Composite positive electrode material and preparation method and application thereof

Abstract

This invention provides a composite cathode material, its preparation method, and its applications. The composite cathode material comprises a lithium manganese iron phosphate core, and an inorganic coating layer and an organic coating layer sequentially coated on the surface of the lithium manganese iron phosphate core; wherein the inorganic coating layer comprises lithium niobium tantalate, and the organic coating layer comprises polypyrrole and polyolefin. This invention employs an inorganic-organic composite coating strategy. On the one hand, the high stability of lithium niobium tantalate serves as the core support, suppressing the volume expansion of the cathode material during charge and discharge, enhancing the structural stability of the material under high temperature and high voltage environments, and reducing safety hazards. On the other hand, the high conductivity of polypyrrole improves the problem of low intrinsic conductivity of the cathode material, effectively improving the rate performance of the battery. Simultaneously, the interfacial protection effect of polyolefin suppresses interfacial side reactions and the dissolution of metal ions such as Mn and Fe, extending the cycle life of the battery.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a composite cathode material, its preparation method, and its application. Background Technology

[0002] Lithium manganese iron phosphate (LMFP) cathode material has shown promising application prospects in the field of lithium-ion batteries due to its high theoretical specific capacity, moderate operating voltage, and abundant lithium resources, and is considered one of the potential cathode materials for next-generation high-energy-density batteries. However, LMFP still faces many technical bottlenecks in practical applications, which limits its commercialization process.

[0003] First, interfacial side reactions easily occur between the LMFP cathode material and the electrolyte, causing metal ions such as Mn and Fe in the material to dissolve and deposit on the negative electrode surface. This disrupts the stability of the electrode structure and continuously consumes active Li. + Firstly, the low intrinsic conductivity of LMFP cathode material hinders electron transport within and between material particles, resulting in poor rate performance and difficulty in meeting the demands of high-power applications. Secondly, during charging and discharging, the LMFP cathode material undergoes volume expansion due to lithium ion insertion and extraction, which can easily lead to particle breakage under long-term cycling, causing electrode structure collapse and further exacerbating capacity decay. In addition, when the battery operating voltage is too high, the interfacial stability between the LMFP cathode material and the electrolyte decreases, and the electrolyte is prone to decomposition reactions, producing gases and byproducts, which not only reduce battery energy efficiency but may also pose safety hazards.

[0004] Therefore, how to suppress interfacial side reactions of LMFP cathode materials, improve electronic conductivity, alleviate volume expansion, and enhance structural stability, thereby improving their electrochemical performance and stability, has become a research hotspot in the field of lithium-ion battery cathode materials. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a composite cathode material, its preparation method, and its applications. This invention employs an inorganic-organic composite coating strategy. On one hand, it leverages the high stability of lithium niobium tantalate as a core support; its excellent chemical stability suppresses volume expansion of the cathode material during charge and discharge, reducing the risk of particle breakage. Simultaneously, it enhances the structural stability of the material under high temperature and high voltage conditions, reducing safety hazards caused by electrolyte decomposition. On the other hand, it utilizes the high conductivity of polypyrrole to address the low intrinsic conductivity of the cathode material, accelerating electron transport within and between material particles, effectively improving the battery's rate performance. Furthermore, it utilizes the interfacial protection effect of polyolefins to form a dense organic coating layer on the surface of the cathode material, hindering direct contact with the electrolyte, thereby suppressing interfacial side reactions and the dissolution of metal ions such as Mn and Fe, extending the battery's cycle life. Therefore, this invention, through the synergistic effect of the inorganic and organic coating layers, achieves improved cycle stability and rate performance of the composite cathode material under high temperature and high voltage conditions, while simultaneously improving its structural stability and safety performance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a composite cathode material comprising a lithium manganese iron phosphate core, and an inorganic coating layer and an organic coating layer sequentially coating the surface of the lithium manganese iron phosphate core.

[0008] The inorganic coating layer includes lithium niobate tantalate, and the organic coating layer includes polypyrrole and polyolefin.

[0009] This invention employs an inorganic-organic composite coating strategy. On one hand, it leverages the high stability of lithium niobium tantalate material, whose excellent chemical stability suppresses volume expansion of the cathode material during charge and discharge, reducing the risk of particle breakage. Simultaneously, it enhances the structural stability of the material under high temperature and high voltage conditions, mitigating safety hazards caused by electrolyte decomposition. On the other hand, it utilizes the high conductivity of polypyrrole to address the low intrinsic conductivity of the cathode material, accelerating electron transport within and between material particles, effectively improving the battery's rate performance. Furthermore, it utilizes the interfacial protection effect of polyolefins to form a dense organic coating layer on the surface of the cathode material, hindering direct contact with the electrolyte and thus suppressing interfacial side reactions and the dissolution of metal ions such as Mn and Fe, extending the battery's cycle life. Therefore, this invention, through the synergistic effect of the inorganic and organic coating layers, achieves improved cycle stability and rate performance of the composite cathode material under high temperature and high voltage conditions, while simultaneously improving its structural stability and safety performance.

