Study On Synthesis And Performance Of LiMnPO₄ As Lithium Ion Battery Cathode Materials

Olivine structure Li Mn PO4 is considered to be one of the most promising lithium ion battery cathode material,which has rich raw materials, green environmental protection, high discharge voltage, high theoretical specific capacity, high energy density, and the advantages of good stability. However, poor ion and electron conductivity have an effect on the performance of the material ratio seriously, which limits its further commercialization. In this paper, Li Mn PO4 are synthesized by two different methods, and modified by coating surface carbon, doping Fe2+, compositing Li3V2(PO4)3. Then structure and electrochemical performance are studied. Main work includes:(1) Li Mn PO4/C was synthesized by two step method with citric acid as carbon resource at different reaction time and temperature, and Li Mn PO4/C was doped with Fe2+, doping Fe2+ can improve the internal electrical conductivity of materials, so as to improve the electrochemical properties of materials. The electrochemical results show that Li Mn PO4/C with the best electrochemical performance was synthesized at 700 ℃, 12 h with citric acid as carbon resource,with discharge specific capacity of 74.4 m Ah?g-1 at the rate of 0.05 C, after doped with Fe2+, electrochemical performance of material was significantly improved, when the doped iron content is 0.3, discharge specific capacity of Li Mn0.7Fe0.3PO4/C was up to 160.2 m Ah?g-1 at 0.05 C, close to the theoretical capacity.(2) Li Mn PO4/C was synthesized by the sol-gel method employing DMF(N, N-dimethylformamide) as a dispersing agent, at different reaction time and temperature, and Li Mn PO4/C was doped with Fe2+ and composited with Li3V2(PO4)3, doping Fe2+ can improve the internal electrical conductivity of materials, compositing Li3V2(PO4)3 can improve the ionic conductivity of the material. The electrochemical results show that Li Mn PO4/C with the best electrochemical performance was synthesized at 700 ℃, 12 h with glucose as carbon resource.the results show that doping with Fe2+ can improve the electrochemical performance and cycling stability significantly, capacity of Li Mn PO4/C have few decrease after cycling 50 times, the result of EIS values shows that the composite materials have much lower charge transfer resistance and Warburg impedance, when the doped iron content is 0.5, Li Mn0.5Fe0.5PO4/C delivers the best discharge specific capacity of 161.5m Ah?g-1 at 0.05 C, close to the theoretical capacity, and 132.0 m Ah?g-1 at 1 C. Compositing with Li3V2(PO4)3 can improve the electrochemical performance of material, because of improved ion conductivity,and also improved Li3V2(PO4)3 multi-platform shortcomings. when value of x is 0.5, Li Mn0.5Fe0.5PO4/C delivers the best electrochemical performance with a discharge specific capacity of 150.5 m Ah?g-1 at 0.05 C, 115.7 m Ah?g-1 at 1 C.(3) x Li Mn PO4?y Li Fe PO4?z Li3V2(PO4)3/C composites were synthesized by the sol-gel method employing DMF(N,N-dimethylformamide) as a dispersing agent. The results shows that 0.5Li Mn PO4?0.2Li Fe PO4?0.3Li3V2(PO4)3/C delivers excellent rate performance and cycling stability with a discharge specific capacity of 155.0 m Ah?g-1 at 0.05 C. CV curve shows that the material has better reversibility, is a promising new type of lithium ion battery cathode material.