Multifunctional carbon sources and their preparation methods and applications, carbon-coated lithium iron phosphate main materials and their preparation methods, lithium-ion batteries

Abstract

This invention relates to the field of lithium-ion battery cathode material modification additives, specifically to a multifunctional carbon source and its preparation method and application, carbon-coated lithium iron phosphate main material and its preparation method, and lithium-ion batteries. The multifunctional carbon source is polymerized from one or more of vinyl aromatic ring monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers; wherein the number-average molecular weight of the multifunctional carbon source is 6000~60000 Da, and the molecular weight distribution coefficient is ≤1.8. Through precise control of monomer types, ratios, and molecular weights in a molecular-scale directional design, a triple synergistic system of lubrication and dispersion, high residual carbon film formation, and nitrogen-doped conductivity is constructed. A quantitative correlation is established between molecular structure, carbon layer characteristics, slurry performance, and electrochemical performance, achieving simultaneous improvement in lithium iron phosphate milling efficiency, carbon layer quality, conductivity, and rate cycling performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material modification additives, specifically to a multifunctional carbon source and its preparation method and application, carbon-coated lithium iron phosphate main material and its preparation method, and lithium-ion batteries. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become the mainstream cathode material in the fields of power batteries and energy storage due to its outstanding advantages such as low raw material cost, excellent thermal stability, long cycle life, and environmental friendliness, and its market share continues to rise. However, lithium iron phosphate has the problem of extremely poor intrinsic electronic conductivity, with a room temperature conductivity of only about 10. -9 ~10 -10 The low S / cm and slow lithium-ion diffusion rate limit its high-rate performance. Meanwhile, in the wet process of lithium iron phosphate preparation, the precursor slurry generally suffers from low solid content, long grinding time, easy particle agglomeration, and large viscosity fluctuations. This not only significantly reduces production efficiency and increases production energy consumption, but also leads to uneven distribution of the subsequent carbon coating layer, further aggravating the degradation of the rate and cycle performance of lithium iron phosphate, becoming an industry bottleneck for the development of high-end lithium iron phosphate products.

[0003] Carbon coating is a core modification method for improving the conductivity of lithium iron phosphate (LFP). The performance of the carbon source directly determines the coating effect and the processing characteristics of LFP slurry. Existing industrially used carbon sources have significant functional defects: sugar-based carbon sources, although inexpensive, have a high-temperature residual carbon rate of ≤25%, and the carbon layer formed after pyrolysis is loose and porous, lacking lubrication and modification effects, thus failing to optimize slurry processing performance; inorganic carbon sources have excellent conductivity, but are prone to agglomeration, and their addition exacerbates the viscosity spike in the slurry, worsening processing characteristics; ordinary polymer carbon sources are mostly single-component designs, possessing only single carbon formation or dispersion functions, lacking in-situ nitrogen doping conductivity modification capabilities, and making it difficult to simultaneously achieve both the slurry processing performance and electrochemical performance of LFP.

[0004] While some functional carbon sources exist in current technologies, they are mostly simple modifications of single components. They lack a structure-property relationship between the carbon source's molecular structure and the processing and electrochemical performance of lithium iron phosphate (LFP), failing to achieve synergistic optimization of multiple properties. Furthermore, their preparation processes are complex, with harsh reaction conditions and poor compatibility with existing industrial production lines, hindering large-scale application. Therefore, developing a multifunctional carbon source with controllable structure, diverse functions, the ability to simultaneously optimize LFP processing and electrochemical performance, and compatibility with existing industrial processes is crucial to overcoming the current bottlenecks in LFP modification technology. Summary of the Invention

[0005] To address the shortcomings of existing carbon sources, such as limited functionality, ambiguous structure-property relationships, inability to simultaneously achieve both processing and electrochemical performance in lithium iron phosphate slurries, and poor industrial applicability, this invention provides a multifunctional carbon source. Through homopolymerization, binary copolymerization, and ternary copolymerization structural design, it precisely controls the types, ratios, and molecular weights of monomers at the molecular scale, constructing a synergistic system of lubrication and dispersion, high residual carbon film formation, and nitrogen-doped conductivity. This establishes a quantitative correlation between molecular structure, carbon layer characteristics, slurry performance, and electrochemical performance, achieving simultaneous improvement in lithium iron phosphate milling efficiency, carbon layer quality, conductivity, and rate cycling performance. Furthermore, the preparation method for this multifunctional carbon source is controllable, mild, low-waste, and has a high monomer conversion rate, fully compatible with existing lithium iron phosphate industrial production lines, and possesses strong industrial application value.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a multifunctional carbon source, which is polymerized from one or more of vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers; The number-average molecular weight of the multifunctional carbon source is 6000~60000 Da, and the molecular weight distribution coefficient is ≤1.8.

[0007] Preferably, the side chains of the vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers all contain polymerizable double bonds.

[0008] Preferably, the vinyl aromatic monomer is selected from one or more of styrene, p-methylstyrene, vinylnaphthalene, and vinylbiphenyl.

[0009] Preferably, the vinyl nitrogen-containing heterocyclic monomer is selected from one or more of 2-vinylpyridine, 4-vinylpyridine, N-vinylimidazolium and N-vinylpyrrole.

[0010] Preferably, the polyether vinyl monomer is selected from one or more of polyethylene glycol monomethyl ether acrylate, polyethylene glycol monovinyl ether, polypropylene glycol monoacrylate, and allyl polyoxyethylene ether.

[0011] Preferably, the number average molecular weight of the polyethylene glycol monomethyl ether acrylate is 500-2000; and the number average molecular weight of the polyethylene glycol monovinyl ether is 400-1500.

[0012] Preferably, the monomer composition of the multifunctional carbon source, based on the molar percentage of monomers, is as follows: 1) Homopolymer: polymerized from a single vinyl aromatic monomer, a single vinyl nitrogen-containing heterocyclic monomer, or a single polyether vinyl monomer, with a corresponding monomer molar percentage of 100%; 2) Binary copolymer type: 30-70% vinyl aromatic monomers combined with 30-70% vinyl nitrogen-containing heterocyclic monomers; 40-75% vinyl aromatic monomers combined with 25-60% polyether vinyl monomers; 35-70% vinyl nitrogen-containing heterocyclic monomers combined with 30-65% polyether vinyl monomers; 3) Ternary copolymer type: 25~65% vinyl aromatic ring monomers, 5~35% vinyl nitrogen-containing heterocyclic monomers and 10~40% polyether vinyl monomers; The sum of the molar percentages of the corresponding monomers in the binary copolymer and ternary copolymer is 100%.

[0013] Secondly, the present invention provides a method for preparing a multifunctional carbon source as described in the present invention. The preparation method includes: dissolving a monomer in a solvent to obtain a monomer solution, removing oxygen, prepolymerizing, adding an initiator dropwise to carry out a polymerization reaction, cooling after the reaction is completed, adjusting the pH to 6.8~7.5, concentrating, drying, and pulverizing at low temperature to obtain a powdered multifunctional carbon source. The monomer is selected from one or more of vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers; The monomer solution has a mass concentration of 25-45 wt%.

[0014] Preferably, the vinyl aromatic monomer is selected from one or more of styrene, p-methylstyrene, vinylnaphthalene, and vinylbiphenyl.

[0015] Preferably, the vinyl nitrogen-containing heterocyclic monomer is selected from one or more of 2-vinylpyridine, 4-vinylpyridine, N-vinylimidazolium and N-vinylpyrrole.

[0016] Preferably, the polyether vinyl monomer is selected from one or more of polyethylene glycol monomethyl ether acrylate, polyethylene glycol monovinyl ether, polypropylene glycol monoacrylate, and allyl polyoxyethylene ether.

[0017] Preferably, the number average molecular weight of the polyethylene glycol monomethyl ether acrylate is 500-2000; and the number average molecular weight of the polyethylene glycol monovinyl ether is 400-1500.

[0018] Preferably, the solvent is selected from one or more of ethanol, aqueous ethanol solution, and aqueous isopropanol solution.

[0019] Preferably, when the monomer solution is a ternary copolymer system, a phase separation solvent system is used, wherein the phase separation solvent system is an aqueous ethanol solution with a volume fraction of 40-60% or an aqueous isopropanol solution with a volume fraction of 40-60%.

[0020] Preferably, the initiator is selected from one or more of ammonium persulfate, potassium persulfate, azobisisobutyronitrile, and azobisisoheptanenitrile.

[0021] Preferably, the amount of the initiator is 0.6 to 2.2% of the total mass of the monomer.

