Sintering and shaping treatment method for improving mechanical strength of lithium iron phosphate particles

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a sintering and shaping treatment method for improving the mechanical strength of lithium iron phosphate particles. The invention aims to solve the problems of easy breakage and poor cycle stability caused by grain coarsening, high porosity and high carbon layer brittleness of lithium iron phosphate particles in the prior art. The method comprises the following steps: preparing a nano precursor with a high specific surface area; polyvinyl alcohol is introduced to be blended and coated with the carbon nanotubes so as to construct a toughness-enhanced carbon network and a conductive skeleton; performing three-section gradient sintering and cooperatively controlling atmosphere and micro-positive pressure; polyacrylonitrile or polyvinylpyrrolidone is used as a polymer carbon source to strengthen interface bonding; an aluminum oxide passivation layer of 2-5 nm is formed through atomic layer deposition; and finally performing low-energy airflow shaping. Through full-chain structure regulation and control, the mechanical strength, the structural stability and the electrochemical performance of the particles are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of battery material technology, and specifically relates to a sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles. Background Technology

[0002] Lithium iron phosphate (LFP), as a highly representative olivine-structured compound in current lithium-ion battery cathode materials, has been widely used in power batteries and energy storage systems due to its excellent thermal stability, cycle life, and environmental friendliness. With the continuous pursuit of high safety, long range, and fast charging capabilities in electric vehicles, and the stringent requirements for cycle durability and manufacturing costs in energy storage power stations, the comprehensive performance optimization of LFP materials has gradually shifted from improving single electrochemical indicators to a multi-dimensional synergistic design encompassing structural integrity, process adaptability, and long-term service reliability. Against this backdrop, the intrinsic mechanical strength of LFP particles is increasingly important, as it not only directly affects the stability of the process window during electrode processing but also has a deeper connection to the structural evolution behavior and capacity retention capability of the battery during long-term cycling.

[0003] In existing technologies, the preparation of lithium iron phosphate (LFP) generally employs a composite process route combining high-temperature solid-state or liquid-phase synthesis with subsequent sintering and carbon coating. The core logic of this method lies in: using high-temperature sintering to induce a crystal phase transformation in the precursor, forming a highly ordered olivine crystal structure; simultaneously, introducing organic carbon sources (such as glucose or sucrose) for in-situ carbonization under an inert atmosphere to construct a conductive network to compensate for the low intrinsic electronic conductivity of LFP. This technological approach effectively solved key bottlenecks such as poor material conductivity and weak rate performance in its early development stages, significantly improving the reversible capacity and power output capability of the battery, thus becoming the mainstream process in the industry. Specifically, conventional sintering typically involves holding at temperatures above 700°C for several hours to ensure sufficient crystallization; while carbon coating relies on the amorphous carbon layer formed by the pyrolysis of the carbon source physically adhering to the particle surface, establishing electronic pathways.

[0004] However, with the continuous development of related technologies and the increasingly stringent requirements for performance indicators in application scenarios, some inherent characteristics of the above-mentioned technical solutions at the principle level have gradually revealed their limitations in addressing new challenges. Fundamentally, this stems from the lack of systematic consideration in the synergistic optimization of "electrochemical performance" and "mechanical performance" in traditional processes, resulting in the unintentional sacrifice of particle structural robustness while improving conductivity. Firstly, in the sintering stage, the high-temperature, long-time process adopted to pursue high crystallinity, while beneficial for establishing lattice integrity, easily leads to excessive grain coarsening, significantly reducing the number of grain boundaries within the particles. Since grain boundaries are key interfaces for stress buffering and crack deflection, their absence increases material brittleness, macroscopically manifesting as a decrease in compressive and shear strength. Furthermore, if volatile components (such as water vapor and carbon dioxide) released by the precursor during heating are not promptly removed through a reasonable heating rate and atmosphere flow, they can easily form closed pores within the particles. These micropores become stress concentration sources during subsequent electrode rolling or battery cycling, inducing the initiation and propagation of microcracks. In addition, conventional processes are difficult to control particle morphology effectively, often generating secondary particles formed by hard agglomeration of primary grains. Their internal bonding mainly relies on van der Waals forces or weak chemical bonds. During high-shear pulping or high-pressure rolling, they are prone to disintegration along the agglomeration interface, generating a large amount of fine powder, which in turn leads to problems such as slurry sedimentation, uneven coating, and electrode powder shedding.

