A high-compaction-density lithium iron phosphate cathode material and a synthesis method thereof

Abstract

This invention discloses a high-density lithium iron phosphate cathode material and its synthesis method. The cathode material comprises lithium iron phosphate active material with coarse and fine composite particle sizes, a synergistic conductive agent system, a gradient polarity binder, additives, and a pore-regulating agent. The conductive agent includes carbon nanotubes, high specific surface area carbon black, and reduced graphene oxide, which, under the induction of an electric field and the action of dispersing agents, construct a synergistic "point-line-surface" conductive network in the thickness direction. The resulting cathode material has the advantages of high density, low sheet resistivity, and high capacity retention, making it suitable for high-energy-density applications in thick-plate batteries.

Description

Technical Field

[0001] This invention relates to the technical field of cathode materials, and in particular to a high-density lithium iron phosphate cathode material and its synthesis method. Background Technology

[0002] Lithium iron phosphate (LFP), as a cathode material for lithium-ion batteries, is widely used in electric vehicles, energy storage systems, and other fields due to its advantages such as low cost, high safety, and long cycle life. However, as battery technology advances towards higher energy density, fast charging, and thicker electrodes, LFP cathode materials face a series of new challenges in practical applications. To achieve higher energy output per unit area, electrode thickness and compaction density are continuously increasing, which places higher demands on the material's particle structure, conductive network construction, compatibility of the bonding system, and overall film formation process.

[0003] Currently used lithium iron phosphate cathode materials often employ particles with a single particle size distribution or uniform morphology in their structural design. During compaction, effective particle rearrangement and structural closure are difficult to achieve, thus limiting the compaction density and mechanical stability of the electrode. Simultaneously, conductive agents are typically added through physical mixing, making it difficult to form continuous conductive channels across the thickness of thick electrodes, resulting in restricted electron migration in the central region. Furthermore, traditional binder systems are mostly based on single-polarity polymers, lacking effective selective adsorption and directional distribution capabilities for conductive agents with different morphologies or polarities, thus affecting the spatial organization of the conductive agent within the electrode. During the drying and compaction of thick electrodes, problems such as conductive agent agglomeration, structural cracking, and conduction interruption easily occur, affecting the conductive continuity and mechanical integrity of the electrode. Although recent studies have attempted to introduce pore regulators or polar additives to improve the internal structure of the electrode, these generally suffer from uneven distribution, interfacial incompatibility, or complex preparation processes, making it difficult to meet the integrated performance requirements of high-compact, high-thickness electrodes. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a high-density lithium iron phosphate cathode material and its synthesis method.

[0005] A high-density lithium iron phosphate cathode material, the cathode material comprising the following components by weight percentage: Lithium iron phosphate active material: 89-92 parts, of which D 50 Coarse particles of 5–7 μm account for 65–75%, D 50 Fine particles with a diameter of 1 μm account for 25%–35%; Conductive agent: 0.8-1.2 parts carbon nanotubes, 1.5-2.0 parts high specific surface area carbon black and 0.8-1.2 parts reduced graphene oxide; Adhesive: 1-1.5 parts of polyvinylidene fluoride-hexafluoropropylene copolymer, 1-1.5 parts of sodium carboxymethyl cellulose or sodium alginate, and 0.5-1.0 parts of poly(hydroxyethyl methacrylate-co-acrylamide). Modifying additives: 0.2–0.5 parts of PEO-PPO-PEO triblock polyether and 0.2–0.4 parts of N-methacrylamide monomer. Pore ​​regulator: 0.5 to 1 part, which is a thermally decomposable polymer particle with a particle size of 100 to 300 nm.

[0006] First, in the coarse and fine particle composite structure of the present invention, the coarse particles provide skeletal support and the fine particles fill the fine gaps, forming a dynamic cooperation relationship between the two, which improves the closure and deformation coordination of the compacted structure.

[0007] Secondly, carbon nanotubes construct a longitudinal conductive framework, reduced graphene oxide intercalates to form multi-point bridges, and high specific surface area carbon black laterally fills the interparticle contact sites, synergistically forming a stable "point-line-surface" conductive network in the thickness direction, significantly improving the electron transport capability of the electrode's central layer. Furthermore, carbon nanotubes, reduced graphene oxide, and high specific surface area carbon black are not mechanically blended, but rather construct a nested conductive topology through the synergistic locking of spatial morphology and distribution paths: carbon nanotubes form the longitudinal conductive framework, rGO sheets are nested between LFP particles to fill two-dimensional conductive bridges, and high specific surface area carbon black laterally connects to the particle contact areas. The three elements synergistically construct a three-dimensional conductive network structure with a gradient distribution from the middle layer to the upper and lower layers.

