The co-doping effects of Zr and Co on structure and electrochemical properties of LiFePO4 cathode materials

In this work, the band conduction mechanism of Zr and Co co-doped LiFePO4 is studied by a first-principle calculation, which demonstrates Zr, Co co-doping can decrease the band gap of LiFePO4 indicating the electrochemical properties can be improved by Zr and Co co-doping. The pristine LiFePO4 and Li0.99Zr0.0025Fe1-xCoxPO4 (x = 0.005, 0.01, 0.015, 0.02) cathode materials are synthesized by a conventional high-temperature solid-state method. All the samples exhibit a highly crystallized olivine structure of LiFePO4, which demonstrates Zr and Co co-doping does not destroy the olivine structure. A small amount of Co and Zr co-doping can modify the morphology of LiFePO4 cathode. Combining the improved structure with the modified particle morphology, the co-doped Li0.99Zr0.0025Fe1-xCoxPO4 cathodes exhibit the enhanced electrochemical performance. The rate capacity and cycle stability of Li0.99Zr0.0025Fe1-xCoxPO4 cathodes enhanced with the Co content and reached the largest values at x = 0.01, then decreased with the further increase of Co content. The Li0.99Zr0.0025Fe0.99Co0.01PO4 composites show the best rate capability and cycle stability, which have a discharge capacity of 139.9 mAh/g at 0.1 C with a capacity retention ratio of 85% after 50 cycles. The calculated results indicate that the Zr and Co co-doped LiFePO4 have a low lithium ion diffusion energy barrier and high lithium ion diffusion coefficient, which attributes to the enhanced electrochemical performance.