Methods to improve the overcharge performance of lithium iron phosphate cathode materials, lithium iron phosphate cathode materials and their applications

Abstract

This application belongs to the field of battery technology, and particularly relates to a method for improving the overcharge performance of lithium iron phosphate (LFP) cathode materials, LFP cathode materials, and their applications. The method includes the following steps: sintering LFP cathode materials that fail overcharge performance tests in a protective gas atmosphere at a temperature of 350℃~550℃. Using this method, the overcharge performance of LFP cathode materials can be significantly improved without damaging the bulk structure of the LFP olivine crystal and the carbon-coated conductive system, achieving a targeted modification effect of "improving overcharge performance without affecting the main material properties."

Description

Technical Field

[0001] This application belongs to the field of battery technology, and in particular relates to a method for improving the overcharge performance of lithium iron phosphate cathode materials, lithium iron phosphate cathode materials and their applications. Background Technology

[0002] Lithium iron phosphate (LFP) cathode materials are widely used in power lithium batteries and large-scale energy storage batteries due to their advantages such as excellent cycle life, high safety and stability, low raw material cost, and environmental friendliness. As safety standards for energy storage and new energy vehicles continue to be upgraded, battery overcharge safety performance has become a core control indicator, and the structural stability of the cathode material itself, surface active impurities, and iron dissolution behavior are key factors determining battery overcharge safety.

[0003] In existing processes for preparing lithium iron phosphate (LFP) cathode materials, a two-stage sintering process is typically employed. This involves dispersing precursors such as iron, phosphorus, lithium, and carbon sources in a liquid medium, followed by drying and then two sintering processes. Some existing technologies also employ a three-stage sintering process. Sintering the precursor mixture effectively improves the material's crystallinity, optimizes the conductive network structure, and enhances its cycle performance and rate capability.

[0004] However, in actual large-scale production, the production yield of LFP cathode materials is difficult to reach 100%, and inevitably there will be some batches of products that fail to meet the overcharge performance requirements.

[0005] To improve the overcharge performance of LFP cathode materials, existing technologies mainly rely on chemical modification methods such as metal element doping, composite coating, and surface passivation. These methods typically require the introduction of additional modifying reagents, resulting in complex processes and high production costs. More importantly, these modification methods are prone to introducing impurities and may, to some extent, sacrifice the material's capacity performance, compaction density, cycle performance, and other core electrochemical indicators, and even affect its processing performance.

[0006] Therefore, there is an urgent need to develop a new processing technology that can effectively improve the overcharge performance of LFP cathode materials without sacrificing other core properties. Summary of the Invention

[0007] The purpose of this application is to provide a method for improving the overcharge performance of LFP cathode materials, LFP cathode materials and their applications, which can improve the overcharge performance of LFP cathode materials without destroying the LFP olivine crystal structure and carbon-coated conductive system, achieving the directional modification effect of "improving overcharge performance without affecting the main material properties".

[0008] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows: In a first aspect, this application provides a method for improving the overcharge performance of LFP cathode materials, comprising the following steps: In a protective gas atmosphere, LFP cathode materials that fail the overcharge performance test are sintered at a temperature of 350℃~550℃, preferably 450℃~520℃.

[0009] The second aspect of this application provides an LFP cathode material obtained by the above method.

[0010] A third aspect of this application provides a secondary battery, including a positive electrode sheet comprising the aforementioned LFP positive electrode material.

[0011] The technical solution of this application has the following beneficial effects: This application involves subjecting LFP cathode materials that fail overcharge performance tests to low-temperature sintering at 350℃~550℃ under a protective atmosphere. This utilizes the low-temperature thermal effect to selectively decompose the amorphous carbon on the surface of the LFP cathode material, eliminate highly reactive oxygen-containing active groups, and effectively passivate iron active sites and reduce the content of iron phosphide impurities. This suppresses the dissolution of iron elements and the side reactions between the LFP cathode material and the electrolyte under high-voltage overcharge conditions, thereby giving the LFP cathode material better high-voltage overcharge tolerance.

[0012] Meanwhile, with a carbon coating layer on the surface of the LFP cathode material, low-temperature sintering at 350℃~550℃ will not cause excessive graphitization of the carbon coating layer, thereby reducing the risk of electrolyte interface side reactions caused by excessive graphitization of the carbon coating layer under high-temperature sintering, alleviating electrolyte decomposition and gas generation during overcharging, and thus improving the overcharge performance of the LFP cathode material.