[0010] In this invention, polypyrrole and polyolefin work together as an organic coating layer to form an integrated "conductive-protective" coating layer. The interfacial protection of polyolefin provides a stable substrate for polypyrrole, preventing it from losing its conductivity due to structural damage during cycling. Meanwhile, the conductive network of polypyrrole compensates for the electron transport obstacles that may be caused by the insulation of polyolefin, ensuring that the coating layer does not affect electron conduction and can effectively perform its protective function. This synergistic effect can help the composite cathode material improve rate performance while simultaneously enhancing cycle stability, forming a triple optimization effect of "structural stability-electron conduction-interfacial protection" with lithium niobate tantalate in the inorganic coating layer.

[0011] Preferably, the particle size D50 of the lithium manganese iron phosphate core is 100-500nm, for example, it can be 100nm, 200nm, 300nm, 400nm or 500nm.

[0012] Preferably, the thickness of the inorganic coating layer is 2-20 nm, for example, it can be 2 nm, 5 nm, 10 nm, 15 nm or 20 nm.

[0013] Preferably, the thickness ratio of the inorganic coating layer to the organic coating layer is 1:(2-5), for example, it can be 1:2, 1:3, 1:4 or 1:5, etc.

[0014] In this invention, a suitable thickness ratio allows the inorganic coating layer to fully function while avoiding excessive obstruction of ion / electron transport. The organic coating layer, on the other hand, meets the requirements for interface protection and conductivity without adding extra to the overall impedance of the material. This thickness ratio ensures a tight bond between the inorganic and organic coating layers. This thickness ratio also coordinates the performance stability of the composite cathode material under high temperature and high voltage, achieving a long-term improvement in electrochemical performance.

[0015] Preferably, in the organic coating layer, the mass ratio of polypyrrole to polyolefin is (1-2):(2-1), wherein the polypyrrole selection range "1-2" can be, for example, 1, 1.2, 1.4, 1.6, 1.8 or 2, and the polyolefin selection range "2-1" can be, for example, 1, 1.2, 1.4, 1.6, 1.8 or 2.

[0016] In this invention, the mass ratio allows the high conductivity of polypyrrole and the interfacial protection of polyolefin to work synergistically, which not only improves the rate performance and cycle stability of the composite cathode material, but also forms a stable organic coating layer, enhances the bonding with the inorganic coating layer, and ultimately synergistically improves the overall performance of the composite cathode material.

[0017] Preferably, the polyolefin includes polypropylene and / or polyethylene.

[0018] Preferably, the general chemical formula of the lithium niobate tantalate material is LiNb.x Ta 1-x O3, where 0 < x < 1, for example, can be 0.2, 0.4, 0.6 or 0.8, etc.

[0019] In a second aspect, the present invention provides a method for preparing the composite cathode material as described in the first aspect, the method comprising the following steps:

[0020] The raw materials for preparing lithium niobate tantalate and lithium manganese iron phosphate are first mixed, and then subjected to a first heat treatment to form an inorganic coating layer on the surface of the lithium manganese iron phosphate material, thus obtaining an intermediate.

[0021] The intermediate and the raw materials for preparing the organic coating layer are mixed for a second time, and then subjected to a second heat treatment to form an organic coating layer on the surface of the inorganic coating layer, thereby obtaining the composite cathode material; wherein, the organic coating layer includes polypyrrole and polyolefin.

[0022] Preferably, the raw materials for preparing the lithium niobate tantalate material include a niobium source, a tantalum source, and a lithium source.

[0023] Preferably, the niobium source includes any one or a combination of at least two of niobium pentoxide, niobium oxalate, ammonium niobate, or niobium pentaethoxy.

[0024] Preferably, the tantalum source includes any one or a combination of at least two of tantalum pentoxide, tantalum oxalate, ammonium tantalate, or tantalum pentapropoxy.

[0025] Preferably, the lithium source includes any one or a combination of at least two of lithium carbonate, lithium nitrate, lithium acetate, or lithium hydroxide.