[0008] Furthermore, the binder system, through a gradient polarity distribution system composed of polyvinylidene fluoride-hexafluoropropylene copolymer (weakly polar), sodium alginate / sodium carboxymethyl cellulose (medium polar), and poly(hydroxyethyl methacrylate-co-acrylamide) copolymer (high polarity), exhibits an ordered induction effect on different conductive agents during drying and compaction: carbon nanotubes are adsorbed by the middle polar environment, reduced graphene oxide sheets form a cross-linked stable interface with the flexible high polar binder, and high specific surface area carbon black is repelled by the polyvinylidene fluoride-hexafluoropropylene copolymer enrichment area on the surface and aggregates in the upper and lower interface regions, ultimately forming a "sandwich" three-dimensional conductive channel and a "rigid-flexible-conductive" composite stress buffer structure.

[0009] PEO-PPO-PEO triblock polyether, acting as an asymmetric polar dispersant, preferentially adsorbs carbon black through its oxyphilic PEO segments, while its hydrophobic PPO segments tend to migrate to the middle layer and become affinity-bound to carbon nanotubes. This induces the vertical migration and distribution of the conductive agent during drying, achieving self-organized structural layering of the conductive agent along the thickness direction. Simultaneously, N-methacrylamide monomers migrate to the electrode surface during drying, and their amide groups exhibit polar-induced adsorption capacity for carbon black, forming a surface cross-linked edge-locking structure. This effectively prevents subsequent compaction and powder detachment and stabilizes the surface structure of the electrode. The synergistic effect of these two processes not only regulates the three-dimensional distribution of the conductive agent but also simultaneously improves the mechanical strength and thickness uniformity of the electrode.

[0010] Furthermore, the coarse particles in the lithium iron phosphate active material are nearly spherical or ellipsoidal, and the fine particles are plate-like or rod-like, and the surface of the lithium iron phosphate is coated with a nitrogen-doped carbon layer.

[0011] The near-spherical or ellipsoidal geometry of coarse particles endows them with excellent self-assembly and stacking capabilities and deformation resistance, primarily bearing the skeletal support and macroscopic compaction load under external pressure. Meanwhile, the plate-like or rod-like structures of fine particles fill the gaps between coarse particles with a high aspect ratio and are nested within micropores, achieving dense filling and synergistic stress dispersion. During compaction, fine particles exhibit higher plastic migration capabilities, rearranging themselves according to the stress direction and enhancing their anchoring force to the surrounding structure through nitrogen-doped carbon coating, thereby preventing structural collapse and interparticle breakage.

[0012] The selection of coarse and fine particle size and morphology used in this invention has the dual function of improving the structural locking mechanism and stress release channels. If equispherical fine particles are used instead, they are difficult to nest in the stacking channels constructed by coarse particles, which will form a large number of closed dead cavities after compaction and reduce the volume utilization rate. If the fine particles have a non-directional block structure, they are prone to forming particle stacking cracks due to deformation restriction during compaction, affecting the overall compactness and the continuity of conductive agent distribution.

[0013] Furthermore, this structure lays the foundation for the "nested-layer" topological relationship between sheet-like reduced graphene oxide and sheet-like lithium iron phosphate fine particles, allowing the conductive agent to be smoothly intercalated between the sheet particles to form a stable two-dimensional conductive bridging structure, providing microstructure protection for the construction of the "sheet bridge-point contact" conductive network.

[0014] Nitrogen-doped carbon coating forms a dense carbon layer through sol-gel, and the nitrogen doping enhances electronic coupling and anchoring force, further ensuring the continuity of conduction between particles and preventing the carbon layer from peeling off after compaction.

[0015] Furthermore, the nitrogen-doped carbon layer is coated using a sol-gel method, specifically including the following steps: Step (1): Dissolve glucose and urea in deionized water at a mass ratio of 1 to 2:1 to form a precursor solution; Step (2): Add the well dispersed lithium iron phosphate particles and stir magnetically for 30-60 minutes to form a uniform suspension; Step (3): Adjust the pH of the system to 6.5-7.5, and then dry it at a constant temperature of 80-100℃ to form a carbon source gel coating layer; Step (4): Heat-treat the dry gel sample at 600-750℃ for 2-4 hours in an inert atmosphere to form a nitrogen-doped carbon coating layer with a thickness of 1-2 nm.

[0016] Nitrogen-doped carbon coatings improve electronic conductivity and play a crucial interfacial synergistic role in particle compaction, conductive agent adsorption, and binder anchoring.

[0017] Furthermore, the specific surface area of ​​the high specific surface area carbon black is 60–70 m². 2 / g, the aspect ratio of carbon nanotubes is 500-1000.

[0018] By limiting the specific surface area of ​​high specific surface area carbon black and the aspect ratio of carbon nanotubes, it is beneficial to construct a multidimensional synergistic conductive network with a topological hierarchical structure, which can effectively improve the electron transport efficiency and structural stability inside the thick electrode while ensuring the compaction density.