[0013] Furthermore, the method of this application does not require chemical modification methods such as doping or coating, and does not introduce impurities; and the low temperature of 350℃~550℃ does not damage the LFP olivine crystal structure and carbon-coated conductive system. Therefore, it can improve the overcharge performance of LFP cathode material without damaging the core performance such as compaction density, capacity, and cycle performance, and achieve the directional modification effect of "only improving overcharge without affecting the main material indicators".

[0014] The method described in this application can significantly improve the overcharge performance of LFP cathode materials without damaging the bulk structure of the LFP olivine crystal and the carbon-coated conductive system. This achieves a targeted modification effect that "improves overcharge performance without affecting the main material properties," overcoming the drawbacks of traditional overcharge performance improvement processes such as high cost and performance degradation. Furthermore, this method only requires adding a low-temperature sintering step to the existing LFP production process, without introducing complex equipment. It can be completed using existing sintering production lines, exhibiting good process compatibility. It is suitable for mass production quality optimization and defective product repair of LFP materials for power batteries and energy storage batteries, and is suitable for rework and quality upgrades of defective products in large-scale mass production. Attached Figure Description

[0015] Figure 1 Capacity-voltage curves of the LFP cathode materials of Example 1 and Comparative Example 1 before and after improvement; Figure 2 The images are transmission electron microscope (TEM) images of the LFP cathode material in Example 1 before (a) and after (b) the improvement. Figure 3 The C 1s high-resolution XPS spectra of the LFP cathode material in Example 1 before (a) and after (b) the improvement are shown. Figure 4 The high-resolution XPS spectra of Fe 2p of the LFP cathode material in Example 1 before (a) and after (b) the improvement are shown. Figure 5 The capacity-voltage curves of the LFP cathode material in Example 2 before and after improvement are shown. Figure 6 The image shows the capacity-voltage curves of the LFP cathode material in Example 3 before and after improvement. Detailed Implementation

[0016] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0017] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0018] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0019] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0020] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0021] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass described in the embodiments of this application can be a well-known unit of mass in the chemical industry, such as µg, mg, g, or kg.

[0022] The first aspect of this application provides a method for improving the overcharge performance of LFP cathode materials, comprising the following steps: In a protective gas atmosphere, LFP cathode materials that fail the overcharge performance test are sintered at a temperature of 350℃~550℃, preferably 450℃~520℃.

[0023] This application involves subjecting LFP cathode materials that fail overcharge performance tests to low-temperature sintering at 350℃~550℃ under a protective atmosphere. This utilizes the low-temperature thermal effect to selectively decompose the amorphous carbon on the surface of the LFP cathode material, eliminate highly reactive oxygen-containing active groups, and effectively passivate iron active sites and reduce the content of iron phosphide impurities. This suppresses the dissolution of iron elements and the side reactions between the LFP cathode material and the electrolyte under high-voltage overcharge conditions, thereby giving the LFP cathode material better high-voltage overcharge tolerance.

[0024] Meanwhile, with a carbon coating layer on the surface of the LFP cathode material, low-temperature sintering at 350℃~550℃ will not cause excessive graphitization of the carbon coating layer, thereby reducing the risk of electrolyte interface side reactions caused by excessive graphitization of the carbon coating layer under high-temperature sintering, alleviating electrolyte decomposition and gas generation during overcharging, and thus improving the overcharge performance of the LFP cathode material.

[0025] Furthermore, the method of this application does not require chemical modification methods such as doping or coating, and does not introduce impurities; and the low temperature of 350℃~550℃ does not damage the LFP olivine crystal structure and carbon-coated conductive system. Therefore, it can improve the overcharge performance of LFP cathode material without damaging the core performance such as compaction density, capacity, and cycle performance, and achieve the directional modification effect of "only improving overcharge without affecting the main material indicators".

[0026] The method described in this application can significantly improve the overcharge performance of LFP cathode materials without damaging the bulk structure of the LFP olivine crystal and the carbon-coated conductive system. This achieves a targeted modification effect that "improves overcharge performance without affecting the main material properties," overcoming the drawbacks of traditional overcharge performance improvement processes such as high cost and performance degradation. Furthermore, this method only requires adding a low-temperature sintering step to the existing LFP production process, without introducing complex equipment. It can be completed using existing sintering production lines, exhibiting good process compatibility. It is suitable for mass production quality optimization and defective product repair of LFP materials for power batteries and energy storage batteries, and is suitable for rework and quality upgrades of defective products in large-scale mass production.

