Research On Preparation Of Lithium Iron Phosphate By Hydrothermal Method Based On Optimization Model And Its Modification

In recent years,lithium-ion batteries have been widely used as electronic energy storage and conversion systems in electronic products,and the growing demand for renewable energy and efficient energy storage indicates that lithium-ion batteries will continue to play a key role in the future.Lithium iron phosphate?LiFePO₄?has been extensively studied because its low price,environmental protection,high theoretical capacity,long cycle life and excellent safety performance.However,in high-rate applications,it is limited by slow lithium ion and electron transport kinetics.In this paper,lithium iron phosphate was synthesized by hydrothermal method,and the modification of carbon material coating and Cerium doping was carried out to improve low conductivity and low lithium ion diffusion rate of LiFePO₄ cathode material.The conditions of hydrothermal preparation of LiFePO₄ were optimized by factorial design of experiment.The heating time,heating temperature and ascorbic ac id dosage were set as the variable factors and the initial discharge specific capacity of 0.1 C was used as the response to establish an effective model.In the experimental setting range,the ascorbic acid dosage has the greatest influence on the discharge specific capacity,and the heating time and heating temperature have the same effect on the discharge specific capacity.The interaction effect on the discharge specific capacity between ascorbic acid dosage and heating time is the largest,and the interaction between heating temperature and heating time is second,and the interaction between ascorbic acid dosage and heating temperature is minimal.The optimum conditions for the preparation of LiFePO₄ by hydrothermal method were heating time of 9 h,heating temperature of 210? and ascorbic acid dosage of 1.5 mmol.LiFePO₄ was prepared based on optimized conditions,and then modified by ball milling to introduce different amounts of carbon nanotubes or graphene for coating modification.Under the test of 2.5?4.2 V,the effect of LiFePO₄ coated with 1 wt% carbon nanotubes was good,the specific discharge capacity of 0.1 C was 159.3 m Ah·g^-1,and the 5 C was maintained at 124.8 m Ah·g^-1.After for 500 cycles,the capacity retention of 2 C was 100%.The effect of LiFePO₄ containing 3 wt% graphene coating was outstanding.The discharge specific capacity of 0.1 C was 163.2 m Ah·g^-1,5 C was maintained at 130.3 m Ah·g^-1.After for 500 cycles,the capacity retention of 2 C was 100%.In the cerium-doping modification study,2% cerium-doped LiFePO₄ had a specific discharge capacity of 158.3 m Ah·g^-1 at 0.1 C,and at 5 C was 111.7 m Ah·g^-1 and the capacity retention rate was 97.42% after 500 times at 2 C.Compared with the unmodified material,the discharge properties of the carbon material coating and the cerium doped modified LiFePO₄ were improved.The electrochemical properties of the materials were evaluated by different characterization methods and electrochemical test methods.The results show that both carbon nanotube and graphene coating modification and cerium-doping modification are effective to improve the conductivity and lithium ion diffusion rate of LiFePO₄.