A method for preparing lithium iron phosphate battery cathode material based on pre-lithiation precursor

Abstract

This invention provides a method for preparing lithium iron phosphate battery cathode materials based on pre-lithiated precursors. By controlling lithium deficiency in the precursor during the liquid phase and introducing a nanoscale second lithium source and an organic chelating agent, the second lithium source is enriched on the particle surface during spray drying, forming a lithium concentration gradient from the outside to the inside. Sufficient lithium replenishment is achieved during the sintering stage, resulting in a high-occupancy, impurity-free lithium iron phosphate material. The obtained product exhibits high initial capacity, high rate performance, and excellent high-temperature long-cycle stability.

Description

Technical Field

[0001] This invention relates to the technical field of lithium iron phosphate battery cathode materials, and in particular to a method for preparing lithium iron phosphate battery cathode materials based on pre-lithiation precursors. Background Technology

[0002] Since its inception, olivine-type lithium iron phosphate (LiFePO4) has become one of the mainstream cathode materials in the fields of power batteries and large-scale energy storage due to its excellent structural stability, thermal safety, low cost, and environmental friendliness. After more than 20 years of continuous optimization, its synthesis process has gradually developed from the early single high-temperature solid-state method to a variety of routes including co-precipitation, hydrothermal, spray pyrolysis, and sol-gel methods. The modification methods have also expanded from the initial simple carbon coating to a comprehensive strategy including bulk ion doping, surface fast ion conductor coating, particle morphology nano-sizing, and crystal facet control, which has significantly improved its overall electrochemical performance and enabled large-scale industrial application in the global power battery market.

[0003] However, existing preparation processes all struggle to avoid lithium volatilization and sublimation losses during the high-temperature sintering stage, leading to a deviation of the final product from the ideal stoichiometry and the formation of lithium vacancy defects (Li). 1-X FePO4). This type of lithium vacancy defect directly reduces the reversible lithium insertion sites in the material on the one hand, and significantly increases the Li-on-carbon intercalation sites on the other. + The migration energy barrier in the one-dimensional channels of the olivine structure significantly reduces the ion diffusion coefficient, resulting in intensified polarization at high current densities, rapid collapse of the discharge plateau, severe capacity decay at low temperatures, and insufficient capacity retention during high-temperature cycling. To alleviate these problems, existing technologies generally employ a passive compensation method of adding excess lithium source at the feed end. However, excess lithium salt readily reacts with phosphate ions at high temperatures to form electrochemically inert Li3PO4, or remains on the particle surface to form a high-resistivity interface layer. This not only fails to fundamentally achieve uniform lithium distribution in the lattice but also introduces new irreversible capacity losses and reduces the initial coulombic efficiency. At the same time, the introduction of excess lithium source also increases raw material costs and the difficulty of treating lithium-containing wastewater. Furthermore, existing lithium replenishment strategies mostly rely on a one-size-fits-all approach of over-adding lithium in a single step, lacking precise control over the precursor structure, lithium loss kinetics during sintering, and the lithium distribution state within the crystal. This results in significant deficiencies in the structural stability and ion transport kinetics of the materials under harsh conditions such as fast charging, high-temperature storage, or long cycling, making it difficult to meet the higher requirements of next-generation high-energy-density, ultra-long-life, and extreme-temperature-adaptable power batteries for cathode materials.

[0004] Therefore, there is an urgent need to develop a novel pre-lithiation technology approach that enables active and controllable lithium replenishment from the precursor design stage. This would fundamentally eliminate lithium vacancy defects caused by high-temperature sintering, while avoiding side reactions and interface problems caused by traditional over-lithiation. In this way, while maintaining the inherent safety advantages of lithium iron phosphate, it would simultaneously improve its ion / electron conductivity and crystal structure stability, providing a more efficient and precise solution for the next generation of high-performance lithium iron phosphate cathode materials. Summary of the Invention

[0005] This application provides a method for preparing lithium iron phosphate cathode material based on a pre-lithiation precursor, including the following steps:

[0006] Step S1. An intermediate precursor A is prepared by reacting an iron source, a phosphorus source and a first lithium source in the liquid phase, wherein the molar amount of the first lithium source is such that the molar ratio of Li to Fe in the intermediate precursor A is 0.90 to 0.98.

[0007] Step S2. A second lithium source, an organic chelating-dispersant, and a carbon source are added to the intermediate precursor A for pre-lithiation coating treatment to obtain a pre-lithiation precursor B. The second lithium source is selected from at least one of LiOH·H2O, Li2CO3, lithium acetate, and nano Li2O. The amount of the second lithium source added is such that the total lithium amount added to the first lithium source and the second lithium source in steps S1 and S2 is 1.005 to 1.065 relative to the stoichiometric Li / Fe molar ratio. The organic chelating-dispersant is selected from at least one of citric acid, ascorbic acid, tartaric acid, and EDTA.

