Lithium iron phosphate material with gradient distribution of anti-site defects and preparation method and application thereof

Abstract

The application provides a lithium iron phosphate material with gradient distribution of anti-site defects and a preparation method and application thereof. The lithium iron phosphate material comprises primary particles, the primary particles are bulk regions, the bulk regions comprise surface layer regions, the surface layer regions are regions from the outer surface of the primary particles to the inner surface by 5-50 nm, the Li-Fe anti-site defect concentration of the surface layer regions is lower than the Li-Fe anti-site defect concentration of the bulk regions, the ratio of the Li-Fe anti-site defect concentration of the surface layer regions to the Li-Fe anti-site defect concentration of the bulk regions is 0.08-0.60:1, the lithium iron phosphate material is subjected to heat treatment to construct bulk controlled anti-sites, then is subjected to surface layer lithium supplement and rapid rearrangement annealing to form gradient distribution of anti-site defects. The lithium iron phosphate material has good industrial application prospect and is suitable for high-power and long-life lithium ion batteries while maintaining intrinsic performance, excellent rate performance and cycle stability.

Description

Technical Field

[0001] This invention belongs to the technical field of lithium-ion battery materials, and particularly relates to lithium iron phosphate materials with antisite defect gradient distribution, their preparation methods and applications. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is a cathode material with an olivine structure, where lithium ions mainly migrate along one-dimensional channels. The Li-Fe antisite defects formed by the exchange of Li and Fe sites can block the one-dimensional diffusion channels. Therefore, current technologies usually emphasize minimizing the overall antisite defect content of the material.

[0003] However, in actual LiFePO4 particle systems, the impact of antisite defects on electrochemical performance depends not only on their average content but also on their spatial distribution within the particles. If a high level of antisite defects exists at the inlet of the particle surface channels, even if the overall average antisite content of the material is not high, it may still lead to a decrease in rate performance. On the other hand, if the level of antisite defects in the bulk region of the particles is too low, the bulk phase transition front may exhibit stronger localized concentrated propagation characteristics during rapid lithium insertion / extraction and long-term cycling, thereby exacerbating local lithium concentration unevenness and lattice strain accumulation, increasing the risk of stress concentration within the particles and affecting structural stability, ultimately leading to accelerated cycle capacity decay.

[0004] Therefore, there is an urgent need in the existing technology for a lithium iron phosphate structure design that takes into account both rapid surface transport and bulk phase change buffering, so that the control of antisite defects can be transformed from simply pursuing absolute reduction to spatial optimization, thereby taking into account rate performance, cycle life and process feasibility. Summary of the Invention

[0005] The main objective of this invention is to provide a lithium iron phosphate material with a gradient distribution of antisite defects, its preparation method, and its application, aiming to solve the problem that it is difficult to balance rate performance, phase interface migration stability, and long cycle life when antisite defects are uniformly distributed in existing lithium iron phosphate materials.

[0006] To achieve the above objectives, the present invention provides a lithium iron phosphate material with a gradient distribution of antisite defects, wherein the lithium iron phosphate material comprises primary particles, and the primary particles are bulk regions; The bulk region includes a surface region, which is a region of 5nm to 50nm extending inward from the outer surface of the primary particle. The concentration of Li-Fe antisite defects in the surface region is lower than that in the bulk region, and the ratio of the concentration of Li-Fe antisite defects in the surface region to that in the bulk region is 0.08 to 0.60:1.

[0007] Furthermore, the concentration of Li-Fe antisite defects in the surface region is 0.2% to 1.5%; and the concentration of Li-Fe antisite defects in the bulk region is 1.5% to 5.0%.

[0008] Furthermore, the equivalent particle size D50 of the primary particles is 80 nm to 2 μm.

[0009] Furthermore, the outer surface of the primary particle is also provided with a carbon coating layer, the thickness of which is 1 nm to 10 nm.

[0010] This invention also provides a method for preparing lithium iron phosphate materials with antisite defect gradient distribution, comprising the following steps: S1. Heat-treat the precursor containing lithium source, iron source, phosphorus source and carbon source, and cool it to room temperature to obtain lithium iron phosphate particles; S2. The lithium iron phosphate particles are immersed in a lithium salt solution for lithium replenishment, dried, and then annealed to obtain lithium iron phosphate material.

[0011] Further, in step S1, the molar ratio of Li to Fe in the precursor is 0.970 to 0.995:1; The concentration of Li-Fe antisite defects in the lithium iron phosphate particles is 1.8% to 6.0%. The heat treatment is carried out under an inert atmosphere, with a heating rate of 3℃ / min to 6℃ / min, a heat treatment temperature of 620℃ to 760℃, and a holding time of 2h to 10h. The cooling rate to room temperature is 5°C / min to 20°C / min.

