High-power lithium iron phosphate cathode material and preparation method thereof

Abstract

The application relates to the field of battery positive electrode materials, in particular to a high-power lithium iron phosphate positive electrode material and a preparation method thereof.In the application, the lithium iron phosphate positive electrode material is prepared through a carbon thermal sintering reduction method of iron, lithium, phosphorus, a conductive precursor and a reducing agent, the coating performance between the metal and carbon is improved through the conductive precursor, and better electrochemical performance is obtained.

Description

Technical Field

[0001] This application relates to the field of battery cathode materials, and in particular to a high-power lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium iron phosphate batteries are currently the core solution for power batteries, with high safety and long cycle life, and have already occupied a large market share.

[0003] Currently, the preparation methods for lithium iron phosphate battery cathode materials include two main routes: solid-state and liquid-state methods, with the solid-state method accounting for over 70% of market output. The solid-state method includes carbothermal reduction and high-temperature solid-state reaction. The carbothermal reduction method involves sintering raw materials (including iron, lithium, phosphorus, and carbon sources) at high temperatures. A conductive precursor is used at high temperatures to reduce ferric iron to ferrous iron, while pyrolysis of carbon forms a conductive coating layer on the particle surface, thus creating the conductive layer. This method uses simple raw materials, has a mature process, and offers significant convenience for mass production, making it the primary production method at present.

[0004] In the above process, the key to the carbothermic reduction method lies in the uniformity of the coating layer, which directly affects the resistance and achievable power of the lithium iron phosphate battery. To improve uniformity, repeated grinding is usually required during granulation. However, this process is difficult to control the particle size distribution, which leads to a low yield rate in the carbothermic reduction method and thus affects product quality. Summary of the Invention

[0005] To improve the stability of the carbothermal reduction method for lithium iron phosphate cathode materials and achieve high power, high cycle life, and high retention rate, this application provides a high-power lithium iron phosphate cathode material and its preparation method.

[0006] First, the preparation method of the high-power lithium iron phosphate cathode material in this application specifically includes the following steps: Raw material mixing and precipitation: Iron source, lithium source, phosphorus source, conductive precursor (0.05-0.1 times the mass of iron), and reducing agent (0.05-0.1 times the mass of iron) are mixed according to the molar ratio of iron, lithium, and phosphorus of 1:0.8-1.2:0.8-1.2. Solvent A is added. The iron source can be selected as iron oxide or iron(II,III) oxide, the lithium source can be selected as any number of lithium carbonate, lithium chloride, lithium nitrate, lithium acetate, and lithium hydroxide, and the phosphorus source can be any number of ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate. The conductive precursor is a resin with a carbon content exceeding 70 wt%. Grinding: The mixture obtained in the raw material mixing and sedimentation step is ball-milled to obtain a powder slurry; Granulation: The powder slurry is spray-granulated to obtain spherical precursors; Sintering: The spherical precursor is sintered in an oxygen-free environment at a temperature not exceeding 800℃ to form a coating layer.

[0007] Overall, similar to existing technologies, this application uses a reducing agent to reduce ferric iron and then forms a coating layer on its surface to provide higher compaction density and better particle uniformity. It also further reduces the ash content in the system and reduces its impact on electrical resistance.

[0008] Building upon the above, this application incorporates a small amount of organic resin into the conductive precursor. The organic resin binds and coats the particles within the system, facilitating a more direct reduction reaction on the surface of the carbon particles and thus providing a better coating level. The conductive particles can be selected from carbon powder, graphite powder, or graphene of specific sizes, while the conductive precursor should possess a certain degree of binding properties to achieve higher compaction density during subsequent sintering, while its branching properties enhance overall conductivity.

[0009] Overall, the organic resin should be selected with components having a higher carbon content, which can form less ash and higher adhesion after sintering, and also have a higher compaction density. Preferably, the conductive precursor is any number of epoxy resins and phenolic resins. The above two resins can bind inorganic components such as iron source, lithium source, and phosphorus source in the system, further improving the uniformity of distribution and the conductivity of the system.