[0026] Preferably, the first mixing method includes:

[0027] (a1) A sol is obtained by mixing a niobium source, a tantalum source, a lithium source, a complexing agent, and a solvent.

[0028] (b1) Disperse lithium manganese iron phosphate material in the sol to form a gel, and then dry it to form a precursor powder.

[0029] The present invention employs the aforementioned mixing method, which ensures the uniform dispersion of the precursor components of lithium niobium tantalate at the molecular level, laying the foundation for the subsequent formation of a homogeneous inorganic coating layer. Dispersing lithium manganese iron phosphate in a sol allows the inorganic precursor components to adhere uniformly to the surface of the lithium manganese iron phosphate particles due to the sol's fluidity, avoiding the problem of uneven coating layer distribution in traditional mixing methods. Simultaneously, the gelation process further fixes the relative positions of the inorganic precursor components and lithium manganese iron phosphate, reducing agglomeration or detachment. This "sol-gel" mixing and dispersion method not only ensures the uniformity and integrity of the inorganic coating layer but also, through subsequent composite with an organic coating layer, ultimately achieves a synergistic improvement in the electrochemical performance and structural stability of the lithium manganese iron phosphate material.

[0030] Preferably, the atmosphere for the first heat treatment is an inert atmosphere. For example, it could be argon or nitrogen.

[0031] Preferably, the temperature of the first heat treatment is 300-600℃, for example, it can be 300℃, 400℃, 500℃ or 600℃.

[0032] Preferably, the first heat treatment time is 3-6 hours, for example, it can be 3 hours, 4 hours, 5 hours or 6 hours.

[0033] Preferably, the raw materials for preparing the organic coating layer include a mixed emulsion of polypyrrole and polyolefin.

[0034] Preferably, the second mixing method includes:

[0035] (a2) Mix polypyrrole, polyolefin, emulsifier and initiator to obtain a mixed emulsion of polypyrrole and polyolefin.

[0036] (b2) The intermediate is added to the mixed emulsion of the polypyrrole and polyolefin.

[0037] This invention employs the aforementioned mixing method, enabling polypyrrole and polyolefin to form a stable dispersion system in the emulsion. This avoids stratification or aggregation due to differences in hydrophilicity and hydrophobicity, ensuring initial mixing of the two polymers at the molecular level. This lays the foundation for the subsequent formation of a synergistic "conductive-protective" organic coating layer. Secondly, when the intermediate is added to the mixed emulsion, the emulsion's fluidity allows polypyrrole and polyolefin to uniformly wet the intermediate surface. The interfacial activity of the emulsifier helps them adhere to the inorganic coating layer surface, avoiding the problems of localized accumulation or incomplete coverage of the organic coating layer in traditional physical mixing. Simultaneously, the initiator further promotes the in-situ curing of polypyrrole and polyolefin on the intermediate surface during mixing, enhancing the bonding force between the organic and inorganic coating layers and reducing the detachment of the organic coating layer during subsequent processing. The synergistic dispersion of the two polymers in the emulsion allows for the formation of a continuous and functionally balanced organic coating layer on the intermediate surface. Therefore, this emulsion dispersion-in-situ coating mixing method, combined with the aforementioned inorganic coating layer preparation process, ultimately constructs an integrated coating structure of "inorganic stable layer-organic functional layer" on the surface of lithium manganese iron phosphate, giving full play to the synergistic effect between the components.

[0038] Preferably, before the intermediate is added to the mixed emulsion of polypyrrole and polyolefin, the intermediate is first immersed in an adhesion promoter solution for modification treatment.

[0039] The purpose of modifying the intermediate in this invention is to improve the interfacial bonding force between the intermediate surface and polypyrrole and polyolefin, thereby solving the problem of loose bonding that may be caused by the difference in hydrophilicity and hydrophobicity between the inorganic coating layer and the organic coating layer, reducing the risk of peeling off the organic coating layer in subsequent charge and discharge cycles, ensuring the long-term effectiveness of the integrated coating structure of "inorganic stable layer-organic functional layer", and thus maintaining the conductivity and interfacial protection performance of the composite cathode material in long-term cycling.

[0040] Preferably, the adhesion promoter solution comprises a silane coupling agent solution. For example, it could be an ethanol solution of a silane coupling agent (KH550), etc.

[0041] Preferably, the mass concentration of the silane coupling agent solution is 0.5-5%, for example, it can be 0.5%, 1%, 2%, 3%, 4% or 5%, etc.

[0042] Preferably, the atmosphere for the second heat treatment is an inert atmosphere. For example, it could be argon or nitrogen.