[0008] Step S3. The pre-lithiated precursor B is sintered at high temperature to obtain lithium iron phosphate cathode material.

[0009] It should be noted that in step S1, by intentionally controlling the first lithium source to a significantly lithium-deficient state (Li / Fe = 0.90–0.98) during the liquid-phase reaction stage, a large number of structural lithium vacancies and Fe-O and PO active sites suspended on the surface are formed inside the intermediate precursor A. In step S2, when the nanoscale second lithium source is added together with an organic chelating-dispersing agent (citric acid, ascorbic acid, etc.), the chelating agent rapidly forms a multidentate coordination layer on the surface of the intermediate precursor A, which significantly reduces the lithium deficiency. + The surface migration energy barrier, and the steric hindrance effect suppresses Li + Rapid diffusion into the depths of the particles; simultaneously, during the subsequent rapid dehydration process of spray drying, strong capillary convection and shelling effects occur within the droplets / slurry, preferentially enriching the second lithium source in the outer layer of the particles, thus naturally constructing a lithium concentration gradient from the surface to the interior in the pre-lithiation precursor B; by the high-temperature sintering stage in step S3, this gradient becomes Li +The continuous chemical site driving force for migration into the particle interior enables lithium to achieve orderly and sufficient lattice occupancy in the one-dimensional channels of the olivine structure. Ultimately, this not only compensates for the lithium volatilization loss during the sintering process, but also avoids the precipitation of the inert Li3PO4 phase caused by the traditional one-step excessive lithium addition. This results in an ideal LiFePO4 crystal structure with extremely high lithium occupancy and no residual lithium salts on the surface or in the bulk phase, thereby simultaneously and significantly improving the ion diffusion coefficient, electronic conductivity, and high-temperature high-rate cycling stability of the material.

[0010] As a preferred technical solution for the preparation method of lithium iron phosphate cathode material based on pre-lithiation precursor, step S2 adopts a spray drying process.

[0011] It should be noted that after the slurry containing the slightly lithium-deficient intermediate precursor A, the nano-sized second lithium source, and the organic chelating-dispersant is atomized at high speed, it rapidly loses water in the high-temperature hot air. The droplets quickly shell and generate strong capillary convection, which allows the second lithium source and the Li captured by the chelating agent to be concentrated in the slurry. + The chelating agent preferentially accumulates on the particle surface; simultaneously, it firmly pins Li through multidentate coordination. + This significantly inhibits the diffusion of lithium into the deeper layers of the particles, thus naturally forming a continuous lithium concentration gradient from the surface to the interior within a very short time after drying. This gradient becomes Li during subsequent sintering. + The persistent driving force for directional migration into the particle interior enables the complete elimination of lattice lithium vacancies and the complete eradication of Li3PO4 impurity phases with extremely low total lithium input.

[0012] In a preferred technical solution for the preparation of lithium iron phosphate cathode material based on a pre-lithiation precursor, the amount of the organic chelating-dispersant is 0.4 to 3.5 wt% of the mass of the intermediate precursor A.

[0013] It should be noted, and specifically emphasized, that the amount of organic chelating-dispersant should preferably be 0.4–3.5 wt% of the intermediate precursor A. Its function is to provide an appropriate number of multidentate coordination sites, which can effectively capture and pin Li in the second lithium source. + This significantly inhibits excessive diffusion of Li into the deeper layers of particles during spray drying, while also preventing excessively thick coordination layers from hindering subsequent sintering due to excessive dosage. + The normal lattice occupancy ensures that the final lithium concentration gradient is continuous and controllable, neither too steep nor too gentle, thereby achieving complete lattice lithium occupancy and completely eliminating the formation of the Li3PO4 inert phase with extremely low total lithium input.

[0014] As a preferred technical solution for the preparation of lithium iron phosphate cathode material based on pre-lithiation precursor, the high-temperature sintering is carried out under a protective atmosphere, with a sintering temperature of 670-785 °C and a holding time of 5-14 h.