[0010] Preferably, the reducing agent comprises the following components by mass percentage: Hydroxypropyl methylcellulose 5-20%; Aliphatic terminal dicarboxylic acids 0.1%–1%; Residual reducing sugars.

[0011] In the above scheme, in addition to the basic reducing sugar, the following two structures are added: First, hydroxypropyl methylcellulose, which improves dispersibility and system uniformity. Second, compared to ordinary fiber regions, the hydroxypropyl groups on its surface, after substitution, can coordinate with metal ions, thus controlling the sites and effects of the reduction reaction after micelle formation. Third, an aliphatic terminal dicarboxylic acid can control the spacing between metal ions through its two-terminal coordination, reducing metal ion aggregation and improving system uniformity. Furthermore, the solubilizing effect of hydroxypropyl methylcellulose further enhances its dispersibility within the system, resulting in a slurry with better uniformity and stability, and good coating properties after sintering. Preferably, the aliphatic terminal dicarboxylic acid is any number of 1,6-adipic acid, 1,7-heptanoic acid, or 1,8-octanoic acid, which provides good control over the distance to metal ions, resulting in a cathode material with better capacitance performance.

[0012] Based on the above, hydroxypropyl methylcellulose with a degree of substitution of 1.6 to 2.4 is preferred, as it has a better coordination effect. The weight-average molecular weight of hydroxypropyl methylcellulose is preferably 50K to 100K, which can better control the viscosity of the system. If a higher molecular weight is used, it will lead to excessive slurry viscosity, which will have an adverse effect on subsequent grinding. If the molecular weight is too low, it will lead to poor dispersibility of the system, which will have a certain adverse effect on electrochemical performance.

[0013] Based on the above, solvent A is preferably a mixture of water and alcohol, wherein the alcohol is selected from any amount of methanol, ethanol, ethylene glycol, propanol, and propylene glycol, and the mass ratio of water to alcohol is 1:0.1 to 0.2. By adopting the above technical solution, solubilization is achieved in the system using a small amount of alcohol, further improving the dispersibility of the system.

[0014] Preferably, in the sintering step, preheating sintering is first performed at a temperature of 300–400°C for 2–4 hours, followed by high-temperature sintering at 700–800°C for 5–10 hours. In this sintering process, the solvent is first evaporated during pre-sintering, and the reducing agent initially reduces the metal ions. Then, high-temperature sintering is performed, and after sufficient reduction, a coating layer forms on the surface of the carbon particles. Further high-temperature carbonization is then carried out, during which the conductive precursor gradually transforms into carbon black, thus achieving conductivity. Simultaneously, the reaction process of lithium iron phosphate also occurs in this step, resulting in a good overall coating effect.

[0015] Preferably, in the grinding step, the viscosity of the ground powder slurry is 3500–5000 mPa·s. Using a slightly higher viscosity powder slurry achieves better overall uniformity. Due to the presence of conductive precursors in the system, the resulting decrease in flowability means that a higher viscosity after grinding will not affect the subsequent granulation process.

[0016] In addition, this application also protects the lithium iron phosphate cathode material prepared by the above preparation method.

[0017] In summary, this application provides a high-power lithium iron phosphate cathode material and its preparation method. By adjusting the reducing agent and adding a certain amount of resin as a conductive precursor to the system, the overall uniformity of carbon coating on the surface of metal ions is improved, and the aggregation of metal ions is also improved. Therefore, it provides better uniformity and higher compaction density, and also significantly improves the electrochemical performance of the cathode material. Detailed Implementation

[0018] The technical solution of this application will be further described through the following specific embodiments.

