly improving the material's intrinsic conductivity. Therefore, this application, through its unique dual-layer structure design of the composite coating layer, solves the technical problems of high residual alkali, low conductivity, and poor air stability in Li5FeO4 lithium replenishing agents, thus obtaining a lithium replenishing agent material with excellent storage stability, high conductivity, and good electrochemical performance.

[0007] Secondly, embodiments of this application provide a method for preparing the composite lithium supplement as described in the first aspect, comprising the following steps: Li5FeO4 lithium supplement powder was mixed evenly with a fluoropolymer in an organic solvent to obtain a slurry; The slurry is dried to obtain the precursor composite material; The precursor composite material was heat-treated under an inert atmosphere to obtain a composite lithium supplement.

[0008] In the technical solution of this application embodiment, the in-situ construction of the coating layer is achieved by mixing Li5FeO4 with a fluoropolymer and then subjecting it to a one-step heat treatment. The active fluorine species generated by the pyrolysis of the fluoropolymer directly react with the residual alkali on the material surface to generate a LiF layer. Simultaneously, the fluoropolymer itself can form a fluorine-doped carbon layer through carbonization, ensuring a strong chemical bond and dense physical coverage between the coating layer and the core material. Furthermore, this preparation method is simple, low-cost, and has great potential for commercial application.

[0009] In some embodiments, the fluoropolymer is one or more of polyvinylidene fluoride, polytetrafluoroethylene, fluorinated ethylene propylene copolymer, and polyvinyl fluoride; and / or, the organic solvent is one or more of N-methylpyrrolidone, dimethylacetamide, and acetone.

[0010] In this embodiment, the selection of a stable fluoropolymer ensures a reliable supply and reactivity of both the fluorine and carbon sources, while the use of an efficient solvent guarantees uniform dispersion of the materials, resulting in a high-quality precursor. The optimized selection of raw materials ensures the stability and reproducibility of the product's performance. Furthermore, the wide availability and controllable cost of the raw materials further enhance the economic viability and feasibility for large-scale production of the entire preparation method.

[0011] In some embodiments, the mass ratio of the Li5FeO4 lithium supplement powder to the fluoropolymer is 100:(3~20).

[0012] In this embodiment, by using a suitable mass ratio of Li5FeO4 to fluoropolymer, the thickness and composition of the coating layer can be effectively controlled, thereby optimizing the material's performance.

[0013] In some embodiments, the D50 particle size of the Li5FeO4 lithium supplement powder is 0.1~20μm.

[0014] In this embodiment, limiting the median particle size of the Li5FeO4 lithium supplement powder helps to optimize the balance between coating effect and overall material performance. This particle size range ensures that the Li5FeO4 particles have sufficient specific surface area, which is conducive to uniform coating and full reaction of fluoropolymers, thereby forming a dense and complete fluorinated composite carbon layer. At the same time, it avoids the serious agglomeration caused by excessively fine particles or the uneven coating caused by excessively coarse particles, ensuring that the modified material has excellent air stability, conductivity and electrochemical consistency.

[0015] In some embodiments, the drying temperature is 110~130°C, and the drying time is 10~15h.

[0016] In this embodiment, suitable drying temperature and drying time help to fully remove organic solvents, obtain a uniform precursor composite material without residue, avoid solvent residue problems caused by excessively low temperature or insufficient time, and at the same time, prevent material pre-decomposition or agglomeration that may be caused by excessively high temperature or excessive time, laying a solid foundation for the subsequent formation of a high-quality, uniform and dense fluorinated composite carbon layer.

[0017] In some embodiments, the heat treatment temperature is 400~700℃, and the holding time is 2~8h.

[0018] In this embodiment, a suitable heat treatment temperature helps to fully pyrolyze the fluoropolymer, releasing active fluorine species that react with residual alkali on the surface to form a dense LiF layer, and further carbonization to form a fluorine-doped carbon layer. A suitable holding time helps ensure the complete progress of the reaction and the uniform growth of the coating layer, thereby efficiently constructing an ideal fluorinated composite carbon layer and ultimately obtaining a high-performance composite lithium supplement material.

[0019] In some embodiments, the heat treatment is carried out under an argon or nitrogen atmosphere, with a heating rate of 1~5°C / min.