[0015] It should be noted that the higher sintering temperature and sufficient heat preservation provide a persistent chemical potential driving force for the lithium concentration gradient from the surface to the interior in the pre-lithiation precursor B, enabling the Li in the outer lithium-rich region to... + This allows for directional migration into the interior of the particles along the one-dimensional channels of the olivine structure, ultimately achieving highly intact occupancy of lithium sites in the lattice. Simultaneously, the extended holding time ensures complete pyrolysis of the carbon source to form a continuous conductive network and completely decomposes residual organic chelating agents, preventing the introduction of impurities. If the temperature is too low or the holding time too short, the gradient drive will be insufficient, and Li... + The internal vacancies cannot be completely filled, resulting in lithium vacancies and Li3PO4 precipitation, and the technical effect is significantly lost.

[0016] As a preferred technical solution for the preparation method of lithium iron phosphate cathode material based on pre-lithiation precursor, the carbon source is selected from at least one of glucose, sucrose, citric acid, and polyvinylpyrrolidone, and the residual carbon content is 1.0 to 3.2 wt%.

[0017] It should be noted that these carbon sources can maintain mild and uniform pyrolysis characteristics even at higher sintering temperatures and longer holding times, thus forming thin and continuous splines in situ on the particle surface and near-surface region. 2 / sp 3 The hybrid conductive carbon layer effectively bridges primary particles and significantly improves electronic conductivity. Furthermore, its low residual carbon content avoids the excessively thick coating layer associated with traditional high-residual-carbon sources (such as asphalt and phenolic resins) that negatively impacts Li. + The blockage of the diffusion channels, thus perfectly synergizing with the lithium concentration gradient from the surface to the interior of the invention, allows Li... + It can still rapidly insert and extract lithium while maintaining low lithium vacancies and high lattice integrity, achieving both high-rate discharge capacity and excellent high-temperature cycling stability.

[0018] A preferred technical solution for preparing lithium iron phosphate cathode material based on a pre-lithiation precursor is provided, wherein the iron source is a soluble divalent iron salt and the phosphorus source is phosphoric acid or ammonium dihydrogen phosphate.

[0019] It should be noted that these two types of raw materials can rapidly form a uniform, loose, and amorphous micro-lithium-deficient Fe-PO intermediate precursor A with the first lithium source during the liquid phase reaction. The particle surface is rich in a large number of highly active Fe-O and PO dangling bonds and structural vacancies, which provide sufficient active sites for the rapid adsorption and pinning of the organic chelating-dispersant and the second lithium source in step S2, as well as the surface enrichment during subsequent spray drying, thereby ensuring the stable formation of the lithium concentration gradient.

[0020] This invention achieves sufficient compensation and high site occupancy stabilization of lithium sites in the lithium iron phosphate material lattice through the synergistic effect of "structuring lithium vacancies using a lithium-deficient precursor + pre-lithiation enrichment of the second lithium source surface + shell formation effect by spray drying". This results in a cathode material with higher crystal integrity, lower electron / ion migration impedance and better structural stability. Therefore, it significantly outperforms the comparative samples in key performance indicators such as initial discharge specific capacity, initial coulombic efficiency, 5C high-rate performance and 60 ℃ high-temperature long cycle life. The initial capacity is increased by about 15-25 mAh / g, the high-rate capacity is increased by about 10-20 mAh / g, and the high-temperature 1500-cycle retention rate is improved to 95.7%-97.3%. It effectively solves the technical bottlenecks of traditional "one-step lithium addition" process, such as easy formation of Li3PO4 impurity phase, insufficient lithium addition and rapid high-temperature cycle decay. It has obvious comprehensive performance advantages and industrial application value. Attached Figure Description

[0021] Figure 1 Here are SEM images of the lithium iron phosphate cathode material prepared in Example 1;

[0022] Figure 2 The image shows the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1. Detailed Implementation

[0023] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0024] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0025] Secondly, the term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single embodiment or an embodiment selectively excluded from other embodiments.

[0026] Example

[0027] Example 1

[0028] This embodiment provides a method for preparing lithium iron phosphate cathode material based on a pre-lithiation precursor, specifically including the following:

[0029] Step S1. Take 78.5 kg of industrial-grade ferrous sulfate heptahydrate (FeSO4·7H2O) and 32.6 kg of 85 wt% phosphoric acid (H3PO4), and add them to a 5 m container containing 350 L of deionized water. 3 In a stainless steel reactor, the stirrer was started (280 rpm), and high-purity nitrogen gas was introduced for protection (oxygen content <10 ppm). After heating to 80 ℃, 11.78 kg of lithium hydroxide monohydrate (LiOH·H2O, battery grade) was slowly added (the feeding time was controlled at about 90 min). Throughout the process, the pH in the reactor was kept stable at 4.5-5.0. After the feeding was completed, the reaction was continued at the temperature for 4 h. During this period, nitrogen gas was continuously introduced and a small amount of deionized water was added to maintain the liquid level. After the reaction was completed, the resulting light blue slurry was filtered through a plate and frame filter press and washed three times with deionized water until the conductivity of the filtrate was <80 μS / cm. The filter cake was then re-pulped, and the solid content was adjusted to 38 wt% to obtain the slightly lithium-deficient intermediate precursor A slurry (at this time, the Li / Fe molar ratio was 0.90).