[0022] Preferably, the dissolution conditions include: a temperature of 15~30℃, a stirring rate of 200~400rpm, and a time of 30~60min.

[0023] Preferably, the deoxygenation conditions include: introducing nitrogen gas into the monomer solution for a deoxygenation time of 25-45 minutes.

[0024] Preferably, the prepolymerization conditions include: a temperature of 65~85℃ and a stirring speed of 220~320rpm.

[0025] Preferably, the polymerization conditions include a temperature of 65~85℃ and a time of 4.5~8h.

[0026] Thirdly, the present invention provides an application of the multifunctional carbon source as described in the present invention in the preparation of lithium iron phosphate main material.

[0027] Fourthly, this invention provides a method for preparing carbon-coated lithium iron phosphate as the main material. The preparation method employs a combined process of wet milling, spray drying, and high-temperature sintering, specifically including the following steps: S1. Mix the lithium iron phosphate precursor, multifunctional carbon source, conventional carbon source and deionized water, put them into a high-speed stirring tank and stir at high speed until the slurry is evenly dispersed. S2. Grinding and refining: The slurry described in step S1 is fed into a closed sand mill and ground until D50 = 0.4~0.6μm; S3. Spray drying: The slurry described in step S2 is fed into a spray drying tower for spray drying to obtain precursor composite powder; S4. Sintering and forming: The precursor composite powder described in step S3 is pre-fired and then sintered under an inert atmosphere, cooled and crushed and sieved to obtain carbon-coated lithium iron phosphate main material. The multifunctional carbon source mentioned in step S1 is the multifunctional carbon source described in this invention.

[0028] Preferably, in step S1, the mass ratio of the lithium iron phosphate precursor, the multifunctional carbon source, the conventional carbon source, and the deionized water is 40:0.4~0.6:9~11:48~50.

[0029] Preferably, in step S1, the conventional carbon source is selected from one or more of sucrose, glucose, starch, and glycerol.

[0030] Preferably, in step S1, the conditions for high-speed stirring include: a stirring rate of 600~800 rpm and a stirring time of 60~90 min.

[0031] Preferably, in step S2, the milling conditions include: using high-purity zirconia beads with a particle size of 0.3~0.8μm, rotating at a speed of 2100~2600 rpm, and maintaining the system temperature at 15~45℃ with water cooling throughout the process.

[0032] Preferably, in step S3, the conditions for spray drying include: an inlet air temperature of 170~230℃ and an outlet air temperature of 75~105℃.

[0033] Preferably, in step S4, the inert atmosphere is a nitrogen atmosphere and / or an argon atmosphere.

[0034] Preferably, in step S4, the pre-firing conditions include: a temperature of 320~420℃ and a time of 1.5~3.5h.

[0035] Preferably, in step S4, the sintering conditions include: a temperature of 620~820℃ and a time of 3.5~8.5h.

[0036] Fifthly, the present invention provides a carbon-coated lithium iron phosphate main material prepared by the preparation method described in the present invention, wherein the resistivity of the carbon-coated lithium iron phosphate main material is ≤1200Ω·cm, preferably 150~300Ω·cm; the 0.1C specific capacity is ≥158mAh / g, preferably 163~169mAh / g; and the 5C rate capacity retention is ≥72%, preferably 83~90%.

[0037] In a sixth aspect, the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery comprises the carbon-coated lithium iron phosphate main material described in the present invention, and the positive electrode is prepared by mixing the carbon-coated lithium iron phosphate main material, a conductive agent and a binder into a slurry and then coating it onto the surface of a current collector.

[0038] In the above technical solution, the multifunctional carbon source of the present invention integrates three functional units—vinyl aromatic rings, vinyl nitrogen-containing heterocycles, and polyethers—through molecular-scale directional design, achieving synergistic coupling of three functions: lubrication and dispersion, high residual carbon film formation, and nitrogen-doped conductivity. This solves the technical defects of existing carbon sources with single functions and can simultaneously improve the solid content, milling time, conductivity, and electrochemical performance of lithium iron phosphate sand. A quantitative correlation has been established between the carbon source molecular structure, carbon layer characteristics, slurry performance, and electrochemical performance.

[0039] Furthermore, the number-average molecular weight of the multifunctional carbon source of this invention is precisely controlled within the range of 6000~60000 Da, with a molecular weight distribution coefficient ≤1.8. This balances the solubility, dispersibility, and pyrolysis stability of the polymer. Moreover, the residual carbon rate is as high as 38% under nitrogen atmosphere at 800℃, which is far higher than that of traditional sugar-based carbon sources. After high-temperature pyrolysis, it can form a continuous and dense amorphous carbon layer, which can block the agglomeration of lithium iron phosphate particles, effectively reduce the resistivity of lithium iron phosphate powder, and build an efficient electron transport network.

[0040] Meanwhile, the preparation method of the multifunctional carbon source of the present invention adopts solution free radical polymerization, which has mild reaction conditions, high monomer conversion rate, and high product purity. By precisely controlling the monomer ratio, initiator dosage, and polymerization process parameters, the structure and performance of the carbon source can be controlled and adjusted. The obtained product is a free-flowing powder, which is convenient for storage and application.

[0041] Furthermore, the preparation method of carbon-coated lithium iron phosphate (LFP) main material of this invention is fully compatible with existing wet-process industrial production lines for LFP, requiring no equipment modification. The process is mild, low-waste, and simple to operate. The resulting carbon-coated LFP main material exhibits low resistivity, high specific capacity, and excellent rate performance, with a 0.1C specific capacity ≥158 mAh / g and a 5C rate capacity retention ≥72%. The overall technical solution of this invention is simple, cost-controllable, and highly adaptable to industrialization. The prepared multifunctional carbon source and carbon-coated LFP main material can meet the performance requirements of LFP materials in various fields such as power batteries and energy storage batteries, and has broad market application prospects.

[0042] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0043] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0044] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0045] In a first aspect, the present invention provides a multifunctional carbon source, wherein the multifunctional carbon source is polymerized from one or more of vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers and polyether vinyl monomers; The multifunctional carbon source has a number-average molecular weight of 6000~60000 Da and a molecular weight distribution coefficient of ≤1.8, avoiding problems such as uneven solubility and asynchronous pyrolysis caused by an excessively wide molecular weight distribution.

[0046] In this invention, the side chains of the vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers all contain polymerizable double bonds, exhibiting high polymerization activity and strong copolymerization uniformity, thus ensuring the smooth progress of the polymerization reaction and the uniformity of the product structure.

[0047] In this invention, the vinyl aromatic ring monomer is selected from one or more of styrene, p-methylstyrene, vinylnaphthalene and vinylbiphenyl. The conjugated structure of the aromatic ring can improve the graphitization degree and electronic conductivity of the carbon layer, and is the core unit to ensure a high residual carbon rate of the carbon source.

[0048] In this invention, the vinyl nitrogen-containing heterocyclic monomer is selected from one or more of 2-vinylpyridine, 4-vinylpyridine, N-vinylimidazolium and N-vinylpyrrole. The nitrogen atom in the heterocyclic structure exists in the form of pyridine nitrogen and pyrrole nitrogen. After doping, the carbon layer defect energy barrier can be significantly reduced and carrier transport sites can be introduced.

[0049] In this invention, the polyether vinyl monomer is selected from one or more of polyethylene glycol monomethyl ether acrylate, polyethylene glycol monovinyl ether, polypropylene glycol monoacrylate, and allyl polyoxyethylene ether. Preferably, the number average molecular weight of the polyethylene glycol monomethyl ether acrylate is 500-2000; the number average molecular weight of the polyethylene glycol monovinyl ether is 400-1500, and the polyether chain length is moderate, which can balance hydrophilic lubricity and polymerization reactivity.

[0050] In this invention, the monomer composition of the multifunctional carbon source, based on the molar percentage of monomers, is as follows: 1) Homopolymer: It is polymerized from a single vinyl aromatic ring monomer, a single vinyl nitrogen-containing heterocyclic monomer or a single polyether vinyl monomer, with a corresponding monomer molar percentage of 100%, a single side chain structure and a highly targeted function; 2) Binary copolymer type: 30-70% vinyl aromatic monomers combined with 30-70% vinyl nitrogen-containing heterocyclic monomers; 40-75% vinyl aromatic monomers combined with 25-60% polyether vinyl monomers; 35-70% vinyl nitrogen-containing heterocyclic monomers combined with 30-65% polyether vinyl monomers; 3) Ternary copolymer type: 25~65% vinyl aromatic ring monomers, 5~35% vinyl nitrogen-containing heterocyclic monomers and 10~40% polyether vinyl monomers; In this process, the sum of the molar percentages of the corresponding monomers in the binary copolymer and ternary copolymer are both 100%, and the random copolymer structure ensures that each functional unit is evenly distributed, thereby achieving synergistic efficiency.

