Impact of Particle Size on the Non-Equilibrium Phase Transition of Lithium-Inserted Anatase TiO₂

The phase transformation behavior in Li-ion battery electrodes is critical for the electrode kinetics and cycle life. Here we reveal unexpected particle size-dependent phase transformation behavior in anatase TiO 2 by in situ X-ray diffraction. The equilibrium voltage measured by the galvanostatic intermittent titration technique decreases progressively with a decrease in particle size, which can attributed to the difference in the surface energy of the pristine and lithiated phases. On the basis of the evolution of the domain size and phase fraction of the two phases, we conclude that the first-order phase transition proceeds by continuous nucleation upon lithium insertion. For all particle sizes, the phase boundary is found to migrate under nonequilibrium conditions even under very slow (dis)charge conditions, as reflected by a distinct deviation from the Li solubility limit during the phase transformation. Remarkably, the degree of nonequilibrium increases with a decrease in particle size, which is rationalized by the difference in the observed phase transition behavior between small and large particles. The absence of phase coexistence in smaller particles in combination with the sluggish ionic transport rationalizes the better electrochemical performance of the nanostructured anatase TiO 2 compared to that of the microsized material. These results suggest a very low nucleation barrier for the formation and movement of the phase boundary in combination with sluggish ionic migration. Therefore, strategies for improving the rate performance of nanostructured anatase TiO 2 should concentrate on improving the interstitial diffusion, for instance by appropriate doping.