Preparation method and application of high-temperature-resistant lithium iron phosphate positive electrode material based on solid solution reaction

Abstract

The invention discloses a preparation method of a high-temperature-resistant lithium iron phosphate positive electrode material based on solid solution reaction, relates to a preparation method of a lithium ion battery positive electrode material, and aims to solve the problems that the lithium iron phosphate positive electrode material of a lithium ion battery is poor in high-temperature cycling stability and cannot operate at ultrahigh temperature. The method comprises the following steps: uniformly mixing a niobium source and a tungsten source in a mortar, placing the mixture in a muffle furnace, carrying out gradient heating, calcining, and carrying out gradient cooling to obtain Nb12WO33 powder; and ball-milling and uniformly mixing the ball-milled Nb12WO33 fine powder with a lithium source, a carbon source and an iron source, and pre-sintering and finally calcining in a tubular furnace to obtain the high-temperature-resistant lithium iron phosphate positive electrode material. The capacity retention ratio of the material after 240 cycles at 100 DEG C and 10C multiplying power is 90%, and the material can be used in the field of high-temperature-resistant lithium ion batteries.

Description

Technical Field

[0001] The invention relates to a preparation method and application of a lithium iron phosphate positive electrode material, and belongs to the field of synthetic preparation of electrode materials. Background Art

[0002] Lithium-ion batteries have become the core power source for new energy vehicles and large-scale energy storage. Lithium iron phosphate (LiFePO4), the most widely installed cathode material in the power battery field, has become the preferred system for large-scale energy storage due to its cost advantages, environmental friendliness, and excellent safety. However, its high-temperature stability currently limits the large-scale application of lithium iron phosphate batteries in tropical regions, heavy-load conditions, and fast-charging scenarios. When the battery operates at temperatures above 60°C, irreversible damage to the material structure is exacerbated, leading to a sudden drop in capacity, a surge in impedance, and even the risk of thermal runaway. Statistics show that for every 10°C increase in temperature, the battery cycle life decay rate increases by approximately 200%. To date, the problem of lithium iron phosphate's sudden drop in cycle stability under high-temperature conditions has not been improved.

[0003] Current mainstream solutions focus on electrolyte additives and surface coatings. While these two methods can suppress interfacial side reactions in the short term, they cannot eradicate the degradation of the material's intrinsic structure. In particular, in lithium iron phosphate systems, traditional carbon coating improves electronic conductivity but struggles to address the core issue of decreased lithium ion diffusion at high temperatures. Summary of the Invention

[0004] The present invention aims to solve the technical problem of poor high-temperature stability of existing lithium iron phosphate positive electrode materials, and to provide a preparation method and application of high-temperature resistant lithium iron phosphate positive electrode materials based on solid solution reaction. The present invention prepares a double high-valent metal oxide precursor in advance, and adopts thermal diffusion doping in the process of preparing lithium iron phosphate by carbon thermal reduction process to achieve high-valent cation co-doping of lithium iron phosphate. Through high-valent cation co-doping, high-valent metal ions replace part of the iron position, change the intrinsic properties of lithium iron phosphate, weaken the high localization of electrons caused by the intrinsic olivine structure of lithium iron phosphate, reduce the energy barrier of lithium iron phosphate solid solution formation, induce lithium iron phosphate to transport lithium ions through the solid solution reaction path, realize the solid solution reaction of lithium iron phosphate at high temperature and high current density, reduce the volume change caused by phase change during the reaction process, weaken the irreversible damage of the material during high-temperature charge and discharge, thereby improving the high-temperature stability of the material. This method can effectively improve the high-temperature cycle stability of lithium iron phosphate, and the process flow is simple, which is convenient for large-scale production and application.

