Modification of LiFePo4 by Citric Acid Coating and Nb5+ Doping
In order to improve the electrochemical performance of LiFePO4, Li0.99Nb0.01FePO4/C composite materials were synthesized with citric acid coating and Nb2O5 doping. The physical chemistry and electrochemical performances of Li0.99Nb0.01FePO4/C were investigated by X-Ray diffraction (XRD) and Transmission electron microscope (TEM). The results show that Li0.99Nb0.01FePO4/C has smooth charge-discharge voltage platform, with first capacity of 151.6 mAh•g-1 (0.1C) and no obvious capacity fading after 16 cycles. The materials have favorable high rate discharge performances, with the first capacity of 131.6 mAh•g-1 at 0.5C, 119.8 mAh•g-1 at 1.0C and 106.2 mAh•g-1 at 2.0C. Equivalent circuits analysis shows that the impedance, especially electrode reaction resistance of Li0.99Nb0.01FePO4/C are significantly reduced compared with the pure LiFePO4 and this helps to improve the electric conductivity, discharge capacity and cycle performance. The diffusion coefficient of Li+ in Li0.99Nb0.01FePO4/C is 2.51×10-12 cm2•s-1 increased two orders of magnitude in comparison with the pure LiFePO4. Carbon-coating makes a more remarkable contribution to lithium diffusion than Nb5+ ion doping.
Xianjun Lu and Jun Qiu
D. Y. Zhang et al., "Modification of LiFePo4 by Citric Acid Coating and Nb5+ Doping", Advanced Materials Research, Vol. 158, pp. 167-173, 2011