[0012] Further, in step S2, the molar ratio of lithium element in the lithium salt solution to lithium iron phosphate particles is 0.001-0.02:1; The liquid-to-solid ratio of the lithium salt solution to the lithium iron phosphate particles is 0.5 mL / g to 1.0 mL / g; The lithium salt solution is selected from at least one of lithium hydroxide, lithium acetate, and lithium dihydrogen phosphate. The temperature for lithium impregnation is 20℃~40℃, and the time is 20min~60min; The drying temperature is 60℃~140℃, and the time is 0.5h~12h; The annealing temperature is 450℃~700℃, and the annealing time is 1min~120min.

[0013] The lithium iron phosphate material provided by this invention can be used in lithium-ion batteries.

[0014] The present invention provides a lithium-ion battery positive electrode sheet, comprising a current collector and a positive electrode coating coated on the current collector, wherein the positive electrode coating comprises the aforementioned lithium iron phosphate material.

[0015] The present invention provides a lithium-ion battery comprising the above-mentioned lithium-ion battery positive electrode sheet.

[0016] The beneficial effects of this invention are as follows: This invention improves the charge-discharge performance of lithium iron phosphate materials under medium-to-high rate conditions by designing low-concentration Li-Fe antisite defects in the surface region of primary particles, thereby reducing the degree of blockage at the lithium-ion diffusion channel inlet and improving the lithium insertion / extraction kinetics on the particle surface.

[0017] This invention retains an appropriate concentration of controlled antisite defects in the bulk region of the primary particles, enabling smoother phase interface migration behavior within the particles during charging and discharging. This helps to mitigate local stress concentration caused by rapid phase transitions and phase interface migration during long cycles. Simultaneously, through the synergistic design between low surface antisite defects and controlled bulk antisite defects, both surface transport efficiency and internal structural stability are considered, thereby improving the rate performance, cycle life, and high-temperature stability of the material.

[0018] The present invention features a gradient distribution of antisite defects within the same particle, without relying on an additional inert thick coating layer. Therefore, it has good compatibility with existing electronic conductive network construction methods and high-density wafer fabrication processes, which is beneficial for maintaining the intrinsic properties of lithium iron phosphate materials while taking into account the adaptability of electrode processing technology.

[0019] This invention combines heat treatment and annealing with a surface lithium replenishment process. The process path is clear and the operation is controllable, making it easy to connect with the existing industrial production route of lithium iron phosphate and showing good prospects for industrial application. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0021] Figure 1 This is a STEM image of the surface region of the lithium iron phosphate material prepared in Example 1 of the present invention; Figure 2 This is a STEM image of the bulk region of the lithium iron phosphate material prepared in Example 1 of the present invention; Figure 3 The X-ray diffraction pattern of the lithium iron phosphate material prepared in Example 1 of this invention is shown. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0024] When numerical ranges are given in the embodiments, it should be understood that, unless otherwise stated in the present invention, both endpoints of each numerical range and any value between the two endpoints may be selected. Unless otherwise defined, all technical and scientific terms used in this invention, as well as the prior art known to those skilled in the art and the description of this invention, may be implemented using any prior art methods, devices, and materials similar to or equivalent to those described, used, or made of materials in the embodiments of this invention.

[0025] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of this application.

[0026] The technical terms and parameters used in this application are defined as follows: Surface region: refers to the area within 5nm to 50nm from the outer surface of the primary lithium iron phosphate particle inward; Bulk region: refers to the entire lithium iron phosphate primary particle itself.

[0027] Surface antisite concentration: refers to the concentration of Li-Fe antisite defects in the surface region, which is obtained by high-resolution STEM combined with cross-sectional statistical analysis; Bulk antisite concentration: refers to the concentration of Li-Fe antisite defects in the bulk region, which is obtained by characterizing the overall antisite concentration using X-ray diffraction combined with Rietveld refinement; Surface / Bulk Inverse Defect Ratio: This refers to the ratio of the Li-Fe inverse defect concentration in the surface region to the Li-Fe inverse defect concentration in the bulk region. Li-Fe antisite defect concentration: refers to the percentage of Fe sites occupied by Li sites relative to the total number of Li sites.

[0028] Both 1C and 5C specific capacities are based on the mass of the positive electrode active material; the 45℃ cycle capacity retention rate refers to the percentage of the battery's discharge capacity relative to its initial stable discharge capacity after a cycle test at 45℃.

[0029] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods, and all solvents used are of analytical grade.