[0051] In the multifunctional carbon source, vinyl aromatic ring units are embedded in the polymer backbone to form a rigid conjugated structure. After high-temperature sintering, they can form a continuous and dense amorphous carbon layer, blocking the agglomeration of lithium iron phosphate particles. Vinyl nitrogen-containing heterocyclic units are distributed in the form of side chains. During sintering, nitrogen atoms are incorporated into the carbon lattice in situ to form n-type doped conductive channels, which significantly reduces the resistivity of lithium iron phosphate powder. Polyether units are grafted onto the backbone to form flexible long branches, providing steric hindrance and hydrophilic lubricity, effectively reducing slurry viscosity, increasing the solid content of sand milling, shortening the sand milling time, and enhancing the interfacial bonding force between the carbon source and the lithium iron phosphate precursor.

[0052] Secondly, the present invention provides a method for preparing a multifunctional carbon source as described in the present invention. The preparation method includes: dissolving a monomer in a solvent to obtain a monomer solution, removing oxygen, prepolymerizing, adding an initiator dropwise to carry out a polymerization reaction, cooling after the reaction is completed, adjusting the pH to 6.8~7.5, concentrating, drying, and pulverizing at low temperature to obtain a powdered multifunctional carbon source. The monomer is selected from one or more of vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers; The monomer solution has a mass concentration of 25-45 wt%, which ensures production efficiency without causing excessive system viscosity, thus affecting mass and heat transfer in the polymerization reaction.

[0053] The preparation method of this invention adopts solution free radical polymerization, with mild reaction conditions, monomer conversion rate ≥95%, high product purity, and the obtained multifunctional carbon source is a free-flowing, non-caking powder, which is convenient for subsequent storage and application.

[0054] In this invention, the selection of vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers is consistent with the limitations in the above-mentioned multifunctional carbon sources, and will not be repeated here.

[0055] In this invention, the solvent is selected from one or more of ethanol, aqueous ethanol solution, and aqueous isopropanol solution. Preferably, when the monomer solution is a ternary copolymerization system, a phase separation solvent system is used. The phase separation solvent system is an aqueous ethanol solution or an aqueous isopropanol solution with a volume fraction of 40-60%, which can adapt to the copolymerization reaction of monomers with different polarities and ensure monomer miscibility and copolymerization uniformity.

[0056] In this invention, the initiator is selected from one or more of ammonium persulfate, potassium persulfate, azobisisobutyronitrile, and azobisisoheptanenitrile. The amount of the initiator is 0.6 to 2.2% of the total mass of the monomer. By precisely controlling the amount of initiator, the molecular weight and distribution coefficient of the carbon source can be effectively controlled.

[0057] In this invention, in order to ensure that the monomer and solvent are fully miscible and form a uniform and transparent monomer solution, the dissolution conditions include: a temperature of 15~30℃, a stirring rate of 200~400rpm, and a time of 30~60min.

[0058] In this invention, the deoxygenation conditions include: introducing high-purity nitrogen gas into the monomer solution for a deoxygenation time of 25-45 minutes to completely remove oxygen from the system and prevent polymerization inhibition side reactions from occurring.

[0059] In this invention, the prepolymerization conditions include: a temperature of 65~85℃ and a stirring speed of 220~320rpm.

[0060] In this invention, the polymerization conditions include: a temperature of 65~85℃ and a time of 4.5~8h. This temperature range can ensure the efficient decomposition of the initiator and the stable polymerization of the monomer. The stirring rate can ensure the uniform heat and mass transfer of the system. The reaction time can ensure the complete polymerization of the monomer and that the molecular weight of the product meets the target.

[0061] Thirdly, the present invention provides an application of the multifunctional carbon source as described in the present invention in the preparation of lithium iron phosphate main material.

[0062] Fourthly, this invention provides a method for preparing carbon-coated lithium iron phosphate as the main material. The preparation method employs a combined process of wet milling, spray drying, and high-temperature sintering, specifically including the following steps: S1. Mix the lithium iron phosphate precursor, multifunctional carbon source, conventional carbon source and deionized water, put them into a high-speed stirring tank and stir at high speed until the slurry is evenly dispersed. S2. Grinding and refining: The slurry described in step S1 is fed into a closed sand mill and ground until D50 = 0.4~0.6μm; S3. Spray drying: The slurry described in step S2 is fed into a spray drying tower for spray drying to obtain precursor composite powder; S4. Sintering and forming: The precursor composite powder described in step S3 is pre-fired and then sintered under an inert atmosphere, cooled and crushed and sieved to obtain carbon-coated lithium iron phosphate main material. The multifunctional carbon source mentioned in step S1 is the multifunctional carbon source described in this invention.

[0063] The multifunctional carbon source of this invention adopts a wet blending method, which is compatible with the sand milling process of lithium iron phosphate precursor. It does not require changes to the existing lithium iron phosphate production process and has strong industrial adaptability.

[0064] In this invention, in step S1, in order to balance the slurry processing performance and the final electrochemical performance of lithium iron phosphate, the mass ratio of the lithium iron phosphate precursor, multifunctional carbon source, conventional carbon source and deionized water is 40:0.4~0.6:9~11:48~50.

[0065] In this invention, in step S1, the conventional carbon source is selected from one or more of sucrose, glucose, starch and glycerol, preferably food-grade sucrose, which has low impurity content and uniform pyrolysis. When used in combination with the multifunctional carbon source of this invention, it can balance production cost and modification effect.

[0066] In this invention, the conditions for high-speed stirring in step S1 include: a stirring rate of 600~800 rpm and a stirring time of 60~90 min, to ensure that the slurry does not agglomerate or separate, and to achieve uniform dispersion of each component.

[0067] In this invention, the sand milling conditions in step S2 include: using high-purity zirconia beads with a particle size of 0.3~0.8μm, rotating at a speed of 2100~2600 rpm, and maintaining the system temperature at 15~45℃ with water cooling throughout the process to ensure the system temperature is stable during the sand milling process and avoid the deterioration of the slurry properties due to excessively high temperature.

[0068] In this invention, the spray drying conditions in step S3 include: an inlet air temperature of 170~230℃, an outlet air temperature of 75~105℃, and a stable and controllable feed rate, which can produce precursor composite powder with high sphericity and excellent flowability, and avoid secondary agglomeration of powder.

[0069] In this invention, in step S4, the inert atmosphere is a nitrogen atmosphere and / or an argon atmosphere to prevent the lithium iron phosphate and carbon layer from being oxidized during the sintering process.

[0070] In this invention, the pre-calcination conditions in step S4 include a temperature of 320~420℃ and a time of 1.5~3.5h, which can effectively remove small molecules and organic solvents from the carbon source, laying the foundation for subsequent carbon layer formation.

[0071] In this invention, the sintering conditions in step S4 include: a temperature of 620~820℃ and a time of 3.5~8.5h, which can complete the complete growth of lithium iron phosphate crystal and the pyrolysis film formation of carbon source, forming a continuous and dense nitrogen-doped carbon coating layer.

[0072] Fifthly, the present invention provides a carbon-coated lithium iron phosphate main material prepared by the preparation method described in the present invention, wherein the resistivity of the carbon-coated lithium iron phosphate main material is ≤1200Ω·cm, preferably 150~300Ω·cm; the 0.1C specific capacity is ≥158mAh / g, preferably 163~169mAh / g; and the 5C rate capacity retention is ≥72%, preferably 83~90%.

[0073] The performance of carbon-coated lithium iron phosphate main materials modified by different types of multifunctional carbon sources exhibits gradient differences. The main material modified by ternary copolymer carbon source has the best comprehensive performance, the main material modified by binary copolymer carbon source can take into account two core performances, and the main material modified by homopolymer carbon source has outstanding single performance. It can be flexibly selected according to the needs of different application scenarios.

[0074] In a sixth aspect, the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery comprises the carbon-coated lithium iron phosphate main material described in the present invention, and the positive electrode is prepared by mixing the carbon-coated lithium iron phosphate main material, a conductive agent and a binder into a slurry and then coating it onto the surface of a current collector.

[0075] The carbon-coated lithium iron phosphate main material of this invention has excellent conductivity, rate capability, and cycle performance. The lithium-ion battery prepared with this material has the characteristics of high capacity, high rate capability, and long cycle life, and can be widely used in power batteries, energy storage batteries and other fields.