[0020] In this embodiment, heat treatment under an inert atmosphere such as argon or nitrogen effectively prevents the Li5FeO4 material from being oxidized at high temperatures, ensuring the stability of the core structure and the integrity of its lithium replenishment activity. Simultaneously, controlling the heating rate within a suitable range ensures uniform heat transfer within the material, avoiding particle sintering or uneven coating caused by localized overheating. This facilitates the formation of a uniform, dense, and firmly bonded fluorinated composite carbon layer.

[0021] In some embodiments, after the composite lithium replenishing agent is exposed to an environment of 25°C and 50% relative humidity for 24 hours, the lithium replenishing capacity decay rate is less than 5%; and / or, the powder resistivity of the composite lithium replenishing agent is less than or equal to 100 Ω·cm; and / or, the residual alkali value of the composite lithium replenishing agent is less than or equal to 1.0 wt%.

[0022] In this embodiment, the residual alkali value is calculated based on Li2CO3. The composite lithium supplement prepared in this application exhibits excellent comprehensive performance and simultaneously solves several problems of high residual alkali, low conductivity and poor air stability of Li5FeO4 lithium supplement, which has important practical significance and application value.

[0023] Thirdly, embodiments of this application provide a positive electrode sheet, including the composite lithium supplement agent as described in the first aspect or the composite lithium supplement agent prepared by the preparation method described in the second aspect.

[0024] In this embodiment, the positive electrode contains the aforementioned composite lithium supplement, thus possessing advantages such as excellent processing performance, high rate performance, and long cycle life.

[0025] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0027] Figure 1 This is a SEM image of the composite lithium supplement obtained in Example 1 of this application; Figure 2 The XRD pattern of the composite lithium supplement obtained in Example 1 of this application; Figure 3 SEM image of the composite lithium supplement obtained in Comparative Example 1; Figure 4 The XRD pattern of the composite lithium supplement obtained in Comparative Example 1 is shown. Figure 5 SEM image of the composite lithium supplement obtained in Comparative Example 3; Figure 6 The image shows the XRD pattern of the composite lithium supplement obtained in Comparative Example 3. Detailed Implementation

[0028] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0030] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0031] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0033] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0034] Li5FeO4, as a high-capacity lithium replenisher, can compensate for the initial irreversible capacity loss in lithium-ion batteries. However, its application faces severe challenges. High residual alkali content on the material surface easily leads to slurry gelation. Furthermore, its extremely low intrinsic conductivity limits electrochemical activity, and it is sensitive to air, exhibiting poor stability. While traditional carbon coating can improve conductivity, its poor compatibility with strongly alkaline surfaces makes it difficult to form a stable interface, limiting its effectiveness in preventing residual alkali migration and moisture penetration, resulting in unsatisfactory long-term stability.

[0035] To address the technical problems of high residual alkali on the surface of Li5FeO4 lithium supplements, low intrinsic conductivity, and poor air stability, this application provides a composite lithium supplement, its preparation method, and its application. Specifically, by constructing a composite coating layer in situ on the surface of Li5FeO4 particles, consisting of an inner LiF interface layer and an outer fluorine-doped carbon layer, the technical effects of effectively inhibiting surface residual alkali migration, isolating moisture erosion, and significantly improving electronic conductivity can be achieved. Consequently, the initial coulombic efficiency, cycle stability, and rate performance of the positive electrode are also improved.

[0036] In a first aspect, embodiments of this application provide a composite lithium replenishing agent, comprising Li5FeO4 particles and a coating layer at least partially coating the surface of the Li5FeO4 particles. The coating layer comprises a LiF interface layer and a fluorine-doped carbon layer sequentially disposed outward from the surface of the Li5FeO4 particles. The total thickness of the coating layer is 5~50 nm. The total fluorine content in the composite lithium replenishing agent is 0.5~5.0 at.%.