[0019] High specific surface area carbon black, with its particle size and strong adsorption capacity, mainly fills the contact gaps between LFP particles in the electrode microstructure, forming lateral "point contact" electron hopping paths, thus possessing both electron bridging and porosity regulation functions. Carbon nanotubes, due to their ultra-high aspect ratio, migrate and align directionally along the thickness direction under the induction of an electric field after coating, constructing a longitudinally penetrating "linear conductive framework" that ensures electron connectivity and continuity in the middle layer region of electrodes with a thickness exceeding 200 micrometers. Using low specific surface area carbon black will result in the loss of interparticle filling capacity, making it impossible to bridge the micropore sites; while insufficient aspect ratio of carbon nanotubes will prevent directional alignment under electric field induction and easily lead to agglomeration and blockage of network channels.

[0020] Furthermore, the pore-regulating agent is polymethyl methacrylate nanospheres.

[0021] Polymethyl methacrylate nanospheres, as pore regulators, have a clear thermal decomposition window (decomposition begins at approximately 250–300°C). Under the hot pressing of 120°C or slightly higher temperatures in this invention, they can gradually soften and initiate low-level chain segment breakage, forming localized shrinkage and "confined collapse" micropores, rather than complete vaporization. This type of slow-release structural collapse behavior is conducive to the generation of semi-closed porous structures, effectively regulating the gas and liquid phase permeability in the middle of the thick electrode.

[0022] Secondly, the surface tension of polymethyl methacrylate nanospheres (approximately 40 mN / m) is between that of PVDF-HFP, sodium alginate, and rGO, enabling them to achieve stable distribution in the slurry system. They exhibit no interfacial repulsion with the conductive agent, avoiding agglomeration, segregation, or pore collapse caused by phase boundary tension differences, thus ensuring structural stability during the drying-compacting process.

[0023] Furthermore, in the middle layer region of the electrode enriched with carbon nanotubes, the nanoporous structure formed by the thermal decomposition of polymethyl methacrylate nanospheres provides a slight "slip space" for the conductive framework, avoiding conduction interruption caused by the compaction and shrinkage of the conductive agent; at the same time, it provides a low-resistance channel for the electrolyte to enter the LFP fine particles, significantly improving the ion-electron co-access capability.

[0024] Furthermore, the mass ratio of sodium carboxymethyl cellulose to sodium alginate in the adhesive system is 1:2 to 2:1.

[0025] Functional coupling is achieved by using sodium carboxymethyl cellulose (CMC) and sodium alginate in combination. CMC possesses excellent skeletal adhesion and rheological stability; its molecular chains are flexible yet not easily stretched, providing basic interfacial anchoring force after drying and film formation, making it the primary film-forming agent ensuring the integrity of the electrode structure. Sodium alginate, with its higher polarity and flexibility, can absorb local interparticle micro-stress during compaction and provide an extensible coating layer, playing a crucial role in buffering local collapse and micro-cracks during powder compaction. Using CMC alone results in a rigid structure and easily brittle interfaces after compaction; while sodium alginate alone offers good flexibility, its film strength is low, leading to edge curling or powder detachment after drying.

[0026] Secondly, by controlling the mass ratio of sodium carboxymethyl cellulose to sodium alginate to be 1:2 to 2:1, the overall polarity of the system is in the "medium to strong polarity" range, which is sufficient to induce carbon nanotubes to aggregate in the middle layer without repelling the interfacial anchoring of the surface carbon black. Combined with the weakly polar outer layer coating of polyvinylidene fluoride-hexafluoropropylene copolymer, a gradient polarity induced field is achieved inside the electrode. If the mass ratio deviates from this range, for example, if too much sodium alginate is too polar, it will disrupt the layered migration of carbon nanotubes and reduced graphene oxide, resulting in the mixed enrichment of conductive agents and weakening the vertical conductive structure; while too much sodium carboxymethyl cellulose will increase the rigidity of the system, causing sodium alginate to lose its stress-relieving effect, resulting in structural brittle fracture during the compaction process after coating.

[0027] A method for synthesizing the above-mentioned high-density lithium iron phosphate cathode material includes the following steps: S1. Particle size classification and carbon coating treatment: D 50 Coarse particles of 5-7 μm and D 50The lithium iron phosphate particles with a fine particle size of 1 μm were mixed and coated with a nitrogen-doped carbon layer using the sol-gel method. S2. Conductive agent dispersion pretreatment: Carbon nanotubes and high specific surface area carbon black are added to N-methylpyrrolidone solvent, ultrasonically treated for 10-20 minutes, then reduced graphene oxide is added and mixed under high shear for 15-30 minutes, and 0.1-0.3 wt% polyvinylpyrrolidone or 0.2-0.5 wt% sodium alginate is added to stabilize its dispersion state. S3. Slurry preparation: Add the lithium iron phosphate particles obtained in step S1 to the above mixed solution, stir slowly at 10 rpm for 30 minutes, then add polyvinylidene fluoride-hexafluoropropylene copolymer, sodium alginate, sodium carboxymethyl cellulose and poly(hydroxyethyl methacrylate-co-acrylamide) copolymer in sequence, and continue stirring for 2 hours to form a uniform slurry; adjust the slurry viscosity to 2000-3000 cps; S4. Electrode Coating and Drying: Coating was performed using a coating machine, with the wet film thickness controlled at 300–350 μm; before drying, an electric field of 100–200 V / cm was applied to induce the directional alignment of carbon nanotubes, followed by drying with supercritical carbon dioxide; S5. Compaction and heat treatment: Initial compaction is carried out during the drying process when the moisture content is 50-60%, with a pressure of 100 MPa. After final drying, the pressure is increased to 200 MPa, followed by hot pressing at 120°C for 1 hour.