[0076] The present invention will be described in detail below through examples. In the following examples, the pharmaceuticals and agents are all conventional commercially available products.

[0077] Example 1: Preparation of styrene homopolymer carbon source (ST-H) The specific steps for preparing a homopolymeric multifunctional carbon source in this embodiment are as follows: 1) Monomer dissolution: Take 100 mol% of styrene monomer and use ethanol as solvent to prepare a monomer solution with a mass concentration of 30 wt%. Stir at a stirring rate of 250 rpm for 40 min until the monomer is completely dissolved to obtain a transparent monomer solution. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above monomer solution for 30 minutes to remove oxygen completely from the system; 3) Polymerization reaction: Heat the system to 75℃, adjust the stirring speed to 260 rpm, slowly add 1.2% of the total monomer mass of azobisisobutyronitrile initiator, and keep the reaction at 75℃ for 6 hours to complete the polymerization; 4) Post-processing: After the reaction is completed, the system is naturally cooled to room temperature, the pH of the system is adjusted to 7.0 with dilute hydrochloric acid, most of the ethanol is removed by rotary evaporation and concentration, and then the system is thoroughly dehydrated by vacuum drying and pulverized at low temperature to obtain styrene homopolymer carbon source powder.

[0078] The resulting multifunctional carbon source has the molecular formula -[CH2-CH(C6H5)] n - A linear regular polymer with moderate main chain flexibility and monocyclic aromatic rigid side chains. The molecule is nonpolar and has no hydrophilic groups or nitrogen doping sites. The number average molecular weight is 15000 Da. The molecular weight distribution coefficient is 1.6. The degree of polymerization is moderate. The aromatic rings cyclize in an orderly manner during pyrolysis. The residual carbon rate is 38% according to TG detection. However, the carbon layer has high surface energy and poor compatibility with aqueous precursors.

[0079] Example 2 Preparation of N-vinylimidazolium homopolymer carbon source (VI-H) The specific steps for preparing a homopolymeric multifunctional carbon source in this embodiment are as follows: 1) Monomer dissolution: Take 100 mol% of N-vinylimidazolium monomer and use a 1:1 water-ethanol aqueous solution as the solvent to prepare a monomer solution with a mass concentration of 35 wt%. Stir at 240 rpm for 35 min until the monomer is completely dissolved. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above monomer solution for 35 minutes to remove oxygen; 3) Polymerization reaction: Heat the system to 70℃, adjust the stirring speed to 240 rpm, slowly add 1.0% of the total monomer mass of ammonium persulfate initiator, and keep the reaction at 70℃ for 5 hours; 4) Post-processing: After the reaction is completed, the mixture is naturally cooled to room temperature, the pH is adjusted to 7.2 with sodium bicarbonate solution, concentrated by rotary evaporation, freeze-dried and then pulverized at low temperature to obtain N-vinylimidazolium homopolymer carbon source powder.

[0080] The resulting multifunctional carbon source has the molecular formula -[CH2-CH(C3H3N2)] n - A water-soluble linear polymer with imidazole heterocyclic side groups on the main chain. Nitrogen atoms are uniformly distributed in the form of pyrrole nitrogen, exhibiting strong polarity and good dispersibility in the aqueous phase. The number average molecular weight is 12000 Da, the molecular weight distribution coefficient is 1.5, the chain segments are short and densely arranged, but the heterocyclic thermal stability is poor, and it is prone to decomposition and weight loss at high temperatures. The TG residual carbon rate is only 22%, and the carbon layer is severely fragmented.

[0081] Example 3: Preparation of Polyethylene Glycol Monomethyl Ether Acrylate Homopolymer Carbon Source (PEGA-H) In this embodiment, a homopolymer multifunctional carbon source is prepared. The number average molecular weight of polyethylene glycol monomethyl ether acrylate is 1000. The specific steps are as follows: 1) Monomer dissolution: Take 100 mol% of polyethylene glycol monomethyl ether acrylate monomer, use water as solvent, prepare a monomer solution with a mass concentration of 40 wt%, and stir at a stirring speed of 280 rpm for 50 min until the monomer is completely dissolved. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above monomer solution for deoxygenation for 40 minutes; 3) Polymerization reaction: Heat the system to 80℃, adjust the stirring speed to 280 rpm, slowly add 1.5% of potassium persulfate initiator by mass of total monomers, and keep the reaction at 80℃ for 7 hours; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.1, the water is removed by rotary evaporation, and the mixture is vacuum dried and pulverized to obtain polyethylene glycol monomethyl ether acrylate homopolymer carbon source powder.

[0082] The resulting multifunctional carbon source has the following molecular formula: -[CH2-CH(COO(CH2CH2O)] n CH3)] n - A water-soluble comb-like polymer with long-chain polyether flexible groups as side chains. It has large steric hindrance, extremely strong hydrophilic lubrication, no aromatic rings or nitrogen heterocycles in the molecule, extremely low pyrolysis temperature, number-average molecular weight of 20,000 Da, molecular weight distribution coefficient of 1.7, carbon residue of 8%, and almost no carbonization conductivity.

[0083] Example 4: Preparation of carbon source for styrene-N-vinylimidazol binary copolymer (ST-VI-1) This embodiment prepares a binary copolymer multifunctional carbon source with monomers consisting of 60 mol% styrene and 40 mol% N-vinylimidazole. The specific steps are as follows: 1) Monomer dissolution: Weigh styrene and N-vinylimidazole according to the above molar ratio, and prepare a mixed monomer solution with a mass concentration of 32wt% using a water-ethanol aqueous solution with a volume ratio of 1:1.2 as the solvent. Stir at a stirring speed of 250 rpm for 45 min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas was continuously introduced into the above mixed monomer solution for 32 minutes to remove oxygen; 3) Polymerization reaction: Heat the system to 78℃, adjust the stirring speed to 250 rpm, slowly add 1.3% of the total monomer mass of azobisisobutyronitrile initiator, and keep the reaction at 78℃ for 6.5 h; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.0, concentrated by rotary evaporation, and dried and pulverized under vacuum to obtain styrene-N-vinylimidazolium binary copolymer carbon source powder.

[0084] The resulting multifunctional carbon source has the following molecular formula: -[(CH2-CH(C6H5)) x -(CH2-CH(C3H3N2)) y[-(x:y=6:4)], random copolymer, aromatic and nitrogen-containing units are uniformly arranged, without segment aggregation; number average molecular weight is 16000 Da, molecular weight distribution coefficient is 1.6, benzene ring ensures thermal stability, imidazole ring introduces nitrogen doping sites, no polyether segments, molecular hydrophilicity is average, TG residual carbon rate is 32%, carbon layer continuity is better than homopolymer nitrogen-containing carbon source.

[0085] Example 5: Preparation of styrene-N-vinylimidazolium binary copolymer carbon source (ST-VI-2) This embodiment prepares a binary copolymer multifunctional carbon source with monomers consisting of 40 mol% styrene and 60 mol% N-vinylimidazole. The specific steps are as follows: 1) Monomer dissolution: Weigh styrene and N-vinylimidazole according to the above molar ratio, and prepare a mixed monomer solution with a mass concentration of 30wt% using a water-ethanol aqueous solution with a volume ratio of 1:1 as the solvent. Stir at a stirring speed of 240rpm for 40min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above mixed monomer solution for deoxygenation for 30 minutes; 3) Polymerization reaction: Heat the system to 75℃, adjust the stirring speed to 240 rpm, slowly add 1.1% of the total monomer mass of ammonium persulfate initiator, and keep the reaction at 75℃ for 5.5 h; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.3, concentrated by rotary evaporation, and dried and pulverized under vacuum to obtain styrene-N-vinylimidazolium binary copolymer carbon source powder.

[0086] The resulting multifunctional carbon source has the following molecular formula: -[(CH2-CH(C6H5)) x -(CH2-CH(C3H3N2)) y [-(x:y=4:6)], a random copolymer with increased nitrogen-containing unit ratio, increased nitrogen doping site density, and increased proportion of pyridine nitrogen and pyrrole nitrogen; the number average molecular weight is 14000 Da, the molecular weight distribution coefficient is 1.5, the chain segments are shorter, but the reduced benzene ring content leads to decreased thermal stability, the TG residual carbon rate drops to 27%, and the carbon layer density deteriorates.