[0037] In this application, a composite coating layer consisting of an inner LiF interface layer and an outer fluorine-doped carbon layer is constructed in situ on the surface of Li5FeO4 particles. The inner LiF layer chemically passivates the surface and permanently immobilizes residual alkali, fundamentally improving the material's air stability and processing performance. The outer fluorine-doped carbon layer not only constructs a dense physical barrier but also forms a highly efficient electronic conductivity network, significantly enhancing the material's intrinsic conductivity. Therefore, this application, through its unique dual-layer structure design, solves the core problems of high residual alkali, low conductivity, and poor air stability in existing Li5FeO4 lithium replenishing agents, thus obtaining a composite lithium replenishing agent material with excellent storage stability, high conductivity, and good electrochemical performance.

[0038] The total thickness of the coating layer is 5-50 nm. If the total thickness of the coating layer is too low, the material's air stability and processing performance will be poor. If the total thickness of the coating layer is too high, the electrochemical performance of the material will be affected, and the preparation cost will also increase significantly. The total fluorine content in the composite lithium supplement is 0.5-5.0 at.%. If the fluorine content is too low, an effective FC structure cannot be formed, and it will be difficult to form a coating on the surface of the LiF passivation material. If the fluorine content is too high, the strong electronegativity of fluorine will excessively bind electrons, leading to a decrease in the electronic conductivity of the material.

[0039] Specifically, in this application, the total thickness of the coating layer can be any value within the range of 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, or 5~50nm. The fluorine content in the composite lithium supplement can be any value within the range of 0.5at.%, 1.0at.%, 1.5at.%, 2.0at.%, 2.5at.%, 3.0at.%, 3.5at.%, 4.0at.%, 4.5at.%, 5.0at.%, or 0.5~5.0 at.%.

[0040] Secondly, embodiments of this application provide a method for preparing the composite lithium supplement as described in the first aspect, comprising the following steps: S10. Mix the Li5FeO4 lithium supplement powder with the fluoropolymer in an organic solvent to obtain a slurry. S20. The slurry is dried to obtain the precursor composite material; S30. The precursor composite material is heat-treated under an inert atmosphere to obtain a composite lithium supplement.

[0041] In this process, Li5FeO4 powder is thoroughly mixed with a fluoropolymer in an organic solvent to ensure that the polymer molecules are uniformly dispersed and tightly adhered to the surface of each particle. Subsequently, the organic solvent is completely removed through drying, resulting in a structurally stable precursor composite material that avoids interference from solvent residues in subsequent pyrolysis reactions. Furthermore, heat treatment under an inert atmosphere allows the active fluorine species released by the fluoropolymer to react in situ with residual alkali on the Li5FeO4 surface, generating a dense LiF interface layer. This layer fundamentally "passivates" the material surface, permanently fixing the residual alkali and preventing it from reacting with external moisture / CO2. Based on this, the remaining fluoropolymer, acting as a carbon source, forms a fluorine-doped carbon layer with strong physical barrier properties and high chemical inertness after pyrolysis. The introduction of fluorine atoms not only enhances the density of the carbon layer but also optimizes its electronic structure, constructing a highly efficient electronic conductivity network. This application combines physical coating with chemical modification, constructing a structurally stable and functionally synergistic composite coating layer in a one-step process, effectively improving the overall performance of the material.

[0042] Furthermore, in some embodiments, the fluoropolymer is one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene copolymer (FEP), and polyvinyl fluoride (PVF); and / or, the organic solvent is one or more of N-methylpyrrolidone (NMP), dimethylacetamide (DMAC), and acetone.

[0043] In this application, a suitable fluoropolymer can provide the fluorine and carbon skeleton required to form the LiF interface layer and the fluorine-doped carbon layer during pyrolysis, and has good solubility or dispersibility in highly polar organic solvents. This ensures that the polymer can be uniformly coated on the surface of Li5FeO4 particles at the molecular or nanoscale level to form a uniform precursor slurry, laying the foundation for the subsequent formation of a uniform, dense and thickness-controllable composite coating layer.

[0044] Furthermore, in some embodiments, the mass ratio of Li5FeO4 lithium supplement powder to fluoropolymer is 100:(3~20).

[0045] In this application, an appropriate mass ratio helps to optimize the thickness, structure and composition of the coating layer. If the amount of fluoropolymer is too small, it is difficult to form a continuous and complete coating layer, resulting in insufficient passivation of residual alkali and difficulty in effectively establishing a conductive network. If the amount of fluoropolymer is too large, an excessively thick coating layer will be formed, which will not only increase the proportion of inactive materials and reduce the overall lithium replenishment capacity of the material, but may also hinder the extraction of lithium ions and affect its electrochemical activity.

