Morphological Impact on Sodium-Ion Storage in TiO ₂ (B) Nanostructures

Among the various anodes considered for sodium ion batteries, titania polymorphs have often been regarded as one of the more appealing candidates given its theoretical specific capacity of 335 mAh·g–1. TiO2(B), in particular, has received considerable attention owing to its open framework structure that is believed to allow sodium insertion. However, the existing literature does not provide adequate evidence to draw conclusions on either the sodium capacity of TiO2(B) or the nature of the redox mechanism occurring upon sodiation. In this study, we systematically compare 0D, 1D, and 2D nanostructures of TiO2(B) to determine the extent of sodium insertion in relation to preferentially exposed crystal facets and lattice strain. We demonstrate experimentally that a clear insertion of Na+ at 1.2 V vs Na/Na+ can only be achieved when introducing lattice expansion via nanostructuring, which extends to a thermodynamic limit of 0.25 Na+ mol per Ti4+ center. Our results consist of electrochemical measurements in combination with high-resolution synchrotron techniques and are supported by computational calculations. A major outcome of this study is that despite Na+ insertion being thermodynamically favorable, its diffusion in practical time frames can only be made possible via strain generation aimed at expanding the crystal lattice.