Study On The Electgrochemical Performances Of Carbon Coating And Mg²⁺, Cr³⁺,Ti⁴⁺ Doped LiFePO₄ Used As Cathode Materials

Olivine-type LiFePO₄ has many advantages compared with other conventional cathode materials. LiFePO₄ has high theoretical capacity, rich resources, high safety, non-toxcity and good thermal stability. Due to the advantages of those, the material is a new promising cathode material for lithium-ion batteries. But the main disadvantages of LiFePO₄ are its low electronic conductivity and Li-ions diffusion coefficient. Due to those, the charge-discharge capacity and the high rates capacity of the LiFePO₄ will decrease greatly. To improve electronic conductivity and Li-ions diffusion coefficient of LiFePO₄, many researchers adopted the methods of LiFePO₄ added with carbon and doped with metal ions.LiFePO₄ cathode material was prepared by high temperature solid-state method and modified by carbon surface treatment, which used glucose, sucrose and phenolic resin as carbon source. The result shows that the crystalstructure of LiFePO₄ added with carbon is indexed as olivinic structure by XRD diffraction analysis. When the mass percentage of carbon which uses phenolic resin as carbon source is 3wt%, the charge capacity at 0.1C rate is 148.97mAh/g, the discharge capacity is 147.49mAh/g, and the charge-discharge efficiency is 99.02%. When the mass percentage of carbon which uses phenolic resin as carbon source is 5wt%, it shows the bset electrochemical performance. Its charge capacity at 0.1C rate is 150.75mAh/g, the discharge capacity is 150.69mAh/g, and the charge-discharge efficiency is 99.96%. The discharge capacity at 0.5C and 1C rate is 121.97mAh/g, 92.85mAh/g.LiFePO₄ cathode material was synthesize by high temperature solid-state method and doped with metal ions (Mg²⁺, Cr³⁺, Ti⁴⁺). The result shows that the crystalstructure of LiFePO₄ doped with metal ions is also the olivinic structure by XRD diffraction analysis. Ti⁴⁺-doped LiFePO₄ shows the best electrochemistry performance. When the mole fraction of the atomic number of Ti/the atomic number of Ti and Fe is 3%, sample’s interfacial resistance is 33.68?, which shows the best electrochemical impedance capacity. Its discharge capacity at 0.1C rate is 132.70mAh/g. When the mole fraction of the atomic number of Ti/the atomic number of Ti and Fe is 2%, sample’s discharge capacity is 98.74mAh/g and 83.08mAh/g at 1.0C and 3.0C rate.Carbon surface treatment and metal ions doping can improve the electronic conductivity and tap density of LiFePO₄. It is conducive to improve the electrochemistry capacity of LiFePO₄.