[0046] Specifically, the mass ratio of Li5FeO4 lithium supplement powder to fluoropolymer can be any value within the range of 100:3, 100:4, 100:5, 100:6, 100:7, 100:8, 100:9, 100:10, 100:11, 100:12, 100:13, 100:14, 100:15, 100:16, 100:17, 100:18, 100:19, 100:20, or 100:(3~20).

[0047] Furthermore, in some embodiments, the D50 particle size of the Li5FeO4 lithium supplement powder is 0.1~20μm.

[0048] In this application, controlling the median particle size of Li5FeO4 powder helps optimize the uniformity of the coating layer and the overall electrochemical performance of the material. If the particle size of the Li5FeO4 lithium supplement powder is too small, the particle specific surface area is too large, making it difficult to be uniformly coated by the limited fluoropolymer. Furthermore, the excessively high surface energy may lead to severe agglomeration, resulting in uneven coating and performance degradation. Conversely, if the particle size of the Li5FeO4 lithium supplement powder is too large, it prolongs the lithium-ion diffusion path, hindering the rapid development of its lithium supplementation activity. Simultaneously, its smaller specific surface area reduces the effective contact area with the coating layer.

[0049] Specifically, the D50 particle size of the Li5FeO4 lithium supplement powder can be 0.1μm, 1μm, 3μm, 5μm, 7μm, 9μm, 11μm, 13μm, 15μm, 17μm, 19μm, 20μm or any value in the range of 0.1~20μm.

[0050] Furthermore, in some embodiments, the drying temperature is 110~130°C, and the drying time is 10~15h.

[0051] In this application, a suitable drying temperature provides sufficient heat energy to efficiently and thoroughly remove organic solvents, avoiding defects such as pores and cracks in the coating layer during pyrolysis caused by solvent residue. Simultaneously, it prevents pre-decomposition or oxidation of the fluoropolymer due to excessively high temperatures, ensuring that it reacts as expected in subsequent heat treatment steps. Sufficient time ensures that heat completely penetrates the material, achieving uniform and slow solvent evaporation, preventing particle agglomeration or uneven coating caused by rapid drying, ultimately obtaining a precursor with a uniform structure and stable performance.

[0052] Specifically, the drying temperature can be 110℃, 115℃, 120℃, 125℃, 130℃, or any value within the range of 110~130℃. The drying time can be 10h, 11h, 12h, 13h, 14h, 15h, or any value within the range of 10~15h.

[0053] Furthermore, in some embodiments, the heat treatment temperature is 400~700℃, and the holding time is 2~8h.

[0054] In this application, by precisely controlling the temperature and time of heat treatment, the pyrolysis process and extent of the fluoropolymer can be regulated, achieving precise construction of the fluorinated composite carbon layer structure. If the temperature is too low, the fluoropolymer will not decompose completely, failing to form a continuous and effective carbon layer and LiF interface; if the temperature is too high, the Li5FeO4 core material may undergo phase transformation or sintering, destroying its electrochemical activity and causing energy waste. If the time is too short, the coating layer may be thin and not dense enough, resulting in insufficient protection; if the time is too long, the carbon layer may become over-graphitized or the particles may grow, which is not conducive to the formation of the conductive network.

[0055] Specifically, the heat treatment temperature can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, or any value within the range of 400~700℃.

[0056] Preferably, the heat treatment temperature is 500~650℃ and the holding time is 3~6h, which helps the pyrolysis reaction of the fluoropolymer to proceed fully and stably, ensuring that both the LiF interface layer and the fluorine-doped carbon layer can achieve better crystallinity and density.

[0057] Furthermore, in some embodiments, the heat treatment is carried out under an argon or nitrogen atmosphere at a heating rate of 1~5°C / min.