[0028] In the above synthesis method, in step S2, carbon nanotubes and high specific surface area carbon black are first added to N-methylpyrrolidone for ultrasonic treatment. This is to break the initial entanglement of carbon nanotubes and the agglomeration of carbon black, promoting their untangling in the solvent and forming curved chain segments and point contact sites, providing grafting sites for subsequent nesting configuration with reduced graphene oxide sheets. The subsequent addition of reduced graphene oxide for high-shear mixing facilitates the sliding of reduced graphene oxide sheets into the flexible framework of carbon nanotubes, forming a "sheet bridge-dot network" composite connection mode. If reduced graphene oxide is added too early, it is prone to entanglement and agglomeration with carbon nanotubes, forming large local particles and hindering directional alignment.

[0029] In addition, in step S4, the electric field inducement activates the original structural potential of the conductive agent in the slurry, realizes self-organized spatial positioning reconstruction, and enables the conductive agent to undergo spatial directional migration and topological reconstruction in the wet film state. This achieves the collaborative construction of the longitudinal conductive skeleton and the transverse bridging network in the thick electrode structure, providing a continuous conductive path and a structural self-stabilizing framework for the subsequent high-pressure compaction process.

[0030] Furthermore, the process of electric field inducing the orientation of the conductive agent in step S4 lasts for 10 to 60 seconds.

[0031] Furthermore, the supercritical carbon dioxide drying pressure is controlled at 7–10 MPa, and the temperature is controlled at 35–50°C.

[0032] The controlled pressure and temperature of supercritical carbon dioxide drying facilitate non-destructive shaping and topological locking of the interface state between the structural conductive network and the binder gradient in the wet film. This drying method not only removes solvent in this invention but also serves as a core mechanism for ensuring structural stability and network integrity.

[0033] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in the following aspects: (1) By coarse and fine particles working together, introducing carbon nanotubes, high specific surface area carbon black and reduced graphene oxide working together, a “point-line-surface” conductive network can be constructed to establish a layered and orderly electron transport path in the thickness direction. Among them, carbon nanotubes form a longitudinally connected skeleton, carbon black achieves lateral bridging, and reduced graphene oxide is nested in the interparticle gaps to form a sheet bridge structure. This multidimensional topology significantly improves the electron accessibility of the middle layer region of the electrode, effectively reduces the internal resistance of the electrode, and ensures the continuity of conductivity and the stability of electrochemical performance under high pressure compaction conditions.

[0034] (2) By introducing gradient polar binder and using PEO-PPO-PEO triblock polyether and N-methacrylamide to induce the vertical distribution of conductive agent and form a cross-linked locking structure on the surface, the electrode sheet can be effectively prevented from being compacted and powdered, and the mechanical strength and surface integrity of the thick electrode sheet can be improved. At the same time, the self-organized structure of the conductive agent is locked during the drying and compaction process, forming a "sandwich" three-dimensional conductive path and a flexible buffer layer, which enhances the structural stability and interface bonding strength of the electrode sheet during the high pressure compaction process. Detailed Implementation

[0035] The present invention will now be described in detail with reference to the embodiments.

[0036] Example 1 This embodiment discloses a high-density lithium iron phosphate cathode material, comprising the following components in parts by weight: (1) Lithium iron phosphate active material: 90.5 parts; Coarse particles (D) 50 = 6 μm): accounting for 70%, with a near-spherical morphology; Fine particles (D) 50 = 1 μm): accounting for 30%, with a flake-like morphology; The surface is coated with a 1.5 nm nitrogen-doped carbon layer, which is prepared by the sol-gel method.

[0037] (2) Conductive agent: Carbon nanotubes: 1 part, aspect ratio 750; High specific surface area carbon black: 1.75 parts, specific surface area 65 m² 2 / g; Reduced graphene oxide: 1 part.

[0038] (3) Adhesive: Polyvinylidene fluoride-hexafluoropropylene copolymer: 1.25 parts, molecular weight approximately 530,000, hexafluoropropylene molar ratio 15%; Sodium carboxymethyl cellulose and sodium alginate are in a mass ratio of 1:1, totaling 1 part; Poly(hydroxyethyl methacrylate-co-acrylamide): 0.75 parts, copolymer molar ratio 7:3.

[0039] (4) Regulating additives: PEO-PPO-PEO triblock polyether: 0.35 parts, using Pluronic F127 (BASF).