[0005] Correspondingly, existing technologies also present deep-seated contradictions in the carbon coating process. The carbon layers formed by the pyrolysis of traditional small-molecule carbon sources (such as glucose and sucrose) are mostly amorphous structures, whose mechanical properties are greatly affected by pyrolysis conditions. They typically have limited interfacial bonding strength and relatively high brittleness, and the bonding with the lithium iron phosphate core is mainly physical adsorption, resulting in limited interfacial bonding strength. During battery charging and discharging, the volume change (approximately 6.8%) caused by lithium-ion insertion and extraction in lithium iron phosphate accumulates periodic stress within the particles. If the carbon layer cannot effectively transfer and dissipate this stress, it can easily lead to carbon shell cracking or detachment from the core, disrupting the continuity of the conductive network, deactivating some active materials, forming "dead lithium," and causing irreversible capacity decay. Simultaneously, the unevenness of the carbon layer thickness further exacerbates performance fluctuations: excessively thick areas hinder lithium-ion diffusion kinetics, while excessively thin areas cannot provide sufficient mechanical constraint, making it difficult to suppress particle pulverization. Therefore, existing carbon coating strategies are essentially passive compensation mechanisms that prioritize electrical conductivity over mechanical properties. Their enhancement effect is limited by the mechanical properties of the carbon layer itself and the interfacial compatibility, making it difficult to fundamentally solve the problem of structural failure of particles under complex working conditions.

[0006] In summary, the core challenge in current lithium iron phosphate (LFP) material preparation processes lies in how to simultaneously construct particle bulks with high density, fine-grained structure, and strong interfaces while ensuring high electrochemical activity, thereby achieving a synergistic leap in mechanical strength and cycle stability. The root of this challenge is not insufficient adjustment of a single process parameter, but rather stems from the lack of integrated design for the multi-scale structure of particles (from the grain interior to the surface shell) in the traditional binary separation approach of "sintering-coating." Therefore, how to achieve full-chain control from core densification and interface strengthening to shell toughening through innovative sintering regimes and composite shaping and coating strategies to obtain LFP particles that meet both high-rate requirements and excellent anti-breakage capabilities has become a key challenge and an urgent technical problem for those skilled in the art. Summary of the Invention

[0007] To overcome the problems in the prior art, this invention provides a sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles. It aims to solve the technical problems in the prior art, such as grain coarsening, high internal porosity, weak secondary particle bonding, and brittle carbon coating layer with poor interfacial bonding, leading to easy particle breakage and poor cycle stability. To achieve the above-mentioned objective, this invention constructs a full-chain structural control system from core densification and interface strengthening to shell toughening, forming a lithium iron phosphate particle composite structure with high strength, high toughness, and excellent electrochemical stability.

[0008] The method described in this invention includes four core process steps: precursor preparation, composite coating, multi-stage gradient sintering, and surface passivation. Each step works synergistically with the other in terms of parameter setting and material selection, and together they affect the microstructure evolution of lithium iron phosphate particles.

[0009] First, in the precursor preparation stage, nanoscale lithium iron phosphate precursors with concentrated particle size distribution, regular morphology, and a specific surface area greater than 30 m² / g are prepared using co-precipitation or sol-gel methods. The average primary grain size is controlled within the range of 50 to 150 nanometers to ensure good reactivity and densification potential during subsequent sintering. This precursor does not contain alkali metal impurity ions, and the iron element exists stably in a divalent state, avoiding redox side reactions during subsequent heat treatment.

[0010] Secondly, in the composite coating and particle shaping process, a synergistic coating mechanism of polymer binder and conductive reinforcing material is introduced. Specifically, before sintering, lithium iron phosphate precursor and polyvinyl alcohol (PVA) are uniformly blended as binder and carbon nanotubes as conductive reinforcing phase, with PVA accounting for 0.2% to 0.5% by mass and carbon nanotubes accounting for 0.1% to 0.3% by mass. After spray drying or mechanical fusion, the mixture forms spherical secondary particles, the interior of which consists of primary nanocrystals bridged by a polymer network.