[0030] The lithium iron phosphate material with a gradient distribution of antisite defects provided by this invention is composed of primary particles, each of which is a bulk region. The bulk region includes a surface region, which is a 5nm to 50nm area extending inward from the outer surface of the primary particle. The surface region has a low Li-Fe antisite defect concentration, while the bulk region retains a certain level of controlled antisite defects. Furthermore, the Li-Fe antisite defect concentration in the surface region is lower than that in the bulk region; the ratio of Li-Fe antisite defect concentration in the surface region to that in the bulk region is 0.08 to 0.60:1.

[0031] In some embodiments of the present invention, the surface region ranges from 10 to 35 nm, and further from 10 to 20 nm, 15 to 20 nm, 15 to 35 nm, or 20 to 35 nm.

[0032] In some embodiments of the present invention, the Li-Fe antisite defect concentration ratio between the surface region and the bulk region is 0.15 to 0.35:1.

[0033] In this invention, the concentration of Li-Fe antisite defects in the surface region is 0.2%–1.5%, preferably 0.3%–1.0%; the concentration of Li-Fe antisite defects in the bulk region is 1.5%–5.0%, preferably 2.0%–3.5%. This invention, through the synergistic design of low surface antisite concentration and controlled antisite defects in the bulk region, achieves a better balance between surface ion transport, bulk phase transition buffering, and cycle stability.

[0034] In this invention, the equivalent particle size D50 of the primary particles is 80 nm to 2 μm, preferably 0.4 μm to 2 μm.

[0035] In this invention, a carbon coating layer is also provided on the outer surface of the primary particle. The thickness of the carbon coating layer is 1nm to 10nm, preferably 2nm to 5nm, and the carbon coating layer does not change the distribution of Li-Fe antisite defects in the surface region and the bulk region of the primary particle.

[0036] The method for preparing lithium iron phosphate material with antisite defect gradient distribution provided by the present invention includes the following steps: S1. The precursor containing lithium, iron, phosphorus and carbon sources is heat-treated and cooled to room temperature to obtain crystallized lithium iron phosphate particles, while retaining 1.8~6.0% of Li-Fe antisite defects in the primary particle bulk region.

[0037] It should be noted that the present invention achieves controlled reverse defects in the bulk region by adjusting at least one of the following parameters during the heat treatment process: initial lithium-iron ratio, heat treatment temperature, holding time, heating rate, and cooling rate.

[0038] In this invention, the molar ratio of Li to Fe in the precursor containing lithium, iron, phosphorus, and carbon sources is 0.970–0.995:1, preferably 0.980–0.992:1. The iron source is at least one of iron phosphate or ferrous oxalate; the lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; and the carbon source is a carbon-containing precursor, but not limited thereto.

[0039] In this invention, the heat treatment is carried out under an inert atmosphere, with a heating rate of 3℃ / min to 6℃ / min, preferably 3℃ / min to 5℃ / min; the heat treatment temperature is 620℃ to 760℃, and the holding time is 2h to 10h; the cooling rate to room temperature is 5℃ / min to 20℃ / min, preferably 5℃ / min to 10℃ / min. This invention, by setting appropriate heat treatment temperature, holding rate, and cooling rate, suppresses excessive recovery of inversion defects during heat treatment, thus retaining a suitable amount of Li-Fe inversion defects in the bulk region.

[0040] The concentration of Li-Fe antisite defects in the lithium iron phosphate particles is 1.5% to 5.0%, preferably 2.0% to 3.5%.

[0041] In this invention, during the heat treatment stage, it is necessary to ensure the full crystallization of lithium iron phosphate particles while avoiding excessive annealing conditions that would completely homogenize and restore Li-Fe sites. By controlling the initial lithium-iron ratio to a slightly underlithiated state, and by controlling the heat treatment temperature, holding time, heating rate, and cooling rate, the particles retain a certain level of Li-Fe antisite defects in the bulk region after the olivine phase crystallization is completed. The slightly underlithiated conditions (the molar ratio of Li to Fe in the precursor is 0.970–0.995:1) are beneficial for increasing the Li-Fe site exchange tendency, while rapid cooling can suppress the full recovery of some disordered structures at high temperatures, thus helping to retain controlled antisite defects in the bulk region of the particles.

[0042] S2. The lithium iron phosphate particles are immersed in a lithium salt solution for lithium replenishment, so that the lithium salt is distributed on the particle surface and near-surface region; the immersed material is dried; and then annealed to reduce the concentration of Li-Fe antisite defects in the particle surface region, thus obtaining lithium iron phosphate material. In this immersion and lithium replenishment stage, the principle of preferential rearrangement on the surface is adopted, and the immersion-drying method is used to avoid excessive reduction of antisite defects in the bulk region of the lithium iron phosphate material, thereby weakening its effect of buffering phase transition strain.