[0043] Preferably, the temperature of the second heat treatment is 50-90°C, for example, it can be 50°C, 60°C, 70°C, 80°C or 90°C.

[0044] Preferably, the second heat treatment time is 2-4 hours, for example, it can be 2 hours, 3 hours or 4 hours.

[0045] Preferably, the preparation method includes the following steps:

[0046] (1) Under stirring conditions, a niobium source, a tantalum source, a lithium source, a complexing agent and a solvent are mixed to obtain a sol; wherein the complexing agent includes any one or a combination of at least two of citric acid, ethylenediaminetetraacetic acid, tartaric acid, ethylene glycol or acetylacetone, and the solvent includes any one or a combination of at least two of water, anhydrous ethanol, isopropanol, ethylene glycol methyl ether or N,N-dimethylformamide.

[0047] Under ultrasonic stirring, lithium manganese iron phosphate material is dispersed in the sol, then the pH is adjusted to 7.1-9 (e.g., 7.1, 7.5, 8, 8.5 or 9, etc.), and heated to form a gel, which is then dried to form a precursor powder.

[0048] The precursor powder is subjected to a first heat treatment in an inert atmosphere at a heating rate of 2-10℃ / min (e.g., 2℃ / min, 4℃ / min, 6℃ / min, 8℃ / min, or 10℃ / min, etc.) to 300-600℃, and the first heat treatment time is 3-6h, so that an inorganic coating layer is formed on the surface of the lithium manganese iron phosphate material to obtain an intermediate.

[0049] (2) Under stirring conditions, polypyrrole and polyolefin are mixed, and then an emulsifier and an initiator are added to obtain a mixed emulsion of polypyrrole and polyolefin; wherein, the emulsifier includes anionic emulsifiers (e.g., sodium dodecyl sulfate or sodium dodecylbenzene sulfonate) and / or nonionic emulsifiers (e.g., polyoxyethylene octylphenyl ether or polyethylene glycol sorbitan fatty acid ester), and the initiator includes oxidative initiators (e.g., ammonium persulfate or potassium persulfate) and / or free radical initiators (e.g., azobisisobutyronitrile or benzoyl peroxide).

[0050] The intermediate is immersed in an adhesion promoter solution for modification. Then, the modified intermediate is added to the mixed emulsion of polypyrrole and polyolefin and subjected to a second heat treatment at 50-90°C for 2-4 hours. After that, it is centrifuged and dried to obtain the composite cathode material. The adhesion promoter solution is a silane coupling agent ethanol solution with a mass concentration of 0.5-5%.

[0051] Thirdly, the present invention provides an application of the composite cathode material as described in the first aspect in the field of secondary batteries.

[0052] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] This invention employs an inorganic-organic composite coating strategy. On one hand, it leverages the high stability of lithium niobium tantalate material, whose excellent chemical stability suppresses volume expansion of the cathode material during charge and discharge, reducing the risk of particle breakage. Simultaneously, it enhances the structural stability of the material under high temperature and high voltage conditions, mitigating safety hazards caused by electrolyte decomposition. On the other hand, it utilizes the high conductivity of polypyrrole to address the low intrinsic conductivity of the cathode material, accelerating electron transport within and between material particles, effectively improving the battery's rate performance. Furthermore, it utilizes the interfacial protection effect of polyolefins to form a dense organic coating layer on the surface of the cathode material, hindering direct contact with the electrolyte and thus suppressing interfacial side reactions and the dissolution of metal ions such as Mn and Fe, extending the battery's cycle life. Therefore, this invention, through the synergistic effect of the inorganic and organic coating layers, achieves improved cycle stability and rate performance of the composite cathode material under high temperature and high voltage conditions, while simultaneously improving its structural stability and safety performance. Detailed Implementation

[0055] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0056] It should be noted that the room temperature below refers to 25℃.

[0057] Example 1

[0058] This embodiment provides a composite cathode material, which includes a lithium manganese iron phosphate core and an inorganic coating layer and an organic coating layer sequentially coated on the surface of the lithium manganese iron phosphate core.

[0059] The lithium manganese iron phosphate core has a particle size D50 of 300 nm and a chemical formula of LiNb. x Ta 1-x O3, where x = 0.5; the inorganic coating layer includes lithium niobate tantalate material; the thickness of the inorganic coating layer is 10 nm; the organic coating layer includes polypyrrole and polyolefin in a mass ratio of 1:1; the thickness ratio of the inorganic coating layer to the organic coating layer is 1:3.5.