[0011] Furthermore, in the sintering process, a three-stage gradient heating and atmosphere pressure control strategy is implemented. The first stage, a low-temperature sintering range, is set at 350 to 450 degrees Celsius, with a holding time of 60 to 90 minutes. This is used to thoroughly remove crystal water and organic residues from the precursor and promote the conversion of the mesophase to the lithium iron phosphate precursor, preventing the rapid release of volatile gases at high temperatures that could cause closed pores inside the particles. The second stage, a medium-temperature sintering range, is set at 550 to 650 degrees Celsius, with a holding time of 40 to 60 minutes. This stage is a critical window for the nucleation and initial growth of lithium iron phosphate crystals. By limiting the upper temperature limit and extending the holding time, abnormal grain growth is effectively suppressed, resulting in a uniform and fine initial grain structure. The third stage, a high-temperature sintering range, is set at 680 to 720 degrees Celsius, with the holding time strictly controlled to not exceed 30 minutes. This is to complete the final ordering of the olivine crystal structure and eliminate residual impurities, while minimizing grain coarsening caused by grain boundary migration. Throughout the sintering process, the furnace atmosphere was maintained as a mixture of inert and reducing gases, with the hydrogen concentration precisely controlled between 2% and 5% by volume, and the oxygen partial pressure maintained below 5 × 10⁻⁻⁻⁻⁶. 5 Atmospheric pressure levels are maintained to ensure the chemical stability of Fe²⁺ throughout the heat treatment cycle and to prevent the formation of Fe³⁺ from adversely affecting electrochemical performance. Furthermore, a slightly positive pressure environment is applied within the sintering chamber, with the pressure value controlled between 1.05 and 1.15 standard atmospheres, to promote the expulsion of gases from within the particles and improve overall density.

[0012] During sintering, polyvinyl alcohol undergoes in-situ carbonization, generating an amorphous carbon network with enhanced toughness. This carbon network not only fills the gaps between primary particles but also forms localized "welded" structures at particle contact points, significantly strengthening the bonding strength within the secondary particles. Simultaneously, carbon nanotubes maintain structural integrity at high temperatures and are oriented along particle boundaries, forming a three-dimensional flexible conductive framework. This provides continuous electron transport paths and endows the particles with excellent shear and compressive strength.

[0013] Furthermore, to enhance the mechanical properties and interfacial bonding strength of the surface carbon layer, this invention employs a polymer carbon source instead of a traditional small-molecule carbon source. The polymer carbon source is selected from one or a combination of polyacrylonitrile or polyvinylpyrrolidone. Its molecular chain structure can form a carbon layer with a certain tendency to graphitize during pyrolysis. Compared to the carbon layer obtained from the pyrolysis of glucose or sucrose, this carbon layer exhibits higher elongation at break and interfacial adhesion. During carbonization, the polymer carbon source undergoes a dehydration condensation reaction with the hydroxyl groups on the lithium iron phosphate surface, enhancing the interfacial bonding strength between the carbon layer and the lithium iron phosphate surface. This achieves a strong chemical bond between the carbon shell and the core material, effectively suppressing carbon shell peeling caused by volume changes during charging and discharging.

[0014] Furthermore, this invention incorporates atomic layer deposition (ALD) for surface passivation of the particles after sintering. Using trimethylaluminum and deionized water as precursors, a thin alumina film with a thickness of 2 to 5 nanometers is deposited layer by layer on the surface of lithium iron phosphate particles. This alumina layer exhibits high density and chemical inertness, effectively preventing electrolyte erosion of the particle surface while simultaneously increasing surface hardness and inhibiting the initiation and propagation of microcracks under cyclic stress. The alumina passivation layer is complete, uniform in thickness, and does not impede the migration kinetics of lithium ions at the solid-liquid interface.

[0015] Finally, in the post-processing shaping stage, the sintered and coated lithium iron phosphate agglomerates undergo gentle depolymerization and surface trimming. Using an air jet mill or high-speed vortex shaping equipment, the particles are surface-smoothed under low energy input conditions, removing loose adhering substances and sharp burrs without damaging the already formed dense structure and carbon network connections within the particles. This shaping process controls the feed rate and airflow velocity to ensure that the collision energy between particles is below their breakage threshold, guaranteeing that the final product has good sphericity and surface smoothness while maintaining high mechanical strength properties.