[0043] In this invention, the molar ratio of lithium element to lithium iron phosphate particles in the lithium salt solution is 0.001 to 0.020:1, preferably 0.002 to 0.010:1.

[0044] In this invention, the liquid-to-solid ratio of the lithium salt solution to the lithium iron phosphate particles is 0.5 mL / g to 1.0 mL / g.

[0045] In this invention, the lithium salt solution is selected from at least one of lithium hydroxide, lithium acetate, and lithium dihydrogen phosphate. In this invention, the drying temperature is 60℃~140℃, and the drying time is 0.5h~12h. The drying method is at least one of evaporation, spray drying, vacuum drying, or oven drying.

[0046] In this invention, the mixing method in the lithium impregnation process is one of stirring mixing, ultrasonic dispersion mixing, or vacuum impregnation mixing.

[0047] In this invention, the temperature for lithium impregnation is 20°C to 40°C, more specifically 20°C to 30°C. The time for lithium impregnation is 20 min to 60 min, more specifically 20 min to 40 min, 20 min to 30 min, or 30 min to 40 min.

[0048] In this invention, the annealing temperature is 450℃~700℃, preferably 520℃~620℃; the annealing time is 1min~120min, preferably 10min~40min.

[0049] In some embodiments of the present invention, in order to further improve the electronic conductivity of lithium iron phosphate material, a carbon coating layer can be provided on the surface of the lithium iron phosphate material. That is, during the heat treatment stage, at least one carbonizable organic material selected from sucrose, glucose, asphalt or phenolic resin is introduced simultaneously, and a carbon coating layer is formed after heat treatment. At the same time, the annealing temperature and time should be controlled within a range that does not cause significant homogenization.

[0050] The lithium iron phosphate material with an anti-site defect gradient distribution of the present invention is constructed through a two-step method. Specifically, in the heat treatment stage of step S1, by controlling the lithium-iron ratio, heat treatment temperature, holding time, and cooling process, a certain level of controlled Li-Fe anti-site defects is formed and retained in the bulk region of the particles. Subsequently, the lithium iron phosphate particles are surface-added with lithium using an impregnation method, that is, lithium salt solution is mixed with lithium iron phosphate particles, so that lithium salt is distributed on the particle surface and near-surface region. After drying, annealing treatment is performed, so that lithium addition and defect rearrangement preferentially occur in the near-surface region of the primary particles, thereby reducing the anti-site defect concentration in the surface region, while the bulk anti-site defect is retained at a higher level, ultimately forming a structure in which the anti-site defect distribution gradually increases from the surface region of the primary particles to the core of the primary particles.

[0051] It should be noted that the lithium iron phosphate material of the present invention forms a Li-Fe antisite defect variation with a low surface region and a high bulk region within the same lithium iron phosphate particle, without the need to form an independent coating layer or obvious secondary phase interface. Therefore, the lithium iron phosphate material structure formed by the present invention is compatible with carbon coating, particle size distribution and high-pressure solid electrode process, and does not introduce obvious interface impedance.

[0052] The lithium iron phosphate material provided by this invention can be used in lithium-ion batteries.

[0053] The present invention provides a lithium-ion battery positive electrode sheet, comprising a current collector and a positive electrode coating coated on the current collector, wherein the positive electrode coating comprises lithium iron phosphate material provided by the present invention.

[0054] The present invention provides a lithium-ion battery comprising the above-mentioned lithium-ion battery positive electrode sheet.

[0055] The following describes the lithium iron phosphate material with antisite defect gradient distribution of the present invention, its preparation and application, with reference to specific embodiments.

[0056] Example 1 A method for preparing lithium iron phosphate materials with antisite defect gradient distribution includes: Lithium carbonate, iron phosphate, and glucose were added to deionized water at a Li:Fe:P:C molar ratio of 0.988:1:1:0.08 to prepare a slurry with a solid content of 35 wt%. After ball milling for 2 hours, the slurry was spray-dried (inlet temperature 180℃, outlet temperature 90℃) to obtain the precursor powder. The precursor powder had an equivalent particle size D50 of 0.9 μm and a specific surface area of ​​13.4 m². 2 / g, moisture content ≤0.5wt%.

[0057] The precursor powder was heated to 700℃ at a rate of 3℃ / min under a nitrogen atmosphere and held for 6h, and then cooled to room temperature at a rate of 5℃ / min to obtain a fully crystallized lithium iron phosphate precursor (i.e., lithium iron phosphate particles) with a bulk region Li-Fe antisite defect concentration of 2.6%.