[0060] This embodiment also provides a method for preparing the above-mentioned composite cathode material, the method comprising the following steps:

[0061] (1) Under stirring conditions, niobium pentoxide, tantalum pentoxide, nitric acid, citric acid and anhydrous ethanol are mixed to obtain a sol.

[0062] Under ultrasonic stirring, lithium manganese iron phosphate material is dispersed in the sol, then the pH is adjusted to 8, and the mixture is heated to form a gel, which is then dried to form a precursor powder.

[0063] The precursor powder was subjected to a first heat treatment at a heating rate of 5°C / min to 450°C in a nitrogen atmosphere for 4.5 hours, so that an inorganic coating layer was formed on the surface of the lithium manganese iron phosphate material, thus obtaining an intermediate.

[0064] (2) Under stirring conditions, polypyrrole and polyethylene are mixed, and then sodium dodecyl sulfate and ammonium persulfate are added and dispersed in deionized water to obtain a mixed emulsion of polypyrrole and polyolefin.

[0065] The intermediate was immersed in a 3% (w / w) ethanol solution of silane coupling agent (KH550) and modified at room temperature. After drying, the modified intermediate was added to the mixed emulsion of polypyrrole and polyolefin and subjected to a second heat treatment at 70°C in a nitrogen atmosphere for 3 hours. Then, it was centrifuged and vacuum dried to obtain the composite cathode material.

[0066] Example 2

[0067] This embodiment provides a composite cathode material, which includes a lithium manganese iron phosphate core and an inorganic coating layer and an organic coating layer sequentially coated on the surface of the lithium manganese iron phosphate core.

[0068] The lithium manganese iron phosphate core has a particle size D50 of 300 nm and a chemical formula of LiNb. x Ta 1-x O3, where x = 0.5; the inorganic coating layer includes lithium niobate tantalate material; the thickness of the inorganic coating layer is 2 nm; the organic coating layer includes polypyrrole and polyolefin in a mass ratio of 1:2; the thickness ratio of the inorganic coating layer to the organic coating layer is 1:2.

[0069] This embodiment also provides a method for preparing the above-mentioned composite cathode material, the method comprising the following steps:

[0070] (1) Under stirring conditions, niobium pentoxide, tantalum pentoxide, nitric acid, citric acid and anhydrous ethanol are mixed to obtain a sol.

[0071] Under ultrasonic stirring, lithium manganese iron phosphate material is dispersed in the sol, then the pH is adjusted to 7.5, heated to form a gel, and then dried to form a precursor powder.

[0072] The precursor powder was subjected to a first heat treatment at a heating rate of 5°C / min to 300°C in a nitrogen atmosphere for 3 hours, so that an inorganic coating layer was formed on the surface of the lithium manganese iron phosphate material, thus obtaining an intermediate.

[0073] (2) Under stirring conditions, polypyrrole and polyethylene are mixed, and then sodium dodecyl sulfate and ammonium persulfate are added and dispersed in deionized water to obtain a mixed emulsion of polypyrrole and polyolefin.

[0074] The intermediate was immersed in a 1% (w / w) ethanol solution of silane coupling agent (KH550) and modified at room temperature. After drying, the modified intermediate was added to the mixed emulsion of polypyrrole and polyolefin and subjected to a second heat treatment at 50°C in a nitrogen atmosphere for 4 hours. Then, it was centrifuged and vacuum dried to obtain the composite cathode material.

[0075] Example 3

[0076] This embodiment provides a composite cathode material, which includes a lithium manganese iron phosphate core and an inorganic coating layer and an organic coating layer sequentially coated on the surface of the lithium manganese iron phosphate core.

[0077] The lithium manganese iron phosphate core has a particle size D50 of 300 nm and a chemical formula of LiNb. x Ta 1-x O3, where x = 0.5; the inorganic coating layer includes lithium niobate tantalate material; the thickness of the inorganic coating layer is 20 nm; the organic coating layer includes polypyrrole and polyolefin in a mass ratio of 3:1; the thickness ratio of the inorganic coating layer to the organic coating layer is 1:5.

[0078] This embodiment also provides a method for preparing the above-mentioned composite cathode material, the method comprising the following steps:

[0079] (1) Under stirring conditions, niobium pentoxide, tantalum pentoxide, nitric acid, citric acid and anhydrous ethanol are mixed to obtain a sol.

[0080] Under ultrasonic stirring, lithium manganese iron phosphate material is dispersed in the sol, then the pH is adjusted to 8.5, heated to form a gel, and then dried to form a precursor powder.

[0081] The precursor powder was subjected to a first heat treatment at a heating rate of 5°C / min to 600°C in a nitrogen atmosphere for 6 hours, so that an inorganic coating layer was formed on the surface of the lithium manganese iron phosphate material, thus obtaining an intermediate.