[0005] The preparation method of the high temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction of the present invention is carried out according to the following steps:

[0006] Step 1: Add niobium source and tungsten source in a molar ratio of (6.5-7.5):1 into a mortar and grind and mix evenly. Then put it into a muffle furnace and heat it to 400-600 °C at a heating rate of 6-10 °C / min and keep it for 2-4 hours for the first sintering. Then heat it to 800-1200 °C at a heating rate of 2-5 °C / min and keep it for 8-12 hours for the final calcination. Then cool it to 300-500 °C at a cooling rate of 2-5 °C / min and keep it for 3-6 hours. Then cool it naturally to room temperature to obtain niobium tungsten oxide powder, whose chemical formula is Nb 12 WO 33 This step adopts a gradient heating and stage cooling strategy. The purpose of the first stage of gradient heating is to remove moisture, residual solvents and low-melting-point organic matter adsorbed by the raw materials, and to avoid powder splashing or composition deviation caused by rapid escape of volatiles in the high-temperature stage. The second stage of heating adopts a slow heating rate to avoid uneven diffusion of niobium and tungsten ions due to different ionic radii, ensure their full diffusion, and avoid powder cracking or local composition segregation due to thermal stress. The purpose of heating is to drive niobium and tungsten ions to achieve uniform atomic-level mutual dissolution in the oxide lattice; the gradient cooling adopts slow cooling, and at the same time, the temperature is kept warm after cooling to a certain temperature to optimize the integrity of the crystal structure and reduce lattice defects and internal stress.

[0007] Step 2: The niobium tungsten oxide powder obtained in step 1 is placed in a planetary ball mill using zirconium oxide balls as grinding beads, and ball milled for 3 to 6 hours at a ball-to-material ratio of (6 to 12):1 and a rotation speed of 300 to 1000 rpm to obtain a fine niobium tungsten oxide powder. This step reduces the particle size of the niobium tungsten oxide powder to 2 to 6 μm, ensuring high uniformity when subsequently mixed with the lithium iron phosphate precursor.

[0008] Step 3: Mix the lithium source with anhydrous ferric phosphate, then add the carbon source and niobium tungsten oxide fine powder. Use zirconia balls as grinding beads in a planetary ball mill, use a ball-to-material ratio of (12-18):1, and rotate at 300-1000 rpm for 8-12 hours to obtain a mixed powder. This step ensures that all components are evenly mixed at the molecular level.

[0009] Step 4: Place the mixed powder in a tube furnace, and under the protection of flowing inert gas, heat it to 200~350℃ at a heating rate of 5~10℃ / min and keep it for 2~4 hours for pre-sintering, then heat it to 550~750℃ at a heating rate of 5~10℃ / min and keep it for 8~15 hours for calcining. Under these conditions, the carbon source is fully pyrolyzed and the carbon thermal reduction reaction is sufficient. Finally, it is naturally cooled to room temperature to obtain a high-temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction. In this step, the purpose of the first stage of gradient heating is to remove volatile substances, make the material initially solidified, prevent the loss of components during subsequent high-temperature sintering, and initially form LiFePO4 crystal nuclei. The purpose of the second stage is to evenly disperse Nb 12 WO 33 The solid solution powder dissolves at high temperature, where niobium and tungsten ions are activated in a reducing atmosphere and diffuse into the growing lithium iron phosphate lattice, dispersing and evenly replacing the Fe²⁺ sites, ensuring that the prepared lithium iron phosphate cathode material can achieve solid solution phase transition at high temperature and high current density.

[0010] Furthermore, the niobium source in step 1 is niobium pentoxide.

[0011] Furthermore, the tungsten source in step 1 is tungsten trioxide.

[0012] Furthermore, the lithium source in step three is one or a mixture of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium dihydrogen phosphate.

[0013] Furthermore, the carbon source in step three is one or a mixture of glucose, sucrose, citric acid, oxalic acid, ascorbic acid, polyacrylonitrile, and phenolic resin.

[0014] Furthermore, the molar ratio of lithium in the lithium source described in step 3 to iron in anhydrous ferric phosphate is (0.9-1.3):1.

[0015] Furthermore, in the mixed powder described in step 3, the mass of the carbon source is 5% to 15% of the mass of the mixed powder.