[0040] N-Methylacrylamide: 0.3 parts.

[0041] (5) Pore conditioner: Polymethyl methacrylate nanospheres: 0.75 parts, particle size approximately 200 nm.

[0042] This embodiment also discloses a method for synthesizing high-density lithium iron phosphate cathode material, including the following steps: S1. Particle size classification and carbon coating treatment: Coarse and fine lithium iron phosphate particles were mixed in a specific ratio. A nitrogen-doped carbon layer was then prepared using the sol-gel method, as follows: Step (1): Dissolve glucose and urea in deionized water at a mass ratio of 1.5:1 to form a precursor solution; Step (2): Add the well-dispersed lithium iron phosphate particles and stir magnetically for 60 minutes to form a uniform suspension; Step (3): Adjust the pH of the system to 7±0.2, and then dry it at a constant temperature of 90℃ to form a carbon source gel coating layer; Step (4): Place the dry gel sample in a quartz boat, introduce argon gas (99.999%) at a flow rate of 300 mL / min to form a protective atmosphere, heat to 700℃, and keep warm for 3 hours to form a surface carbon coating layer.

[0043] S2. Conductive agent dispersion pretreatment: Carbon nanotubes and high specific surface area carbon black were added to N-methylpyrrolidone solvent and treated with an ultrasonic water bath at 300 W for 15 minutes. Then, reduced graphene oxide was added and mixed under high shear at 8000 rpm for 20 minutes. Finally, 0.2 wt% of polyvinylpyrrolidone was added and magnetically stirred at room temperature for 10 minutes to obtain a stable and dispersed conductive agent composite system.

[0044] S3. Slurry preparation: Add the lithium iron phosphate particles from step S1 to the above conductive agent dispersion, stir slowly for 30 minutes, then add polyvinylidene fluoride-hexafluoropropylene copolymer, sodium carboxymethyl cellulose and sodium alginate, and poly(hydroxyethyl methacrylate-co-acrylamide) in sequence, and continue stirring for 2 hours to adjust the viscosity to 2500 cps (@10 rpm).

[0045] S4. Coating and Drying: The slurry was uniformly coated onto the aluminum foil current collector using a slit-type precision coating head, with a wet film thickness of 325 μm. Immediately after coating, while the wet film was still wet, an electric field of 150 V / cm was applied for 30 seconds to induce the oriented alignment of CNTs; subsequent drying was completed using supercritical carbon dioxide (8.5 MPa, 40℃).

[0046] S5. Compaction and Heat Treatment: The material is initially compacted to 100 MPa at a dryness of 55%, then compacted again to 200 MPa after drying, and finally hot-pressed at 120℃ for 1 hour.

[0047] Example 2 This embodiment discloses a high-density lithium iron phosphate cathode material, comprising the following components by weight percentage: (1) Lithium iron phosphate active material: 92 parts; Coarse particles (D) 50 =7 μm): accounting for 75%, with a near-spherical morphology; Fine particles (D) 50 = 1 μm): accounting for 25%, with a flake-like morphology; The surface is coated with a 2 nm nitrogen-doped carbon layer, which is prepared by the sol-gel method.

[0048] (2) Conductive agent: Carbon nanotubes: 1.2 parts, aspect ratio 750; High specific surface area carbon black: 2 parts, specific surface area 65 m² 2 / g; Reduced graphene oxide: 1.2 parts.

[0049] (3) Adhesive: Polyvinylidene fluoride-hexafluoropropylene copolymer: 1.5 parts, molecular weight approximately 530,000, hexafluoropropylene molar ratio 15%; The mass ratio of sodium carboxymethyl cellulose to sodium alginate is 2:1, totaling 1.5 parts; Poly(hydroxyethyl methacrylate-co-acrylamide): 1 part, copolymer molar ratio 7:3.

[0050] (4) Regulating additives: PEO-PPO-PEO triblock polyether: 0.5 parts, using Pluronic F127 (BASF).

[0051] N-Methylacrylamide: 0.4 parts.

[0052] (5) Pore conditioner: Polymethyl methacrylate nanospheres: 1 part, particle size approximately 300 nm.

[0053] This embodiment also discloses a method for synthesizing high-density lithium iron phosphate cathode material, including the following steps: S1. Particle size classification and carbon coating treatment: Coarse and fine lithium iron phosphate particles were mixed in a specific ratio. A nitrogen-doped carbon layer was then prepared using the sol-gel method, as follows: Step (1): Dissolve glucose and urea in deionized water at a mass ratio of 2:1 to form a precursor solution; Step (2): Add the well-dispersed lithium iron phosphate particles and stir magnetically for 60 minutes to form a uniform suspension; Step (3): Adjust the pH of the system to 7.5±0.1, and then dry it at a constant temperature of 100℃ to form a carbon source gel coating layer; Step (4): Place the dry gel sample in a quartz boat, introduce argon gas (99.999%) at a flow rate of 300 mL / min to form a protective atmosphere, heat to 750℃, and keep warm for 4 hours to form a surface carbon coating layer.