[0082] (2) Under stirring conditions, polypyrrole and polypropylene are mixed, and then sodium dodecyl sulfate and ammonium persulfate are added and dispersed in deionized water to obtain a mixed emulsion of polypyrrole and polyolefin.

[0083] The intermediate was immersed in a 5% (w / w) ethanol solution of silane coupling agent (KH550) and modified at room temperature. After drying, the modified intermediate was added to the mixed emulsion of polypyrrole and polyolefin and subjected to a second heat treatment at 90°C under a nitrogen atmosphere for 2 hours. Then, it was centrifuged and vacuum dried to obtain the composite cathode material.

[0084] Example 4

[0085] The difference between this embodiment and Embodiment 1 is that the thickness ratio of the inorganic coating layer to the organic coating layer is 1:1.

[0086] The remaining preparation methods and parameters are consistent with those in Example 1.

[0087] Example 5

[0088] The difference between this embodiment and Embodiment 1 is that the thickness ratio of the inorganic coating layer to the organic coating layer is 1:8.

[0089] The remaining preparation methods and parameters are consistent with those in Example 1.

[0090] Example 6

[0091] The difference between this embodiment and Embodiment 1 is that the mass ratio of polypyrrole to polyolefin in the organic coating layer is 1:3.

[0092] The remaining preparation methods and parameters are consistent with those in Example 1.

[0093] Example 7

[0094] The difference between this embodiment and Embodiment 1 is that the mass ratio of polypyrrole to polyolefin in the organic coating layer is 3:1.

[0095] The remaining preparation methods and parameters are consistent with those in Example 1.

[0096] Example 8

[0097] The difference between this embodiment and embodiment 1 is that no modification treatment is performed on the intermediate in step (2).

[0098] The remaining preparation methods and parameters are consistent with those in Example 1.

[0099] Comparative Example 1

[0100] The difference between this comparative example and Example 1 is that step (1) is omitted, and instead, lithium manganese iron phosphate material is directly used to replace the intermediate in step (2), so that the surface of the lithium manganese iron phosphate core is only coated with an organic coating layer.

[0101] The remaining preparation methods and parameters are consistent with those in Example 1.

[0102] Comparative Example 2

[0103] The difference between this comparative example and Example 1 is that step (2) is omitted, so that the surface of the lithium manganese iron phosphate core is only coated with an inorganic coating layer.

[0104] The remaining preparation methods and parameters are consistent with those in Example 1.

[0105] Comparative Example 3

[0106] The difference between this comparative example and Example 1 is that in step (2), polyethylene is replaced with an equal mass of polypyrrole.

[0107] The remaining preparation methods and parameters are consistent with those in Example 1.

[0108] Performance testing

[0109] Lithium-ion batteries were prepared based on the composite cathode materials provided in the above embodiments and comparative examples: The composite cathode material, conductive carbon black, and polyvinylidene fluoride prepared above were weighed in a mass ratio of 8:1:1 and added to N-methylpyrrolidone to form a slurry. The slurry was then coated onto aluminum foil, dried, and sliced ​​to obtain a cathode sheet. A lithium metal sheet was used as the counter electrode, a polypropylene microporous membrane was used as the separator, and an electrolyte with a solute concentration of 1 mol / L was prepared (wherein, the solvent was a mixed solution of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1, and the solute was LiPF6). The batteries were assembled into CR2025 button batteries in an argon glove box.

[0110] Cycle performance and rate performance were tested on the CR2025 button cell.

[0111] Cyclic performance test conditions: After the assembled battery is left to stand for 2 hours, it is subjected to 300 cycles at 25℃, voltage range of 2.5-4.5V, and 2C rate to obtain the cycle capacity retention rate.

[0112] Test conditions for rate performance: After the assembled battery is left to stand for 2 hours, it is cyclically charged 5 times each at 0.2C, 0.5C, 1C, 2C and 5C at 25℃ and a voltage range of 2.5-4.5V.

[0113] The test results are shown in Table 1.

[0114] Table 1

[0115]

[0116]

[0117] analyze

[0118] As shown in Table 1, this invention adopts an inorganic-organic composite coating strategy. On the one hand, the high stability of lithium niobium tantalate material serves as the core support, suppressing the volume expansion of the cathode material during charging and discharging, enhancing the structural stability of the material under high temperature and high voltage environments, and reducing safety hazards. On the other hand, the high conductivity of polypyrrole improves the problem of low intrinsic conductivity of the cathode material, effectively improving the rate performance of the battery. At the same time, the interfacial protection effect of polyolefin is utilized to suppress interfacial side reactions and the dissolution of metal ions such as Mn and Fe, extending the cycle life of the battery.