[0016] In summary, this invention achieves full-scale structural optimization of lithium iron phosphate particles from the core to the shell through the systematic integration of multiple technologies, including precursor nano-design, multi-stage gradient sintering, polymer-assisted carbon network construction, polymer carbon source interface strengthening, and atomic layer deposition surface passivation. The resulting lithium iron phosphate particles possess a fine and uniform grain structure, high density, strong internal bonding, a continuous and stable conductive network, and a robust and wear-resistant surface protective layer. This significantly improves their structural stability and service life during electrode processing and battery cycling while ensuring high electrochemical activity. Detailed Implementation

[0017] This invention provides a sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles. The technical solution integrates four core process steps: precursor preparation, composite coating, multi-stage gradient sintering, and surface passivation. This achieves full-chain structural control of lithium iron phosphate particles, from core densification and interface strengthening to shell toughening. The technical solution of this invention will be described in detail below with reference to specific embodiments.

[0018] In the precursor preparation stage, lithium iron phosphate precursor was synthesized using a co-precipitation method. Analytical grade ferrous sulfate heptahydrate, ammonium dihydrogen phosphate, lithium nitrate, and citric acid were dissolved in deionized water at a stoichiometric ratio of Fe:P:Li = 1:1:1.05 to prepare a mixed solution with a total metal ion concentration of 1.2 mol / L. Under a nitrogen atmosphere, this solution was pumped into the reactor at a flow rate of 30 mL / min, while 2.0 mol / L ammonia was added as a pH adjuster to maintain the pH of the reaction system at 6.8 ± 0.2. The reaction temperature was maintained at 55°C, the stirring speed at 400 rpm, and the reaction time was 4 hours. After the reaction, the resulting slurry was filtered, washed with deionized water until the conductivity of the filtrate was below 50 μS / cm, and then dried in a vacuum drying oven at 80°C for 12 hours to obtain a light green powdered precursor. BET surface area analysis showed that the precursor had a specific surface area of ​​35.6 m² / g. X-ray diffraction analysis confirmed that it was an amorphous iron phosphate phase, with no impurity peaks detected. Scanning electron microscopy showed that the primary particles were spherical with a concentrated particle size distribution and an average particle size of 98 nm. X-ray photoelectron spectroscopy indicated that iron existed in the form of Fe²⁺, with no Fe³⁺ signal detected. Inductively coupled plasma atomic emission spectrometry showed that the contents of alkali metal impurities such as sodium and potassium were all below 10 ppm.

[0019] In a preferred embodiment of the present invention, the precursor can also be prepared using the sol-gel method. Ethylene glycol and citric acid are mixed in a molar ratio of 2:1 and heated to 80°C to form a transparent complex solution. Then, lithium nitrate, ferrous nitrate, and triethyl phosphate are added sequentially, with a molar ratio of Li:Fe:P = 1.05:1:1. The mixture is continuously stirred at 120°C to evaporate moisture, forming a viscous gel, which is then spontaneously combusted at 180°C to form a fluffy precursor powder. This precursor also meets the technical requirements of a specific surface area greater than 30 m² / g, a primary grain size in the range of 50 to 150 nanometers, absence of alkali metal impurities, and stable iron content in a divalent state.

[0020] In the composite coating and particle shaping stage, a synergistic coating mechanism of polymer binder and conductive reinforcing material is introduced after precursor preparation and before sintering. Specifically, the dried precursor powder, polyvinyl alcohol (molecular weight 85,000, degree of hydrolysis 98%), and multi-walled carbon nanotubes (outer diameter 10–20 nm, length 1–5 μm) are weighed at a mass ratio of 100:0.35:0.2. First, polyvinyl alcohol is dissolved in deionized water to prepare a 2wt% aqueous solution. Then, carbon nanotubes are added and dispersed for 30 minutes under ultrasonic power of 300 W and frequency of 40 kHz to form a uniform and stable dispersion. The precursor powder is slowly added to this dispersion and mixed for 15 minutes at 8000 rpm in a high-speed shear emulsifier to ensure uniform adhesion of the polymer and carbon nanotubes to the surface of the precursor particles. The mixed slurry is then fed into a spray drying tower with an inlet air temperature of 220°C, an outlet air temperature of 110°C, and an atomization pressure of 0.3 MPa to obtain spherical secondary particles. Scanning electron microscopy revealed that the secondary particles were regularly spherical with a D50 diameter of 12.5 micrometers, a smooth surface, and were composed of primary nanocrystals bridged by a polymer network.