[0058] Using lithium acetate as the lithium source, the amount of lithium added was controlled at a molar ratio of lithium element provided by lithium acetate to lithium iron phosphate precursor of 0.006:1, and lithium acetate solution was added at a liquid-to-solid ratio of 0.8 mL / g. The mixture was stirred at 300 r / min for 30 min at room temperature to distribute lithium salt on the surface and near-surface region of the particles. Then, the mixture was evaporated at 80 °C for 1 h until no visible free liquid was found, and then dried at 110 °C for 2 h. Finally, the mixture was annealed at 560 °C for 20 min at a rate of 5 °C / min to induce preferential lithium addition and rearrangement on the surface of the particles. The mixture was then cooled to room temperature in the furnace to obtain lithium iron phosphate material.

[0059] Testing revealed that the surface layer thickness of the primary particles in the lithium iron phosphate material is 20 nm. For example... Figure 1 The image shown is a STEM image of the surface region. The image indicates that the lattice atoms are arranged in an ordered manner, with iron atoms occupying only a small number of lithium sites, demonstrating a low antisite concentration. Statistical analysis shows that the surface antisite concentration is 0.6%. Figure 2 As shown, the STEM image of the bulk region reveals an increase in the number of lithium sites occupied by iron atoms compared to the surface region. This indicates a higher antisite concentration in the bulk region compared to the surface region, forming a concentration gradient. After XRD refinement and fitting, the antisite concentration in the bulk region is 2.4%, as shown in Table 1 below. Figure 3 The image shows the X-ray diffraction pattern (Cu) of the lithium iron phosphate material obtained in Example 1. It matches the standard card PDF#01-081-1173. The positions and relative intensities of the diffraction peaks are consistent, indicating that the obtained product is a single lithium iron phosphate crystal phase (olivine structure), and the diffraction peaks are sharp and have high crystallinity.

[0060] Table 1. Atomic occupancy analysis of refined lithium iron phosphate material in Example 1 Example 2 A method for preparing lithium iron phosphate materials with antisite defect gradient distribution includes: A solution of ferric oxide and phosphoric acid was prepared at a Fe:P molar ratio of 1:1. Lithium carbonate was added, and the Li:Fe molar ratio was controlled at 0.985:1. After co-precipitation, washing, and drying, the solution was mixed with a sucrose solution, and the Fe:C molar ratio was controlled at 1:0.09. After drying, the precursor powder was obtained. The equivalent particle size D50 of this precursor powder was 0.6 μm, and the specific surface area was 14.5 m². 2 / g.

[0061] The precursor powder was heated to 680°C at a rate of 4°C / min under an argon atmosphere and held for 4 hours. After the holding period, it was cooled to room temperature at a rate of 8°C / min to form a crystallized lithium iron phosphate precursor with a Li-Fe antisite defect concentration of 3.2% in the bulk region.

[0062] Using LiOH as the lithium source, the amount of lithium added was controlled according to the molar ratio of lithium element provided by LiOH to lithium iron phosphate precursor of 0.008:1, and LiOH solution was added at a liquid-to-solid ratio of 0.9 mL / g. The mixture was stirred at 300 r / min for 20 min at room temperature and then allowed to stand for 20 min to allow the lithium salt to be evenly distributed in the near-surface region of the particles. Subsequently, the mixture was evaporated at 75 °C for 1 h until no visible free liquid was found, and then dried at 100 °C for 1 h. Finally, the mixture was annealed at 540 °C for 15 min at a rate of 10 °C / min and then cooled to room temperature in the furnace to obtain lithium iron phosphate material.

[0063] Testing revealed that the surface region of the lithium iron phosphate material has a thickness of 15 nm, a surface antisite concentration of 0.8%, and a bulk antisite concentration of approximately 3.0%, making it suitable for high-rate applications (such as 2C and above discharge rates).

[0064] Example 3 A method for preparing lithium iron phosphate materials with antisite defect gradient distribution includes: Lithium carbonate, iron phosphate, and glucose were mixed at a Li:Fe:P:C molar ratio of 0.990:1:1:0.08, ball-milled for 2 hours, dried, and sieved to obtain a precursor powder. The precursor powder had an equivalent particle size D50 of 1.3 μm and a specific surface area of ​​12.6 m². 2 / g.

[0065] The precursor powder was heated to 720°C at a rate of 5°C / min under a nitrogen atmosphere and held at that temperature for 8 hours. A segmented cooling method was adopted, that is, the temperature was reduced from 720°C to 550°C at a cooling rate of 6°C / min, and then reduced from 550°C to room temperature at a cooling rate of 10°C / min. This allowed the particles to be fully crystallized while the antisite defect concentration in the bulk region was 2.1%, thus forming a lithium iron phosphate precursor.