[0027] In some embodiments, the sintering time is 1h to 4h, preferably 1.5h to 2.5h. After sintering, the material is naturally cooled to room temperature. Sintering at a low temperature of 350℃ to 550℃ for 1h to 4h can decompose the amorphous carbon on the surface of the LFP cathode material, eliminate highly reactive oxygen-containing active groups, passivate iron active sites, and reduce the content of iron phosphide impurities. This suppresses the side reactions between the LFP cathode material and the electrolyte and the dissolution of iron elements under high voltage overcharge conditions, giving the LFP cathode material better high voltage overcharge tolerance. It can also suppress the damage to the LFP olivine crystal structure and carbon-coated conductive system caused by over-sintering and gently regulate the microstructure of the material surface.

[0028] In some embodiments, the heating rate of the sintering process is 5°C / min to 15°C / min, preferably 5°C / min to 10°C / min, and even more preferably 6°C / min to 8°C / min. Controlling the heating rate to 5°C / min to 15°C / min during sintering gently regulates the microstructure of the material surface, which helps to maintain the LFP cathode material particle size without significant changes after sintering, thereby preventing significant fluctuations in the compaction density; at the same time, it reduces the risk of damage to the LFP olivine crystal bulk structure and the carbon-coated conductive system.

[0029] In some embodiments, the protective gas includes at least one of nitrogen and argon. The purity of the protective gas is greater than or equal to 99.99%, and the oxygen content is less than or equal to 10 ppm. Sintering under a high-purity protective gas can suppress the oxidation of ferrous ions to ferric ions in the LFP cathode material, reduce the iron phosphide content, thereby reducing the iron dissolution of the LFP cathode material, which in turn helps to alleviate side reactions during overcharging and improves the overcharge performance of the LFP cathode material.

[0030] In some embodiments, after sintering, the process further includes a step of mechanically grinding the sintered LFP cathode material in a protective gas atmosphere. The mechanical grinding method can be ball milling. The ball milling speed can be set to 500 r / min to 650 r / min, and the time to be 0.5 h to 2 h. After ball milling, the volume median particle size (Dv50) of the LFP cathode material is 0.8 μm to 2 μm. Performing mechanical grinding after sintering allows the LFP cathode material to achieve the required particle size, thereby improving the compaction density.

[0031] In some embodiments, LFP cathode materials that fail the overcharge performance test refer to LFP cathode materials that, after being processed into batteries according to specific industry standards (such as GB / T 36276, GB 38031, etc.), fail the safety assessment during overcharge testing. More specifically, in the embodiments of this application, the LFP cathode material that fails the overcharge performance test has a charge specific capacity greater than 190 mAh / g at an overcharge voltage of 5.5V and a charging rate of 0.5C.

[0032] For LFP cathode materials, the specific capacity under normal charging voltage and a 0.5C charging rate is typically around 170 mAh / g. However, under high-voltage overcharge conditions, such as when the battery is charged to 5.5V, which is far above the normal cutoff voltage, the LFP cathode material structure may be damaged, and the electrolyte will undergo severe oxidative decomposition, releasing a large amount of heat and gas, easily leading to thermal runaway. This electrolyte decomposition and side reactions will contribute additional capacity, increasing the specific capacity. Therefore, an LFP cathode material with a specific capacity greater than 190 mAh / g at the overcharge cutoff voltage of 5.5V indicates that it has undergone severe side reactions under 5.5V high-voltage overcharge conditions, and is therefore unqualified for overcharge testing. For LFP cathode materials that fail to meet overcharge standards, this application proposes to perform low-temperature sintering treatment, which can eliminate highly reactive oxygen-containing active groups, passivate iron active sites, reduce the content of iron phosphide impurities, and reduce the amount of iron dissolved. This can alleviate the side reactions of LFP cathode materials under high-voltage overcharge conditions, reduce the charging specific capacity of LFP under high-voltage overcharge conditions, and make LFP cathode materials have better high-voltage overcharge tolerance.