[0021] In the sintering process, the precursor is placed in a tube furnace, and a three-stage gradient heating and atmosphere pressure control strategy is implemented. The first stage, a low-temperature sintering zone, is set at 400 degrees Celsius for 75 minutes. A mixture of high-purity argon (99.999% purity) and hydrogen is introduced into the furnace, with hydrogen comprising 3.5% and a total gas flow rate of 200 mL / min. During this stage, the water of crystallization, residual organic matter, and the mesophase ferrous oxalate in the precursor are completely decomposed. The generated gaseous products such as H2O, CO2, and CO are continuously discharged, preventing the formation of closed pores inside the particles due to rapid gas expansion in the subsequent high-temperature stages. The pressure inside the furnace is precisely controlled at 1.10 standard atmospheres via a back pressure valve, creating a slightly positive pressure environment to promote gas diffusion.

[0022] The second stage of intermediate-temperature sintering was set at 600 degrees Celsius for 50 minutes. This stage is a critical window for the nucleation and initial growth of lithium iron phosphate crystals. By limiting the upper temperature limit to below 650 degrees Celsius and extending the holding time, abnormal grain growth was effectively suppressed. At this temperature, Li3PO4 undergoes a solid-state reaction with intermediate phases such as Fe2P2O7, gradually transforming into olivine-type LiFePO4 crystals. X-ray diffraction patterns show that the product at this stage possesses complete characteristic peaks of LiFePO4, but the grain size remains at the sub-micron level. Synchrotron radiation X-ray tomography confirmed that the internal porosity of the particles was significantly reduced, with an average pore size of less than 50 nanometers.

[0023] The third high-temperature sintering stage was set at 700 degrees Celsius, with a holding time strictly controlled at 25 minutes. This stage aimed to achieve the final ordering of the olivine crystal structure, eliminate residual impurities, and optimize lattice integrity. Throughout the sintering process, the furnace atmosphere was maintained as an argon-hydrogen mixture containing 3.5% hydrogen. The oxygen partial pressure was monitored and controlled in real-time by an oxygen sensor, remaining below 5 × 10⁻⁻⁻⁻⁶. 5 Atmospheric pressure ensures the chemical stability of Fe²⁺ throughout the heat treatment cycle. A continuous micro-positive pressure environment (1.10 atm) further compresses residual porosity within the particles, increasing overall density. The relative density of the obtained lithium iron phosphate particles, determined by the Archimedes displacement method, reaches 96.8% of the theoretical density.

[0024] During sintering, polyvinyl alcohol undergoes in-situ carbonization in the 350-450°C range, generating an amorphous carbon network with enhanced toughness. This carbon network not only fills the interparticle spaces of the primary particles but also forms localized "welded" structures at particle contact points. Transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS) surface scanning analysis confirmed that carbon elements are distributed in a continuous network within the secondary particles, particularly enriched in the grain boundary regions. Simultaneously, carbon nanotubes maintain structural integrity at a high temperature of 700°C and are oriented along the particle boundaries, forming a three-dimensional flexible conductive framework. The volume conductivity of this composite particle, measured using the four-probe method, is 2.8 × 10⁻² S / cm, nearly two orders of magnitude higher than the control sample without added carbon nanotubes.

[0025] Furthermore, to improve the mechanical properties and interfacial bonding strength of the surface carbon layer, this invention uses a polymeric carbon source instead of a traditional small-molecule carbon source. In another embodiment, polyacrylonitrile (PAN) is introduced as the carbon source. Specifically, the secondary particles obtained from the aforementioned spray drying are immersed in a 5 wt% polyacrylonitrile / N,N-dimethylformamide solution with a solid-liquid ratio of 1:3 (g / mL), and stirred at 40°C for 2 hours to allow PAN to be fully adsorbed onto the particle surface. Subsequently, they are vacuum dried at 60°C for 12 hours to remove the solvent. During sintering, PAN undergoes cyclization crosslinking at 280 to 320°C, followed by pyrolysis at 500 to 700°C to generate a carbon layer with a certain tendency to graphitize. Raman spectroscopy shows that the I_D / I_G ratio of this carbon layer is 1.05, indicating a high degree of structural order. X-ray photoelectron spectroscopy depth analysis confirmed the presence of a significant CO-Fe bonding signal at the interface between the carbon layer and lithium iron phosphate, with binding energies at 531.2 eV (O 1s) and 709.5 eV (Fe 2p3 / 2). This indicates that the polymeric carbon source undergoes a dehydration condensation reaction with the hydroxyl groups on the surface of lithium iron phosphate during pyrolysis, forming a strong chemical bond.