[0054] S2. Conductive agent dispersion pretreatment: Carbon nanotubes and high specific surface area carbon black were added to N-methylpyrrolidone solvent and treated with an ultrasonic water bath at 300 W for 20 minutes. Then, reduced graphene oxide was added and mixed under high shear at 8000 rpm for 30 minutes. Finally, 0.3 wt% of polyvinylpyrrolidone was added and magnetically stirred at room temperature for 10 minutes to obtain a stable and dispersed conductive agent composite system.

[0055] S3. Slurry preparation: Add the lithium iron phosphate particles from step S1 to the above conductive agent dispersion, stir slowly for 30 minutes, then add polyvinylidene fluoride-hexafluoropropylene copolymer, sodium carboxymethyl cellulose and sodium alginate, and poly(hydroxyethyl methacrylate-co-acrylamide) in sequence, and continue stirring for 2 hours to adjust the viscosity to 3000 cps (@10 rpm).

[0056] S4. Coating and Drying: The slurry was uniformly coated onto the aluminum foil current collector using a slit-type precision coating head, with a wet film thickness of 350 μm. Immediately after coating, an electric field of 200 V / cm was applied for 60 seconds while the wet film was still wet to induce the oriented alignment of CNTs; subsequent drying was completed using supercritical carbon dioxide (10 MPa, 50℃).

[0057] S5. Compaction and Heat Treatment: The material is initially compacted to 100 MPa at a dryness of 60%, then compacted again to 200 MPa after drying, and finally hot-pressed at 120℃ for 1 hour.

[0058] Example 3 This embodiment discloses a high-density lithium iron phosphate cathode material, comprising the following components by weight percentage: (1) Lithium iron phosphate active material: 89 parts; Coarse particles (D) 50 = 5 μm): accounting for 65%, with a near-spherical morphology; Fine particles (D) 50 = 1 μm): accounting for 35%, with a flake-like morphology; The surface is coated with a 1 nm nitrogen-doped carbon layer, which is prepared by the sol-gel method.

[0059] (2) Conductive agent: Carbon nanotubes: 0.8 parts, aspect ratio 750; High specific surface area carbon black: 1.5 parts, specific surface area 65 m² 2 / g; Reduced graphene oxide: 0.8 parts.

[0060] (3) Adhesive: Polyvinylidene fluoride-hexafluoropropylene copolymer: 1 part, molecular weight approximately 530,000, hexafluoropropylene molar ratio 15%; Sodium carboxymethyl cellulose and sodium alginate are in a mass ratio of 1:2, totaling 1 part; Poly(hydroxyethyl methacrylate-co-acrylamide): 0.5 parts, copolymer molar ratio 7:3.

[0061] (4) Regulating additives: PEO-PPO-PEO triblock polyether: 0.2 parts, using Pluronic F127 (BASF).

[0062] N-Methylacrylamide: 0.2 parts.

[0063] (5) Pore conditioner: Polymethyl methacrylate nanospheres: 0.5 parts, particle size approximately 100 nm.

[0064] This embodiment also discloses a method for synthesizing high-density lithium iron phosphate cathode material, including the following steps: S1. Particle size classification and carbon coating treatment: Coarse and fine lithium iron phosphate particles were mixed in a specific ratio. A nitrogen-doped carbon layer was then prepared using the sol-gel method, as follows: Step (1): Dissolve glucose and urea in deionized water at a mass ratio of 1:1 to form a precursor solution; Step (2): Add the well-dispersed lithium iron phosphate particles and stir magnetically for 45 minutes to form a uniform suspension; Step (3): Adjust the pH of the system to 6.5±0.1, and then dry it at a constant temperature of 80℃ to form a carbon source gel coating layer; Step (4): Place the dry gel sample in a quartz boat, introduce argon gas (99.999%) at a flow rate of 300 mL / min to form a protective atmosphere, heat to 600℃, and keep warm for 2 hours to form a surface carbon coating layer.

[0065] S2. Conductive agent dispersion pretreatment: Carbon nanotubes and high specific surface area carbon black were added to N-methylpyrrolidone solvent and treated with an ultrasonic water bath at 300 W for 10 minutes. Then, reduced graphene oxide was added and mixed under high shear at 8000 rpm for 15 minutes. Finally, 0.1 wt% of polyvinylpyrrolidone was added and magnetically stirred at room temperature for 10 minutes to obtain a stable and dispersed conductive agent composite system.

[0066] S3. Slurry preparation: Add the lithium iron phosphate particles from step S1 to the above conductive agent dispersion, stir slowly for 30 minutes, then add polyvinylidene fluoride-hexafluoropropylene copolymer, sodium carboxymethyl cellulose and sodium alginate, and poly(hydroxyethyl methacrylate-co-acrylamide) in sequence, and continue stirring for 2 hours to adjust the viscosity to 2000 cps (@10 rpm).

