| dc.description.abstract | Sodium ion battery is the most suitable candidate to replace lithium ion battery as a large-scale energy storage system due to its absolute cost advantage and abundant natural resources, and the search for anode materials with high lifetime and high theoretical capacity is one of the major challenges. Titanium dioxide is considered as a promising anode material for sodium ion batteries because of its low toxicity, high chemical stability, easy availability and high theoretical capacity, but it has low conductivity due to high band gap.
In this study, we used density functional theory (DFT) to investigate the intercalation and diffusion behavior of sodium ions before and after doping with tantalum atoms from a microscopic perspective based on first principles calculations, and we also analyzed the Hirshfeld population, electron density difference map, and electronic structure. To find the most favorable tantalum doping distribution, we first performed full geometric optimization calculations for three common titanium dioxide polymorphs: anatase, rutile, and TiO2(B). Then, two different diffusion paths were designed according to different diffusion environments, i.e., whether the sodium ion passes through two neighboring tantalum atoms during diffusion, and the effects of different paths and tantalum doping were compared using the climbing image nudged elastic band (CI-NEB) method. The results show that tantalum dopants are not favorable for sodium ion transport due to size effects and higher Hirshfeld charges, and that in the case of anatase and rutile systems, the enhancement of the diffusion energy barrier is up to 9 folds when the sodium ion passes between two neighboring tantalum atoms compared to the pathway where the sodium ion is intercalated without passing through two neighboring tantalum atoms. On the contrary, the diffusion barrier of tantalum-doped TiO2(B) decreases slightly by 0.168 and 0.175 eV compared to that of sodium-intercalated pristine titanium dioxide.
Finally, from the perspective of electronic structure, tantalum doping generates upper spin polarization and forms new localized states, which effectively reduces the bandgap of anatase and TiO2(B) and promotes the electrochemical performance. We suggest that tantalum-doped anatase and TiO2(B) composite materials, and appropriate amount of doping maintains the stability of the diffusion performance of sodium ions, and establishes the potential of tantalum-doped titanium dioxide as a sodium-ion battery. | en_US |