[0016] Furthermore, in the mixed powder described in step 3, the mass of the niobium tungsten oxide fine powder is 0.7% to 1.3% of the mass of the mixed powder.

[0017] Furthermore, the inert gas in step 3 is argon, nitrogen or a mixture of hydrogen and argon;

[0018] Furthermore, during the pre-firing phase described in step 3, the inert gas flow rate is 200-300 mL / min. The high flow rate in the first stage allows the gas to quickly remove the H2O, CO2, CO, and hydrocarbon gases produced by decomposition, preventing these gases from accumulating in the furnace tube, condensing, or re-adsorbing onto the powder surface.

[0019] Furthermore, during the calcination phase described in step 3, the inert gas flow rate is 50-100 mL / min. The second phase uses a low gas flow rate to reduce airflow disturbances, providing a stable gas-solid interface for ion diffusion, while also preventing high-speed airflow from disrupting contact between precursor particles and ensuring the continuity of the solid-phase reaction.

[0020] The beneficial effects of the present invention compared to the prior art are:

[0021] (1) The present invention avoids doping localization by pre-synthesizing a double high-valent metal solid solution precursor, thereby achieving precise control of the amount of substitution in the lattice during the thermal diffusion doping process. By regulating the co-doping of high-valent cations, high-valent metal ions replace part of the iron site, changing the intrinsic properties of lithium iron phosphate, weakening the high localization of electrons caused by the intrinsic olivine structure of lithium iron phosphate, reducing the energy barrier of the lithium iron phosphate solid solution reaction, inducing lithium iron phosphate to transport lithium ions through the solid solution reaction path, and realizing the solid solution reaction of micron-level lithium iron phosphate at high current density and high temperature, effectively reducing the lattice volume change caused by the two-phase transition during the reaction process, weakening the irreversible damage of the material during high-temperature charging and discharging, and significantly improving the cycle stability of lithium iron phosphate batteries at high temperatures. Therefore, the high-temperature resistant lithium iron phosphate cathode material lithium iron phosphate prepared by the present invention can solve the problem that lithium ion batteries cannot be used in tropical areas, heavy-load conditions and fast charging scenarios, realize the larger-scale application of lithium ion batteries, and have high market value.

[0022] (2) The present invention uses the carbon thermal reduction method to add niobium tungsten oxide precursor to lithium iron phosphate for thermal diffusion doping modification. The test results show that the average powder conductivity of lithium iron phosphate prepared by the method of the present invention is 8×10 -3S / cm. Thanks to its excellent electrical conductivity and unique lithium transfer mechanism, the lithium iron phosphate cathode material for lithium-ion batteries prepared by the present invention has excellent cycle stability under high temperature conditions. At a temperature of 60°C, at a rate of 5 C, its capacity retention rate is 88% after 1000 cycles, and at a rate of 10 C, its capacity retention rate is 92% after 1000 cycles; at a temperature of 100°C, at a rate of 5 C, its capacity retention rate is 94% after 120 cycles, and at a rate of 10 C, its capacity retention rate is 90% after 240 cycles. The lithium iron phosphate prepared in the comparative example has a capacity retention rate of 50% after 600 cycles at a rate of 5 C at a temperature of 60°C, and a capacity retention rate of 30% after 300 cycles at a rate of 10 C; at a temperature of 100°C, at a rate of 5 C, its capacity retention rate is 15% after 50 cycles, and at a rate of 10 C, its capacity decays to 0% after 10 cycles. Obviously, the lithium iron phosphate cathode material for lithium-ion batteries prepared by the present invention has excellent high-temperature stability.