[0030] Step S2. Add sequentially to the precursor A slurry obtained in step S1: 4.62 kg of battery-grade LiOH·H2O with an average particle size of 300 nm (to achieve a total Li / Fe molar ratio of 1.065), 1.8 kg of first-grade citric acid (C6H8O7) (3.5 wt% of the dry basis of precursor A), and industrial-grade glucose (C6H... 12 8.5 kg of O6·H2O was mixed with high-speed shear stirring (1200 rpm) for 30 min, then transferred to a storage tank. The slurry was then pumped to a centrifugal spray drying tower using a peristaltic pump. The spray drying process parameters were: inlet air temperature 260 ℃, outlet air temperature 110 ℃, atomizing disc speed 22000 rpm, feed rate 180 L / h, and negative pressure inside the tower -80 Pa. After drying, the light gray powder at the bottom of the tower and in the cyclone separator was collected to obtain the pre-lithiation precursor B.

[0031] Step S3. The pre-lithiation precursor B obtained in step S2 is loaded into a stainless steel boat and pushed into an industrial roller kiln. Under a high-purity nitrogen protective atmosphere (oxygen content < 5 ppm), the temperature is increased from room temperature to 670 ℃ at a heating rate of 5 ℃ / min and held for 14 h. Then, it is naturally cooled to < 80 ℃ and removed from the kiln to obtain a dark gray lithium iron phosphate cathode material, labeled as LFP-1. The residual carbon content is determined to be 1.0 wt% by chemical titration and combustion method.

[0032] Example 2

[0033] This embodiment provides a method for preparing lithium iron phosphate cathode material based on a pre-lithiation precursor, specifically including the following:

[0034] Step S1. Take 81.2 kg of industrial-grade ferrous nitrate hexahydrate (Fe(NO3)2·6H2O) and 40.8 kg of ammonium dihydrogen phosphate (NH4H2PO4), and add them to a 5 m container containing 360 L of deionized water. 3 In a stainless steel reactor, the stirrer was started (260 rpm), and high-purity nitrogen gas was introduced for protection (oxygen content <10 ppm). After heating to 75 ℃, 14.25 kg of battery-grade lithium carbonate (Li2CO3) was slowly added (the feeding time was controlled at about 100 min). Throughout the process, the pH in the reactor was kept stable at 4.8-5.2. After the feeding was completed, the reaction was continued at the temperature for 5 h. During this period, nitrogen gas was continuously introduced and a small amount of deionized water was added to maintain the liquid level. After the reaction was completed, the resulting light green slurry was filtered through a plate and frame filter press and washed three times with deionized water until the conductivity of the filtrate was <75 μS / cm. The filter cake was then re-pulped, and the solid content was adjusted to 40 wt% to obtain the slightly lithium-deficient intermediate precursor A slurry (at this time, the Li / Fe molar ratio was 0.92).

[0035] Step S2. Add 3.88 kg of battery-grade Li₂CO₃ with an average particle size of 250 nm (to make the total Li / Fe molar ratio accurately reach 1.045), 1.1 kg of food-grade ascorbic acid (accounting for 2.2 wt% of the dry basis of precursor A), and 9.2 kg of industrial-grade sucrose to the precursor A slurry obtained in step S1 in sequence. After high-speed shear stirring (speed 1200 rpm) for 35 min, transfer to a storage tank and continue to use a peristaltic pump to transport the slurry to a centrifugal spray drying tower. The spray drying process parameters are: inlet air temperature 270 ℃, outlet air temperature 108 ℃, atomizing disc speed 23000 rpm, feed rate 190 L / h, and negative pressure inside the tower -85 Pa. After drying, collect the grayish-white powder at the bottom of the tower and in the cyclone separator to obtain the pre-lithiation precursor B.

[0036] Step S3. The pre-lithiation precursor B obtained in step S2 is loaded into a stainless steel boat and pushed into an industrial roller kiln. Under a high-purity nitrogen protective atmosphere (oxygen content < 5 ppm), the temperature is raised from room temperature to 690 ℃ at a heating rate of 4 ℃ / min and held for 12 h. Then, it is naturally cooled to < 80 ℃ and removed from the kiln to obtain a dark gray lithium iron phosphate cathode material, labeled as LFP-2. The residual carbon content is determined to be 1.6 wt% by chemical titration and combustion method.

