Mechanisms of sodiation in anatase TiO2 in terms of equilibrium thermodynamics and kinetics

Anatase TiO₂ 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 TiO₂ 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., TiO₂ and NaTiO₂), we study the progressive crystal structure changes of anatase TiO₂ upon step-by-step Na-ion uptake (Na_xTiO₂, x = 0.0625, 0.125, 0.25, 0.5, 0.75, and 1) for the first time. It is found that the anatase TiO₂ goes through a thermodynamically unstable intermediate phase (Na_0.25TiO₂) before reaching crystalline NaTiO₂, confirming the inevitable crystal structure disintegration during sodiation. These combined experimental and theoretical studies provide new insights into the sodium storage mechanisms of TiO₂ and are expected to provide useful information for further improving the performance of TiO₂-based anodes for SIB applications.