[0033] In some embodiments, LFP cathode materials that fail the overcharge performance test exhibit an iron dissolution rate greater than 300 ppm at an overcharge voltage of 5.5V and a charging rate of 0.5C. For LFP cathode materials with an iron dissolution rate greater than 300 ppm, iron ions readily dissolve and migrate to the negative electrode, reducing to elemental iron particles. These iron particles catalyze electrolyte decomposition, disrupt the uniformity of the SEI film, and induce lithium dendrite growth. Simultaneously, iron ions on the positive electrode side exacerbate the oxidative decomposition and structural degradation of the electrolyte. Under high-voltage overcharge conditions, these side reactions are further accelerated, leading to significant gas generation, increased internal pressure, and a surge in heat generation, which can easily trigger internal short circuits and thermal runaway, thereby drastically reducing the battery's overcharge tolerance and safety. In other words, at the aforementioned iron dissolution rate, the LFP cathode material fails the overcharge test. For LFP cathode materials that suffer from overcharge failure due to surface defects, low-temperature sintering can reduce iron leaching, thereby mitigating side reactions under high-voltage overcharge conditions and improving the overcharge tolerance of LFP cathode materials.

[0034] In some embodiments, the iron phosphide content of LFP cathode materials that fail overcharge performance tests is greater than 350 ppb. During battery overcharging, iron phosphide reacts to convert into iron ions, which then enter the electrolyte. That is, a high iron phosphide content increases iron dissolution. After dissolving, the iron ions migrate to the negative electrode and deposit to form iron dendrites, directly piercing the separator and causing internal micro-short circuits. This process not only exacerbates electrolyte decomposition and heat generation but also induces the co-growth of lithium dendrites, causing the battery to fail rapidly under high-voltage overcharge conditions. To address this, this application employs low-temperature sintering treatment on LFP cathode materials with surface defects that induce overcharge failure. This reduces the iron phosphide content, thereby reducing iron dissolution and mitigating side reactions of the LFP cathode material under high-voltage overcharge conditions, resulting in better high-voltage overcharge tolerance.

[0035] In some embodiments, the LFP cathode material that fails the overcharge performance test includes an LFP core and a carbon coating layer disposed on the surface of the LFP core, wherein the content of the carbon coating layer is 1wt% to 1.5wt%.

[0036] In some embodiments, the specific surface area of ​​LFP cathode materials that fail the overcharge performance test is 9m². 2 / g~18m 2 / g.

[0037] A second aspect of this application provides an LFP cathode material obtained by the above method. This LFP cathode material has at least one of the following properties: (1) The specific capacity under overcharge voltage of 5.5V and charging rate of 0.5C is less than or equal to 185mAh / g; (2) The amount of iron dissolved under an overcharge voltage of 5.5V and a charging rate of 0.5C is less than or equal to 130ppm, and optionally less than or equal to 100ppm; (3) The content of iron phosphide impurities is less than or equal to 100 ppb.

[0038] LFP cathode materials that failed the overcharge performance test were treated with low-temperature sintering, which reduced iron phosphide impurities and iron dissolution. The specific capacity under high-voltage overcharge conditions was lower than that before low-temperature sintering, and the overcharge performance was significantly improved.

[0039] In some embodiments, the specific surface area of ​​the LFP cathode material is 8m². 2 / g~12m 2 / g. After treatment, the overall LFP cathode material is reduced compared to before treatment, which reduces the contact area between the LFP cathode material and the electrolyte, thereby reducing interfacial reactions and helping to alleviate side reactions under high voltage overcharge conditions, thus improving overcharge performance.

[0040] In some embodiments, the LFP cathode material includes an LFP core and a carbon coating layer disposed on the surface of the LFP core.

[0041] A third aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode includes the aforementioned LFP positive electrode material.

[0042] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0043] In the following embodiments and comparative examples, the commercial LFP cathode materials that failed the overcharge performance test all included an LFP core, and the surface of the LFP core was coated with a carbon coating layer.

[0044] The performance testing and structural characterization methods for the LFP cathode materials before and after improvement are as follows. Unless otherwise specified, the following testing or analysis methods are performed at 20℃~25℃.

[0045] 1. LFP cathode material structure (1.1) Carbon content Take 0.1g of LFP cathode material sample and use a carbon-sulfur analyzer to test the carbon content of LFP cathode material.

[0046] (1.2) Iron phosphide impurity content Take 1 kg of LFP cathode material, extract the magnetic material with a magnetic rod, add 5% aqua regia to digest it, filter it and perform ICP testing.

