Effect of Nb-Doping at Fe-Site on the Electronic Structure and Property of LiFePO₄

The electronic structures of Nb-doped LiFePO₄ were calculated by density functional theory (DFT) of general gradient approximation (GGA) based on the first principle method. The effects of doping amount on the cell parameters and Fermi levels were analyzed. Nb-doping to LiFePO₄ resulted in the increase of cell parameters, and the cell volume increased from 0.280 8 nm³ to 0.295 2 nm³ when the Nb amount in mole ratio increased from 0 to 0.10. The change of geometrical structure of this system indicated that Nb-doping could improve the electric conductivity of LiFePO₄. The band gap between LUMO and HOMO decreased from 0.75 eV (LiFePO₄) to 0.62 eV (LiFe_0.90Nb_0.10PO₄). 4d5s of Nb and 3d4s of Fe both provided the electron, and Nb-doping improved the electric conductivity. The peak intensity of valence bands and conduction bands increased near the Fermi level. Fermi energies firstly increased and then decreased with increasing Nb amount, and LiFe_0.90Nb_0.05PO₄ received the maximum of 3.871 eV. A small amount of doped Nb had an effect on the electronic structures, indicating that the electrochemical property of LiFePO₄ could be improved. The theoretical result was consistent with the experimental data.