[0087] Example 6: Preparation of carbon source for styrene-polyethylene glycol monomethyl ether acrylate binary copolymer (ST-PEGA) This embodiment prepares a binary copolymer multifunctional carbon source with a monomer composition of 65 mol% styrene and 35 mol% polyethylene glycol monomethyl ether acrylate (Mn=1000). The specific steps are as follows: 1) Monomer dissolution: Weigh styrene and polyethylene glycol monomethyl ether acrylate according to the above molar ratio, and prepare a mixed monomer solution with a mass concentration of 35wt% using ethanol as solvent. Stir at a stirring speed of 270rpm for 50min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above mixed monomer solution for deoxygenation for 35 minutes; 3) Polymerization reaction: Heat the system to 80℃, adjust the stirring speed to 270 rpm, slowly add 1.4% of the total monomer mass of azobisisobutyronitrile initiator, and keep the reaction at 80℃ for 7 hours; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.2, the solvent is removed by rotary evaporation, and the mixture is dried and pulverized under vacuum to obtain styrene-polyethylene glycol monomethyl ether acrylate binary copolymer carbon source powder.

[0088] The resulting multifunctional carbon source has the following molecular formula: -[(CH2-CH(C6H5)) x -(CH2-CH(COO(CH2CH2O) n CH3)) y [-(x:y=6.5:3.5)], comb-like random copolymer, with aromatic carbon-forming units and polyether dispersion units uniformly combined; number average molecular weight is 18000 Da, molecular weight distribution coefficient is 1.7, polyether chain enhances hydrophilicity, benzene ring ensures 34% residual carbon rate, no nitrogen doping sites, carbon layer is pure amorphous carbon, no doping modification effect.

[0089] Example 7 Preparation of N-vinylimidazolium-polyethylene glycol monomethyl ether acrylate binary copolymer carbon source (VI-PEGA) This embodiment prepares a binary copolymer multifunctional carbon source with monomer composition of 50 mol% N-vinylimidazolium and 50 mol% polyethylene glycol monomethyl ether acrylate (Mn=1000). The specific steps are as follows: 1) Monomer dissolution: Weigh N-vinylimidazolium and polyethylene glycol monomethyl ether acrylate according to the above molar ratio, use water as solvent, prepare a mixed monomer solution with a mass concentration of 38wt%, and stir at a stirring speed of 260rpm for 45min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above mixed monomer solution for deoxygenation for 40 minutes; 3) Polymerization reaction: Heat the system to 72℃, adjust the stirring speed to 260 rpm, slowly add 1.2% of potassium persulfate initiator by total monomer mass, and keep the reaction at 72℃ for 6 hours; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.1, and after freeze-drying, it is pulverized to obtain N-vinylimidazolium-polyethylene glycol monomethyl ether acrylate binary copolymer carbon source powder.

[0090] The resulting multifunctional carbon source has the following molecular formula: -[(CH2-CH(C3H3N2)) x -(CH2-CH(COO(CH2CH2O) n CH3)) y -(x:y=1:1) It is a water-soluble random copolymer with nitrogen-doped units and polyether dispersion units arranged in equal proportions. It has extremely strong polarity and excellent dispersibility. However, it lacks aromatic ring structure, has extremely poor thermal stability, a number-average molecular weight of 16,000 Da, a molecular weight distribution coefficient of 1.7, and a TG carbon residue of only 15%. The carbon layer is severely fragmented and cannot form a continuous conductive network.

[0091] Example 8 Preparation of carbon source (T-1) for styrene-N-vinylimidazolium-polyethylene glycol monomethyl ether acrylate terpolymer This embodiment prepares a ternary copolymer multifunctional carbon source with monomer composition of 45 mol% styrene, 15 mol% N-vinylimidazolium, and 40 mol% polyethylene glycol monomethyl ether acrylate (Mn=1000). The specific steps are as follows: 1) Monomer dissolution: Weigh the three types of monomers according to the above molar ratio, and use a water-isopropanol phase separation solvent system with a volume ratio of 1:1.2 as the solvent to prepare a mixed monomer solution with a mass concentration of 33wt%. Stir at a stirring rate of 260rpm for 50min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above mixed monomer solution for deoxygenation for 40 minutes; 3) Polymerization reaction: Heat the system to 76℃, adjust the stirring speed to 260 rpm, slowly add 1.5% of the total monomer mass of the composite initiator (ammonium persulfate: azobisisobutyronitrile = 1:1), and keep the reaction at 76℃ for 7.5 h; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.0, concentrated by rotary evaporation, and dried and pulverized under vacuum to obtain styrene-N-vinylimidazolium-polyethylene glycol monomethyl ether acrylate terpolymer carbon source powder.

[0092] The resulting multifunctional carbon source has the following molecular formula: -[(CH2-CH(C6H5)) x -(CH2-CH(C3H3N2)) y -(CH2-CH(COO(CH2CH2O) n CH3)) z[-(x:y:z=4.5:1.5:4), a ternary random comb copolymer with uniform distribution of three types of functional units without agglomeration; the number average molecular weight is 12000 Da, the molecular weight distribution coefficient is 1.5, the low molecular weight narrow distribution has excellent dispersibility; the benzene ring ensures a 33% residual carbon rate, the imidazole ring achieves uniform nitrogen doping, the polyether chain provides lubrication and steric hindrance, and the carbon layer is continuous and uniformly doped.

[0093] Example 9 Preparation of a vinylnaphthalene-2-vinylpyridine-allyl polyoxyethylene ether ternary copolymer carbon source (T-2) This embodiment prepares a ternary copolymer multifunctional carbon source with monomers consisting of 50 mol% vinylnaphthalene, 12 mol% 2-vinylpyridine, and 38 mol% allyl polyoxyethylene ether. The specific steps are as follows: 1) Monomer dissolution: Weigh the three types of monomers according to the above molar ratio, and use a water-ethanol phase separation solvent system with a volume ratio of 1:1.4 as the solvent to prepare a mixed monomer solution with a mass concentration of 30wt%. Stir at a stirring rate of 290rpm for 60min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas is continuously introduced into the above mixed monomer solution for deoxygenation for 45 minutes; 3) Polymerization reaction: The system was heated to 82℃, the stirring speed was adjusted to 290 rpm, and 1.8% of the total monomer mass of azobisisobutyronitrile initiator was slowly added. The reaction was kept at 82℃ for 8 hours. 4) Post-processing: After the reaction is completed, the mixture is naturally cooled to room temperature, the pH is adjusted to 7.2, the mixture is concentrated under reduced pressure, freeze-dried and pulverized to obtain vinylnaphthalene-2-vinylpyridine-allyl polyoxyethylene ether ternary copolymer carbon source powder.

[0094] The structural formula of the obtained multifunctional carbon source is: -[(CH2-CH(C 10 H7)) x -(CH2-CH(C5H4N)) y -(CH2-CH2CH2O(CH2CH2O) n H)) z [- (x:y:z=5:1.2:3.8), a fused-ring aromatic terpolymer, the fused-ring structure of vinylnaphthalene enhances the rigidity and graphitization of the main chain, the pyridine nitrogen doping of 2-vinylpyridine provides stronger stability, and allyl polyoxyethylene ether is suitable for high solids content slurries; the number average molecular weight is 18000 Da, the molecular weight distribution coefficient is 1.6, the TG residual carbon rate is 37%, and the carbon layer has the best conductivity and continuity.

[0095] Example 10: Preparation of p-methylstyrene-N-vinylpyrrole-polyethylene glycol monomethyl ether acrylate terpolymer carbon source (T-3) This embodiment prepares a ternary copolymer multifunctional carbon source with monomers consisting of 55 mol% p-methylstyrene, 10 mol% N-vinylpyrrole, and 35 mol% polyethylene glycol monomethyl ether acrylate (Mn=1000). The specific steps are as follows: 1) Monomer dissolution: Weigh the three types of monomers according to the above molar ratio, and use a water-isopropanol phase separation solvent system with a volume ratio of 1:1 as the solvent to prepare a mixed monomer solution with a mass concentration of 36wt%. Stir at a stirring speed of 270rpm for 45min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas was continuously introduced into the above mixed monomer solution for 38 minutes to remove oxygen; 3) Polymerization reaction: Heat the system to 78°C, adjust the stirring speed to 270 rpm, slowly add 1.4% of potassium persulfate initiator by total monomer mass, and keep the reaction at 78°C for 7 hours; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.3, and the mixture is vacuum dried and pulverized to obtain p-methylstyrene-N-vinylpyrrole-polyethylene glycol monomethyl ether acrylate terpolymer carbon source powder.