[0047] (1.3) Specific surface area Take 0.3g~1.0g of LFP cathode material powder and load it into a sample tube. Degas the sample under high vacuum at 120℃~150℃ for 4h~8h to completely remove adsorbed moisture and residual solvent. Cool and weigh the sample for later use. Place the sample tube in a liquid nitrogen environment at 77K and purge it with high-purity nitrogen gas. Collect adsorption data in the range of relative partial pressure 0.05Pa~0.35Pa, plot the adsorption isotherm, and obtain the sample specific surface area by BET linear fitting.

[0048] (1.4) Compacted density Take 1.0000g of LFP cathode material powder and add it into a compaction mold. Use a pressure of 1T~3T on a compaction machine to test the compaction density.

[0049] 2. Electrical properties After assembling LFP cathode materials into coin cells according to the following method, the performance of the cells was tested using a Blue Electric workstation.

[0050] LFP cathode material powder, polyvinylidene fluoride (PVDF), and conductive carbon black (SP) were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and stirred until homogeneous to obtain a cathode slurry. The cathode slurry was coated onto carbon-coated aluminum foil, dried, and then cut into small circular pieces with a diameter of 1.4 cm as cathode sheets. These cathode sheets were then used to assemble coin cells in a glove box. The negative electrode of the coin cell was a lithium-ion sheet, and the electrolyte used was Saiwei Technology 2001A electrolyte (electrolyte components include ethylene carbonate EC, dimethyl carbonate DMC, diethyl carbonate DEC, vinylene carbonate VC, and LiPF6).

[0051] (2.1) Overcharge specific capacity The standard constant current and constant voltage charging mode was used for capacity determination. Specifically, the battery was charged at a constant current of 0.5C to 3.75V, and then charged at a constant voltage of 3.75V until the current dropped to 0.05C, at which point the battery was fully charged. The total charging capacity (mAh) at this point was recorded, and divided by the mass of the active material to obtain the specific charging capacity C1 (mAh / g) after 0.5C capacity determination.

[0052] The overcharge current (0.5C current) is calculated using the formula: I = C1 * active material (mg) * 0.5 / 1000000. Constant current charging is then performed using the above I until 5.5V is reached. The charging capacity during the entire overcharge phase (from full charge voltage to 5.5V) is recorded, and divided by the mass of the active material, yields the overcharge specific capacity.

[0053] (2.2) Iron leaching amount After performing the overcharge test according to the method in (2.1), 0.2g of negative electrode was dissolved, filtered, and then subjected to ICP test to obtain the amount of iron dissolved.

[0054] (2.3) 1C charge / discharge capacity and capacity retention The above-mentioned button cell was subjected to constant current charge and discharge at a rate of 1C, with a charging cutoff voltage of 3.75V. The initial charge capacity and initial discharge capacity at 1C were recorded. The cell was then cycled at 1C, and the discharge capacity after 1000 cycles was recorded. Therefore, the capacity retention rate after 1000 cycles = (discharge capacity after 1000 cycles / initial discharge capacity) × 100%.

[0055] Example 1 This embodiment provides a method for improving the overcharge performance of LFP electrode materials, including the following steps: S1. A commercially available LFP cathode material that failed the overcharge performance test was selected. Testing revealed that this material had a charge specific capacity of 198 mAh / g, an iron dissolution rate of 600 ppm, an iron phosphide impurity content of 365 ppb, a carbon content of 1.2 wt%, and a specific surface area of ​​11.68 m² under a 5.5V overcharge test. 2 / g, compacted density is 2.61g / cm³ 3 The 1C charge / discharge capacity is 159.1 / 139.2 mAh / g, and the capacity retention rate is 89.2% after 1000 cycles.

[0056] S2. The LFP cathode material that failed the overcharge performance test was placed in a nitrogen atmosphere with a purity of 99.99% and heated to 500℃ at a heating rate of 8℃ / min for sintering treatment, with a holding time of 2 hours. After sintering, it was naturally cooled to room temperature and then ball-milled for 1 hour to obtain the improved LFP cathode material.

[0057] The results are shown in Tables 1-3 and Figures 1-4 .in Figure 1 The image shows the capacity-voltage curves of the LFP cathode material before and after improvement. Figure 2 The images show transmission electron microscopy (TEM) images of the LFP cathode material before (a) and after (b) the improvement. Figure 3 High-resolution XPS spectra of C 1s for LFP cathode material before (a) and after (b) improvement. Figure 4 High-resolution XPS spectra of Fe 2p in LFP cathode material before (a) and after (b) improvement.