[0026] Furthermore, this invention incorporates atomic layer deposition (ALD) to passivate the particle surface after sintering. Sintered and cooled to room temperature lithium iron phosphate particles are loaded into a rotating fluidized bed ALD reaction chamber. The particles are uniformly dispersed by controlling the rotation rate and gas flow rate. Trimethylaluminum (TMA) and deionized water (H2O) are used as precursors, and the deposition temperature is 150°C. Each ALD cycle includes a 0.1-second TMA pulse, a 10-second purge, a 0.15-second H2O pulse, and a 10-second purge. By controlling the number of cycles to 30, an alumina film with a thickness of approximately 3.2 nanometers is obtained. High-resolution transmission electron microscopy (HRTEM) observation shows that this alumina layer is continuous, dense, and pinhole-free, covering the entire particle surface. Angular-resolved X-ray photoelectron spectroscopy analysis shows that the Al 2p peak intensity increases with decreasing probe angle, confirming uniform film coating. This alumina layer exhibits high chemical inertness, effectively preventing corrosive components such as HF in the electrolyte from eroding the lithium iron phosphate surface, while simultaneously improving surface hardness. Nanoindentation tests showed that the Vickers hardness of the particle surface increased from the original 4.2 GPa to 6.8 GPa after ALD treatment.

[0027] In the post-processing shaping stage, the sintered and coated lithium iron phosphate agglomerates underwent gentle deagglomeration and surface trimming. An air jet mill was used, with a feed rate of 1.5 kg / h, compressed air pressure of 0.45 MPa, and classifier wheel speed of 3600 rpm. Under these low-energy input conditions, elastic collisions and slight friction occurred between particles, effectively removing loosely attached carbon fragments and sharp burrs without damaging the already formed dense crystalline structure and carbon network connections within the particles. Laser particle size analyzer measurements showed that the D50 of the shaped product was 11.8 μm, the D90 was 18.5 μm, and the particle size distribution Span value was 0.92, indicating a concentrated particle size distribution. Scanning electron microscopy images showed smooth particle surfaces, rounded contours, and no obvious breakage or cracks. The tap density measured by a tapper was 1.28 g / cm³, an increase of 8.5% compared to before shaping, indicating that improved particle sphericity effectively improved packing efficiency.

[0028] To verify the technical effect of the present invention, the following embodiments and comparative examples were set up for comparative experiments.

[0029] In one specific embodiment, lithium iron phosphate cathode material was prepared according to the complete process described above: the precursor was prepared by co-precipitation, with an average primary grain size of 98 nm and a specific surface area of ​​35.6 m² / g; the composite coating consisted of 0.35% polyvinyl alcohol and 0.2% carbon nanotubes, and was formed by spray drying; sintering was carried out using a three-stage gradient process (400℃ / 75 min → 600℃ / 50 min → 700℃ / 25 min) in an atmosphere of Ar-3.5%H2 at a pressure of 1.10 atm; the surface carbon source was polyacrylonitrile; 3.2 nm Al2O3 was deposited by ALD; and finally, the material was shaped by air jet milling. The resulting material is referred to as Example 1.

[0030] In one specific embodiment, lithium iron phosphate cathode material was prepared according to the complete process described above: the precursor was prepared by co-precipitation, with an average primary grain size of 98 nm and a specific surface area of ​​35.6 m² / g; the composite coating consisted of 0.2% polyvinyl alcohol and 0.1% carbon nanotubes, and was spray-dried; sintering was performed using a three-stage gradient process (400℃ / 75 min → 600℃ / 50 min → 700℃ / 25 min) in an atmosphere of Ar-3.5%H₂ at a pressure of 1.10 atm; the surface carbon source was polyacrylonitrile; 3.2 nm Al₂O₃ was deposited by ALD; and finally, the material was shaped by air jet milling. The resulting material is referred to as Example 2.

[0031] In one specific embodiment, lithium iron phosphate cathode material was prepared according to the complete process described above: the precursor was prepared by co-precipitation, with an average primary grain size of 98 nm and a specific surface area of ​​35.6 m² / g; the composite coating consisted of 0.5% polyvinyl alcohol and 0.3% carbon nanotubes, and was spray-dried; sintering was performed using a three-stage gradient process (400℃ / 75 min → 600℃ / 50 min → 700℃ / 25 min) in an atmosphere of Ar-3.5%H₂ at a pressure of 1.10 atm; the surface carbon source was polyacrylonitrile; 3.2 nm Al₂O₃ was deposited by ALD; and finally, the material was shaped by air jet milling. The resulting material is referred to as Example 3.

