Preparation And Electrochemical Properties Of LiFePO₄ Cathode Materials

In recent years,lithium-ion batteries（LIBs）with high energy density have been widely used in portable electronic devices,electric vehicles（EVs）,and grid energy storage.LiFePO₄ cathode materials with olivine structure in Li-ion batteries have attracted extensive attention due to environmental friendliness,safety,and long cycle life.However,the large-scale application of LiFePO₄ cathode materials is limited by low electronic conductivity,ionic conductivity,and green preparation.In this article,starting from the efficient synthesis process of FePO₄·2H₂O with flake morphology,a new method for the synthesis of iron phosphate precursors was developed.The performance of LiFePO₄ cathode material was improved by optimizing the primary particle morphology and particle size of the precursor.The specific research results are as follows:Using iron powder as the iron source,FePO₄·2H₂O is synthesized by a short-flow liquid phase precipitation.No harmful gas is generated during the synthesis process,excess phosphoric acid is recovered and recycled,and no waste water is generated,which is environmentally friendly.By changing the amount of phosphoric acid in the synthesis process,the effects on the structure,morphology and electrochemical properties of the synthesized materials were studied.The results show that the iron phosphate prepared by adding 3 times the theoretical amount of phosphoric acid has a dispersed lamellar morphology and a uniform primary particle size.The LiFePO₄/C material prepared with this material inherits the iron phosphate morphology and has optimized electrochemical performance.The material has a specific discharge capacity of 149 m Ah/g at a rate of 1 C,and a capacity retention rate of98.9%after 200 cycles.In order to further reduce the cost,a cheaper magnetite is used as the iron source.The research shows that the removal rate of Si and Al is93.01%and 53.3%when Na OH is used for pre-removal.By comparing different synthesis processes by various means of testing,the synthesis route of transformation precipitation was determined.The study found that the FePO₄ with flake morphology was prepared by the transformation precipitation method,and the removal rate of Si,Al,Ca,Mg impurity ions in the finished iron phosphate was 97.73%,79.33%,93.24%,95.10%.At the same time,by collecting the phosphoric acid in the filtrate for recycling,the feasibility of the phosphoric acid recovery and recycling process was explored and the synthesis process was improved.The synthesized LiFePO₄/C material has nano-scale primary particles,showing excellent electrochemical performance,the discharge specific capacity reaches 141.6 m Ah/g at 1 C rate.The iron-containing zinc oxide raffinate was selected,and a new slag-free route for removing iron ions in the zinc oxide raffinate by iron phosphate precipitation was explored,which improved the utilization value of iron.In order to reduce the impurity content and improve the utilization rate of iron ions,a p H-regulated conversion synthesis process was designed.The content of Zn in iron phosphate is 4200 ppm,the content of S is only 27 ppm,and the utilization rate of iron ions reaches99.23%.At the same time,the effects of different sintering temperatures on the synthesized LiFePO₄/C materials are compared.It is found that when the sintering temperature is too low,the primary particles will crystallize incompletely,the grain boundaries are not obvious;if the temperature is too high,the primary particles will grow up,which is not conducive to the diffusion of lithium ions.When the sintering temperature is 650°C,the material has the smallest dispersed particles and shows the best electrochemical performance.The lower discharge specific capacity reaches 126 m Ah/g at 1 C rate.38 figures,10 tables and 108 references...