[0096] The resulting multifunctional carbon source has the following molecular formula: -[(CH2-CH(C6H4CH3)) x -(CH2-CH(C4H4N)) y -(CH2-CH(COO(CH2CH2O) n CH3)) z [- (x:y:z=5.5:1:3.5), methyl-substituted aromatic ring terpolymer, methyl enhances monomer polarity and slurry solubility, N-vinylpyrrole has strong pyrrole nitrogen doping stability, polyether chain ensures dispersibility; number average molecular weight is 22000 Da, molecular weight distribution coefficient is 1.7, steric hindrance is large, TG residual carbon rate is 35%, carbon layer is uniform and dense.

[0097] Example 11 Preparation of styrene-4-vinylpyridine-polypropylene glycol monoacrylate terpolymer carbon source (T-4) This embodiment prepares a ternary copolymer multifunctional carbon source with monomers consisting of 35 mol% styrene, 25 mol% 4-vinylpyridine, and 40 mol% polypropylene glycol monoacrylate. The specific steps are as follows: 1) Monomer dissolution: Weigh the three types of monomers according to the above molar ratio, and use a water-ethanol phase separation solvent system with a volume ratio of 1:1.3 as the solvent to prepare a mixed monomer solution with a mass concentration of 34wt%. Stir at a stirring speed of 280rpm for 55min until the monomers are completely mixed. 2) Deoxygenation: High-purity nitrogen gas was continuously introduced into the above mixed monomer solution for deoxygenation for 42 minutes; 3) Polymerization reaction: Heat the system to 80℃, adjust the stirring speed to 280rpm, slowly add 1.6% of the total monomer mass of the composite initiator, and keep the reaction at 80℃ for 7.5h; 4) Post-processing: After the reaction is completed, the mixture is allowed to cool naturally to room temperature, the pH is adjusted to 7.1, the solvent is removed by rotary evaporation, and the mixture is pulverized at low temperature to obtain styrene-4-vinylpyridine-polypropylene glycol monoacrylate terpolymer carbon source powder.

[0098] The structural formula of the obtained multifunctional carbon source is: -[(CH2-CH(C6H5)] x -(CH2-CH(C5H4N)) y -(CH2-CH(COO(CH2CH(CH3)O) n H)) z [- (x:y:z=3.5:2.5:4), a high-nitrogen comb-like terpolymer, with 4-vinylpyridine increasing the nitrogen doping content, and polypropylene glycol chain lubrication superior to polyethylene glycol, suitable for high-viscosity slurries; number average molecular weight is 25000 Da, molecular weight distribution coefficient is 1.8, the comb-like structure is dense, TG residual carbon rate is 30%, and the nitrogen doping uniformity is excellent.

[0099] Application Example 1: Using the multifunctional carbon sources prepared in Examples 1-11 as modifying agents, carbon-coated lithium iron phosphate main materials were prepared. The specific steps are as follows: S1. Slurry preparation: Weigh the lithium iron phosphate precursor, the multifunctional carbon source of Examples 1-11, food-grade sucrose and deionized water in a mass ratio of 40:0.5:10:49.5, put them into a high-speed stirring tank, and stir at 700 rpm for 75 min until the slurry is free from agglomeration, layering and uniform dispersion. S2. Sand milling and refining: The mixed slurry is fed into a closed sand mill, using 0.5μm high-purity zirconia beads. The sand mill speed is 2300rpm, and the slurry particle size D50=0.5μm is sand milled. The temperature is controlled by circulating water cooling throughout the process to keep the system temperature at 45℃. S3. Spray drying: The qualified slurry is fed into the spray drying tower, the inlet air temperature is controlled at 200℃, the outlet air temperature is controlled at 90℃, the feeding rate is stable, and a precursor composite powder with high sphericity and excellent flowability is obtained. S4. Sintering and forming: The precursor powder is placed in an atmosphere sintering furnace and protected by nitrogen inert atmosphere. It is first pre-fired at a low temperature of 380℃ for 2.5h, and then heated to a high temperature of 720℃ for 6h. After natural cooling to room temperature, it is crushed and sieved to obtain carbon-coated lithium iron phosphate main material.

[0100] Comparative Example 1: The method of Application Example 1 was carried out, except that food-grade sucrose was used as the sole carbon source, and the amount added was the same as the total amount of multifunctional carbon source and food-grade sucrose in Application Example 1. The remaining preparation steps were the same as in Application Example 1, and carbon-coated lithium iron phosphate main material was obtained.

[0101] Test Example 1: To comprehensively verify the intrinsic performance of the multifunctional carbon sources prepared in Examples 1-11 of this invention and their modification effect on lithium iron phosphate main material, the multifunctional carbon sources prepared in Examples 1-11 above, as well as the carbon-coated lithium iron phosphate main materials prepared by each carbon source modification, and the sucrose carbon source and sucrose-modified lithium iron phosphate material in Comparative Example 1 were used as test samples. The same test standards and operating procedures were adopted to ensure the accuracy and comparability of the test results. The test methods are as follows, and the test results are shown in Table 1.

[0102] • Residual carbon rate test: Thermogravimetric analysis (TGA) was used. 5 mg of functional carbon source sample was taken and heated from room temperature to 800℃ at a heating rate of 10℃ / min under nitrogen atmosphere, and kept at this temperature for 2 hours. The percentage of the remaining solid mass to the initial mass of the sample was calculated, which is the residual carbon rate. • Sand-milled solids content test: Using the gravimetric method, take 10g of the slurry after sand milling of each sample, place it in an oven at 105℃ and dry it to constant weight. Calculate the percentage of the mass of solids after drying to the initial mass of the slurry, which is the sand-milled solids content.

[0103] • Sand milling time test: The test endpoint is set at the particle size of the slurry reaching D50=0.5±0.1μm. The time required from starting the sand mill to reaching this particle size is recorded as the sand milling time.

[0104] • Powder resistivity test: The powder resistivity of each sample was tested using the standard four-probe method at a pressure of 20 MPa.

[0105] • 0.1C gram capacity test: The carbon-coated lithium iron phosphate main material prepared in Application Example 1 and Comparative Example 1 was mixed with acetylene black and polyvinylidene fluoride (PVDF) at a mass ratio of 97:2:1. N-methylpyrrolidone (NMP) was added to form a uniform slurry. The slurry was coated on copper foil and dried to form a coin cell. A constant current charge-discharge test was performed at a 0.1C rate. The charging voltage was set to 3.6V and the discharging voltage to 2.0V. The first discharge capacity of the battery was recorded. The 0.1C gram capacity was obtained by dividing the first discharge capacity by the mass of the active material.

[0106] • 5C rate capacity retention test: The coin cell prepared using the above steps is first activated by charging and discharging at a 0.1C rate three times, and then cycled 50 times in a 5C rate discharge and 0.1C rate charge mode. The percentage of the capacity after the 50th discharge to the capacity after the first 0.1C discharge is recorded, which is the 5C rate capacity retention rate.

[0107] Table 1

[0108] As shown in Table 1, the core structure of the styrene homopolymer carbon source prepared in Example 1 is a linear aromatic conjugated chain without hydrophilic or nitrogen-doped groups. Its performance is characterized by high residual carbon, low dispersion, and high resistivity, resulting in only basic carbon formation and extremely poor processing performance. The aromatic conjugated structure ensures a high residual carbon rate, forming a continuous carbon layer after sintering, which initially improves electronic conductivity; however, it lacks hydrophilic lubrication and nitrogen doping functions, leading to severe particle agglomeration in the slurry. The sand milling solids content is only 43% with a milling time of 3.2 hours, resulting in high resistivity. It can only achieve basic carbon coating, with a rate retention rate of only 74.2%, failing to balance processing and electrochemical performance.

[0109] The N-vinylimidazolium homopolymer carbon source prepared in Example 2 has a nitrogen-containing heterocyclic linear chain as its core structure, lacking aromatic carbon-forming and lubricating groups. Its performance corresponds to excellent conductivity and extremely low residual carbon in nitrogen doping, resulting in improved conductivity but no improvement in processing, and a discontinuous carbon layer. Nitrogen doping sites can effectively reduce the defect energy barrier of the carbon layer, lowering the resistivity to 32 Ω·cm and improving electron transport; however, the excessively low residual carbon rate leads to a broken conductive network, resulting in rapid capacity decay at high rates. Furthermore, the lack of dispersing lubricating groups does not improve slurry processing performance. With a sand milling solids content of 42% and a grinding time of 3.0 h, the rate retention rate is 75.8%, indicating improved conductivity but significant processing limitations.