[0037] Example 3

[0038] This embodiment provides a method for preparing lithium iron phosphate cathode material based on a pre-lithiation precursor, specifically including the following:

[0039] Step S1. Take 72.3 kg of industrial-grade ferrous sulfate heptahydrate (FeSO4·7H2O) and 33.1 kg of 85 wt% phosphoric acid (H3PO4), and add them to a 5 m container containing 340 L of deionized water. 3 In a stainless steel reactor, the stirrer was started (270 rpm), and high-purity nitrogen gas was introduced for protection (oxygen content <10 ppm). After heating to 85 ℃, 20.86 kg of battery-grade lithium acetate dihydrate (LiAc·2H2O) was slowly added (the feeding time was controlled at about 85 min). Throughout the process, the pH in the reactor was kept stable at 4.6-5.1. After the feeding was completed, the reaction was continued at the temperature for 3.5 h. During this period, nitrogen gas was continuously introduced and a small amount of deionized water was added to maintain the liquid level. After the reaction was completed, the resulting light blue slurry was filtered through a plate and frame filter press and washed three times with deionized water until the conductivity of the filtrate was <70 μS / cm. The filter cake was then re-slurryed and the solid content was adjusted to 42 wt% to obtain the slightly lithium-deficient intermediate precursor A slurry (at this time, the Li / Fe molar ratio was 0.96).

[0040] Step S2. Add 2.15 kg of nano-Li₂O with an average particle size of 200 nm (to make the total Li / Fe molar ratio accurately reach 1.025), 0.75 kg of analytical grade tartaric acid (accounting for 1.5 wt% of the dry basis mass of precursor A), and 10.5 kg of polyvinylpyrrolidone (K30) to the precursor A slurry obtained in step S1 in sequence. After high-speed shear stirring (speed 1200 rpm) for 40 min, transfer to a storage tank and continue to use a peristaltic pump to transport the slurry to a centrifugal spray drying tower. The spray drying process parameters are: inlet air temperature 255℃, outlet air temperature 112℃, atomizing disc speed 24000 rpm, feed rate 185 L / h, and negative pressure inside the tower -90 Pa. After drying, collect the gray powder at the bottom of the tower and in the cyclone separator to obtain the pre-lithiated precursor B.

[0041] Step S3. The pre-lithiation precursor B obtained in step S2 is loaded into a stainless steel boat and pushed into an industrial roller kiln. Under a high-purity nitrogen protective atmosphere (oxygen content < 5 ppm), the temperature is increased from room temperature to 750 ℃ ​​at a heating rate of 6 ℃ / min and held for 8 h. Then, it is naturally cooled to < 80 ℃ and removed from the kiln to obtain a dark gray lithium iron phosphate cathode material, labeled as LFP-3. The residual carbon content is determined to be 2.4 wt% by chemical titration and combustion method.

[0042] Example 4

[0043] This embodiment provides a method for preparing lithium iron phosphate cathode material based on a pre-lithiation precursor, specifically including the following:

[0044] Step S1. Take 70.8 kg of industrial-grade ferrous chloride tetrahydrate (FeCl2·4H2O) and 41.5 kg of ammonium dihydrogen phosphate (NH4H2PO4), and add them to a 5 m³ container containing 370 L of deionized water. 3 In a stainless steel reactor, the stirrer was started (265 rpm), and high-purity nitrogen gas was introduced for protection (oxygen content <10 ppm). After heating to 70 ℃, 18.52 kg of battery-grade lithium hydroxide monohydrate (LiOH·H2O) was slowly added (the feeding time was controlled at about 95 min). Throughout the process, the pH in the reactor was kept stable at 5.0-5.3. After the feeding was completed, the reaction was continued at the temperature for 6 h. During this period, nitrogen gas was continuously introduced and a small amount of deionized water was added to maintain the liquid level. After the reaction was completed, the resulting light green slurry was filtered through a plate and frame filter press and washed three times with deionized water until the conductivity of the filtrate was <80 μS / cm. The filter cake was then re-pulped, and the solid content was adjusted to 44 wt% to obtain the slightly lithium-deficient intermediate precursor A slurry (at this time, the Li / Fe molar ratio was 0.98).

