Preparation method of lithium iron phosphate positive electrode and battery

Abstract

The application provides a preparation method of a lithium iron phosphate positive electrode and a battery, and comprises a lithium iron phosphate positive electrode material, a preparation method of the lithium iron phosphate positive electrode material, and the preparation method comprises the following steps: grading a lithium iron phosphate material semi-finished product into a coarse powder part and a fine powder part; performing surface activation pretreatment on the fine powder part; preparing a composite functional agent comprising a conductive agent and a bonding component; mixing the fine powder part subjected to the surface activation pretreatment and the composite functional agent to form a functionalized fine powder unit; and mixing the functionalized fine powder unit and the coarse powder part to obtain the lithium iron phosphate positive electrode material. Since the positive electrode material itself has an optimized conductive network and bonding structure, the amount of additional conductive agent and bonding agent can be reduced when preparing a positive electrode sheet, so that the proportion of active substances is increased under the premise of maintaining the performance of the electrode, and the energy density of the battery is further improved.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery cathode materials, specifically to a method for preparing a lithium iron phosphate cathode and a battery thereof. Background Technology

[0002] Lithium iron phosphate (LFP) has become one of the most widely used cathode materials in power batteries and energy storage due to its advantages such as low cost, high safety, and long cycle life. In the preparation of LFP cathode materials, the particle size distribution and packing method of the powder have a significant impact on the energy density, rate performance, and cycle stability of the electrode. Rationally utilizing LFP particles of different sizes for graded packing is an important technical approach to improve the compaction density and electrochemical performance of the cathode material.

[0003] Currently, the preparation of lithium iron phosphate cathode materials typically employs single-size powder or a simple coarse-fine powder mixing process. However, in simple mixing processes, the fine powder particles are unevenly distributed in the gaps between the coarse powder particles, resulting in poor continuity of the conductive network and insufficient bonding strength. This leads to problems such as active material shedding and increased contact resistance during charge and discharge, thus affecting the battery's rate performance and cycle life. Therefore, improving the interfacial bonding and conductive connectivity between coarse and fine powders is an important direction for improvement in the current lithium iron phosphate cathode material preparation process.

[0004] In coarse-fine powder gradation systems, fine powder particles have high surface energy and are prone to agglomeration, resulting in weak interfacial bonding with coarse powder. Traditional mixing processes struggle to construct a uniform conductive and binding functional layer on the surface of fine powder particles. This prevents fine powder particles from effectively performing conductive bridging and space-filling functions when filling gaps in coarse powder, hindering further improvements in the compaction density and electrochemical performance of the cathode material. Achieving uniform pre-coating of conductive agents and binders on the surface of fine powder particles, and enabling efficient macroscopic assembly of the functionalized fine and coarse powders, is a key technical challenge in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, this application provides a method for preparing a lithium iron phosphate cathode and a battery.

[0006] The first aspect of this application provides a method for preparing a lithium iron phosphate cathode material, comprising the following steps: classifying a semi-finished lithium iron phosphate material into a coarse powder portion and a fine powder portion; performing a surface activation pretreatment on the fine powder portion; preparing a composite functional agent containing a conductive agent and a binder component; mixing the surface-activated fine powder portion with the composite functional agent to form a functionalized fine powder unit; and mixing the functionalized fine powder unit with the coarse powder portion to obtain the lithium iron phosphate cathode material.

[0007] This method first achieves the separate processing and functionalization of particles of different sizes by classifying the semi-finished lithium iron phosphate material into coarse and fine powders. Surface activation pretreatment of the fine powder improves the surface state of the particles, reduces their agglomeration tendency, and enhances the wettability and bonding force between the fine powder particles and subsequent composite functional agents. The composite functional agent contains both conductive agents and binders; mixing it with the activated fine powder forms a uniform conductive-binder composite coating layer on the surface of the fine powder particles, making each fine powder particle a functionalized fine powder unit with both conductive bridging and bonding functions. Macroscopic assembly and mixing of the functionalized fine powder units with the coarse powder allows the functionalized fine powder units to effectively fill the gaps between the coarse powder particles. Simultaneously, the conductive layer on their surface establishes continuous electron transport channels between the coarse powder particles, and the binder layer on their surface enhances the interfacial bonding force between the fine and coarse powders. This "functionalization first, assembly later" process strategy, compared to the traditional one-time mixing process, can significantly improve the compaction density, conductivity uniformity and structural stability of the cathode material, thereby improving the energy density, rate performance and cycle life of lithium-ion batteries.