[0110] The core structure of the polyethylene glycol monomethyl ether acrylate homopolymer carbon source prepared in Example 3 is a comb-like flexible polyether chain, without carbon-forming or doped groups. Its performance corresponds to excellent lubrication and dispersion, but lacks carbon-forming conductivity, resulting in superior processing but deteriorated electrochemical performance. The long polyether chain significantly reduces slurry viscosity, breaks up particle agglomeration, increases the sand milling solids content to 47%, shortens the grinding time to 2.6 hours, and exhibits excellent slurry dispersion stability. However, after sintering, there is no continuous conductive carbon layer, and the resistivity soars to 60 Ω·cm. Its rate performance is lower than the blank sample, and it can only optimize processing performance, not achieve conductive modification.

[0111] The styrene-N-vinylimidazolium binary copolymer carbon source prepared in Example 4 and the styrene-N-vinylimidazolium binary copolymer carbon source prepared in Example 5 both have aromatic carbon-forming and nitrogen-doped dual units in their core structure, without lubricating groups. Their performance shows a significant improvement in conductivity but no improvement in processing performance. Increased nitrogen content leads to decreased resistivity, but also a simultaneous decrease in residual carbon content, resulting in a decline in rate performance. ST-VI-1 balances high residual carbon film formation with nitrogen-doped conductivity, reducing resistivity to 21 Ω·cm, achieving a 0.1C capacity of 162.3 mAh / g and a rate retention of 79.5%, with a significant improvement in conductivity. However, lacking a polyether dispersion unit, the slurry viscosity remains high, requiring 46% solids content and 2.8 hours of sand milling, resulting in no significant improvement in processing performance. The increased nitrogen content of ST-VI-2 further reduced the resistivity to 18 Ω·cm, resulting in better conductivity; however, the decrease in residual carbon content led to insufficient integrity of the conductive network, and the 5C rate retention rate dropped to 78.3%, with no improvement in processing performance. The sand grinding solid content was 45% and the grinding time was 2.7 hours. The dual performances worked synergistically, but the overall performance was mediocre.

[0112] The styrene-polyethylene glycol monomethyl ether acrylate binary copolymer carbon source prepared in Example 6 has a core structure of aromatic carbonization and lubrication dispersion dual units, without nitrogen doping. Its performance is characterized by significantly improved processing efficiency, relatively high resistivity, and excellent dispersibility, but without doping modification, its conductivity is noticeably weak. The polyether chain significantly optimizes the slurry rheological properties, increasing the sand milling solids content to 52% and shortening the time to 2.1 hours, resulting in a significant improvement in processing efficiency. However, without nitrogen doping functionality, the carbon layer has a high intrinsic resistivity (29 Ω·cm) and a rate retention rate of 77.6%, indicating excellent processing performance, but its conductivity needs further improvement.

[0113] The N-vinylimidazolium-polyethylene glycol monomethyl ether acrylate binary copolymer carbon source prepared in Example 7 has a core structure of nitrogen-doped and lubricating dispersion dual units, without aromatic carbon formation. Its performance corresponds to optimal processing performance, extremely low residual carbon, and a broken conductive network, resulting in poor overall electrochemical performance. The slurry processing performance reaches the peak of the binary system, with a sand milling solids content of 55% and a time of 2.0 hours; however, the excessively low residual carbon rate leads to conductive network failure. Although nitrogen doping has an effect, the resistivity still reaches 26 Ω·cm, with a rate retention rate of 76.4%, indicating that processing and conductivity performance cannot be simultaneously achieved.

[0114] The styrene-N-vinylimidazolium-polyethylene glycol monomethyl ether acrylate ternary copolymer carbon source prepared in Example 8 has a core structure of random comb-like chains with a balanced ratio of three types of units. Its performance is comprehensively balanced in terms of processing, conductivity, and electrochemistry, with no obvious shortcomings, making it suitable for general industrial production. The triple functions work synergistically: the polyether chain optimizes the slurry rheology, achieving a sand milling solids content of 58% for 1.2 hours; nitrogen doping reduces the resistivity to 11 Ω·cm; the 0.1C capacity is 166.7 mAh / g; the 5C rate retention is 86.3%; and the processing and electrochemical performance are comprehensively balanced.

[0115] The vinylnaphthalene-2-vinylpyridine-allyl polyoxyethylene ether ternary copolymer carbon source prepared in Example 9 has a core structure of fused-ring aromatic rings, pyridine nitrogen, and long-chain polyether. Its performance in terms of residual carbon content, conductivity, and dispersibility all reach peak values, exhibiting optimal overall performance and making it suitable for high-rate power batteries. The fused-ring aromatic rings significantly improve the conductivity of the carbon layer, reducing the resistivity to 7 Ω·cm, while maintaining 89.7% of the 5C rate. The polyether chains achieve optimal dispersion, with a sand milling solids content of 62% and a milling time of only 0.8 hours, resulting in top-tier overall performance.

[0116] The p-methylstyrene-N-vinylpyrrole-polyethylene glycol monomethyl ether acrylate ternary copolymer carbon source prepared in Example 10 has a methyl-substituted aromatic ring and a stable pyrrole nitrogen as its structural core. Its performance corresponds to excellent slurry dispersibility and strong cycle stability, making it suitable for long-life energy storage batteries. Improved monomer solubility leads to more uniform slurry dispersion; milling time is 1.0 h, solid content is 56%; resistivity is 13 Ω·cm, 0.1C capacity is 165.3 mAh / g, rate retention is 84.8%, and long-term cycle stability is excellent.

[0117] It is also evident that the carbon-coated lithium iron phosphate main material modified with the multifunctional carbon source prepared in Examples 1-11 of this invention exhibits significantly better performance indicators than the blank control sample modified with the traditional sucrose carbon source. Furthermore, the overall residual carbon rate of the carbon source is superior to that of the sucrose carbon source (≤25%). This fully demonstrates that the multifunctional carbon source of this invention can achieve modification and improvement from multiple dimensions, including the carbon-forming properties of the carbon source itself, the sand milling process of lithium iron phosphate, conductivity, and electrochemical performance. This solves the technical defects of traditional carbon sources, such as single function and low residual carbon rate.

[0118] The residual carbon content of the carbon source is positively correlated with the performance of lithium iron phosphate main material: the residual carbon content of the carbon source containing vinyl aromatic ring units is significantly higher, the residual carbon content of fused ring aromatic rings is larger than that of monocyclic aromatic rings, and the higher the proportion of aromatic ring units, the higher the residual carbon content; the higher the residual carbon content of the carbon source, the better the continuity of the carbon layer of the modified lithium iron phosphate main material, the lower the powder resistivity, and the better the electrochemical performance.

[0119] The performance of lithium iron phosphate main materials modified by multifunctional carbon sources with different structural types showed obvious gradient differences: homopolymer carbon sources (Examples 1-3) had outstanding single performance, aromatic ring homopolymer carbon sources had excellent carbonization properties, nitrogen-containing heterocyclic homopolymer carbon sources had excellent conductivity, and polyether homopolymer carbon sources had excellent dispersibility; binary copolymer carbon sources (Examples 4-7) achieved synergistic performance of two core properties, aromatic ring-polyether copolymerization was the optimal solution for processing performance in binary systems, and aromatic ring-nitrogen-containing heterocyclic copolymerization was the optimal solution for electrochemical performance in binary systems; ternary copolymer carbon sources (Examples 8-11) achieved simultaneous optimization of residual carbon rate, sand milling performance, conductivity and electrochemical performance through the synergistic coupling of three functional units: vinyl aromatic ring, nitrogen-containing heterocyclic ring and polyether, making it the modification solution with the best comprehensive performance.

[0120] Among the ternary copolymer carbon sources of this invention, the vinylnaphthalene-2-vinylpyridine-allyl polyoxyethylene ether ternary copolymer carbon source prepared in Example 9 has the best modification effect, with a residual carbon rate of 37%, corresponding to a solid content of 62% in the lithium iron phosphate main material after sand milling, a sand milling time of only 0.8h, a powder resistivity as low as 7Ω·cm, a 0.1C specific capacity of 168.9mAh / g, and a 5C rate capacity retention rate of 89.7%, which is fully adapted to the high performance requirements of high-end power batteries and energy storage batteries for lithium iron phosphate materials.

[0121] The preparation processes of all carbon sources and the application preparation methods of lithium iron phosphate main materials in this invention are compatible with existing industrial production lines, require no equipment modification, have a monomer conversion rate of ≥95%, are mild and low-waste, are easy to operate, and have controllable costs, thus possessing extremely strong industrial application value.