[0023] (3) The raw materials required for the preparation method of the present invention are widely available and low in cost. The process flow is simple and the product has high reproducibility. It can be produced in large quantities at one time and meet the needs of practical applications of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The conductivity test graph of the lithium iron phosphate positive electrode materials prepared in Example 1 and Comparative Example 1;

[0025] Figure 2 X-ray diffraction patterns of the lithium iron phosphate positive electrode materials prepared in Example 1 and Comparative Example 1;

[0026] Figure 3 This is a transmission electron micrograph of the lithium iron phosphate positive electrode material prepared in Comparative Example 1;

[0027] Figure 4 Element distribution diagram of the lithium iron phosphate positive electrode material prepared in Comparative Example 1;

[0028] Figure 5 This is a transmission electron micrograph of the lithium iron phosphate positive electrode material prepared in Example 1;

[0029] Figure 6 Element distribution diagram of the lithium iron phosphate positive electrode material prepared in Example 1;

[0030] Figure 7 This is a transmission electron microscope image of the lithium iron phosphate positive electrode material prepared in Comparative Example 2;

[0031] Figure 8 This is the element distribution diagram of the lithium iron phosphate positive electrode material prepared in Comparative Example 2;

[0032] Figure 9 In-situ X-ray diffraction patterns of the lithium iron phosphate positive electrode materials prepared in Example 1 and Comparative Example 1 at 5C rate charge and discharge;

[0033] Figure 10 The graph shows the cycle performance of the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 1 under a high temperature environment of 60°C and a charge and discharge rate of 5C;

[0034] Figure 11 The graph shows the cycle performance of the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 1 under a high temperature environment of 60°C and a charge and discharge rate of 10C;

[0035] Figure 12 The graph is a cycle performance diagram of the lithium iron phosphate positive electrode materials prepared in Comparative Example 1, Example 1, and Comparative Example 2 at a charge and discharge rate of 5C in a high temperature environment of 100°C;

[0036] Figure 13 The graph is a cycle performance diagram of the lithium iron phosphate positive electrode materials prepared in Comparative Example 1, Example 1, and Comparative Example 2 under a high temperature environment of 100°C and a charge and discharge rate of 10C. DETAILED DESCRIPTION

[0037] The beneficial effects of the present invention are demonstrated with the following examples.

[0038] Example 1: The preparation method of the high-temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction of this embodiment is carried out according to the following steps:

[0039] Step 1: Weigh 4.79g of niobium pentoxide and 0.6954g of tungsten trioxide into a mortar and grind them manually with a pestle for 30 minutes to mix the niobium source and tungsten source evenly. Then put the mixed powder into a muffle furnace and heat it to 500℃ at a heating rate of 10℃ / min and keep it for 4 hours for the first sintering. Then heat it to 1000℃ at a heating rate of 5℃ / min and keep it for 9 hours for the final calcination. Then cool it to 500℃ at a cooling rate of 5℃ / min and keep it for 4 hours. Then cool it naturally to room temperature to obtain niobium tungsten oxide powder, whose chemical formula is Nb 12 WO 33 This step adopts a gradient heating and stage cooling strategy, which is conducive to achieving atomic-level mutual solubility during the synthesis of niobium tungsten oxide, while ensuring the integrity of the lattice structure and reducing lattice defects;

[0040] Step 2: The niobium tungsten oxide powder obtained in step 1 is placed in a planetary ball mill using zirconium oxide balls as grinding beads. Dry milling is performed for 5 hours at a ball-to-material ratio of 8:1 and a rotation speed of 1000 rpm to obtain a fine niobium tungsten oxide powder. This step reduces the particle size of the niobium tungsten oxide powder to 2-6 μm, ensuring high uniformity when subsequently mixed with the lithium iron phosphate precursor.

[0041] Step 3: Mix 0.4899 g of lithium carbonate with 2.00 g of anhydrous ferric phosphate, then add 0.2896 g of sucrose and 0.03 g of the fine niobium tungsten oxide powder prepared in Step 2. In a planetary ball mill, using zirconia balls as grinding beads, dry-mill for 9 hours at a ball-to-material ratio of 17:1 and a rotation speed of 1000 rpm to obtain a mixed powder. This step ensures that all components are uniformly mixed at the molecular level.