[0066] Using lithium dihydrogen phosphate as the lithium source, the amount of lithium added was controlled at a molar ratio of lithium element provided by lithium dihydrogen phosphate to lithium iron phosphate precursor of 0.005:1, and lithium dihydrogen phosphate solution was added at a liquid-to-solid ratio of 0.7 mL / g. The mixture was stirred at 300 r / min for 40 min at room temperature to allow the lithium salt to fully wet the surface and near-surface area of ​​the particles. Subsequently, the mixture was evaporated at 85 °C for 1 h until no visible free liquid was found, and then dried at 120 °C for 3 h. The mixture was then annealed at 600 °C for 30 min at a rate of 5 °C / min, and then cooled to room temperature in the furnace to obtain lithium iron phosphate material.

[0067] Testing revealed that the primary particles in the lithium iron phosphate material have a surface layer thickness of 35 nm, a surface antisite concentration of 0.4%, and a bulk antisite concentration of 1.7%, making them more suitable for balancing cycle life and rate performance.

[0068] Comparative Example 1 Unlike Example 1, in the impregnation and lithium replenishment process of Comparative Example 1, the molar ratio of lithium element in lithium acetate to lithium iron phosphate precursor is 0.012:1; the annealing temperature is 620°C and the annealing time is 180 min; other steps and parameters are the same as in Example 1.

[0069] Testing revealed that the surface antisite concentration of the primary particles in the lithium iron phosphate material was 0.7%, while the bulk antisite concentration was 0.9%, indicating a uniform and low antisite defect concentration in both the surface and bulk regions. This is because the lithium addition amount in this comparative example was high, and the annealing time was significantly extended. Lithium elements were no longer confined to the near-surface region but diffused further into the particle interior, promoting the recovery of overall Li-Fe antisite defects. This significantly reduced the difference in antisite defects between the surface and bulk regions, ultimately resulting in a near-uniform, low-antisite distribution of lithium iron phosphate material.

[0070] Comparative Example 2 Unlike Example 2, the Li / Fe molar ratio in the precursor of Comparative Example 2 is 0.978, and no impregnation, lithium replenishment, drying, or annealing treatment is performed, thus obtaining a crystallized lithium iron phosphate precursor. Other steps and parameters are the same as in Example 2.

[0071] Testing revealed that the surface antisite concentration of the primary particles in the lithium iron phosphate material was 2.8%, while the bulk antisite concentration was 3.0%, indicating a uniform and high antisite defect concentration in both the surface and bulk regions. The initial system (precursor) of Comparative Example 2 was in a slightly under-lithiated state, and no further surface lithium replenishment or short-term rearrangement was performed to repair defects in the near-surface region after heat treatment. Therefore, both the surface and bulk regions of the particles retained a high level of Li-Fe antisite defects, ultimately resulting in lithium iron phosphate particles with a relatively uniform high antisite distribution.

[0072] Comparative Example 3 The lithium iron phosphate material prepared in Comparative Example 1 was used as the matrix material. This matrix material was mixed with a water / ethanol mixture at a liquid-to-solid ratio of 0.8 mL / g, where the volume ratio of water to ethanol in the mixture was 1:1. Then, 1 wt% of iron phosphate precursor surface treatment solution (based on the matrix material) was added to create a relatively low lithium activity environment on the particle surface. The mixture was stirred at 300 r / min for 20 min at room temperature, then evaporated to dryness at 80 °C for 1 h until no visible free liquid remained. The mixture was then annealed at 580 °C for 25 min without introducing any lithium replenishment source. The furnace was then cooled to room temperature to obtain the lithium iron phosphate material. The iron phosphate precursor surface treatment solution was prepared by dissolving ferric chloride and phosphoric acid in deionized water at a molar ratio of Fe:P = 1:1 to prepare a 0.05 mol / L (based on FePO4) lithium salt-free iron phosphate precursor solution, which was used as the surface treatment solution.

[0073] Testing revealed that the surface antisite concentration of the primary lithium iron phosphate material was 2.4%, while the bulk antisite concentration was 1.1%. Comparative Example 3 involved post-processing the lithium iron phosphate material prepared in Comparative Example 1. During this post-processing, the surface of the lithium iron phosphate particles did not receive lithium compensation, and localized lithium loss and Li-Fe re-dislocation were more likely to occur on the particle surface during annealing, resulting in an increased antisite defect concentration in the surface region. In contrast, the bulk region, maintaining the aforementioned low-antisite matrix structure, exhibited a relatively low antisite defect level, thus forming a reverse distribution of high antisite on the surface and low antisite in the bulk.

[0074] The structural parameters of the lithium iron phosphate materials prepared in Examples 1-3 and Comparative Examples 1-3 were tested, and the test results are shown in Table 2 below.