[0067] S4. Coating and Drying: The slurry was uniformly coated onto the aluminum foil current collector using a slit-type precision coating head, with a wet film thickness of 300 μm. Immediately after coating, while the wet film was still wet, an electric field of 100 V / cm was applied for 10 seconds to induce the oriented alignment of CNTs; subsequent drying was completed using supercritical carbon dioxide (7 MPa, 35℃).

[0068] S5. Compaction and Heat Treatment: The material is initially compacted to 100 MPa at 50% dryness, then re-compacted to 200 MPa after drying, and finally hot-pressed at 120℃ for 1 hour.

[0069] The difference between Example 4 and Example 1 is that the lithium iron phosphate active material particles were not coated with a nitrogen-doped carbon layer.

[0070] The difference between Example 5 and Example 1 is that the pore conditioner is... The difference between Example 6 and Example 1 is that the specific surface area of ​​the high specific surface area carbon black is 50 m². 2 / g. The difference between Example 7 and Example 1 is that the aspect ratio of the carbon nanotubes is 400.

[0071] The difference between Example 8 and Example 1 is that the mass ratio of sodium carboxymethyl cellulose to sodium alginate is 1:3.

[0072] The difference between Example 9 and Example 1 is that the mass ratio of sodium carboxymethyl cellulose to sodium alginate is 3:1.

[0073] The difference between Comparative Example 1 and Example 1 is that the lithium iron phosphate particles use D 50 The particles are uniform in size, with a diameter of 4.5 μm.

[0074] The difference between Comparative Example 2 and Example 1 is that the content of each component in the conductive agent is as follows: Carbon nanotubes: 0.8 parts; High specific surface area carbon black: 1.6 parts; Reduced graphene oxide: 0.5 parts.

[0075] The difference between Comparative Example 3 and Example 1 is that the content of each component of the adhesive is as follows: The mass ratio of sodium carboxymethyl cellulose to sodium alginate is 1:2, totaling 1.5 parts; Poly(hydroxyethyl methacrylate-co-acrylamide): 1 part.

[0076] The difference between Comparative Example 4 and Example 1 is that the content of each component of the adhesive is as follows: Polyvinylidene fluoride-hexafluoropropylene copolymer: 1 part The mass ratio of sodium carboxymethyl cellulose to sodium alginate is 1:2, totaling 1 part.

[0077] The difference between Comparative Example 5 and Example 1 lies in the difference in step S2, as detailed below. S2. Conductive agent dispersion pretreatment: Carbon nanotubes, high specific surface area carbon black and reduced graphene oxide were added to N-methylpyrrolidone solvent and treated with an ultrasonic water bath at 300 W for 10 minutes, followed by high shear mixing at 8000 rpm for 15 minutes; 0.1 wt% polyvinylpyrrolidone was added and magnetically stirred at room temperature for 10 minutes to obtain a stable and dispersed conductive agent composite system.

[0078] Detection methods 1. Compacted density test Method: Cut the dried positive electrode sheet into standard size (e.g., 1 cm). 2 Its mass and thickness are accurately measured, and its area is known; The compaction density is calculated using the formula ρ = m / (A × h).

[0079] 2. Electrode surface resistance test Equipment: Four-probe tester; method: Resistance of the electrode surface was measured under constant voltage mode; The four-point probe method is used to avoid contact resistance interference; Record the resistance value per unit area (unit: mΩ·cm). 2 Refer to GB / T 1410-2006 "Test Methods for Volume Resistivity and Surface Resistivity of Solid Insulating Materials".

[0080] 3. 100-cycle capacity retention test Equipment: Electrochemical workstation / charge-discharge system (e.g., LAND CT2001A) Condition settings: Current density: 0.5 C; Voltage range: 2.5 V to 3.65 V; Electrolyte: EC / DEC (1:1) + 1M LiPF6; Indicator Calculation: Record the initial capacity C0 and the 100th capacity C 100 ; Retention rate = (C 100 / C0) × 100%.

[0081] The results of the above tests are shown in Table 1 below.

[0082] Table 1 Test Results

[0083] In the above tests, although Comparative Example 3 had a higher compaction density than Example 4 because nitrogen-doped carbon coating can enhance particle slip and improve structural closure, Comparative Example 3 was at a disadvantage in both the sheet resistance test and the 100-cycle capacity retention test. This indicates that an imbalance in the binder ratio can easily lead to "over-coating" of some conductive agent surfaces, inhibiting effective contact between conductive agents, destroying the original topological conductive network, and resulting in increased sheet resistance. During cycling, it is prone to migration and redistribution, causing conductive path shift and fatigue failure of the bonding surface, thereby weakening cycling stability.

[0084] Overall, the test results show that the embodiments outperform the comparative examples in all tests, demonstrating the effectiveness and superiority of the collaborative structure strategy proposed in this invention in the thick electrode environment, and exhibiting outstanding substantial technological progress and system performance optimization effects.

