In addition to well-known H + and Li + systems, pseudocapacitive materials can also reversibly store charge in “beyond-lithium” systems including Na +, K +, and Zn 2+ 4, 5, 6, 7, 8, 9, 10. Pseudocapacitive materials store charge through Faradaic reactions at rapid rates, offering a route for achieving both high energy and power densities 1, 2, 3. 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. The surface-redox properties of TiO 2(A) result in excellent rate capability, cycling stability and low overpotentials. 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. 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. 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. 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. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO 2(A)). However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium.

Tbot tio2 core shell