[0119] As can be seen from the comparison between Example 1 and Examples 4-5, if the thickness ratio of the inorganic coating layer to the organic coating layer is too large, the inorganic coating layer will be relatively too thick, which will significantly increase the diffusion resistance of lithium ions inside the material, resulting in a decrease in battery rate performance. At the same time, the excessively thick inorganic layer is prone to cracking during charge and discharge volume expansion due to excessive rigidity, which will reduce structural stability. If the thickness ratio of the inorganic coating layer to the organic coating layer is too small, the organic coating layer will be relatively too thick, which is prone to wrinkling or falling off during cycling due to excessive flexibility, and cannot stably perform the interface protection function, resulting in aggravated interface side reactions and decreased cycle stability.

[0120] A comparison of Examples 1 and 6-7 shows that if the mass ratio of polypyrrole to polyolefin in the organic coating layer is too small, the proportion of polypyrrole is insufficient, making it difficult to form a continuous conductive network and effectively improve the electronic conductivity of the material, resulting in a deterioration in the battery rate performance. If the mass ratio of polypyrrole to polyolefin in the organic coating layer is too large, the proportion of polyolefin is insufficient, making it difficult to form a complete and dense interfacial protective layer on the material surface, making it difficult to block the direct contact between the material and the electrolyte, and exacerbating interfacial side reactions and metal ion dissolution, leading to a decrease in cycle stability.

[0121] As can be seen from the comparison between Example 1 and Example 8, if the intermediate is not modified in step (2), the adhesion of the inorganic coating layer to the organic coating layer will decrease, resulting in poor interfacial bonding between the two, which is prone to delamination or peeling, which is not conducive to the stability of the composite coating structure, and ultimately causes the degradation of battery cycle stability and rate performance.

[0122] As can be seen from the comparison between Example 1 and Comparative Example 1, if the surface of the lithium manganese iron phosphate core is only coated with an organic coating layer, it lacks the rigid support of the inorganic coating layer, cannot effectively resist the volume expansion during the charging and discharging process, is easily damaged under long-term cycling, has poor structural stability, and electrolyte decomposition and safety hazards are difficult to avoid.

[0123] As can be seen from the comparison between Example 1 and Comparative Example 2, if the surface of the lithium manganese iron phosphate core is only coated with an inorganic coating layer, it cannot improve the problem of low intrinsic conductivity of the core material. The obstruction of electron transport leads to poor rate performance. In addition, without the interface protection of the organic coating layer, the core and electrolyte are still prone to interfacial side reactions, resulting in poor cycle life.

[0124] As can be seen from the comparison between Example 1 and Comparative Example 3, if polyethylene is replaced with an equal mass of polypyrrole in step (2), that is, the organic coating layer contains only polypyrrole, then the interfacial protection of polyolefin is lacking, and it is difficult to form a dense physical barrier. This will lead to direct contact between the core material and the electrolyte, and the problem of metal ion dissolution in the interfacial side reaction will be difficult to suppress, thus shortening the battery cycle life. At the same time, polypyrrole has poor film-forming properties and it is difficult to form a continuous and complete coating layer when coated alone. Some core materials are easily exposed to the electrolyte, which further aggravates the capacity decay. In addition, polypyrrole is not stable enough in high temperature environment. It is easy to undergo structural decomposition under high temperature conditions for a long time, lose its conductivity, and cause the battery performance to drop sharply.

[0125] It should be noted that the technical solution of the present invention is illustrated through the above embodiments, but the present invention is not limited to the above embodiments, that is, it does not mean that the present invention must rely on the above embodiments to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A composite cathode material, characterized in that, The composite cathode material includes a lithium manganese iron phosphate core, and an inorganic coating layer and an organic coating layer sequentially coated on the surface of the lithium manganese iron phosphate core. The inorganic coating layer includes lithium niobate tantalate, and the organic coating layer includes polypyrrole and polyolefin.

2. The composite cathode material according to claim 1, characterized in that, The particle size D50 of the lithium manganese iron phosphate core is 100-500nm.

3. The composite cathode material according to claim 1, characterized in that, The thickness of the inorganic coating layer is 2-20 nm; The thickness ratio of the inorganic coating layer to the organic coating layer is 1:(2-5).

4. The composite cathode material according to claim 1, characterized in that, In the organic coating layer, the mass ratio of polypyrrole to polyolefin is (1-2):(2-1).