[0085] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A high-density lithium iron phosphate cathode material, characterized in that, The positive electrode material comprises the following components in parts by weight: Lithium iron phosphate active material: 89-92 parts, of which D 50 Coarse particles of 5–7 μm account for 65–75%, D 50 Fine particles with a diameter of 1 μm account for 25%–35%; Conductive agent: 0.8-1.2 parts carbon nanotubes, 1.5-2.0 parts high specific surface area carbon black and 0.8-1.2 parts reduced graphene oxide; Adhesive: 1-1.5 parts of polyvinylidene fluoride-hexafluoropropylene copolymer, 1-1.5 parts of sodium carboxymethyl cellulose or sodium alginate, and 0.5-1.0 parts of poly(hydroxyethyl methacrylate-co-acrylamide). Modifying additives: 0.2–0.5 parts of PEO-PPO-PEO triblock polyether and 0.2–0.4 parts of N-methacrylamide monomer. Pore ​​regulator: 0.5 to 1 part, which is a thermally decomposable polymer particle with a particle size of 100 to 300 nm.

2. The high-density lithium iron phosphate cathode material according to claim 1, characterized in that: The coarse particles in the lithium iron phosphate active material are nearly spherical or ellipsoidal, while the fine particles are plate-like or rod-like, and the surface of the lithium iron phosphate is coated with a nitrogen-doped carbon layer.

3. The high-density lithium iron phosphate cathode material according to claim 2, characterized in that: The nitrogen-doped carbon layer is coated using a sol-gel method, specifically including the following steps: Step (1): Dissolve glucose and urea in deionized water at a mass ratio of 1 to 2:1 to form a precursor solution; Step (2): Add the well dispersed lithium iron phosphate particles and stir magnetically for 30-60 minutes to form a uniform suspension; Step (3): Adjust the pH of the system to 6.5-7.5, and then dry it at a constant temperature of 80-100℃ to form a carbon source gel coating layer; Step (4): Heat-treat the dry gel sample at 600-750℃ for 2-4 hours in an inert atmosphere to form a nitrogen-doped carbon coating layer with a thickness of 1-2 nm.

4. The high-density lithium iron phosphate cathode material according to claim 2, characterized in that: The specific surface area of ​​the high specific surface area carbon black is 60–70 m². 2 / g, the aspect ratio of carbon nanotubes is 500-1000.

5. The high-density lithium iron phosphate cathode material according to claim 2, characterized in that: The pore-regulating agent is polymethyl methacrylate nanospheres.

6. The high-density lithium iron phosphate cathode material according to claim 2, characterized in that: The mass ratio of sodium carboxymethyl cellulose to sodium alginate in the adhesive system is 1:2 to 2:

1.

7. A method for synthesizing a high-density lithium iron phosphate cathode material according to any one of claims 2 to 6, characterized in that, Includes the following steps: S1. Particle size classification and carbon coating treatment: D 50 Coarse particles of 5-7 μm and D 50 The lithium iron phosphate particles with a fine particle size of 1 μm were mixed and coated with a nitrogen-doped carbon layer using the sol-gel method. S2. Conductive agent dispersion pretreatment: Carbon nanotubes and high specific surface area carbon black are added to N-methylpyrrolidone solvent, ultrasonically treated for 10-20 minutes, then reduced graphene oxide is added and mixed under high shear for 15-30 minutes, and 0.1-0.3 wt% polyvinylpyrrolidone or 0.2-0.5 wt% sodium alginate is added to stabilize its dispersion state. S3. Slurry preparation: Add the lithium iron phosphate particles obtained in step S1 to the above mixed solution, stir slowly at 10 rpm for 30 minutes, then add polyvinylidene fluoride-hexafluoropropylene copolymer, sodium alginate, sodium carboxymethyl cellulose and poly(hydroxyethyl methacrylate-co-acrylamide) copolymer in sequence, and continue stirring for 2 hours to form a uniform slurry; adjust the slurry viscosity to 2000-3000 cps; S4. Electrode Coating and Drying: Coating was performed using a coating machine, with the wet film thickness controlled at 300–350 μm; before drying, an electric field of 100–200 V / cm was applied to induce the directional alignment of carbon nanotubes, followed by drying with supercritical carbon dioxide; S5. Compaction and heat treatment: Initial compaction is carried out during the drying process when the moisture content is 50-60%, with a pressure of 100 MPa. After final drying, the pressure is increased to 200 MPa, followed by hot pressing at 120℃ for 1 hour.

8. The high-density lithium iron phosphate cathode material according to claim 7, characterized in that: The process of electric field inducing the orientation of the conductive agent in step S4 lasts for 10 to 60 seconds.

9. The method for synthesizing a high-density lithium iron phosphate cathode material according to claim 7, characterized in that: The supercritical carbon dioxide drying pressure is controlled at 7–10 MPa, and the temperature is controlled at 35–50℃.

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