[0008] Furthermore, the surface activation pretreatment involves contacting the fine powder portion with an organic acid solution. The organic acid solution can undergo a mild chemical reaction with the alkaline sites or inert oxide layer on the surface of the lithium iron phosphate fine powder particles, removing the inert layer and impurities from the particle surface and exposing a fresh active surface, thereby significantly improving the chemical affinity and physical wettability between the fine powder particles and the composite functional agent. Specifically, the organic acid can be selected from at least one of citric acid, tartaric acid, malic acid, oxalic acid, and acetic acid, and the concentration of the organic acid solution can be selected from any value among 0.5wt%, 1wt%, 2wt%, 3wt%, and 5wt%.

[0009] Furthermore, the organic acid is at least one of citric acid, tartaric acid, or malic acid. Citric acid, tartaric acid, and malic acid are all polyhydroxy acids, containing multiple carboxyl and hydroxyl groups in their molecules. These can form multi-point chelate coordination with the surface of lithium iron phosphate particles, effectively removing the surface inert layer and introducing hydrophilic functional groups onto the particle surface, further improving the interfacial compatibility between the fine powder particles and the water-based composite functional agent. Specifically, the organic acid is preferably citric acid.

[0010] Furthermore, the composite functional agent comprises sodium carboxymethyl cellulose and conductive carbon black, with a mass ratio of sodium carboxymethyl cellulose to conductive carbon black of 1:2. Sodium carboxymethyl cellulose, as a water-soluble binder, possesses excellent film-forming properties and adhesion, enabling the formation of a uniform adhesive film on the surface of fine powder particles. Conductive carbon black, with its high specific surface area and excellent electronic conductivity, can construct a conductive network within the adhesive film. When used in a 1:2 mass ratio, the content and uniformity of the conductive agent can be maximized while ensuring adhesive strength, thereby obtaining a composite coating layer with balanced conductivity and adhesion. Specifically, the mass ratio of sodium carboxymethyl cellulose to conductive carbon black can be selected from any value among 1:1, 1:1.5, 1:2, 1:2.5, and 1:3, and the conductive carbon black can be selected from any one of Super P, acetylene black, and Ketjen black.

[0011] The second aspect of this application provides a lithium iron phosphate cathode material prepared by the above-described preparation method. This cathode material has a unique gradient stacking structure, where coarse particles serve as a framework providing the bulk filling and compaction density, and functionalized fine powder units fill the gaps between the coarse particles. Through a composite coating layer on the surface, these units form a tight conductive-bonding interface with the coarse particles, resulting in a cathode material with high compaction density, a uniform conductive network, and excellent structural integrity.

[0012] Furthermore, the cathode material has a gradient structure, comprising coarse particles as a framework and functionalized fine powder units filling the gaps between the coarse particles. This gradient structure can effectively utilize the spatial complementary effect of particles of different sizes to achieve higher packing density and more uniform stress distribution. Specifically, the median particle size D50 of the coarse particles can be selected from any value among 8μm, 10μm, 12μm, and 15μm, and the median particle size D50 of the fine powder particles can be selected from any value among 1μm, 2μm, 3μm, and 4μm.

[0013] Furthermore, the functionalized fine powder unit includes lithium iron phosphate fine powder particles and a composite coating layer covering their surface. The composite coating layer transforms the fine powder particles from simple fillers into active units with conductive and binding functions, enabling them to actively participate in electron transport and structural maintenance within the electrode.

[0014] Furthermore, the composite coating layer includes a binder and a conductive agent. The coexistence of the binder and the conductive agent enables the composite coating layer to simultaneously possess the dual functions of interfacial bonding and electronic conductivity, avoiding the problem of uneven distribution of binders and conductive agents during macroscopic mixing in traditional processes.

