Surface-redox sodium-ion storage in anatase titanium oxide.

Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO 2 (A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO 2 (A) becomes amorphous but still undergoes Ti 4+ /Ti 3+ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO 2 (A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g -1 , at high charge/discharge rates. Kinetic studies of TiO 2 (A) nanoparticles indicate that sodium-ion storage is due to a surface-redox mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO 2 (A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO 2 (A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, tailoring the surface-redox mechanism enables thick electrodes of TiO 2 (A) to retain high rate properties, and represents a promising direction for high-power sodium-ion storage.