Promoting amorphization of commercial TiO2 upon sodiation to boost the sodium storage performance

Anatase TiO2 is a promising anode material for sodium-ion batteries, yet the low electronic and ionic conductivities are the main obstacles for its practical application. Even though the amorphization of TiO2 upon sodiation has already been observed, its underneath mechanisms are not fully elucidated. Herein, a low-cost nitrogen-containing carbon source of polyacrylonitrile is adopted to modify commercial anatase TiO2 by a convenient and nontoxic ball-milling technique combined with subsequent annealing treatment. In particular, the employment of a nitrogen-doping approach accompanied by nitrogen-doped carbon coating, results in a greatly improved conductivity, overall leading to a high reversible capacity of about 260 mA h g−1 at 25 mA g−1, superior rate capabilities, and an ultra-stable capacity of about 186 mA h g−1 after 1600 cycles at 500 mA g−1. Detailed characterizations denote that the improved conductivity as well as the small size of the synthesized TiO2 grains play a key role in the TiO2 amorphization upon sodiation, with the TiO2/C nanocomposite undergoing a complete amorphization in just few cycles. Finally, the irreversible amorphization of TiO2 is confirmed to be a crucial ingredient facilitating the Na+ diffusion kinetics and pseudocapacitive behavior, thus boosting the sodium storage performance.