[0045] Step S2. Add 1.28 kg of battery-grade lithium acetate with an average particle size of 350 nm (to make the total Li / Fe molar ratio accurately reach 1.005), 0.36 kg of analytical grade EDTA (accounting for 0.4 wt% of the dry basis of precursor A), and 12.8 kg of citric acid + sucrose composite carbon source (mass ratio 1:1) to the precursor A slurry obtained in step S1 in sequence. After high-speed shear stirring (speed 1200 rpm) for 45 min, transfer to a storage tank and continue to use a peristaltic pump to transport the slurry to a centrifugal spray drying tower. The spray drying process parameters are: inlet air temperature 245 ℃, outlet air temperature 115 ℃, atomizing disc speed 25000 rpm, feed rate 195 L / h, and negative pressure inside the tower -75 Pa. After drying, collect the dark gray powder at the bottom of the tower and in the cyclone separator to obtain the pre-lithiated precursor B.

[0046] Step S3. The pre-lithiation precursor B obtained in step S2 is loaded into a stainless steel boat and pushed into an industrial roller kiln. Under a high-purity nitrogen protective atmosphere (oxygen content < 5 ppm), the temperature is increased from room temperature to 785 ℃ at a heating rate of 3 ℃ / min and held for 5 h. Then, it is naturally cooled to < 80 ℃ and removed from the kiln to obtain a dark gray lithium iron phosphate cathode material, labeled as LFP-4. The residual carbon content is determined to be 3.2 wt% by chemical titration and combustion method.

[0047] Comparison Example

[0048] Comparative Example 1

[0049] The only difference from Example 1 is that in step S1, the amount of LiOH·H2O added is increased to 16.40 kg, so that the Li / Fe molar ratio is directly 1.065 (i.e., the total lithium amount is the same as in Example 1, but all of it is added in step S1), and the second lithium source supplementation in S2 is no longer performed. The rest of the operation is exactly the same. The resulting material is labeled CE-1.

[0050] Comparative Example 2

[0051] The only difference from Example 1 is that in step S2, 4.62 kg of LiOH·H2O is no longer added; instead, 1.8 kg of citric acid and 8.5 kg of glucose are added for spray drying. The total Li / Fe molar ratio remains at 0.90 as in S1, and all other operations are exactly the same. The resulting material is labeled CE-2.

[0052] Comparative Example 3

[0053] The only difference from Example 1 is that in step S2, 1.8 kg of citric acid was not added; instead, 4.62 kg of LiOH·H2O and 8.5 kg of glucose were added for spray drying. The rest of the operation was exactly the same. The resulting material was labeled CE-3.

[0054] Comparative Example 4

[0055] The only difference from Example 1 is that the slurry obtained in step S2 is not spray-dried, but directly dried in a 120°C forced-air oven for 12 hours before being sieved through a 200-mesh sieve. The rest of the operation is exactly the same. The resulting material is labeled CE-4.

[0056] Application examples

[0057] Application Example 1

[0058] First, the positive electrode sheet was prepared: The lithium iron phosphate positive electrode materials prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were weighed and mixed with conductive carbon black (Super P) and PVDF binder at a mass ratio of 80:10:10. NMP solvent was added and stirred for 4 h to form a uniform slurry with a solid content of 40 wt%. The slurry was coated onto an aluminum foil current collector (wet film thickness 100 μm), dried in a vacuum oven at 80 ℃ for 8 h, and then cut into circular electrode sheets with a diameter of 14 mm. The sheets were then compacted to a loading of 2.5 mg / cm² using a roller mill. 2 The thickness is 50 μm. The negative electrode is a lithium metal foil (1 mm thick). The electrolyte is a mixed solvent of 1 mol / L LiPF6 dissolved in EC / DMC / EMC (volume ratio 1:1:1). The separator is a Celgard 2400 polypropylene microporous membrane. CR2032 coin cells were assembled in an argon glove box with a water and oxygen content of <0.1 ppm.

[0059] Performance testing

[0060] 1. Testing of the battery's first discharge specific capacity and first efficiency: The Blue Electric CT3001A testing system was used to perform constant current charge and discharge at a rate of 0.1C (17 mA / g) in the range of 2.0 to 4.2 V under constant temperature conditions of 25±0.5 ℃. The first discharge specific capacity (mAh / g) and the first coulombic efficiency (%) were recorded.

[0061] 2. Rate performance (5C discharge specific capacity) test: Under constant temperature conditions of 25±0.5 ℃, after activation for 3 weeks by constant current charge and discharge at a rate of 0.2C, it was charged to 4.2 V by constant current at 0.2C, then switched to constant voltage charging until the current <0.02C, and then directly discharged to 2.0 V by constant current at a rate of 5C (850 mA / g), and the discharge specific capacity (mAh / g) was recorded.

