Synthesis And Electrochemical Performance Of High Power Density LiFePO₄Cathode Materials

As a new kind of lithium-ion battery cathode material, LiFePO₄has high specific capacity, goodsafety performance, low costs and environmental friendliness. It has been industrially producted bysome companies. In order to meet the new requirements of lithium-ion battery, we need to improvethe methods of synthesising LiFePO₄or modify it to enhance its electrochemical properties. Variousapproaches have been used to solve this problem, which include: i) decreasing the particle size, ii)doping LiFePO₄with foreign atoms and iii) surface coating or admixing with electronicallyconductive materials. In this paper, LiFePO₄prepared with the assistance of hydrothermal method hasspecial morphologies, which is further modified by high conductive materials. The details are asfollows:（1）The carbon coated LiFePO₄with porous structure has been successfully prepared byhydrothermal method and following annealing processes. The effects of pH values, temperatures andannealing times on the electrochemical capacitance of the C-LiFePO₄composite were investigateddetailedly. The results demonstrated that the LiFePO₄composite obtained from pH=7.0and annealingat650°C for15h exhibits best electrochemical performance.（2）LiFePO₄was modified by multi-wall carbon nanotubes which have high electricalconductivity. The CNTs contacted with the carbon formed a3D conducte network, which helps toimprove the electrochemical performance of LiFePO₄. The CNTs-C-LiFePO₄composite showsdischarge capacities of161mAh/g at0.2C and97mAh/g at5C. The CNTs-C-LiFePO₄compositehad excellent cyclability with the capacity retention about92.1%after80cycles at1C.（3） LiFePO₄composite was modified by graphene which has larger surface area. Its sheetstructure reduced the reunion between LiFePO₄particals. Graphene has high electrical conductivity soit can contacted with the carbon formed3D conducte network, improved the electrochemicalperformance of LiFePO₄. The results showed that the LiFePO₄composite discharge capacity of162mAh/g at0.2C. At10C the discharge capacity is95mAh/g exhibited a higher discharge capacity.The LiFePO₄composite had excellent cyclability with the capacity retention about89.5%after200cycles at10C rate.（4） Graphene modified LiFePO₄microspheres were successfully synthesized by usingNH₄H₂PO₄as starting material, the composite was used as the cathode material for aqueous lithiumbatteries. The addition of graphene improved the electrochemical performance of LiFePO₄anddecreased the side reactions. So, the LiFePO₄/graphene composite had excellent cyclability.