[0122] In summary, the multifunctional carbon source of this invention achieves precise control of the molecular weight and distribution coefficient of the carbon source by molecularly-scale directional design of homopolymer / copolymer structures of three types of functional monomers. Combined with a controllable synthesis process using solution free radical polymerization and a combined application process of wet sand milling-spray drying-high-temperature sintering, a polymer carbon source with three functions—lubrication and dispersion, high residual carbon film formation, and nitrogen-doped conductivity—is obtained. This results in a carbon-coated lithium iron phosphate main material with a dense carbon layer, excellent conductivity, and superior processing performance. This meets the high-performance requirements of lithium iron phosphate materials in fields such as power batteries and energy storage batteries, demanding high conductivity, high rate capability, and long cycle life. Simultaneously, it is compatible with existing industrial production processes for lithium iron phosphate, ensuring cost control and efficiency improvement. It possesses core advantages such as controllable structure, synergistic functions, excellent performance, and strong industrial adaptability. This effectively solves the industry's technical bottlenecks of traditional carbon sources, such as single function, low residual carbon rate, and difficulty in simultaneously achieving good processing and electrochemical performance of lithium iron phosphate. It demonstrates significant technological innovation and industrial application value.

[0123] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0124] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0125] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A multifunctional carbon source, characterized by, The multifunctional carbon source is polymerized from one or more of vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers; The number-average molecular weight of the multifunctional carbon source is 6000~60000 Da, and the molecular weight distribution coefficient is ≤1.

8.

2. The multifunctional carbon source according to claim 1, wherein The side chains of the vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers all contain polymerizable double bonds; The vinyl aromatic monomers are selected from one or more of styrene, p-methylstyrene, vinylnaphthalene, and vinyl biphenyl; The vinyl nitrogen-containing heterocyclic monomer is selected from one or more of 2-vinylpyridine, 4-vinylpyridine, N-vinylimidazolium and N-vinylpyrrole; The polyether vinyl monomer is selected from one or more of polyethylene glycol monomethyl ether acrylate, polyethylene glycol monovinyl ether, polypropylene glycol monoacrylate, and allyl polyoxyethylene ether; Preferably, the number average molecular weight of the polyethylene glycol monomethyl ether acrylate is 500-2000; and the number average molecular weight of the polyethylene glycol monovinyl ether is 400-1500.

3. The multifunctional carbon source according to claim 1 or 2, characterized in that, The monomer composition of the multifunctional carbon source, based on the molar percentage of monomers, is as follows: 1) Homopolymer: polymerized from a single vinyl aromatic monomer, a single vinyl nitrogen-containing heterocyclic monomer, or a single polyether vinyl monomer, with a corresponding monomer molar percentage of 100%; 2) Binary copolymer type: 30-70% vinyl aromatic monomers combined with 30-70% vinyl nitrogen-containing heterocyclic monomers; 40-75% vinyl aromatic monomers combined with 25-60% polyether vinyl monomers; 35-70% vinyl nitrogen-containing heterocyclic monomers combined with 30-65% polyether vinyl monomers; 3) Ternary copolymer type: 25~65% vinyl aromatic ring monomers, 5~35% vinyl nitrogen-containing heterocyclic monomers and 10~40% polyether vinyl monomers; The sum of the molar percentages of the corresponding monomers in the binary copolymer and ternary copolymer is 100%.

4. A method for preparing a multifunctional carbon source as described in any one of claims 1-3, characterized in that, The preparation method includes: dissolving the monomer in a solvent to obtain a monomer solution, removing oxygen, prepolymerizing, adding an initiator dropwise to carry out a polymerization reaction, cooling after the reaction is completed, adjusting the pH to 6.8~7.5, concentrating, drying, and pulverizing at low temperature to obtain a powdered multifunctional carbon source; The monomer is selected from one or more of vinyl aromatic monomers, vinyl nitrogen-containing heterocyclic monomers, and polyether vinyl monomers; The monomer solution has a mass concentration of 25-45 wt%.

5. The preparation method according to claim 4, characterized in that, The vinyl aromatic monomers are selected from one or more of styrene, p-methylstyrene, vinylnaphthalene, and vinyl biphenyl; The vinyl nitrogen-containing heterocyclic monomer is selected from one or more of 2-vinylpyridine, 4-vinylpyridine, N-vinylimidazolium and N-vinylpyrrole; The polyether vinyl monomer is selected from one or more of polyethylene glycol monomethyl ether acrylate, polyethylene glycol monovinyl ether, polypropylene glycol monoacrylate, and allyl polyoxyethylene ether; Preferably, the number average molecular weight of the polyethylene glycol monomethyl ether acrylate is 500-2000; and the number average molecular weight of the polyethylene glycol monovinyl ether is 400-1500. The solvent is selected from one or more of ethanol, aqueous ethanol solution, and aqueous isopropanol solution; Preferably, when the monomer solution is a ternary copolymer system, a phase separation solvent system is used, wherein the phase separation solvent system is an aqueous ethanol solution with a volume fraction of 40-60% or an aqueous isopropanol solution with a volume fraction of 40-60%. The initiator is selected from one or more of ammonium persulfate, potassium persulfate, azobisisobutyronitrile, and azobisisoheptanenitrile; The amount of the initiator is 0.6 to 2.2% of the total mass of the monomer.

6. The preparation method according to claim 4 or 5, characterized in that, The dissolution conditions include: a temperature of 15~30℃, a stirring rate of 200~400rpm, and a time of 30~60min; The deoxygenation conditions include: introducing nitrogen gas into the monomer solution for a deoxygenation time of 25-45 minutes; The prepolymerization conditions include: a temperature of 65~85℃ and a stirring speed of 220~320rpm; The polymerization conditions include a temperature of 65~85℃ and a time of 4.5~8h.

7. The application of a multifunctional carbon source as described in any one of claims 1-3 in the preparation of lithium iron phosphate main material.

8. A method for preparing carbon-coated lithium iron phosphate main material, characterized in that, The preparation method employs a combined process of wet sand milling, spray drying, and high-temperature sintering, specifically including the following steps: S1. Mix the lithium iron phosphate precursor, multifunctional carbon source, conventional carbon source and deionized water, put them into a high-speed stirring tank and stir at high speed until the slurry is evenly dispersed. S2. Grinding and refining: The slurry described in step S1 is fed into a closed sand mill and ground until D50 = 0.4~0.6μm; S3. Spray drying: The slurry described in step S2 is fed into a spray drying tower for spray drying to obtain precursor composite powder; S4. Sintering and forming: The precursor composite powder described in step S3 is pre-fired and then sintered under an inert atmosphere, cooled and crushed and sieved to obtain carbon-coated lithium iron phosphate main material. The multifunctional carbon source mentioned in step S1 is the multifunctional carbon source described in any one of claims 1-3; Preferably, in step S1, the mass ratio of the lithium iron phosphate precursor, the multifunctional carbon source, the conventional carbon source, and the deionized water is 40:0.4~0.6:9~11:48~50; Preferably, in step S1, the conventional carbon source is selected from one or more of sucrose, glucose, starch, and glycerol; Preferably, in step S1, the conditions for high-speed stirring include: a stirring rate of 600~800 rpm and a stirring time of 60~90 min; Preferably, in step S2, the milling conditions include: using high-purity zirconia beads with a particle size of 0.3~0.8μm, rotating at a speed of 2100~2600 rpm, and maintaining the system temperature at 15~45℃ with water cooling throughout the process; Preferably, in step S3, the conditions for spray drying include: an inlet air temperature of 170~230℃ and an outlet air temperature of 75~105℃; Preferably, in step S4, the inert atmosphere is a nitrogen atmosphere and / or an argon atmosphere; Preferably, in step S4, the pre-firing conditions include: a temperature of 320~420℃ and a time of 1.5~3.5h; Preferably, in step S4, the sintering conditions include: a temperature of 620~820℃ and a time of 3.5~8.5h.

9. A carbon-coated lithium iron phosphate main material prepared by the preparation method as described in claim 8, characterized in that, The resistivity of the carbon-coated lithium iron phosphate main material is ≤1200Ω·cm, preferably 150~300Ω·cm; the 0.1C capacity is ≥158mAh / g, preferably 163~169mAh / g; and the 5C rate capacity retention is ≥72%, preferably 83~90%.

10. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery comprises the carbon-coated lithium iron phosphate main material as described in claim 9. The positive electrode is prepared by mixing the carbon-coated lithium iron phosphate main material, conductive agent and binder into a slurry and then coating it onto the surface of the current collector.

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