[0042] Step 4: Place the mixed powder in a tubular furnace, and under the protection of a flowing argon atmosphere with a gas flow rate of 250 mL / min, heat it to 300 ° C at a rate of 5 ° C / min and keep it for 3 hours for pre-sintering. Then, under the protection of a flowing argon atmosphere with a gas flow rate of 100 mL / min, heat it to 650 ° C at a rate of 5 ° C / min and keep it for 9 hours for final calcination. Under these conditions, the carbon source is fully pyrolyzed and the carbon thermal reduction reaction is fully achieved. Finally, it is naturally cooled to room temperature to obtain a high-temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction.

[0043] Comparative Example 1: In this comparative example, lithium iron phosphate is prepared by carbon thermal reduction of an undoped iron phosphate precursor. The specific preparation method is as follows:

[0044] 1. Weigh 2.00 g of iron phosphate, 0.4899 g of lithium carbonate, and 0.2896 g of sucrose and add them to a planetary ball mill. Use zirconia ball milling beads and dry ball mill for 9 h at a ball mill speed of 1000 rpm and a ball-to-material ratio of 17:1 to obtain a mixed powder.

[0045] 2. The mixed powder was transferred into a tubular furnace, and pre-sintered at a heating rate of 5°C / min to 300°C and maintained for 3 h under the protection of a flowing argon atmosphere with a gas flow rate of 250 mL / min. Then, the mixture was finally calcined at a heating rate of 5°C / min to 650°C and maintained for 9 h under the protection of a flowing argon atmosphere with a gas flow rate of 100 mL / min. After natural cooling, the lithium iron phosphate positive electrode material was obtained.

[0046] Comparative Example 2: In this comparative example, a bimetallic oxide precursor is not prepared in advance. A niobium source and a tungsten source are directly added during the carbothermal reduction process to prepare a dual cation-doped lithium iron phosphate positive electrode material. The steps are as follows:

[0047] 1. Weigh 2.00 g of iron phosphate, 0.4899 g of lithium carbonate, 0.2896 g of sucrose, 0.042 g of niobium pentoxide, and 0.012 g of tungsten trioxide, and dry-mill them for 9 h using zirconium oxide ball milling beads at a ball mill speed of 1000 rpm and a ball-to-material ratio of 17:1 to obtain a mixed powder;

[0048] 2. The mixed powder was transferred into a tubular furnace, and pre-sintered at a heating rate of 5°C / min to 300°C and maintained for 3 h under the protection of a flowing argon atmosphere with a gas flow rate of 250 mL / min. Then, the mixture was finally calcined at a heating rate of 5°C / min to 650°C and maintained for 9 h under the protection of a flowing argon atmosphere with a gas flow rate of 100 mL / min. After natural cooling, a high-valent cation co-doped lithium iron phosphate positive electrode material was obtained.

[0049] Figure 1 The conductivity test diagram of the lithium iron phosphate positive electrode material prepared in Example 1 and Comparative Example 1. Figure 1 It can be seen that the average conductivity of the high temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction prepared in Example 1 is 8×10 -3 S / cm, while the average conductivity of lithium iron phosphate prepared in Comparative Example 1 is 5×10 -3 S / cm. In comparison, the electrical conductivity of the high-temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction prepared in Example 1 is significantly higher than that in Comparative Example 1, indicating that the addition of niobium tungsten oxide is beneficial to improving the electrical conductivity of the material.

[0050] Figure 2 The X-ray diffraction patterns of the lithium iron phosphate cathode materials prepared in Example 1 and Comparative Example 1 are shown in FIG. Figure 2 It can be seen that both the samples of Example 1 and Comparative Example 1 can be indexed as lithium iron phosphate LiFePO4 structures. Since the doping is trace, the peaks of the doping elements niobium and tungsten cannot be seen in the X-ray diffraction spectrum.

[0051] Figure 3 This is a scanning electron microscope image of the lithium iron phosphate cathode material prepared in Comparative Example 1. It can be observed that the lithium iron phosphate prepared by the carbothermal reduction method has a particle size of approximately 5 μm. Micron-sized lithium iron phosphate has a higher compaction density than nanoparticles, ensuring the practicality of lithium iron phosphate in commercial applications.