[0015] Furthermore, the binder is sodium carboxymethyl cellulose, and the conductive agent is Super P conductive carbon black. Sodium carboxymethyl cellulose has excellent water solubility and film-forming uniformity, while Super P conductive carbon black has high specific surface area and good electronic conductivity. The combined use of these two materials can form a thin and uniform functional layer on the surface of fine powder particles. Specifically, the binder can also be selected from polyvinylidene fluoride and polyacrylic acid, and the conductive agent can also be selected from acetylene black, Ketjen black, carbon nanotubes, and graphene.

[0016] A third aspect of this application provides a method for preparing a lithium iron phosphate cathode and a battery, using the aforementioned lithium iron phosphate cathode material. Since this cathode material itself possesses an optimized conductive network and bonding structure, the amount of additional conductive agent and binder can be reduced during the preparation of the cathode sheet, thereby increasing the proportion of active material while maintaining electrode performance, and further improving the energy density of the battery.

[0017] The fourth aspect of this application provides the application of the aforementioned lithium iron phosphate cathode material in the preparation of lithium-ion batteries. Lithium-ion batteries prepared using the aforementioned cathode material have higher energy density.

[0018] The present invention has the following beneficial effects:

[0019] This solution achieves precise functionalization of particles of different sizes by classifying lithium iron phosphate semi-finished products into coarse and fine powders and processing them separately. Compared with the traditional one-time mixing process, it can significantly improve the uniformity of the coating of conductive agents and binders on the surface of fine powder particles.

[0020] This method effectively removes the inert layer on the particle surface by pre-treating the fine powder with organic acid surface activation, improves the interfacial bonding between the fine powder and the composite functional agent, and provides a good surface foundation for subsequent functional coating.

[0021] This solution employs a two-step mixing strategy: first functionalizing fine powder, then assembling it with coarse powder. This allows the functionalized fine powder units to simultaneously perform three functions in the gaps between coarse powder: space filling, conductive bridging, and interfacial bonding. This significantly improves the compaction density, conductivity uniformity, and structural stability of the cathode material, thereby enhancing the energy density, rate performance, and cycle life of lithium-ion batteries. Detailed Implementation

[0022] To facilitate understanding of this application, a more complete description will be provided below. This application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0023] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of the application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. In the description of this application, "several" means at least one, such as one, two, etc., unless otherwise explicitly specified.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0025] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0026] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0027] Unless otherwise specified, the percentage content mentioned in this application refers to mass percentage for solid-liquid mixtures and solid-phase-solid mixtures, and volume percentage for liquid-phase-liquid mixtures.

[0028] Unless otherwise specified, all percentage concentrations mentioned in this application refer to the final concentration. The final concentration refers to the proportion of the added component in the system after the addition of that component.

[0029] Unless otherwise specified, the temperature parameters in this application may be either constant temperature processing or processing within a certain temperature range. The constant temperature processing allows for temperature fluctuations within the precision range controlled by the instrument.

[0030] The term "particle" as used in this application, or a substance with a defined particle size distribution, is not necessarily spherical in shape; it may be irregular and can be either primary or secondary particles. The particle size of irregular particles is calculated as the average of their maximum and minimum diameters.

[0031] In some preferred embodiments, the lithium iron phosphate material semi-finished product can be lithium iron phosphate powder synthesized by conventional solid-phase method, hydrothermal method or sol-gel method, and the classification method can be any one of air classification, vibrating sieving or cyclone separation.

[0032] In some preferred embodiments, the median particle size D50 of the coarse powder portion can be selected from any value among 8μm, 10μm, 12μm, and 15μm; the median particle size D50 of the fine powder portion can be selected from any value among 1μm, 2μm, 3μm, and 4μm.

[0033] In some preferred embodiments, the organic acid used in the surface activation pretreatment can be selected from any one or a combination of several of citric acid, tartaric acid, malic acid, oxalic acid or acetic acid; the concentration of the organic acid solution can be selected from any value among 0.5wt%, 1wt%, 2wt%, 3wt%, and 5wt%; and the contact reaction time can be selected from any value among 10min, 20min, 30min, 45min, and 60min.

