Synthesis And Electrochemical Performance Of LiFePO₄as Cathod Material For Lithium Ion Batteries By Two-step Liquid-action Method

Olivine-structured lithium iron phosphate is the most promising positive electrode material for lithium-ion batteries materials, because of various advantages, such as abundance of raw materials, high theoretical capacity, stable crystal structure, excellent cycling performance, preferable safety and environmentally friendly. Although there are various methods to prepare LiFePO4, each method is always has it’s shortcomings.Therefore, based on the LiFePO4as research object, this paper puts forward a more convenient and efficient preparing method: two-step liquid-action method to synthesis LiFePO4, and the best synthesis condition is also researched.Firstly, FePO4xH2O precursor was synthesized from FeSO4·7H2O, NH4H2PO4and oxidant H2O2under various concentrations of solution. LiFePO4cathode material was then synthesized by liquid-action method with the homogeneous mixture of CH3COOL、FePO4·xH2O and reductor. The effect of the solution concentration on the performance of FePO4·xH2O was researched. The structures of FePO4·xH2Oand LiFePO4were characterized by XRD and SEM. The effect of the solution concentration on the electrochemical performance of LiFePO4was also discussed. The results show that no impurities exist in the FePO4·xH2O synthesized at solution concetration from0.01mol/L to1.Omol/L. LiFePO4made from FePO4·xH2O precursor synthesized with the solution concentration of1.Omol/L shows excellent electrochemical performance. It has an initial discharge capacity of157.2mAh·g-1at0.1C and capacity retention remains95.36%after30cycles.Secondly, LiFePO4was synthesized from CH3COOLi, reductor and FePO4·xH2O precursor synthesized with the solution concentration of1.Omol/L. The effects of RA content in the mixture reductor, acalcination temperature and calcination time on LiFePO4were studied in detail. The results show that no impurities exist in the LiFePO4synthesized with the mole ratio of RA in the mixture reductor from0.1to0.5. When the mole ratio of RA was0.1, LiFePO4sintered at600℃for2h in Ar ambience has better crystallinity, uniform particle sizes and electrochemical performance. It has an initial discharge capacity of164.8mAh·g-1at0.1C and capacity retention remains98.91%after30cycles.At last, after the synthesis condition of LiFePO4was optimized, solvent recycling was researched. Results show that solvent water can be re-used directly without any processing5times for preparing the precursor FePO4·xH2O. Although the number of crystallization water contained in the the precursor FePO4·xH2O were different, the final production of LiFePO4synthesized with raw material precursor FePO4xH2O has not much differerence in the structure and electrochemical performance. The solvent ethanol can be recycled after be distilled for preparing LiFePO4. Under the best synthesis condition, LiFePO4made from FePO4·xH2O precursor synthesized with the fifth recycled water and distilled ethanol shows excellent electrochemical performance. It has an initial discharge capacity of156.3mAh·g-1at0.1C and capacity retention remains99.49%after30cycles.