Effect of Nb doping at Fe site on the cycling stability and rate capability of LiFePO4 for lithium-ion batteries

Olivine LiFePO4 structures with long cycle life, low cost, and environment friendly characteristics have widely been used as cathode effective constituents for lithium-ion batteries (LIBs) in China. However, their low electronic conductivity limits their wide applications in high-range electric vehicles. Here, Nb5+ is doped at the Fe site of LiFePO4 on the basis of first-principles calculation, and Nb5+ is doped into the crystal lattice of LiFePO4 to improve the electronic conductivity. The crystal cell parameter of the LFNbPO-0.75 sample (0.75% Nb2O5-doped LiFePO4) is smaller than that of the bare LiFePO4 sample, the micromorphology of LFNbPO-0.75 sample becomes fine and uniform in the process of Nb5+ doping, and the Nb5+ doped into LiFePO4 expands the crystal plane spacing, which is conducive to Li+ diffusion. Amongst all the doped samples, the Li+ diffusion coefficient of LFNbPO-0.75 is the largest, and the redox peak of LFNbPO-0.75 is more symmetrical, sharper and narrower, indicating that the proper amount of Nb5+-modified LiFePO4 can improve the charge and discharge specific capacity with better reversibility and capacity retention. The discharge capacity in LFNbPO-0.75 for LIBs is 169.87 mAh·g−1 in the first cycle at 1.0 C, which is approximately the same as the theoretical capacity of 169 mAh·g−1, and the corresponding capacity retention rate is 99.03% after 100 cycles. These results promote the wide applications of LiFePO4batteries in high-range electric vehicles.