[0019] In this application, the electrode material is verified in the following two aspects: 1. Compacted density: The positive electrode material was tested using a powder compaction density meter. The sheet thickness was 0.185 mm and the pressure was 325 MPa. 2. Electrochemical Performance: The positive electrode materials prepared in each example and preparation example were added to N-methyl-2-pyrrolidone at a mass ratio of 8:1:1 with conductive carbon black and PVDF. After mixing, a slurry was prepared. The slurry was coated onto aluminum foil and dried in an oven at 80°C to form a positive electrode sheet. The positive electrode sheets were assembled into a button cell. The electrolyte was 1 mol / L LiPF6 (EC:DMC=1:1). The negative electrode was a lithium sheet, and the separator was a Celgard 2400 polypropylene porous membrane.

[0020] Initial coulombic efficiency: At 25°C, the battery is charged and discharged at a current of 0.1C with a voltage of 2.5-4.2V. The initial coulombic efficiency is calculated as the discharge capacity / charge capacity × 100%.

[0021] Long-term cycle stability: At 25℃, the battery is charged and discharged using a 3C current. After 1000 cycles, the battery capacity retention rate is calculated.

[0022] Example 1: This example provides a set of high-power lithium iron phosphate cathode materials, the preparation method of which specifically includes the following steps: Raw material mixing and precipitation: Weigh iron oxide, lithium acetate dihydrate, and diammonium hydrogen phosphate according to the iron, lithium, and phosphorus molar ratio of 1:1:1. At the same time, weigh the conductive precursor and reducing agent according to the mass ratio of conductive precursor to iron of 0.1:1 and the mass ratio of reducing agent to iron of 0.1:1. Add 90wt% water and 10wt% ethanol, and control the overall solid content within the range of 40±5%. Mix evenly. Specifically, the conductive precursor is bisphenol A type epoxy resin (theoretical carbon content 74.1%, grade E51). The mass ratio of iron oxide, lithium acetate dihydrate, diammonium hydrogen phosphate, conductive precursor, and reducing agent is 3:4:5:0.21:0.21. The specific mass ratio of the reducing agent is as follows: Sucrose 89.5% Hydroxypropyl methylcellulose 10% 1,6-Adipic acid 0.5% The degree of substitution of hydroxypropyl methylcellulose is 2.2, and the weight-average molecular weight is 80K.

[0023] Grinding: The above system was ball-milled using 0.3-0.5 mm zirconia beads at a linear velocity of 10-12 m / s to obtain a powder slurry with a final viscosity of 4500 mPa·s. Granulation: The powder slurry is spray granulated, with the inlet air temperature controlled at 335℃, the outlet air temperature at 100℃, and the atomization pressure at 0.4MPa, to obtain spherical precursors.

[0024] Primary sintering: The spherical precursor is sintered under an oxygen-free environment with high-purity nitrogen. During the sintering process, pre-sintering is performed first, with the temperature increased to 350℃ at 2℃ / min and held for 3 hours. Then, the temperature is increased to 760℃ at 4℃ / min and sintering is continued for 8 hours. After cooling, the material is discharged and large agglomerates are crushed to obtain cathode material powder.

[0025] Example 2: In this example, the selected conductive precursor was adjusted. While reducing the amount of conductive precursor, an equal mass of reducing agent was added to maintain the overall carbon content. Similarly, while increasing the mass of the conductive precursor, the amount of reducing agent used was reduced. Specifically, the experimental results for Examples 1 and 2 are shown in Table 1.

[0026] A comparison of Examples 1 and 2 reveals that the addition of the conductive precursor reduces the compaction density of the system to some extent. However, the impact is relatively small when using conductive precursors with a carbon content higher than 70%, while a significant impact occurs when the carbon content is lower than 70% (e.g., polyacrylic acid). Furthermore, a lower carbon content also leads to a decrease in the degree of crosslinking after sintering, resulting in some loss of conductive pathways and consequently a decline in overall electrical performance. Additionally, the specific type of conductive precursor chosen also significantly affects the electrochemical performance of the product. For example, using PP or PE results in a reduction in overall electrical performance, possibly due to the ash generated during processing affecting the coating structure between the metal and carbon.

