Improved High Rate Electrochemical Performances Of Metal Oxide And Carbon Co-coated LiFePO₄ Cathode Materials

Three methods were adopted in this work to improve electrochemical properties and to reduce particle size of LiFePO₄ cathode materials based on its low electronic conductivity and lithium-ion diffusion rate. The first was metallic ion doping and carbon coating LiFePO₄; The second is metal oxide and carbon co-coating LiFePO₄; The last is freeze-drying to synthesize the nanometer-sized precursor of LiFePO₄/C. Different metallic ions, carbon sources, metal oxides and synthesis methods are studied, focusing on their influences on charge-discharge property, cycle ability, electrochemical kinetics parameters such as charge transfer resistance, exchange current density i0, diffusion coefficient D and conductivityσ. The microstructure and morphologies of these composites were investigated by XRD, SEM and HRTEM. The electrochemical performances were evaluated by galvanostatic charge-discharge, impedance spectroscopy and cyclic voltammogram.LiFePO₄/(C+La³⁺) and LiFePO₄/(C+Ti⁴⁺) cathode materials were synthesized by microwave heating. The results indicated that the metallic ion doping and carbon coating could greatly enhance the electronic conductivity of LiFePO₄, especially when the radius of doping ion is similar to the ones of Li⁺ and Fe²⁺. The reason is that the distortion of crystalline lattice is relatively small and the structure is stable, accordingly, the electrochemical performances, particularly the high rate cycle performances, were obviously improved.The carbon-coated LiFePO₄ composite was synthesized via sol-gel method and two times firing procedure with keeping 8 h at 600℃, using oxalic acid as carbon source. It was demonstrated that the highest discharge specific capacity of 129.0 mAh·g-1 after 10 cycles at 0.2C rate was obtained for this composite cathode, with the capacity fading being only 1.07%. La_0.7Sr_0.3MnO₃+C co-coated LiFePO₄ and CuO+C co-coated LiFePO₄ were gained by suspension mixing process and precipitation method, respectively. Nanolayer structured La0.7Sr0.3MnO3 or CuO together with the amorphous carbon layer formed an integrate network arranged on the bare surface of LiFePO₄ as corroborated by high resolution transmission electron microscopy. La_0.7Sr_0.3MnO₃+C co-coated LiFePO₄ delivered a high discharge specific capacity of 134.3 mAh·g-1 after 35 cycles at 0.5C rate. CuO+C co-coated LiFePO₄ showed a discharge specific capacity of 125.0 mAhg-1 at 1C, and the capacity remained to be 123.0 mAhg-1 after 20 cycles. The capacity fading is only 1.6%.LiFePO₄/C composite synthesized by freeze-drying method was used as a matrix. Then it was further coated with La0.7Sr0.3MnO3 or ZnO by suspension mixing process or precipitation method, respectively. The results indicated that the particle sizes of co-coated composite were about 50 nm. When the content of La0.7Sr0.3MnO3 was 2%, it delivered the highest discharge capacity of 143.4 mAh/g and 133.6 mAh/g at 0.5C and 1C, respectively. When the content of ZnO was 2%, the materials showed the best electrochemical properties. The discharge capacity reached 130.7 mAh g-1 and 122.6 mAh g-1 at the rates of 1C and 2C, respectively. The cycle performances were also very good. The three cyclic voltammogram curves were almost overlapping after 55 discharge cycles, demonstrating that the continuous integrated conductive layer between LiFePO₄ particles resulted in negligible polarization during cycling.