[0058] [Table 1] Performance test results of LFP cathode material before and after improvement in Example 1

[0059] [Table 2] Relative chemical states of carbon (C) before and after improvement in the LFP cathode material of Example 1

[0060] [Table 3] Relative chemical states of iron (Fe) before and after improvement in the LFP cathode material of Example 1

[0061] Table 1 and Figure 1 The results showed that after treating the LFP cathode material with substandard overcharge performance using the method in Example 1, the 5.5V overcharge specific capacity of the LFP cathode material decreased to 174 mAh / g, and the overcharge performance improved by 12.1%. Simultaneously, the iron dissolution and iron phosphide impurity content were significantly reduced. The carbon content and specific surface area of ​​the improved LFP cathode material were slightly adjusted. Furthermore, the core electrical properties of the LFP cathode material, such as compaction density, charge / discharge capacity, and cycle capacity retention, showed no significant fluctuations before and after the improvement.

[0062] Figure 2 TEM images show that the carbon layer order on the surface of the improved LFP cathode material is increased compared to the unimproved material; in addition, Raman spectroscopy shows that the Ig on the surface of the improved LFP cathode material is higher. D / I G As the value decreases, the degree of graphitization increases, and the thickness of the amorphous carbon layer on the material surface decreases.

[0063] Tables 2-3 and Figures 3-4 XPS analysis results show that the improved LFP cathode material has formed a more ideal passivation state on its surface. Specifically, on the one hand, the proportion of highly conductive graphitic carbon (CC / C=C) in the carbon coating layer is significantly increased (meaning an increased degree of graphitization of the carbon layer, consistent with TEM and Raman test results, which can improve conductivity), while the content of polar oxygen-containing functional groups CO and OC=O is reduced by 18.3%, which is beneficial for constructing a more complete and dense conductive / protective layer; on the other hand, the Fe content of the improved LFP cathode material is significantly increased. 2+ / Fe 3+ The 10.3% increase in the proportion indicates highly catalytically active surface Fe 3+ Impurity phases were largely eliminated. This synergistic effect effectively passivated the surface iron active sites, inhibiting their harmful contact with the electrolyte and side reactions. Therefore, the improved LFP cathode material exhibited higher tolerance under overcharge conditions.

[0064] Example 2 This embodiment provides a method for improving the overcharge performance of LFP electrode materials, including the following steps: S1. LFP cathode material that failed the overcharge performance test was selected. Testing revealed that after reaching 5.5V in the overcharge test, the material had a charging specific capacity of 195mAh / g; simultaneously, its iron dissolution was 396ppm, iron phosphide impurity content was 425ppb, carbon content was 1.11%, and specific surface area was 10.46m². 2 / g, compacted density is 2.592g / cm³ 3 The 1C charge / discharge capacity is 158.8 / 139.8 mAh / g, and the capacity retention rate is 89.4% after 500 cycles.

[0065] S2. The above LFP cathode material is placed in an argon atmosphere with a purity of 99.99% and heated to 550°C at a heating rate of 5°C / min for sintering treatment. The sintering time is controlled to be 1 hour. After sintering, it is naturally cooled to room temperature and then ball-milled for 1.5 hours to obtain the improved LFP cathode material.

[0066] The performance of the improved LFP cathode material was tested according to the test method in Example 1. The results are shown in the table below. Figure 5 As shown.

[0067] [Table 4] Performance test results of LFP cathode material before and after improvement in Example 2

[0068] The results showed that the improved LFP cathode material, after reaching 5.5V in the overcharge test, had a charging specific capacity of 163mAh / g, representing a 19.6% improvement in overcharge performance. Simultaneously, the iron dissolution and iron phosphide impurity content were significantly reduced. Furthermore, the carbon content and specific surface area of ​​the improved LFP cathode material were slightly adjusted. Moreover, the core electrical performance indicators of the LFP cathode material, such as compaction density, charge / discharge capacity, and cycle capacity retention, exhibited weak fluctuations.

[0069] In addition, XPS showed that the content of CO and OC=O functional groups on the surface of the improved LFP cathode material decreased by 10.5% compared with that before the improvement, and Fe... 2+ / Fe 3+ The proportion increased by 11.9% compared to before the improvement; Raman and TEM results show that the improved LFP cathode material has an I... D / I G As the value decreases, the degree of graphitization increases, and the thickness of the amorphous carbon layer on the material surface decreases.