[0052] Figure 4 This is the element distribution diagram of the lithium iron phosphate positive electrode material prepared in Comparative Example 1. It can be observed that the O, Fe, and P elements are evenly distributed in the particles, without obvious vacancies or concentrated areas.

[0053] Figure 5This is a scanning electron microscope image of the high-temperature-resistant lithium iron phosphate cathode material based on a solid solution reaction prepared in Example 1. It can be observed that the particle size of the lithium iron phosphate prepared after doping is approximately 6 μm, indicating that the high-valent cation doping has no effect on the particle size of the lithium iron phosphate, and it can still maintain a high compaction density.

[0054] Figure 6 This is the element distribution diagram of the high-temperature resistant lithium iron phosphate cathode material based on solid solution reaction prepared in Example 1. It can be observed that the Fe and P elements are evenly distributed in the particles, with no obvious vacancies or concentrated areas, indicating that the prepared lithium iron phosphate meets the standards. At the same time, the Nb and W elements are evenly distributed within the particles, without localized element doping, demonstrating the precise control of doping achieved by the present invention.

[0055] Figure 7 This is a scanning electron microscope image of the dual high-valent cation-doped lithium iron phosphate powder prepared in Comparative Example 2. It can be observed that the particle size of the lithium iron phosphate prepared after doping in Example 2 is approximately 5 μm, similar to the particle size of the lithium iron phosphate cathode material prepared in Example 1, indicating that the doping method has little effect on the particle size.

[0056] Figure 8 This is the element distribution diagram of the dual high-valent cation-doped lithium iron phosphate powder prepared in Comparative Example 2. It can be observed that the Fe and P elements are evenly distributed in the particles, with no obvious vacancies or concentrated areas, indicating that the prepared lithium iron phosphate meets the standards. However, the Nb and W elements are unevenly distributed within the particles, with localized Nb doping and a small amount of W distributed within the particles.

[0057] Figure 9 The in-situ X-ray diffraction patterns of the lithium iron phosphate positive electrode materials prepared in Comparative Example 1 and Example 1 during the charge and discharge process at a 5C rate, wherein a is Comparative Example 1 and b is Example 1. Figure 9 It can be observed that the peak position of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 remains consistent after charge and discharge at a rate of 5C, and no shift occurs. However, the peak position of the high-temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction prepared in Example 1 gradually shifts when a 5C current is applied for charge and discharge. The peak position shift is caused by the formation of a solid solution phase during the charge and discharge process. After the charge and discharge are completed, the solid solution phase does not immediately decompose into two phases due to its metastable nature. Therefore, it can be seen that the peak position of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 remains consistent after charge and discharge at a rate of 5C, and no shift occurs. Figure 7 A significant peak shift was observed in the results, which indicates that the co-doping of high-valent cations reduced the formation energy barrier of the lithium iron phosphate solid solution phase and realized the transformation from two-phase opposite strain to solid solution reaction.

[0058] The lithium iron phosphate cathode materials prepared in Example 1, Comparative Example 1 and Comparative Example 2 were prepared into electrodes in the following manner: the active material was mixed with conductive carbon black (Super-p) and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then dropped into an appropriate amount of N-methyl-2-pyrrolidone (NMP) and stirred for 6 hours. Subsequently, the mixture was coated on an aluminum current collector and vacuum dried at 120°C for 6 hours. The dried sample was cut into a circular electrode with a diameter of 14 mm, and the electrode mass loading was controlled to be between 2-3 mg using a coating knife with a thickness of 100 μm. The assembly of CR2032 button cells was carried out in an argon-filled glove box. The working electrode in the 25°C and 60°C tests was the prepared electrode, and lithium metal was used as the negative electrode; the working electrode in the 100°C test was the prepared electrode, and a graphite electrode was used as the negative electrode. Celgard 2500 separators were used in the 25°C test, PI / PP separators were used in the 60°C test, and glass fiber filter paper separators were used in the 100°C test. Commercially available power battery electrolyte (LX-127) was used as the electrolyte. The electrolyte was 1.0 mol L⁻¹ LiPF6 dissolved in a mixed solvent of ethyl carbonate (EC): dimethyl carbonate (DMC) = 1:1 (v / v). The assembled battery was tested on a NEWARE BTS5.3 battery test system, and the charge-discharge cycles were carried out at different rates in the voltage range of 2.5-4.2 V (vs. Li / Li⁺) based on the nominal capacity (170 mA h g⁻¹).