[0075] Table 2 Structural parameters of lithium iron phosphate materials As shown in Table 2, Examples 1-3 all exhibited a significantly lower antisite defect concentration on the surface compared to the bulk region, and the surface / bulk antisite concentration ratio was also low. This indicates that by constructing controlled antisite defects in the bulk region through heat treatment, combined with impregnation for lithium replenishment, evaporation, and short-time annealing, lithium replenishment and defect rearrangement can be preferentially achieved in the near-surface region of the particles, resulting in an antisite defect distribution that gradually increases from the surface to the bulk. Among these, Example 1 achieved a relatively balanced match between the antisite concentration on the surface and the antisite concentration in the bulk region; Example 2, due to its smaller primary particles and lower surface antisite concentration, was more conducive to demonstrating the advantages of high-rate transport; Example 3 had the lowest surface antisite concentration, thus being more beneficial for improving interface stability during cycling.

[0076] In contrast, although Comparative Example 1 had a lower overall level of antisite defects, the difference in antisite defects between the surface region and the bulk region was small, and no obvious gradient distribution was formed. Comparative Example 2 retained a high level of antisite defects in both the surface region and the bulk region, indicating that under the condition of slight lithium deficiency and no second surface lithium replenishment and rearrangement repair, the material as a whole tended to be in a uniform high antisite state. Comparative Example 3 showed that the antisite concentration in the surface region was higher than that in the bulk region, forming an inverse gradient distribution opposite to that of the present invention.

[0077] In summary, the process employed in this invention does not simply reduce the average antisite defect content of the material, but rather achieves the synergistic construction of low antisite in the surface region and controlled antisite in the bulk region by regulating its spatial distribution within the particles, thereby providing a structural basis for subsequent performance improvement.

[0078] Application of the lithium iron phosphate material provided by this invention in lithium-ion batteries: The fabrication of lithium-ion batteries: Positive electrode sheet: The lithium iron phosphate material prepared in Examples 1-3 and Comparative Examples 1-3 of this invention is used as the active material, and mixed with Super P conductive carbon black and binder PVDF at a mass ratio of 80:10:10. NMP solvent is added, and the mixture is stirred evenly to form a positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil current collector, vacuum dried at 80°C for 12 hours, rolled, and punched into positive electrode sheets with a diameter of 12 mm. The loading of the active material is controlled to be 2-3 mg / cm³. 2 ; Negative electrode: Lithium metal sheet; Diaphragm: Polyolefin microporous diaphragm; Electrolyte: 1.0 mol / L LiPF6 / EC:DMC:EMC (volume ratio 1:1:1).

[0079] The positive electrode, negative electrode, separator, and electrolyte were assembled into a CR2032 coin cell (i.e., lithium-ion battery) in an argon-filled glove box. After being left to stand and wet for at least 6 hours, electrochemical tests were performed. The test results are shown in Table 3 below.

[0080] 1C capacity and 5C capacity: measured at 25°C; 45℃ Cyclic Capacity Retention Rate: Under 45℃ conditions, a charge-discharge cycle test was conducted at a 1C rate, and the capacity retention rate was recorded after 1000 cycles. DC internal resistance: Measured using the pulse method at 50% SOC.

[0081] Table 3 Electrochemical performance test results of lithium iron phosphate materials As can be seen from the results in Table 3, Examples 1-3 are generally superior to Comparative Examples 1-3 in terms of 1C discharge specific capacity, 5C discharge specific capacity, 45℃ cycle capacity retention rate, and DC internal resistance. This indicates that the surface low inversion and block region controlled inversion distribution constructed by the present invention are beneficial to taking into account rate performance, cycle stability, and interface transmission performance.

[0082] Specifically, Example 1 exhibits a good overall balance between 1C capacity, 5C capacity, and high-temperature cycle retention, indicating that the simultaneous presence of low antisite sites in the surface region and moderately controlled antisite sites in the bulk region is beneficial for improving the entry and exit processes of lithium ions on the particle surface, and also helps to mitigate the accumulation of stress and structural mismatch during the phase transition propagation process inside the particle. Example 2, due to its smaller primary particle size, demonstrates superior high-rate discharge performance, indicating that a shorter diffusion path and lower surface antisite concentration are beneficial for improving the kinetic response under rapid charge and discharge conditions. Although Example 3's 5C capacity is slightly lower than some examples, it has the highest capacity retention after 1000 cycles at 45°C, indicating that a lower surface antisite concentration is more beneficial for improving structural stability during long-term cycling.

