Mechanisms of sodiation in anatase TiO(2) in terms of equilibrium thermodynamics and kinetics

Anatase TiO(2) is a promising anode material for sodium-ion batteries (SIBs). However, its sodium storage mechanisms in terms of crystal structure transformation during sodiation/de-sodiation processes are far from clear. Here, by analyzing the redox thermodynamics and kinetics under near-equilibrium states, we observe, for the first time, that upon Na-ion uptake, the anatase TiO(2) undergoes a phase transition and then an irreversible crystal structure disintegration. Additionally, unlike previous theoretical studies which investigate only the two end points of the sodiation process (i.e., TiO(2) and NaTiO(2)), we study the progressive crystal structure changes of anatase TiO(2) upon step-by-step Na-ion uptake (Na(x)TiO(2), x = 0.0625, 0.125, 0.25, 0.5, 0.75, and 1) for the first time. It is found that the anatase TiO(2) goes through a thermodynamically unstable intermediate phase (Na(0.25)TiO(2)) before reaching crystalline NaTiO(2), confirming the inevitable crystal structure disintegration during sodiation. These combined experimental and theoretical studies provide new insights into the sodium storage mechanisms of TiO(2) and are expected to provide useful information for further improving the performance of TiO(2)-based anodes for SIB applications.