[0059] Figure 10 、 Figure 11 The cycle performance graphs of the lithium iron phosphate cathode materials prepared in Comparative Example 1 and Example 1 during the charge-discharge process at a temperature of 60°C and at rates of 5C and 10C, respectively. At a temperature of 60°C, when the current density was 5C, the capacity retention rate of the high-temperature-resistant lithium iron phosphate cathode material prepared in Example 1 based on solid solution reaction was 88% after 1000 cycles of charge-discharge; when the current density increased to 10C, the capacity retention rate of the high-temperature-resistant lithium iron phosphate cathode material prepared in Example 1 based on solid solution reaction was still 90% after 1200 cycles of charge-discharge, indicating excellent high-temperature cycle performance. In contrast, when the current density was 5C, the capacity retention rate of the lithium iron phosphate cathode material prepared in Comparative Example 1 had already decayed to 50% after 600 cycles of charge-discharge; when the current density increased to 10C, the capacity retention rate of the lithium iron phosphate cathode material prepared in Comparative Example 1 dropped sharply to 30% after only 300 cycles of charge-discharge, indicating that the high-temperature stability of the lithium iron phosphate material could not meet the actual demand.

[0060] Figure 12 、 Figure 13The following graphs show the cycling performance of the lithium iron phosphate cathode materials prepared in Comparative Example 1, Example 1, and Comparative Example 2 at 100°C and 5C and 10C charge-discharge rates. At 100°C and a current density of 5C, the high-temperature-resistant lithium iron phosphate cathode material based on solid solution reaction prepared in Example 1 maintained a capacity retention rate of 94% after 120 charge-discharge cycles, while maintaining a Coulombic efficiency of 90% throughout the cycle. When the current density was increased to 10C, the high-temperature-resistant lithium iron phosphate cathode material based on solid solution reaction prepared in Example 1 maintained a capacity retention rate of 90% after 240 charge-discharge cycles, while maintaining a Coulombic efficiency of 97%. In contrast, at a current density of 5C, the capacity retention rate of the lithium iron phosphate cathode material prepared in Comparative Example 1 had already declined to 15% after 50 charge-discharge cycles, and the Coulombic efficiency declined significantly during the cycle. When the current density was increased to 10C, the capacity retention rate of the lithium iron phosphate cathode material prepared in Comparative Example 1 declined to 0% after only 10 charge-discharge cycles. When the current density was 5C, the capacity retention rate of the double-high-valent cation-doped lithium iron phosphate prepared in Comparative Example 2 had decayed to 13% after 60 charge-discharge cycles, and the Coulombic efficiency decayed extremely severely during the cycle. When the current density was increased to 10C, the capacity retention rate of the double-high-valent cation-doped lithium iron phosphate prepared in Comparative Example 2 decayed to 84% after 240 charge-discharge cycles. This comparison shows that the lithium iron phosphate cathode material prepared in Example 1 has significantly improved cyclic stability at high temperatures, can adapt to extreme environments, and has good application prospects.

[0061] The present invention avoids localization of the doping process by pre-preparing and pre-synthesizing a bivalent metal precursor, thereby achieving precise control of the bimetallic doping ratio. By co-doping with high-valent Nb and W cations, high-valent metal ions replace iron positions, changing the intrinsic properties of lithium iron phosphate, weakening the high localization of electrons caused by the intrinsic olivine structure of lithium iron phosphate, reducing the energy barrier for the formation of lithium iron phosphate solid solution, and realizing lithium ion transmission through solid solution reaction of micron-level lithium iron phosphate positive electrode materials at high current density and high temperature, effectively reducing the volume change caused by phase change during the reaction process, weakening the irreversible damage to the material during high-temperature charging and discharging, and significantly improving the cycle stability of lithium iron phosphate batteries at high temperatures. The present invention reconstructs the bulk stability of the material through lattice engineering, suppresses phase change stress and ion transmission barriers from the source, and successfully breaks through the bottleneck of high-temperature circulation.

