Investigating lithium intercalation and diffusion in Nb-doped TiO₂ by first principles calculations

1) Background: Recent research has shown growing interest in developing titanium dioxide (TiO₂) based anode for lithium ion batteries due to its high theoretical specific capacity, safety, chemical stability, and abundance. Niobium-doped (Nb-doped) TiO₂ anode was proposed for lithium ion batteries and demonstrated to improve cycling stability and cycling performance. 2) Methods: In this study, first principles calculation based on density functional theory (DFT) was used to reveal and understand the mechanism of Nb-doped TiO₂ outperforming pristine TiO₂ as the anode material of lithium ion batteries. The lithium intercalation energy, lithium diffusion energy barrier, electron density difference mapping, and electronic structure of Nb-doped TiO₂ at dilute lithium concentration were computed and compared to those of pristine TiO₂. 3) Significant Findings: For all three investigated polymorphs: anatase, rutile, and TiO₂(B), Nb-doping enhances the lithium intercalation process by lowering the intercalation energy, but slightly increasing the energy barrier of lithium diffusion due to stronger interaction between the intercalated lithium and polaron induced by Nb dopant in TiO₂. From the analysis of electronic structure, new energy states are formed, induced by doping with Nb. These new states narrow the band gap and shift Fermi levels towards the conduction band, thus facilitating improvement of electronic conductivity for all three phases studied.