[0062] 3. High-temperature and high-rate cycle performance test: The battery was placed in a constant temperature chamber at 60 ± 0.5 ℃. After activation at 0.2C rate for 3 weeks, it was charged at 2C (340 mA / g) rate with constant current to 4.2 V, then switched to constant voltage until the current <0.05C, and then discharged at 2C rate with constant current to 2.0 V for continuous cycling. Capacity calibration was performed every 50 weeks at 0.5C rate. The capacity retention rate (%) was recorded after 1500 cycles.

[0063] Table 1

[0064]

[0065] In conjunction with Example 1 and Figure 1 As can be seen, the samples are uniform quasi-spherical particles with a concentrated overall particle size distribution. Most primary particles are approximately 150-300 nm in size, exhibiting slight aggregation but without forming obvious hard agglomerates. The particle surfaces are smooth with clear boundaries, and a uniform and dense carbon coating layer can be observed, enabling the particles to maintain their intact structure and form a continuous electron conduction network. This morphology is consistent with the characteristics of the process in Example 1, which involved spray drying to construct the outer lithium-rich structure and subsequent high-temperature sintering at 670 °C for 14 h. This indicates that the pre-lithiated precursor successfully maintained good particle morphology stability during sintering, which is beneficial for improving the electron / ion transport efficiency of the material and provides a structural basis for its high-rate performance and excellent cycle life.

[0066] Combining Example 1 and Comparative Example 1, and Figure 2It can be seen that the XRD curve (black) of Example 1 exhibits characteristic diffraction peaks consistent with olivine-type LiFePO4 across the entire range, with sharp peaks and no Li3PO4 impurity phase signal. In contrast, Comparative Example 1 (red) shows obvious Li3PO4 impurity peaks around 25° and 29°, indicating that its "one-step lithium addition" scheme leads to excess lithium reacting with phosphate during sintering to generate an inert impurity phase. In comparison, Example 1, through the "lithium-deficient precursor + second lithium source surface enrichment + spray shelling" method, enables lithium to fully migrate into the particle interior and occupy the site completely during the sintering stage, thereby obtaining a high-purity single-phase structure. This is not only reflected in the high consistency and absence of impurity peaks in the XRD pattern, but also directly corresponds to the significant advantages of Example 1 in terms of capacity, rate capability, and high-temperature cycling performance.

[0067] As can be seen from Examples 1 to 4 and Table 1, the lithium iron phosphate cathode material obtained by this invention exhibits stable and excellent electrochemical performance under different process parameters. Its initial discharge specific capacity remains between 160.0 and 163.8 mAh / g, its initial coulombic efficiency is stable between 94.1% and 94.8%, and its 5C high-rate discharge capacity is between 148.5 and 152.4 mAh / g. This indicates that the "micro-lithium-deficient precursor + second lithium source pre-lithiation + spray-dried surface lithium enrichment" technical route constructed by this invention has a stable and repeatable performance improvement effect. Especially in terms of high-temperature cycling, the capacity retention rate of the sample samples after 1500 cycles at 60 ℃ and 2C is between 95.7% and 97.3%, indicating that the formed high occupancy rate structure and uniform conductive network significantly improve the structural stability and long-term cycling reliability of the material.

[0068] As can be seen from Example 1, Comparative Example 1, and Table 1, the initial discharge specific capacity (163.8 mAh / g), 5C discharge capacity (152.4 mAh / g), and 1500-cycle high-temperature cycle retention rate (97.3%) of Example 1 are significantly better than those of Comparative Example 1 (148.2 mAh / g, 138.6 mAh / g, and 89.2%, respectively). The fundamental reason is that Comparative Example 1 uses a "one-step normal lithium addition" method, where all lithium sources are added in S1. This leads to lithium sublimation during high-temperature sintering, creating vacancies. Furthermore, excess lithium sources readily react with phosphate ions to form the inert Li3PO4 phase, resulting in incomplete lattice occupancy and decreased electron / ion transport performance. In contrast, Example 1 employs a "lithium-deficient S1 + second lithium source S2 + spray-rich lithium" strategy, which enriches the second lithium source on the particle surface and diffuses it inward during sintering, resulting in a high Li site occupancy rate and the absence of the Li3PO4 impurity phase, thereby significantly improving rate performance and high-temperature cycling stability.

