Fluorine Triggered Surface and Lattice Regulation in Anatase TiO2- x Fx Nanocrystals for Ultrafast Pseudocapacitive Sodium Storage.

Sodium-ion batteries (SIBs) have been considered as one of the most promising secondary battery techniques for large-scale energy storage applications. However, developing appropriate electrode materials that can satisfy the demands of long-term cycling and high energy/power capabilities remains a challenge. Herein, a fluorine modulation strategy is reported that can trigger highly active exposed crystal facets in anatase TiO2- x Fx , while simultaneously inducing improved electron transfer and Na+ diffusion via lattice regulation. When tested in SIBs, the optimized fluorine doped TiO2- x Fx nanocrystals exhibit a high reversible capacity of 275 mA h g-1 at 0.05 A g-1 , outstanding rate capability (delivering 129 mA h g-1 at 10 A g-1 ), and remarkable cycling stability with 91% capacity retained after 6000 cycles at 2 A g-1 . Importantly, the optimized TiO2- x Fx nanocrystals are dominated by pseudocapacitive Na+ storage, which can be attributed to the fluorine induced surface and lattice regulation, enabling ultrafast electrode kinetics.