Phases Hybriding and Hierarchical Structuring of Mesoporous TiO₂ Nanowire Bundles for High-Rate and High-Capacity Lithium Batteries

A hierarchical mesoporous TiO₂ nanowire bundles (HM-TiO₂-NB) superstructure with amorphous surface and straight nanochannels has been designed and synthesized through a templating method at a low temperature under acidic and wet conditions. The obtained HM-TiO₂-NB superstructure demonstrates high reversible capacity, excellent cycling performance, and superior rate capability. Most importantly, a self-improving phenomenon of Li⁺ insertion capability based on two simultaneous effects, the crystallization of amorphous TiO₂ and the formation of Li₂Ti₂O₄ crystalline dots on the surface of TiO₂ nanowires, has been clearly revealed through ex situ transmission electron microcopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) techniques during the Li⁺ insertion process. When discharged for 100 cycles at 1 C, the HM-TiO₂-NB exhibits a reversible capacity of 174 mA h g⁻¹. Even when the current density is increased to 50 C, a very stable and extraordinarily high reversible capacity of 96 mA h g⁻¹ can be delivered after 50 cycles. Compared to the previously reported results, both the lithium storage capacity and rate capability of our pure TiO₂ material without any additives are among the highest values reported. The advanced electrochemical performance of these HM-TiO₂-NB superstructures is the result of the synergistic effect of hybriding of amorphous and crystalline (anatase/rutile) phases and hierarchically structuring of TiO₂ nanowire bundles. Our material could be a very promising anodic material for lithium-ion batteries.