[0034] In some preferred embodiments, the binder component in the composite functional agent can be selected from any one of sodium carboxymethyl cellulose, polyvinylidene fluoride, or polyacrylic acid; the conductive agent can be selected from any one of Super P conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, or graphene; and the mass ratio of the binder component to the conductive agent can be selected from any value among 1:1, 1:1.5, 1:2, 1:2.5, and 1:3.

[0035] In some preferred embodiments, the mass ratio of the functionalized fine powder unit to the coarse powder portion can be selected from any value among 2:8, 3:7, and 4:6.

[0036] In some preferred embodiments, the total amount of the composite functional agent added, based on the mass of the fine powder portion, can be selected from any value among 3wt%, 5wt%, 8wt%, and 10wt%.

[0037] Example 1: This example provides a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery.

[0038] The raw materials for preparing lithium iron phosphate cathode materials include: lithium iron phosphate material semi-finished product (solid phase synthesis), citric acid (analytical grade, Sinopharm Group), sodium carboxymethyl cellulose (CMC, viscosity 800-1200 mPa·s, purchased from Thermo Fisher Scientific), Super P conductive carbon black (particle size 40 nm, purchased from Temico), and deionized water.

[0039] The preparation method is as follows:

[0040] 100g of lithium iron phosphate material semi-finished product was classified by an air classifier to obtain 70g of coarse powder (D50=10μm) and 30g of fine powder (D50=2μm).

[0041] Add 30g of fine powder to 150mL of 2wt% citric acid aqueous solution, stir for 30min at room temperature, filter, wash twice with deionized water, and vacuum dry at 80℃ for 4h to obtain surface-activated fine powder.

[0042] Dissolve 0.5g sodium carboxymethyl cellulose in 50mL of deionized water and stir until completely dissolved. Add 1.0g SuperP conductive carbon black (CMC to Super P mass ratio of 1:2) and stir at high speed for 30min to obtain a composite functional agent suspension.

[0043] The surface-activated fine powder was added to the composite functional agent suspension, stirred and mixed for 1 hour, vacuum dried at 80°C for 4 hours, lightly ground and sieved to obtain the functionalized fine powder unit.

[0044] The functionalized fine powder unit and 70g of coarse powder were mixed in a V-type mixer for 30 minutes to obtain lithium iron phosphate cathode material.

[0045] Preparation of positive electrode sheet: The above-mentioned lithium iron phosphate positive electrode material, Super P conductive carbon black and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:2:2. The positive electrode slurry is prepared with N-methylpyrrolidone (NMP) as solvent, coated on aluminum foil current collector, vacuum dried at 120°C for 12 h, and obtained by rolling after rolling. The electrode sheet has an areal density of 18 mg / cm².

[0046] The above positive electrode, negative electrode, separator, and electrolyte are assembled into a coin cell.

[0047] Example 2: This example provides a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery.

[0048] The difference between this embodiment and embodiment 1 is that in step S2, the organic acid is replaced by tartaric acid instead of citric acid, and the concentration of the tartaric acid aqueous solution is 2wt%. The other raw materials and process parameters are the same as those in embodiment 1.

[0049] Example 3: This example provides a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery.

[0050] The difference between this embodiment and embodiment 1 is that in step S2, the organic acid is replaced by malic acid instead of citric acid, and the concentration of the malic acid aqueous solution is 2wt%. The other raw materials and process parameters are the same as those in embodiment 1.

[0051] Example 4: This example provides a lithium iron phosphate cathode material and its preparation method, as well as a lithium-ion battery.

[0052] The difference between this embodiment and Embodiment 1 is that in step S3, the mass ratio of sodium carboxymethyl cellulose to Super P conductive carbon black is adjusted from 1:2 to 1:1 (CMC 0.75g, Super P 0.75g), while the other raw materials and process parameters are the same as in Embodiment 1.