[0083] Although Comparative Example 1 has a lower overall antisite defect level, its high-rate performance and long-cycle stability are still lower than those of the embodiments of the present invention due to the lack of a clear surface / bulk region distribution. This indicates that simply pursuing a low overall antisite defect level does not yield the best overall performance. Comparative Example 2 has high antisite defects in both the surface and bulk regions, resulting in more significant blockage of the lithium-ion diffusion channels. Therefore, its capacity, rate capability, and internal resistance are all poor. Comparative Example 3 has an inverse distribution of high antisite defects in the surface region and low antisite defects in the bulk region. Its 5C capacity, cycle retention, and DC internal resistance are the worst among all samples, indicating that increased antisite defects in the surface region significantly weaken the openness of the lithium-ion channel inlet and have a significant adverse effect on the overall kinetic process.

[0084] In summary, the impact of antisite defects on the performance of lithium iron phosphate materials depends not only on their average content but also on their spatial distribution within the particles. Compared to uniform low antisite, uniform high antisite, antisite gradient, and weak differential distribution, the surface low antisite and bulk region controlled antisite gradient structure constructed in this invention can more effectively balance surface ion transport efficiency, internal phase transition buffering capacity, and long-term cycling stability, thus exhibiting superior overall electrochemical performance.

[0085] The above technical solutions of the present invention are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent transformations made using the contents of the present invention under the technical concept of the present invention, or direct / indirect applications in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A lithium iron phosphate material with a gradient distribution of antisite defects, characterized in that, The lithium iron phosphate material comprises primary particles, wherein the primary particles are bulk regions. The bulk region includes a surface region, which is a region of 5nm to 50nm extending inward from the outer surface of the primary particle. The concentration of Li-Fe antisite defects in the surface region is lower than that in the bulk region, and the ratio of the concentration of Li-Fe antisite defects in the surface region to that in the bulk region is 0.08 to 0.60:

1.

2. The lithium iron phosphate material with an antisite defect gradient distribution according to claim 1, characterized in that, The concentration of Li-Fe antisite defects in the surface region is 0.2% to 1.5%; the concentration of Li-Fe antisite defects in the bulk region is 1.5% to 5.0%.

3. The lithium iron phosphate material with an antisite defect gradient distribution according to claim 1, characterized in that, The equivalent particle size D50 of the primary particles is 80 nm to 2 μm.

4. The lithium iron phosphate material with an antisite defect gradient distribution according to any one of claims 1-3, characterized in that, The outer surface of the primary particle is also provided with a carbon coating layer, the thickness of which is 1 nm to 10 nm.

5. The method for preparing lithium iron phosphate material with antisite defect gradient distribution as described in any one of claims 1-4, characterized in that, Includes the following steps: S1. Heat-treat the precursor containing lithium source, iron source, phosphorus source and carbon source, and cool it to room temperature to obtain lithium iron phosphate particles; S2. The lithium iron phosphate particles are immersed in a lithium salt solution for lithium replenishment, dried, and then annealed to obtain lithium iron phosphate material.

6. The method for preparing lithium iron phosphate material with antisite defect gradient distribution according to claim 5, characterized in that, In step S1, the molar ratio of Li to Fe in the precursor is 0.970 to 0.995:1; The concentration of Li-Fe antisite defects in the lithium iron phosphate particles is 1.8% to 6.0%. The heat treatment is carried out under an inert atmosphere, with a heating rate of 3℃ / min to 6℃ / min, a heat treatment temperature of 620℃ to 760℃, and a holding time of 2h to 10h. The cooling rate to room temperature is 5°C / min to 20°C / min.

7. The method for preparing lithium iron phosphate material with antisite defect gradient distribution according to claim 5, characterized in that, In step S2, the molar ratio of lithium element in the lithium salt solution to lithium iron phosphate particles is 0.001-0.02:1; The liquid-to-solid ratio of the lithium salt solution to the lithium iron phosphate particles is 0.5 mL / g to 1.0 mL / g; The lithium salt solution is selected from at least one of lithium hydroxide, lithium acetate, and lithium dihydrogen phosphate. The temperature for lithium impregnation is 20℃~40℃, and the time is 20min~60min; The drying temperature is 60℃~140℃, and the time is 0.5h~12h; The annealing temperature is 450℃~700℃, and the annealing time is 1min~120min.

8. The application of the lithium iron phosphate material according to any one of claims 1-4 or the lithium iron phosphate material prepared by the preparation method according to any one of claims 5-7 in lithium-ion batteries.

9. A positive electrode sheet for a lithium-ion battery, characterized in that, It includes a current collector and a positive electrode coating coated on the current collector, wherein the positive electrode coating comprises the lithium iron phosphate material according to any one of claims 1-4 or the lithium iron phosphate material prepared by the preparation method according to any one of claims 5-7.

10. A lithium-ion battery, characterized in that, It includes the lithium-ion battery positive electrode sheet as described in claim 9.

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