5. The composite cathode material according to claim 1, characterized in that, The polyolefins include polypropylene and / or polyethylene.

6. The composite cathode material according to claim 1, characterized in that, The general chemical formula of the lithium niobate tantalate material is LiNb. x Ta 1-x O3, where 0 < x < 1.

7. A method for preparing a composite cathode material as described in any one of claims 1-6, characterized in that, The preparation method includes the following steps: The raw materials for preparing lithium niobate tantalate and lithium manganese iron phosphate are first mixed, and then subjected to a first heat treatment to form an inorganic coating layer on the surface of the lithium manganese iron phosphate material, thus obtaining an intermediate. The intermediate and the raw materials for preparing the organic coating layer are mixed for a second time, and then subjected to a second heat treatment to form an organic coating layer on the surface of the inorganic coating layer, thereby obtaining the composite cathode material; wherein, the organic coating layer includes polypyrrole and polyolefin.

8. The preparation method according to claim 7, characterized in that, The raw materials for preparing the lithium niobate tantalate material include niobium source, tantalum source and lithium source.

9. The preparation method according to claim 7, characterized in that, The first mixing method includes: (a1) A sol is obtained by mixing a niobium source, a tantalum source, a lithium source, a complexing agent, and a solvent; (b1) Disperse lithium manganese iron phosphate material in the sol to form a gel, and then dry it to form a precursor powder.

10. The preparation method according to claim 7, characterized in that, The atmosphere for the first heat treatment is an inert atmosphere.

11. The preparation method according to claim 7, characterized in that, The temperature of the first heat treatment is 300-600℃.

12. The preparation method according to claim 7, characterized in that, The first heat treatment lasts for 3-6 hours.

13. The preparation method according to claim 7, characterized in that, The raw materials for preparing the organic coating layer include a mixed emulsion of polypyrrole and polyolefin.

14. The preparation method according to claim 7, characterized in that, The second mixing method includes: (a2) Mix polypyrrole, polyolefin, emulsifier and initiator to obtain a mixed emulsion of polypyrrole and polyolefin; (b2) The intermediate is added to the mixed emulsion of the polypyrrole and polyolefin.

15. The preparation method according to claim 7, characterized in that, Before the intermediate is added to the mixed emulsion of polypyrrole and polyolefin, the intermediate is first immersed in an adhesion promoter solution for modification treatment.

16. The preparation method according to claim 15, characterized in that, The adhesion promoter solution includes a silane coupling agent solution.

17. The preparation method according to claim 7, characterized in that, The atmosphere for the second heat treatment is an inert atmosphere.

18. The preparation method according to claim 7, characterized in that, The temperature of the second heat treatment is 50-90℃.

19. The preparation method according to claim 7, characterized in that, The second heat treatment lasts for 2-4 hours.

20. The preparation method according to claim 7, characterized in that, The preparation method includes the following steps: (1) Under stirring conditions, niobium source, tantalum source, lithium source, complexing agent and solvent are mixed to obtain sol; wherein, the complexing agent includes any one or a combination of at least two of citric acid, ethylenediaminetetraacetic acid, tartaric acid, ethylene glycol or acetylacetone, and the solvent includes any one or a combination of at least two of water, anhydrous ethanol, isopropanol, ethylene glycol methyl ether or N,N-dimethylformamide; Under ultrasonic stirring, lithium manganese iron phosphate material is dispersed in the sol, then the pH is adjusted to 7.1-9, heated to form a gel, and then dried to form a precursor powder; The precursor powder is subjected to a first heat treatment at a heating rate of 2-10℃ / min to 300-600℃ in an inert atmosphere for 3-6 hours, so that an inorganic coating layer is formed on the surface of the lithium manganese iron phosphate material to obtain an intermediate. (2) Under stirring conditions, polypyrrole and polyolefin are mixed, and then emulsifier and initiator are added to obtain a mixed emulsion of polypyrrole and polyolefin; wherein, the emulsifier includes anionic emulsifier and / or nonionic emulsifier, and the initiator includes oxidative initiator and / or free radical initiator; The intermediate is immersed in an adhesion promoter solution for modification. Then, the modified intermediate is added to the mixed emulsion of polypyrrole and polyolefin and subjected to a second heat treatment at 50-90°C for 2-4 hours. After centrifugation and drying, the composite cathode material is obtained. The adhesion promoter solution is a silane coupling agent ethanol solution with a mass concentration of 0.5-5%.

21. The application of a composite cathode material as described in any one of claims 1-6 in the field of secondary batteries.

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