[0053] Example 5: This example provides a lithium iron phosphate cathode material and its preparation method, as well as a lithium-ion battery.

[0054] The difference between this embodiment and Embodiment 1 is that in step S3, the mass ratio of sodium carboxymethyl cellulose to Super P conductive carbon black is adjusted from 1:2 to 1:3 (CMC 0.375g, Super P 1.125g), while the other raw materials and process parameters are the same as in Embodiment 1.

[0055] Example 6: This example provides a lithium iron phosphate cathode material and its preparation method, as well as a lithium-ion battery.

[0056] The difference between this embodiment and embodiment 1 is that in step S3, the conductive agent is replaced by acetylene black (particle size 35-45nm, purchased from Tianjin Xinglongtai), the mass ratio of CMC to acetylene black is 1:2, and the remaining raw materials and process parameters are the same as in embodiment 1.

[0057] Example 7: This example provides a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery.

[0058] The difference between this embodiment and Embodiment 1 is that in step S1, the mass ratio of coarse powder to fine powder is adjusted from 7:3 to 6:4 (60g of coarse powder and 40g of fine powder), and the total amount of composite functional agent in step S3 is adjusted accordingly (0.67g of CMC and 1.33g of Super P, maintaining the mass ratio of CMC to Super P at 1:2). All other process parameters are the same as in Embodiment 1.

[0059] Example 8: This example provides a lithium iron phosphate cathode material and its preparation method, as well as a lithium-ion battery.

[0060] The difference between this embodiment and embodiment 1 is that in step S2, the concentration of the citric acid aqueous solution is adjusted from 2wt% to 5wt%, while the other raw materials and process parameters are the same as in embodiment 1.

[0061] Comparative Example 1: This comparative example did not undergo coarse and fine powder classification.

[0062] 100g of ungraded lithium iron phosphate material semi-finished product (D50=7μm) was directly mixed with 0.5g of sodium carboxymethyl cellulose and 1.0g of Super P conductive carbon black in a V-type mixer for 30min to obtain the positive electrode material. The preparation method of the positive electrode sheet is the same as in Example 1.

[0063] Comparative Example 2: This comparative example did not perform surface activation pretreatment on the fine powder.

[0064] The difference between this comparative example and Example 1 is that step S2 is omitted, and the unactivated fine powder portion is directly mixed with the composite functional agent suspension (step S4). The remaining raw materials and process parameters are the same as in Example 1.

[0065] Comparative Example 3: The composite functional agent in this comparative example does not contain any binder components.

[0066] The difference between this comparative example and Example 1 is that in step S3, only 1.5g of Super P conductive carbon black was dispersed in deionized water, and sodium carboxymethyl cellulose was not added. All other raw materials and process parameters were the same as in Example 1.

[0067] Performance testing methods

[0068] (1) Compacted density test: The cathode material is compacted under a pressure of 200 MPa, and the thickness and mass of the compacted powder are measured. The compacted density is calculated in g / cm³. 3 .

[0069] (2) Powder electronic conductivity test: The positive electrode material powder was pressed into a disc under a pressure of 10 MPa using the four-probe method, and its electronic conductivity was measured in units of S / cm.

[0070] (3) Initial discharge specific capacity test: A coin cell (CR2032) was assembled with a positive electrode as the positive electrode, a lithium metal sheet as the negative electrode, and a 1 mol / L LiPF6 EC / DMC (1:1 v / v) solution as the electrolyte. The first charge and discharge test was carried out at 25℃ at a 0.1C rate in the voltage range of 2.5-3.65V. The initial discharge specific capacity was recorded in mAh / g.

[0071] (4) Rate performance test: In the above button half-cells, charge and discharge tests were conducted at 0.1C and 1C rates respectively. The ratio of 1C discharge capacity to 0.1C discharge capacity was calculated, i.e., 1C / 0.1C capacity retention rate, expressed as a percentage.

[0072] (39) Cyclic performance test: In the above button half-cell, charge and discharge cycle test is carried out at a 1C rate in the voltage range of 2.5-3.65V. The ratio of the discharge capacity after 500 cycles to the initial discharge capacity is recorded, that is, the capacity retention rate after 500 cycles, expressed as a percentage.

