Highly porous TiO₂ hollow microspheres constructed by radially oriented nanorods chains for high capacity, high rate and long cycle capability lithium battery

Hierarchical nanorods chains-constructed TiO<inf>2</inf> hollow microspheres (HNC-TiO<inf>2</inf>-HMSs) have been designed and prepared through a facile one-pot fluorine-free solvothermal alcoholysis route using TiCl<inf>4</inf> and isopropanol reaction system. Owing to the assembly of radially oriented nanorods chains leading to the formation of the shell of the hollow spheres, a large series of straight channels along nanorods chains are formed. Such highly porous hollow microspheres with hollow cavity, straight nanorods chains and straight nanochannels are highly desirable for Li ions batteries because such structure can easily store the electrolyte, facilitate the charge diffusion and Li⁺ insertion and buffer the volume change during the Li⁺ insertion/extraction process. One of the key innovation of the present work is the fine tuning of water amount released from the esterification of alcohol to induce in a well controlled hydrolysis of TiCl<inf>4</inf> and engineer precisely HNC-TiO<inf>2</inf>-HMSs formation. Most importantly, the released Cl^- ions direct the nanorods growing along (001) crystal plane and self-assembling along the radial direction accompanying with nanorods size controlling to form HNC-TiO<inf>2</inf>-HMSs. The obtained TiO<inf>2</inf> anode material with such special structure demonstrates excellent Li⁺ storage capacity with outstanding cycle performance and superior rate capability at different rates over 700 cycles: a reversible capacity of 216mAhg^-1 is obtained after 100 cycles at 1C and a reversible capacity of 112mAhg^-1 is retained after 100 cycles at 10C. The SEM, TEM, HRTEM and in-situ XRD techniques have been utilized to shed light on the Li⁺ insertion process and the phase transformation. Most importantly, a self-improving phenomenon of cycle performance and storage capacity was observed owing to the formation of numerous ~5nm Li<inf>2</inf>Ti<inf>2</inf>O<inf>4</inf> nanocrystals formed on the surface of the nanorods chains. The results reveal that the high performance of the as-prepared HNC-TiO<inf>2</inf>-HMSs in terms of storage capacity, cycle performance and rate capability can be attributed to the synergy of the special structure and the self-improving phenomenon. Our simple reaction system may provide a concrete example on the construction of novel hollow spherical porous anode materials for high performance lithium batteries.