Study On The Electrochemical Properties And Modification Of LiFeO₄Cathode Materials

The intrinsic drawbacks of LiFePO4cathode material, such as:low electronic conductivity, low ionic conductivity, low ion diffusion coefficient and low tap density, signicantly limit its pratical applications. In order to solve these problems, two methods were applied to prepare materials by choosing different raw materials in our study, including Sol-gel and high temperature solid state method. In addition, Li and Fe site doped with exotic metal ion were also investigated.(1) The cathode materials were prepared by high temperature solid state method with Li2CO3and LiNO3as different Lithium sources. SEM results revealed that the sample prepared with Li2CO3as Lithium source had the smaller particles and was more regularity than that of LiNO3, while the particles of the sample prepared with LiNO3presented layer overlap structure. Charge/discharge measurement at0.2C indicated that the specific discharge capacity of samples prepared with Li2CO3and LiNO3were136mA·h/g and117mA·h/g, respectively. The sample prepared with Li2CO3showed higher specific discharge capacity and more excellent high rate discharge ability.(2) Mg-doped LiFePOC composites via Li site replacement were synthesized by Sol-gel method according to the Diagonal principle. The rusults showed the materials’charge transfer impedance increased and the discharge ability at high current rate became weaker by doping with Mg2+. Meanwhile, the cathode materials by doping with Mg2+showed poor cyclic reversibility, which may be attributed to the destruction of stabilized olivine structure of LiFePO4by Mg2+replacement.(3) LiFePO4/C composites were prepared by high temperature solid state method with Ti3+and Ce3+doped via Fe site replacement. The charge transfer impedance of the sample by doping with Ti3+became lower and the diffusion rate of Li ion increased, resulting in better discharge ability of the cell. The sample by doping with0.06Ti3+showed highest discharge capacity, which could reached152mA h/g and125mA·h/g at0.2C and5C respectively. The samples by doping with Ce3+showed excellent high rate discharge ability, but it was difficult to substitute Ce3+for Fe site because of the large difference of atomic radius. Increased the amount of Ce3+was no good for the materials because it didn’t increase the doped amount but it generated CePO4and decreased the amount of PO43-, which resulted in poor specific capacity of the composite. The discharge capacity of the sample by doping with Ce3+was135mA·h/g and111mA·h/g at0.2C and5C, respectively.