[0069] Combining Example 1 and Comparative Example 2 with Table 1, it can be seen that Comparative Example 2, due to the absence of a second lithium source, has an initial capacity of only 138.9 mAh / g, a 5C capacity of 129.4 mAh / g, and a high-temperature cycling retention rate of only 83.7%, resulting in a significant decrease in overall performance. In contrast, Example 1 exhibits high retention rates of 163.8 mAh / g, 152.4 mAh / g, and 97.3%, respectively. This is because Comparative Example 2, under lithium-deficient conditions in S1, did not undergo lithium replenishment. Consequently, the sintered material inevitably contains a large number of lattice lithium vacancies, increasing the ion migration barrier of the one-dimensional channels in the olivine structure. This leads to severe high-rate polarization and gradual structural collapse during cycling. In contrast, Example 1, by replenishing lithium with a second lithium source and forming a lithium-rich surface structure, drives lithium migration during the sintering process, ultimately achieving a high Li lattice occupancy rate and thus obtaining significantly higher capacity and rate performance.

[0070] As can be seen from Example 1, Comparative Example 3, and Table 1, although Comparative Example 3 added a second lithium source, it lacked a chelating agent, resulting in significantly lower initial capacity (145.6 mAh / g), 5C capacity (135.2 mAh / g), and 1500-cycle retention (87.6%) compared to Example 1. This is because, without an organic chelating agent, the second lithium source cannot be effectively "pinned" to the precursor surface in the slurry, and a stable surface enrichment cannot be maintained during spray drying, leading to insufficient and disordered lithium replenishment. In contrast, the chelating agent in Example 1 forms a multi-toothed complex layer on the precursor surface, inhibiting Li... + The diffusion into the deep part of the particles allows the shelling effect generated during spray drying to firmly position the second lithium source on the outer layer of the particles, thereby forming a stable lithium concentration gradient, achieving sufficient lithium replenishment and high Li occupancy rate, resulting in a comprehensive performance that is significantly better than that of Comparative Example 3.

[0071] Combining Example 1 and Comparative Example 4 with Table 1, it can be seen that Comparative Example 4, which uses oven drying instead of spray drying, has significantly lower initial capacity (149.1 mAh / g), 5C capacity (139.8 mAh / g), and cycle retention (90.1%) than Example 1. The fundamental reason is that oven drying cannot produce a droplet shelling effect, nor does it drive capillary convection. This results in a uniform and random distribution of the second lithium source during the drying process, failing to form a lithium-rich structure on the particle surface. Without this gradient, the driving force for lithium migration inward during sintering is insufficient, failing to fully fill lattice vacancies. Furthermore, some unreacted lithium salts may remain on the particle surface, increasing interfacial impedance. In contrast, the spray drying process in Example 1 can rapidly form a shell structure and achieve lithium source enrichment on the surface, thereby achieving sufficient lattice lithium replenishment during sintering, resulting in a significant improvement in the final material's capacity, rate capability, and cycle performance.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing lithium iron phosphate cathode material based on a pre-lithiation precursor, characterized in that, Includes the following steps: Step S1. An intermediate precursor A is prepared by reacting an iron source, a phosphorus source, and a first lithium source in the liquid phase. The molar amount of the first lithium source is such that the molar ratio of Li to Fe in the intermediate precursor A is 0.90 to 0.

98. The iron source is a soluble divalent iron salt, and the phosphorus source is phosphoric acid or ammonium dihydrogen phosphate. These two types of raw materials can rapidly form a uniform, loose, and amorphous slightly lithium-deficient Fe-PO intermediate precursor A with the first lithium source during the liquid phase reaction. Step S2. A second lithium source, an organic chelating-dispersant, and a carbon source are added to the intermediate precursor A for pre-lithiation coating treatment to obtain a pre-lithiation precursor B. The second lithium source is selected from at least one of LiOH·H2O, Li2CO3, lithium acetate, and nano Li2O. The amount of the second lithium source added is such that the total lithium amount added to the first lithium source and the second lithium source in steps S1 and S2 is 1.005 to 1.065 relative to the stoichiometric Li / Fe molar ratio. The organic chelating-dispersant is selected from at least one of citric acid, ascorbic acid, tartaric acid, and EDTA. Step S3. The pre-lithiated precursor B is sintered at high temperature to obtain lithium iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that, Step S2 employs a spray drying process.

3. The preparation method according to claim 2, characterized in that, The amount of the organic chelating-dispersant is 0.4 to 3.5 wt% of the mass of intermediate precursor A.

4. The preparation method according to claim 3, characterized in that, The high-temperature sintering is carried out under a protective atmosphere, with a sintering temperature of 670–785 °C and a holding time of 5–14 h.

5. The preparation method according to claim 4, characterized in that, The carbon source is selected from at least one of glucose, sucrose, citric acid, and polyvinylpyrrolidone, and the residual carbon content is 1.0 to 3.2 wt%.

6. A lithium iron phosphate cathode material prepared by any one of the preparation methods described in claims 1-5.

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