Study On The Preparation And Modification Of High Rate Performance Lithium Iron Phosphate Cathode Material

LiFePO₄ is considered as one of mainstream commercial cathode material for power lithium ion batteries due to its high theoretical capacity,good thermal stability,good cycling performance and environmental friendliness etc.However,the low electronic conductivity and lithium ion diffusion coefficient deteriorate the rate performance of LiFePO₄,which highly limited its practical application in high power lithium ion battery.In this paper,the rate performance of LiFePO₄ is improved by graphene coating and morphology controling,and the energy density of LiFePO₄ is improved by alien ion doping.The effects of graphene or carbon content and feeding sequences in precursor on the structure and electrochemical properties of LiFePO₄ are investigated.The research contents and results are as follows:?1?LiFePO₄/graphene composites are synthesized by surfactant-assisted self-assembly method with the self-produced graphene oxide and commercial LiFePO₄ as primitive material,and the effects of the amount of graphene on the electrochemical performance of LiFePO₄/graphene composites is discussed.The LiFePO₄/graphene composites with graphene content of 6.2wt%exhibits best electrochemical performance,it delivers discharge capacity of132.6 mAh g^-11 at 5 C rate,which is much more improved than that of primitive LiFePO₄,87.5mAh g^-1.Menwhile,the LiFePO₄/graphene composite shows excellent cyclic stability,it shows capacity retention of 82.6%at 5 C rate during 1000 cycles while that of primitive LiFePO₄ is49%.?2?LiFePO₄ nanoparticles are prepared by glycol-based solvothermal method with LiOH,FeSO₄ and H₃PO₄ as precursor materials,the effect of feeding sequences in precursor on the electrochemical performance of the products are investigated.The result shows that LiFePO₄with shorter b-axis can be synthesized by adding H₃PO₄ into the FeSO₄ firstly,which shows better electrochemical performance.It shows specific discharge capacities of 157.3 and 140.1mAh g^-1 at the 0.2 and 5 C rates,respectively.On this basis,the effects of carbon content on LiFePO₄ are studied.The result shows that the electrochemical performance of LiFePO₄ can be improved by carbon coating and the electrochemical performance in high rates can be increased with increase of carbon content.?3?LiFe_0.5Mn_0.5PO₄ nanoparticles are synthesized by glycol-based solvothermal method with LiOH,FeSO₄,MnSO₄ and H₃PO₄ as precursor materials and the carbon coating are carried out.The effects of feeding sequences in precursor on morphology,structure and the electrochemical performance of the products are investigated.The result shows that the smaller particles with short b-axis can be synthesized by adding H₃PO₄ into the mixture of FeSO₄ and MnSO₄ firstly,which increased the energy density of LiFePO₄ under low current density.XRD and SEM analysis of different reaction time products are studied to illustrate the mechanism of the feeding sequence.