[0070] Example 3 This embodiment provides a method for improving the overcharge performance of LFP electrode materials, including the following steps: S1. LFP cathode material that failed the overcharge performance test was selected. Testing revealed that after reaching 5.5V in the overcharge test, the material had a charging specific capacity of 204mAh / g; simultaneously, its iron dissolution was 305ppm, iron phosphide impurity content was 560ppb, carbon content was 1.28%, and specific surface area was 13.46m². 2 / g, compacted density is 2.292g / cm³ 3 The 1C charge / discharge capacity is 159.8 / 151.0 mAh / g, and the capacity retention rate is 89.39% after 500 cycles.

[0071] S2. The above LFP cathode material is placed in a nitrogen-argon mixed atmosphere (volume ratio 1:1) with a purity of 99.99%, and heated to 500°C at a heating rate of 15°C / min for sintering treatment. The sintering time is controlled to be 4h. After sintering, it is naturally cooled to room temperature, and then ball-milled for 0.5h to obtain the improved LFP cathode material.

[0072] The performance of the improved LFP cathode material was tested, and the results are shown in the table below. Figure 6 As shown.

[0073] [Table 5] Performance test results of LFP cathode material before and after improvement in Example 3

[0074] The results show that the improved LFP cathode material, after reaching 5.5V in the overcharge test, exhibits a reduced charging specific capacity of 178mAh / g, representing a 12.7% improvement in overcharge performance. Simultaneously, the iron dissolution rate and iron phosphide impurity content are significantly reduced. Furthermore, the carbon content and specific surface area of ​​the improved LFP cathode material are slightly adjusted. Moreover, the core electrical performance indicators of the LFP cathode material, such as compaction density, charge / discharge capacity, and cycle capacity retention, show relatively weak fluctuations.

[0075] In addition, XPS showed that the content of CO and OC=O functional groups on the surface of the improved LFP cathode material decreased by 9.7% compared with that before the improvement, and Fe... 2+ / Fe 3+ The proportion increased by 11.5% compared to before the improvement; TEM and Raman spectroscopy showed that the graphitization degree of the improved LFP cathode material increased and the thickness of the amorphous carbon layer on the material surface decreased.

[0076] Examples 4-7 The difference between Examples 4-7 and Example 1 lies in the adjustment of the sintering temperature. The sintering temperatures of each example are shown in Table 6.

[0077] Examples 8-10 The difference between Examples 8-10 and Example 1 is that the sintering time is adjusted. The sintering times for each example are shown in Table 6.

[0078] Examples 11-13 The difference between Examples 11-13 and Example 1 is that the heating rate during the sintering process is adjusted. The sintering heating rates for each example are shown in Table 6.

[0079] Comparative Example 1 The difference between this comparative example and Example 1 is that the sintering temperature was adjusted to 700°C.

[0080] Comparative Example 2 The difference between this comparative example and Example 1 is that the sintering temperature was adjusted to 200°C.

[0081] Comparative Example 3 This comparative example follows the method of Example 1, and the LFP cathode material that has already undergone sintering treatment in Example 1 is sintered again.

[0082] Comparative Example 4 In this comparative example, the LFP cathode material of Comparative Example 1, which had already undergone sintering treatment, was sintered again according to the method of Example 1.

[0083] [Table 6] Sintering processes of Examples 1, 4-13 and Comparative Examples 1-2

[0084] The performance test results of the LFP cathode materials after treatment in Examples 1, 4-13 and Comparative Examples 1-4 are shown in the table below.

[0085] [Table 7] Performance test results of LFP cathode materials in Examples 1, 4-13 and Comparative Examples 1-4 before and after improvement.

[0086] The results showed that treating LFP cathode materials with unsatisfactory overcharge performance using the low-temperature sintering process described in Examples 1 and 4-13 could reduce the overcharge specific capacity and effectively improve overcharge performance. Simultaneously, the iron dissolution rate and iron phosphide impurity content were significantly reduced; furthermore, the core electrical properties of the LFP cathode material, such as compaction density, charge / discharge capacity, and cycle capacity retention, showed no significant fluctuations before and after the improvement. Additionally, under the low-temperature sintering process described in Examples 1 and 4-7, the overcharge specific capacity exhibited a non-monotonic change with increasing temperature.

