Research On Modification And Electrochemical Performance Of Cathode Material LiFePO₄/C For Lithium-Ion Batteries

The olivine-type cathode material LiFePO₄ for Lithium ion battery has been found to exhibit rich resources,cheap price,good cycle stability performance,high specific capacity,no memory effect as well as higher safety characteristics,and it is one of lithium ion battery cathode material with broad prospect for development.However,the olivine-type LiFePO₄ of Lithium ion battery suffers from bad tap density,lower chemical diffusion coefficient,poor electric conductivity,which are large obstacles to restrain its application.Optimizing the synthetic process,adding carbon and doping ion in the raw material can improve electrochemical performance of LiFePO₄?In this paper,LiFePO₄ was synthesized from FeC₂O₄·2H₂O,which was as the source of iron,by high temperature solid method,and the physical and electrochemical properties of the material were characterized by X-ray diffraction?XRD?,scanning electron microscope?SEM?and galvanostatic charge-discharge test.The effect of temperature and time of calcinations,lithium source and secondary add carbon source and carbon content on crystal structure,morphology and electrochemical performance of the sample of LiFePO₄ was studied.The results showed that the optimum conditions were LiH₂PO₄ as lithium source and add 10% polyvinyl alcohol?PVA?,T=700? and t=10h.The LiFePO₄ synthesized under the optimum conditions had olivine-type and the crystal structure was complete.Its partical size was uniform.It had excellent electrochemical performance.The results of charge-discharge test showed that the initial discharge capacity of this sample was 118.3mAh/g in the voltage range of 2.4⁴.2V at 1C.And the capacity retention after 60 cycles was 95.31 %.LiFePO₄ cathode material was modified by doping method.The effect of Nb-doping,on crystal structure,morphology and electrochemical performance of the sample of LiFePO₄ was studied.The results showed that Nb-doping didn't change the structure of LiFePO₄ and Nb-doping could improve its charge and discharge capacity.The sample of LiFePO₄ had complete structure.Its partical size was uniform.The results of charge-discharge test exhibiting a good rate performances and cycling stability.The initial discharge capacity of this sample was 140.7mAh/g in the voltage range of 2.4⁴.2V at 1C.And the capacity retention after 60 cycles was 97.74%.Take advantage of Nb-doping can improve the charge and discharge specific capacity of materials and F-doping can improve the ratio of material performance,Nb-F-doping was studied.The results showed that Nb-F-doping didn't change the structure of LiFePO₄ and had complete structure,the sample of LiFe_0.98Nb_0.02PO_0.97F_0.03/C had the best electrochemical properties.The charge-discharge test showed that the initial discharge capacity of this sample was 148.8mAh/g in the voltage range of 2.4⁴.2V at 1C.And the capacity retention after 60 cycles was 98.74%.The sample exhibiting a good rate performances and cycling stability.