[0058] In this application, heat treatment is performed under an inert atmosphere such as argon or nitrogen, which effectively isolates the air and prevents the Li5FeO4 core material from being oxidized at high temperatures. This avoids side reactions between the fluoropolymer and oxygen during pyrolysis, ensuring that the resulting LiF and carbon layers are pure and structurally ideal. Simultaneously, strictly controlling the heating rate within a slow range of 1~5℃ / min helps achieve gentle and uniform heating, allowing the fluoropolymer to undergo uniform and controllable pyrolysis and in-situ reaction on the Li5FeO4 surface, forming a dense and defect-free coating layer. This avoids internal stress concentration, particle cracking, or coating layer peeling caused by excessively rapid heating, ultimately resulting in a high-quality lithium supplement with a complete structure and uniform performance.

[0059] Furthermore, in some embodiments, after the composite lithium replenishing agent is exposed to an environment of 25°C and 50% relative humidity for 24 hours, the lithium replenishing capacity decay rate is less than 5%; and / or, the powder resistivity of the composite lithium replenishing agent is less than or equal to 100 Ω·cm; and / or, the residual alkali value of the composite lithium replenishing agent, calculated as Li2CO3, is less than or equal to 1.0 wt%.

[0060] In this application, the prepared composite lithium replenishing agent material exhibits a lithium replenishing capacity decay rate of less than 5% after 24 hours of exposure in a harsh environment of 25°C and 50% relative humidity. This indicates that the composite coating layer forms an efficient physical and chemical barrier, endowing the material with excellent air stability. Its powder resistivity is less than or equal to 100 Ω·cm, indicating that the outer fluorine-doped carbon layer successfully constructs a continuous electronic conductive network, fundamentally solving the problem of low intrinsic conductivity of Li5FeO4. The residual alkali value (calculated as Li2CO3) is less than or equal to 1.0 wt%, indicating that the inner LiF interface layer effectively reduces the negative impact of the highly active residual alkali on the electrode slurry stability and battery performance by fixing and transforming it in situ.

[0061] Thirdly, embodiments of this application provide a positive electrode sheet, comprising a composite lithium supplement agent as described in the first aspect or a composite lithium supplement agent prepared by the preparation method described in the second aspect.

[0062] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0063] I. Preparation Method Example 1 This embodiment provides a method for preparing a composite lithium supplement, including the following steps: S1. Weigh 10.0g of Li5FeO4 powder with a particle size D50 of 5μm and 1.0g of PVDF powder, add them together to 100mL of NMP solvent, place them in a planetary ball mill, and ball mill at 300rpm for 4h to obtain a uniform and stable slurry. S2. Place the slurry in a vacuum oven at 120°C and dry for 12 hours to completely remove the NMP solvent and obtain the precursor composite material. S3. The precursor composite material was placed in a tube furnace and heated to 550°C at a rate of 3°C / min under an argon atmosphere. It was then held at this temperature for 4 hours and allowed to cool naturally to room temperature to obtain the composite lithium supplement, denoted as LFO@F / C. Its SEM analysis is shown below. Figure 1 As shown, the XRD test is as follows Figure 2 As shown, the fluorinated composite carbon layer was successfully coated on the surface of Li5FeO4 particles, and the coating layer has a uniform structure and pure phase. The coating process did not damage the crystal integrity.

[0064] Example 2 This embodiment provides a method for preparing a composite lithium supplement. The only difference from Example 1 is that the amount of PVDF powder added is 0.3g.

[0065] Example 3 This embodiment provides a method for preparing a composite lithium supplement. The only difference from Example 1 is that the amount of PVDF powder added is 2g.

[0066] Example 4 This embodiment provides a method for preparing a composite lithium supplement. Compared with Example 1, the only difference is that the heat treatment temperature is 400℃ and the holding time is 2h.

[0067] Example 5 This embodiment provides a method for preparing a composite lithium supplement. Compared with Example 1, the only difference is that the heat treatment temperature is 700℃ and the holding time is 8h.

[0068] Example 6 This embodiment provides a method for preparing a composite lithium supplement. Compared with Example 1, the only difference is that the median particle size D50 of the Li5FeO4 lithium supplement powder is 0.1 μm.

[0069] Example 7 This embodiment provides a method for preparing a composite lithium supplement. The only difference from Example 1 is that the median particle size D50 of the Li5FeO4 lithium supplement powder is 20 μm.