[0087] Compared to Example 1, Comparative Example 1 used an excessively high sintering temperature. The LFP cathode material treated with this method exhibited a charge specific capacity of only 188 mAh / g under a 5.5V overcharge test, representing a mere 5.1% improvement in overcharge performance compared to the untreated material. This indicates a limited improvement in the overcharge performance of the LFP cathode material. Furthermore, regarding impurities and surface properties, the carbon content and specific surface area of ​​the LFP cathode material treated in Comparative Example 1 were significantly reduced compared to Example 1, while the iron dissolution and iron phosphide impurity content increased. Simultaneously, the core electrical properties of the LFP cathode material treated in Comparative Example 1, such as compaction density, charge / discharge capacity, and cycle capacity retention, were all significantly reduced compared to the untreated material.

[0088] Comparative Example 2 used an excessively low sintering temperature. The LFP cathode material treated with this material showed an increased charge specific capacity under a 5.5V overcharge test compared to the untreated material, indicating a deterioration in overcharge performance. Furthermore, its carbon content and specific surface area decreased more significantly compared to Example 1, while the iron dissolution rate and iron phosphide impurity content increased. Core electrical properties such as charge / discharge capacity and cycle capacity retention rate also showed a substantial decrease compared to the untreated material.

[0089] Comparative Example 3 subjected the LFP cathode material treated in Example 1 to low-temperature sintering again, but the result was no further improvement in its overcharge performance; instead, the overcharge specific capacity increased. This may be due to the increased impurity content after multiple low-temperature sintering treatments. For example, the iron phosphide impurity content in Comparative Example 3 was higher than that in Example 1, and iron phosphate or other impurities may also be generated, causing performance degradation.

[0090] Comparative Example 4 involved low-temperature sintering of the LFP cathode material treated in Comparative Example 1, but the improvement effect on various properties of the LFP cathode material was not significant. In summary, the results of all analyses show that the method described in this application, used to treat LFP cathode materials that fail the overcharge performance test, can selectively control surface defects in the LFP cathode material. Specifically, it selectively eliminates amorphous carbon and oxygen-containing active functional groups on the material surface, passivates surface iron active sites, reduces iron phosphide impurities and iron dissolution, and addresses the severe side reactions of the electrolyte under high-voltage overcharging at the microscopic interface, thereby improving the overcharge performance of the LFP cathode material. Furthermore, the results show that while improving the overcharge performance of the LFP cathode material, the above method does not decrease compaction density, capacity, or long-cycle performance, achieving a synergistic balance between overcharge, compaction density, capacity, and long-cycle performance.

[0091] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for improving the overcharge performance of lithium iron phosphate cathode materials, characterized in that, Includes the following steps: In a protective gas atmosphere, lithium iron phosphate cathode materials that fail the overcharge performance test are sintered at a temperature of 350°C to 550°C.

2. The method according to claim 1, characterized in that, The sintering temperature is 450℃~520℃.

3. The method according to claim 1 or 2, characterized in that, The sintering process takes 1 to 4 hours. And / or, the heating rate of the sintering process is 5℃ / min to 15℃ / min.

4. The method according to claim 3, characterized in that, The sintering treatment time is 1.5h to 2.5h; and / or, The heating rate of the sintering process is 5℃ / min to 10℃ / min.

5. The method according to claim 1 or 2, characterized in that, The lithium iron phosphate cathode material that fails the overcharge performance test has a charge specific capacity greater than 190 mAh / g at an overcharge voltage of 5.5V and a charging rate of 0.5C.

6. The method according to claim 5, characterized in that, The lithium iron phosphate cathode material that fails the overcharge performance test has an iron leaching amount greater than 300 ppm at an overcharge voltage of 5.5V and a charging rate of 0.5C.

7. The method according to claim 5, characterized in that, The lithium iron phosphate cathode material that fails the overcharge performance test has an iron phosphide content greater than 350 ppb.

8. The method according to claim 5, characterized in that, The lithium iron phosphate cathode material that fails the overcharge performance test includes a lithium iron phosphate core and a carbon coating layer disposed on the surface of the lithium iron phosphate core, wherein the carbon coating layer has a content of 1wt%~1.5wt%; and / or, The lithium iron phosphate cathode material that failed the overcharge performance test had a specific surface area of ​​9m². 2 / g~18m 2 / g.

9. A lithium iron phosphate cathode material, characterized in that, It is obtained by the method described in any one of claims 1 to 8.

10. A secondary battery, characterized in that, It includes a positive electrode sheet, wherein the positive electrode sheet includes the lithium iron phosphate positive electrode material as described in claim 9.

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