Theoretical investigation on niobium doped LiFePO₄ cathode material for high performance lithium-ion batteries

In this work, the first principle study has been conducted to investigate the properties of LiFePO₄ (LFP) cathode material for the Li-ion batteries, and the effects of doping on their crystalline structures with Nb atoms was explored. The obtained results showed that doping of the LFP structure with Nb at the Fe site significantly improved the conductivity of LFP and decreased the band gap from 3.502 to 0.658 eV, which is consistent with the acquired density of state data. Moreover, energy formation results proved that the stability of the system after doping was ideal. The geometrical analysis of the structure showed that the Nb doping leads to widening of the Li passage channel, increased bond length, and reduction in covalent strength among atoms which in turn mitigated energy barrier. Furthermore, our calculation proved that Nb doping has no adverse effect on the theoretical voltage of LFP and, its value remained constant at about 3.1 V. Then, the nudged elastic band method was examined for the different pathways of Li ion diffusion in the LFP crystal. The finding represented that energy barriers of Li migration along all paths were decreased through Nb doping. In addition, the diffusion coefficient of Li-ion in the doped systems was improved by four orders of magnitude compared to undoped LFP. Moreover, the effective mass of electron theory and theoretical voltage of pristine LFP and Nb-doped LFP were evaluated. Finally, the electron localization function theory was implemented, for in-depth study and illustration of the difficulty of Li-ion diffusion and bond characteristics.