[0070] Comparative Example 1 Comparative Example 1 provides a method for preparing a composite lithium supplement, which differs from Example 1 only in that PVDF powder was not added. The SEM test results of the obtained lithium supplement are as follows. Figure 3 As shown, the XRD test is as follows Figure 4 As shown, the surface of the Li5FeO4 particles without any coating treatment is smooth and clean, with no coating layer present. The XRD pattern shows obvious impurity peaks such as Li2CO3 and LiOH in addition to the characteristic diffraction peaks of Li5FeO4, indicating that the sample without PVDF powder has no carbon layer coating and is easily decomposed when directly exposed to air.

[0071] Comparative Example 2 Comparative Example 2 provides a method for preparing a composite lithium supplement, which differs from Example 1 only in that the amount of PVDF powder added is 2.5g.

[0072] Comparative Example 3 Comparative Example 3 provides a method for preparing a composite lithium supplement, which differs from Example 1 only in that glucose is used instead of PVDF powder. The SEM test results of the obtained lithium supplement are as follows. Figure 5As shown, the XRD test is as follows Figure 6 As shown, the carbon layer formed by glucose pyrolysis is unevenly coated, with a rough and non-dense surface. Although the main phase of Li5FeO4 is preserved in the XRD pattern, impurities such as Li2CO3 and LiOH can still be detected. This is because oxygen-containing carbon sources such as glucose generate water during carbonization, and Li5FeO4 reacts with water to form LiOH. Upon contact with air, it also decomposes to form Li2CO3 and LiOH.

[0073] Comparative Example 4 Comparative Example 4 provides a method for preparing a composite lithium supplement, which differs from Example 1 only in that the heat treatment temperature is 300°C.

[0074] Comparative Example 5 Comparative Example 5 provides a method for preparing a composite lithium supplement, which differs from Example 1 only in that the heat treatment temperature is 800°C.

[0075] Comparative Example 6 Comparative Example 6 provides a method for preparing a composite lithium supplement. The only difference from Example 1 is that the median particle size D50 of the Li5FeO4 lithium supplement powder is 0.05 μm.

[0076] Comparative Example 7 Comparative Example 7 provides a method for preparing a composite lithium supplement. The only difference from Example 1 is that the median particle size D50 of the Li5FeO4 lithium supplement powder is 30 μm.

[0077] II. Testing Methods 1. SEM testing The composite lithium supplement prepared in Example 1 was tested using a scanning electron microscope SU8600.

[0078] 2. XRD test The composite lithium supplement prepared in Example 1 was tested using a Shimadzu XRD-6100 microscope.

[0079] 3. Resistivity test The composite lithium supplements obtained in the examples and comparative examples were pressed into sheets under a pressure of 10 MPa, and their resistivity was tested using the four-probe method.

[0080] 4. Residual alkali value test The residual alkali value of the composite lithium supplements obtained in the examples and comparative examples was tested using potentiometric titration.

[0081] 5. Capacity test The composite lithium supplement agent, conductive agent Super-P, and binder PVDF were mixed evenly in N-methylpyrrolidone solvent at a mass ratio of 8:1:1 to form a slurry. The slurry was then uniformly coated on an aluminum foil to form an electrode sheet. The negative electrode sheet was made of lithium metal. A simulated battery was assembled using 1 mol / L LiPF6 / (EC+DEC) mass ratio (1:1) as the electrolyte and charged and discharged at 0.05 C.

[0082] The test results are shown in Table 1.

[0083] Table 1. Detection results of the composite lithium supplement provided in the examples and comparative examples. III. Analysis of Test Results for Each Embodiment and Comparative Example As can be seen from Table 1, the composite lithium replenishing agent materials prepared in each embodiment of this application have significantly reduced powder resistivity to below 100 Ω·cm, and the residual alkali value (calculated as Li2CO3) is effectively controlled within 1.0 wt%. Moreover, the lithium replenishing capacity decay rate after exposure to harsh environment (25°C, 50% RH, 24h) is less than 5%, simultaneously achieving high conductivity, low residual alkali and excellent air stability.