[0073] Table 1 Test results of the examples and comparative examples

[0074]

[0075] As can be seen from Table 1, Examples 1-8 are significantly better than the three comparative examples in terms of compaction density, electronic conductivity, first discharge specific capacity, rate performance and cycle stability. This indicates that the graded-activation-functionalization-assembly process strategy adopted in this scheme has obvious technical advantages in improving the comprehensive performance of lithium iron phosphate cathode materials.

[0076] Example 1 used citric acid, Example 2 used tartaric acid, and Example 3 used malic acid for surface activation of fine powder. The three showed little difference in various performance indicators and all exhibited excellent overall performance. Example 1 (citric acid) showed slightly better performance in all indicators, which may be related to the presence of three carboxyl groups in the citric acid molecule, resulting in stronger chelating and coordination capabilities, more effectively removing the inert layer on the particle surface and introducing more hydrophilic functional groups. Overall, the results indicate that citric acid, tartaric acid, and malic acid, three polyhydroxy acids, can all be effectively used for surface activation treatment of lithium iron phosphate fine powder, with citric acid showing the best performance.

[0077] In Example 4, the mass ratio of CMC to Super P was 1:1, compared to Example 1 (1:2). This reduction in the proportion of conductive agent resulted in an increase in electronic conductivity from 1.2 × 10⁻⁶. -2 S / cm decreased to 8.5×10 -3 The S / cm and 1C / 0.1C capacity retention decreased from 93.2% to 89.5%, indicating that insufficient conductive agent content resulted in an incomplete conductive network on the surface of the fine powder particles, affecting electron transport efficiency and rate performance. In Example 5, the CMC to Super P mass ratio was 1:3, and the electronic conductivity increased to 1.3 × 10⁻⁶. -2 The S / cm ratio was high, but the capacity retention rate after 500 cycles decreased from 96.5% to 93.5%. This was due to the relatively insufficient content of the binder component, which led to a decrease in the bonding strength of the composite coating. During long-term cycling, the interfacial bonding between the functionalized fine powder units and coarse particles deteriorated, resulting in the shedding of some active materials and an increase in contact resistance. In Example 1, the 1:2 ratio of CMC to Super P achieved a good balance between conductivity and adhesion.

[0078] Example 6 uses acetylene black instead of Super P as the conductive agent, and the electronic conductivity is increased from 1.2 × 10⁻⁶. -2 S / cm decreased slightly to 1.0×10 -2The S / cm ratio and other performance indicators are basically similar to those of Example 1. This indicates that different types of conductive carbon black can be used in this scheme, but Super P has certain advantages in constructing continuous conductive networks due to its higher specific surface area and more regular chain aggregate structure.

[0079] In Example 7, the mass ratio of coarse powder to fine powder was adjusted from 7:3 to 6:4, and the compacted density was increased from 2.55 g / cm³. 3 Reduced to 2.45 g / cm³ 3 This is because when the proportion of fine powder is too high, the accumulation of fine powder occupies more space, weakening the supporting effect of the coarse powder skeleton structure and thus reducing the overall packing efficiency. This result indicates that the ratio of coarse to fine powder needs to be controlled within a reasonable range, with 7:3 being the optimal gradation ratio in this system.

[0080] In Example 8, increasing the concentration of the citric acid aqueous solution from 2 wt% to 5 wt% slightly decreased the initial discharge specific capacity from 158.5 mAh / g to 156.5 mAh / g, and the capacity retention rate after 500 cycles decreased from 96.5% to 95.0%. The reason for this is that excessively high concentrations of organic acid may cause excessive erosion of the lithium iron phosphate particle surface, damaging part of the surface crystal structure and leading to increased irreversible capacity loss. Therefore, the concentration of the organic acid solution should be controlled within an appropriate range, with 2 wt% being the optimal concentration in this system.