[0062] The solid solution reaction pathway lithium iron phosphate cathode material prepared by the present invention maintains excellent cycling stability at both 60°C and 100°C. Furthermore, the preparation method of the present invention has a simple process flow, and the product has high reproducibility, enabling one-time mass production, meeting the practical application requirements of lithium-ion batteries.

Claims

1. A method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction, characterized in that: The method proceeds as follows: Step 1: Add niobium source and tungsten source in a molar ratio of (6.5-7.5):1 into a mortar and grind and mix evenly. Then put it into a muffle furnace and heat it to 400-600 °C at a heating rate of 6-10 °C / min and keep it for 2-4 hours for the first sintering. Then heat it to 800-1200 °C at a heating rate of 2-5 °C / min and keep it for 8-12 hours for the final calcination. Then cool it to 300-500 °C at a cooling rate of 2-5 °C / min and keep it for 3-6 hours. Then cool it naturally to room temperature to obtain niobium tungsten oxide powder, whose chemical formula is Nb 12 WO 33 ; Step 2: The niobium tungsten oxide powder obtained in step 1 is placed in a planetary ball mill using zirconia balls as grinding beads, at a ball-to-material ratio of (6-12):1 and a rotation speed of 300-1000 rpm for 3-6 hours to obtain niobium tungsten oxide fine powder; Step 3: Mix the lithium source with anhydrous ferric phosphate, add the carbon source and niobium tungsten oxide fine powder, and use zirconia balls as grinding beads in a planetary ball mill at a ball-to-material ratio of (12-18):1 and a rotation speed of 300-1000 rpm for 8-12 hours to obtain a mixed powder; Step 4: Place the mixed powder in a tubular furnace, and under the protection of flowing inert gas, heat it to 200~350℃ at a heating rate of 5~10℃ / min and keep it for 2~4 hours for pre-sintering, then heat it to 550~750℃ at a heating rate of 5~10℃ / min and keep it for 8~15 hours for calcination, and finally cool it naturally to room temperature to obtain a high-temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction.

2. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1, characterized in that: The niobium source described in step 1 is niobium pentoxide.

3. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1 or 2, characterized in that: The tungsten source described in step 1 is tungsten trioxide.

4. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1 or 2, characterized in that: The lithium source in step 3 is one or a mixture of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium dihydrogen phosphate.

5. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1 or 2, characterized in that: The carbon source in step 3 is one or a mixture of glucose, sucrose, citric acid, oxalic acid, ascorbic acid, polyacrylonitrile, and phenolic resin.

6. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1 or 2, characterized in that: The molar ratio of lithium in the lithium source described in step 3 to iron in anhydrous ferric phosphate is (0.9-1.3):

1.

7. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1 or 2, characterized in that: In the mixed powder described in step 3, the mass of the carbon source is 5% to 15% of the mass of the mixed powder.

8. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1 or 2, characterized in that: In the mixed powder described in step 3, the mass of the niobium tungsten oxide fine powder is 0.7% to 1.3% of the mass of the mixed powder.

9. The method for preparing a high-temperature resistant lithium iron phosphate positive electrode material based on a solid solution reaction according to claim 1 or 2, characterized in that: During the pre-burning stage described in step 3, the flow rate of the inert gas is 200-300 mL / min.

10. The method for preparing a high temperature resistant lithium iron phosphate positive electrode material based on solid solution reaction according to claim 1 or 2, characterized in that: During the calcination stage described in step 3, the flow rate of the inert gas is 50-100 mL / min.

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