[0084] Specifically, combining the data from Examples 1-3 and Comparative Examples 1 and 2, it can be seen that the amount of PVDF powder added has a significant impact on the performance of the prepared composite lithium supplement material. In Comparative Example 1, no PVDF powder was used, and in Example 2, a smaller amount of PVDF powder was used, resulting in a high resistivity of the prepared composite lithium supplement powder. However, as the amount of PVDF powder used increased, the resistivity of the composite lithium supplement prepared in Examples 1 and 3 decreased significantly, and the residual alkali content and capacity decay rate also decreased significantly. Furthermore, as shown in the data from Comparative Example 2, with a further increase in the amount of PVDF powder used, the performance of the prepared composite lithium supplement actually deteriorated.

[0085] Specifically, based on the data from Examples 1, 4, and 5 and Comparative Examples 4 and 5, it can be seen that when the heat treatment temperature is too low, the fluoropolymer does not decompose completely and cannot form an effective conductive network. As the composite coating layer is affected, the powder resistivity, residual alkali value, and capacity decay rate of the prepared composite lithium supplement increase significantly, and the electrochemical performance and air stability are also affected. On the other hand, if the heat treatment temperature is too high, it will destroy the LiF interface layer, resulting in an increase in residual alkali value and capacity decay rate.

[0086] Specifically, based on the data from Examples 1, 6, and 7 and Comparative Examples 6 and 7, it can be seen that when the particle size of Li5FeO4 particles is too small, the coating is uneven and the residual alkali cannot be effectively passivated. In contrast, in Comparative Example 7, the particle size of Li5FeO4 particles is too large, which reduces the specific surface area of ​​the material, affects the formation of the coating layer, and is not conducive to the construction of the conductive network.

[0087] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A composite lithium supplement, characterized in that, It includes Li5FeO4 particles and a coating layer at least partially covering the surface of the Li5FeO4 particles. The coating layer includes a LiF interface layer and a fluorine-doped carbon layer sequentially disposed outward from the surface of the Li5FeO4 particles. The total thickness of the coating layer is 5-50 nm; the total fluorine content in the composite lithium supplement is 0.5-5.0 at.%.

2. A method for preparing the composite lithium supplement as described in claim 1, characterized in that, Includes the following steps: Li5FeO4 lithium supplement powder was mixed evenly with a fluoropolymer in an organic solvent to obtain a slurry; The slurry is dried to obtain the precursor composite material; The precursor composite material was heat-treated under an inert atmosphere to obtain a composite lithium supplement.

3. The method for preparing the composite lithium supplement according to claim 2, characterized in that, The fluoropolymer is one or more selected from polyvinylidene fluoride, polytetrafluoroethylene, fluorinated ethylene propylene copolymer, and polyvinyl fluoride; and / or The organic solvent is one or more of N-methylpyrrolidone, dimethylacetamide, and acetone.

4. The method for preparing the composite lithium supplement according to claim 3, characterized in that, The mass ratio of the Li5FeO4 lithium supplement powder to the fluoropolymer is 100:(3~20).

5. The method for preparing the composite lithium supplement according to claim 2, characterized in that, The D50 particle size of the Li5FeO4 lithium supplement powder is 0.1~20μm.

6. The method for preparing the composite lithium supplement according to claim 2, characterized in that, The drying process is carried out at a temperature of 110~130℃ for 10~15 hours.

7. The method for preparing the composite lithium supplement according to claim 2, characterized in that, The heat treatment temperature is 400~700℃, and the holding time is 2~8h.

8. The method for preparing the composite lithium supplement according to claim 7, characterized in that, The heat treatment is carried out under an argon or nitrogen atmosphere, with a heating rate of 1~5℃ / min.

9. The method for preparing the composite lithium supplement according to claim 2, characterized in that, After being exposed to an environment of 25°C and 50% relative humidity for 24 hours, the lithium replenishment capacity decay rate of the composite lithium replenishment agent is less than 5%; and / or, The powder resistivity of the composite lithium supplement is less than or equal to 100 Ω·cm; and / or, The residual alkali value of the composite lithium supplement is less than or equal to 1.0 wt%.

10. A positive electrode material, characterized in that, This includes the composite lithium supplement as described in claim 1 or the composite lithium supplement prepared by the preparation method described in any one of claims 2-9.

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