[0081] Comparative Example 1 did not perform coarse and fine powder classification; lithium iron phosphate powder of the entire particle size range was directly mixed with CMC and Super P. Its compacted density was only 2.35 g / cm³. 3 It is significantly lower than 2.55 g / cm³ in Example 1. 3 The electronic conductivity is 5.0 × 10⁻⁶. -3 The S / cm ratio was only about 42% of that in Example 1; the initial discharge specific capacity at 0.1C was 150.2 mAh / g, the 1C / 0.1C capacity retention was only 82.0%, and the capacity retention after 500 cycles was 85.5%. This indicates that under ungraded conditions, the distribution of conductive agents and binders among particles of different sizes lacks selectivity, and fine particles are difficult to be effectively functionalized, resulting in poor continuity of the conductive network, weak interparticle bonding, and a significant decrease in overall electrochemical performance.

[0082] Comparative Example 2, although it involved the classification of coarse and fine powders and the preparation of a composite functional agent, omitted the surface activation pretreatment step for the fine powder. Its compacted density was 2.45 g / cm³. 3 The electronic conductivity is 7.5 × 10⁻⁶. -3The S / cm capacity retention rate at 1C / 0.1C was 86.5%, and the capacity retention rate after 500 cycles was 89.0%, both lower than in Example 1. The unactivated fine powder particles had a thick inert layer and adsorbed impurities on their surface, resulting in poor wettability and chemical bonding between the composite functional agent and the fine powder particle surface. This made it difficult to form a uniform and strong composite coating layer, leading to a decrease in the quality of the functionalized fine powder units and consequently affecting their conductive bridging and bonding functions in the gaps between coarse powder particles.

[0083] Comparative Example 3's composite functional agent contained only Super P conductive carbon black and no CMC binder component. Despite its high electronic conductivity (9.0 × 10⁻⁶ S / m³), it still achieved good electronic conductivity. -3 The conductivity (m) was higher than that of Comparative Example 2, but the capacity retention rate after 500 cycles was only 88.5%, which was comparable to Comparative Example 2 and significantly lower than 96.5% of Example 1. This indicates that relying solely on the physical adsorption of the conductive agent on the surface of the fine powder particles, without the fixing effect of the binder component, the conductive layer is prone to detachment and displacement during long-term charge-discharge processes, failing to maintain a stable conductive network and interfacial contact. This result further confirms the necessity of the synergistic effect between the conductive agent and the binder component in the composite functional agent—the binder component provides a carrier and fixed matrix for the conductive agent, ensuring the long-term structural integrity of the functionalized coating layer.

[0084] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0085] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: The semi-finished lithium iron phosphate material is classified into coarse powder and fine powder. The fine powder portion is subjected to surface activation pretreatment; Prepare composite functional agents containing conductive agents and binder components; The surface-activated pretreated fine powder portion is mixed with the composite functional agent to form a functionalized fine powder unit; The functionalized fine powder unit is mixed with the coarse powder portion to obtain the lithium iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that, The surface activation pretreatment involves reacting the fine powder with an organic acid solution.

3. The preparation method according to claim 2, characterized in that, The organic acid is at least one of citric acid, tartaric acid, or malic acid.

4. The preparation method according to claim 1, characterized in that, The composite functional agent comprises sodium carboxymethyl cellulose and conductive carbon black, and the mass ratio of sodium carboxymethyl cellulose to conductive carbon black is 1:

2.

5. A lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method according to any one of claims 1-4.

6. The lithium iron phosphate cathode material according to claim 5, characterized in that, The cathode material has a gradient structure, including coarse particles as a framework and functionalized fine powder units filling the gaps between the coarse particles.

7. The lithium iron phosphate cathode material according to claim 6, characterized in that, The functionalized fine powder unit includes lithium iron phosphate fine powder particles and a composite coating layer covering its surface.

8. The lithium iron phosphate cathode material according to claim 7, characterized in that, The composite coating layer contains an adhesive and a conductive agent.

9. A method for preparing a lithium iron phosphate cathode and a battery, characterized in that, The lithium iron phosphate cathode material according to any one of claims 5-8 is used.

10. The application of the lithium iron phosphate cathode material according to any one of claims 5-8 in the preparation of lithium-ion batteries.

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