[0027] Example 3: In this example, based on Example 1, the hydroxypropyl methylcellulose ether in Example 1 was replaced. Specifically, several different types of cellulose ethers were used in this example, and the types of aliphatic terminal dicarboxylic acids were also completely replaced. The results are shown in Table 2. It should be noted that in Table 2, the addition or subtraction of aliphatic terminal dicarboxylic acids and hydroxypropyl methylcellulose were all made up by sucrose (or other reducing sugars).

[0028] The above experiments clearly demonstrate that the amount and properties of hydroxypropyl methylcellulose (HMC) significantly affect performance. Based on these experiments, it can be seen that when the degree of substitution of HMC is 1.6–2.4 and the molecular weight is within the range of 50–100K, it better balances compaction density and electrical conductivity. In Examples 3-13, where no HMC is added, a significant decrease in compaction density and electrical efficiency is observed. Regarding terminal dicarboxylic acids, a comparison between Examples 3-21 and other examples clearly shows a decrease in density and a certain reduction in electrochemical performance. This demonstrates that polycarboxylic acids play a role in controlling the position of metal ions. Overall, the electrochemical effect initially increases and then decreases with increasing carbon chain length, with 1,6-adipic acid, 1,7-heptanoic acid, or 1,8-octanoic acid exhibiting better results.

[0029] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for preparing a high-power lithium iron phosphate cathode material, characterized in that, Includes the following steps: Raw material mixing and precipitation: Iron source, lithium source, phosphorus source, conductive precursor (0.05-0.1 times the mass of iron), and reducing agent (0.05-0.1 times the mass of iron) are mixed according to the molar ratio of iron, lithium, and phosphorus of 1:0.8-1.2:0.8-1.

2. Solvent A is added. The iron source can be selected as iron oxide or iron(II,III) oxide, the lithium source can be selected as any number of lithium carbonate, lithium chloride, lithium nitrate, lithium acetate, and lithium hydroxide, and the phosphorus source can be any number of ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate. The conductive precursor is a resin with a carbon content exceeding 70 wt%. Grinding: The mixture obtained in the raw material mixing and sedimentation step is ball-milled to obtain a powder slurry; Granulation: The powder slurry is spray-granulated to obtain spherical precursors; Sintering: The spherical precursor is sintered in an oxygen-free environment at a temperature not exceeding 800℃ to form a coating layer. The reducing agent comprises the following components by mass percentage: Hydroxypropyl methylcellulose 5-20%; Aliphatic terminal dicarboxylic acids 0.1%–1%; Residual reducing sugars; The conductive precursor is any number of epoxy resins and phenolic resins.

2. The method for preparing a high-power lithium iron phosphate cathode material according to claim 1, characterized in that, The degree of substitution of the hydroxypropyl methylcellulose is 1.6 to 2.

4.

3. The method for preparing a high-power lithium iron phosphate cathode material according to claim 2, characterized in that, The weight-average molecular weight of the hydroxypropyl methylcellulose is 50K to 100K.

4. The method for preparing a high-power lithium iron phosphate cathode material according to claim 2, characterized in that, Solvent A is a mixture of water and alcohol, wherein the alcohol is selected from any number of methanol, ethanol, ethylene glycol, propanol, and propylene glycol, and the mass ratio of water to alcohol is 1:0.1 to 0.

2.

5. The method for preparing a high-power lithium iron phosphate cathode material according to claim 2, characterized in that, The aliphatic terminal dicarboxylic acid is any number of 1,6-adipic acid, 1,7-heptanoic acid, or 1,8-octanoic acid.

6. The method for preparing a high-power lithium iron phosphate cathode material according to claim 1, characterized in that, In the sintering process, preheating sintering is first carried out at a temperature of 300-400℃ for 2-4 hours, followed by high-temperature sintering at 700-800℃ for 5-10 hours.

7. The method for preparing a high-power lithium iron phosphate cathode material according to claim 1, characterized in that, In the grinding step, the viscosity of the ground powder slurry is 3500–5000 mPa·s.

8. The high-power lithium iron phosphate cathode material prepared by the preparation method of any one of claims 1 to 7.

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