[0032] Comparative Example 1 uses a conventional single-stage sintering process: the same precursor is directly heated to 750°C and held for 5 hours in a nitrogen atmosphere. The remaining steps are the same as in Example 1, but ALD passivation treatment is not performed.

[0033] Comparative Example 2 uses traditional glucose carbon coating: the precursor is mixed with 3 wt% glucose and sintered (700℃ / 5h, N2 atmosphere), without adding polymer binders and carbon nanotubes, and without ALD treatment.

[0034] Comparative Example 3 omits the micro-positive pressure control in multi-stage sintering, and the remaining process parameters are the same as in Example 1.

[0035] The materials described above were mixed with conductive carbon black and polyvinylidene fluoride at a mass ratio of 92:4:4, respectively. A slurry was prepared using N-methylpyrrolidone as a solvent, coated onto aluminum foil, dried, rolled, and cut into sheets to serve as the positive electrode. A CR2032 coin cell was assembled using lithium metal as the counter electrode, a Celgard 2400 membrane as the separator, and 1 mol / L LiPF6 in EC:DMC:EMC (1:1:1, v / v) as the electrolyte. Electrochemical performance was tested at 25°C.

[0036] Mechanical strength testing was performed using a particle crushing force tester. Fifty single particles with a diameter of 10–15 micrometers were selected, and the critical load required for uniaxial compression to breakage was determined. The average value was taken as the particle compressive strength index.

[0037] Cyclic stability tests were conducted at a 0.5 C rate, and the discharge specific capacity was recorded at the 1st and 500th cycles to calculate the capacity retention rate.

[0038] The obtained data is summarized in the table below: Data analysis shows that the lithium iron phosphate particles prepared in Example 1 have significantly higher mechanical strength, with an average compressive strength of 86.4 mN, far exceeding that of the comparative examples. This is attributed to the combined effects of its fine and uniform grain structure, high density, polymer carbon network bridging effect, and carbon nanotube-reinforced three-dimensional conductive framework. Meanwhile, Example 1 exhibits excellent cycle stability, with a capacity retention of 98.7% after 500 cycles, indicating that its structure remains highly stable during long-term charge-discharge processes. The first-cycle coulombic efficiency is also better than the comparative examples, indicating reduced side reactions and improved interface stability. Example 2 has slightly lower mechanical strength due to insufficient carbon network construction, but the lithium-ion transport path is smoother; Example 3 has a slight increase in interface impedance due to excessive carbon source, but its structural support is better. Both examples exhibit typical boundary effects: although the compressive strength and cycle stability are lower than Example 1 (optimal ratio), they are significantly better than all comparative examples, verifying the technical effect gradient of the 0.2%-0.5% polyvinyl alcohol and 0.1%-0.3% carbon nanotube ratio window.

[0039] Furthermore, cross-sectional focused ion beam scanning electron microscopy (FIB-SEM) three-dimensional reconstruction analysis was performed on the material of Example 1. The results showed that the internal porosity of the particles was less than 3.5%, the standard deviation of the primary grain size distribution was less than 15 nm, the carbon network ran through the entire secondary particle, the carbon nanotubes were arranged along the grain boundaries, and the alumina passivation layer completely covered the surface. These microstructural features together constitute the material basis for high strength, high toughness, and high electrochemical stability.

[0040] In another embodiment of the present invention, polyvinylpyrrolidone (PVP) can also be used as the polymer carbon source. PVP (K30, molecular weight 40000) was prepared into a 4 wt% aqueous solution, mixed with the precursor at a solid-liquid ratio of 1:2.5, spray-dried, and then sintered. This also enhances the interfacial bonding strength between the carbon layer and the lithium iron phosphate surface, resulting in a Raman ID / IG ratio of 1.12, a particle compressive strength of 83.7 mN, and a 500-cycle capacity retention of 98.1%. While the performance is slightly lower than the PAN system, it is still significantly superior to traditional small-molecule carbon sources.

[0041] In the ALD passivation process, other metal oxide precursors can also be used, such as titanium tetrachloride reacting with water to deposit TiO2, or diethylzinc reacting with water to deposit ZnO. However, experiments show that Al2O3 has the best overall performance due to its high dielectric constant, excellent chemical stability, and good lattice matching with lithium iron phosphate.

