Synthesis And Electrochemical Properties Of LiFePO₄ Cathode Material

Olive LiFePO₄ is a promising cathode material of lithium-ion battery, especially to the LIB used for EVs and HEVs, due to its relatively large theoretical capacity, low cost, environmental friendliness and safety. In this work, carbon coated and cation doped LiFePO₄ have been synthesized by solid-state (two-step annealing) and sol-gel methods. Influence of synthesis methods, carbon coated, cation doped, and ground followed was investigated. The Li⁺ diffusion coefficient of doped LiFePO₄ at different discharge states was estimated and analyzed.Fine LiFePO₄ were obtained without purity by sol-gel process. However, three kinds of purities, varying with carbon coated and/or cation doped, were found to be mixed with LiFePO₄ solid state synthesized.Cell parameters and performance of Mg-doped LiFePO₄ synthesized from raw materials with stoichiometric amounts in LiFe0.98Mg0.02PO4 and Li0.98Mg0.02FePO4 were compared, and the latter exhibited better electrochemical performance with larger cell parameters. Difference between them was over 10mAh g-1(0.1C). Carbon coated LiFePO₄ with 10wt.% carbon by solid state method and lower 6wt.% carbon by sol-gel process exhibited the best electrochemical performance.Specific capacity in the discharge of the former was 136.4mAh g-1(0.1C) at room temperature and 130.9mAh g-1(0.5C) at 50℃. LiFe0.98Mg0.02PO4 also delivered larger specific capacity of 118.5mAh g-1 at 0.1C with larger cell parameters among Cu-doped materials. In contrast, carbon coated materials exhibited better performance than the M-doped one.Diffusion coefficient (lgDLi≈-10) reduced gradually in the discharge process, and after lgDLi<10.4 the profile would slope severely. Although carbon coated materials could deliver higher specific capacity, the discharge profile of M-doped materials were flatter due to their higher lithium ion conductivity.