[0042] In summary, this invention achieves full-scale structural optimization of lithium iron phosphate particles from the core to the shell through the systematic integration of multiple techniques, including precursor nano-design, polymer-assisted carbon network construction, multi-stage gradient sintering, polymer carbon source interface strengthening, and atomic layer deposition surface passivation. The resulting lithium iron phosphate particles possess a fine and uniform grain structure, high density, strong internal bonding, a continuous and stable conductive network, and a robust and wear-resistant surface protective layer. This significantly improves structural stability and service life during electrode processing and battery cycling while ensuring high electrochemical activity. Those skilled in the art can, based on the disclosed technical content of this invention and in conjunction with specific equipment conditions and raw material characteristics, reasonably adjust the various process parameters within the scope defined in the claims to obtain lithium iron phosphate cathode materials with high mechanical strength and excellent electrochemical performance.

[0043] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made to it in form and detail without departing from the scope defined by the claims of the present invention.

Claims

1. A sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles, characterized in that, Includes the following steps: S1. Prepare lithium iron phosphate precursor with concentrated particle size distribution, specific surface area greater than 30 m² / g, average primary grain size of 50 to 150 nanometers and iron element in divalent state. S2. The precursor is blended with polyvinyl alcohol and carbon nanotubes, wherein the mass addition ratio of polyvinyl alcohol is 0.2% to 0.5% and the mass addition ratio of carbon nanotubes is 0.1% to 0.3%, and spherical secondary particles are formed by spray drying or mechanical fusion. S3. The precursor is subjected to a three-stage gradient sintering in a mixed atmosphere of inert and reducing gases. The first stage is held at 350 to 450°C for 60 to 90 minutes, the second stage at 550 to 650°C for 40 to 60 minutes, and the third stage at 680 to 720°C for no more than 30 minutes. During the sintering process, the furnace pressure is maintained at 1.05 to 1.15 atmospheres, the hydrogen gas fraction is 2% to 5%, and the oxygen partial pressure is below 5 × 10⁻⁻⁻⁻⁻⁻⁵. 5 Atmospheric pressure; S4. After sintering, an alumina passivation layer with a thickness of 2 to 5 nanometers is deposited on the particle surface using an atomic layer deposition process; S5. Perform low-energy airflow shaping on the passivated particles to remove surface burrs without damaging the internal structure.

2. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, The lithium iron phosphate precursor in step S1 is prepared by co-precipitation or sol-gel method and does not contain alkali metal impurity ions.

3. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, The polyvinyl alcohol is carbonized in situ during the sintering process in step S3, forming an amorphous carbon network that fills the gaps between primary particles and connects the particle contact points.

4. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, In step S2, the carbon nanotubes are partially oriented along the particle surface under shear force during spray drying, and after sintering, they form a locally oriented arrangement along the secondary particle boundary, constituting a three-dimensional flexible conductive framework.

5. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, A polymeric carbon source is introduced onto the surface of the precursor or secondary particles. The polymeric carbon source is selected from at least one of polyacrylonitrile or polyvinylpyrrolidone. During pyrolysis, it reacts with the hydroxyl groups on the surface of lithium iron phosphate to enhance the interfacial bonding strength between the carbon layer and the surface of lithium iron phosphate.

6. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 5, characterized in that, The aforementioned polymer carbon source is loaded onto the particle surface by impregnation or blending, and pyrolyzes during sintering to generate a carbon layer with graphitization tendency.

7. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, The atomic layer deposition process in step S4 uses trimethylaluminum and deionized water as precursors to deposit an alumina film at 150°C.

8. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, The airflow shaping in step S5 uses an airflow mill or a high-speed vortex device to control the feed rate and airflow speed, so that the collision energy between particles is lower than their crushing threshold.

9. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, The treated lithium iron phosphate particles have an internal porosity of less than 3.5%, a primary grain size distribution standard deviation of less than 15 nanometers, and a continuous and dense alumina passivation layer on their surface.

10. The sintering and shaping method for improving the mechanical strength of lithium iron phosphate particles according to claim 1, characterized in that, The average compressive strength of the obtained lithium iron phosphate particles is not less than 80 millinewtons, and the capacity retention rate after 500 cycles is not less than 98%.

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