Self-assembled titanium-deficient undoped anatase TiO2 nanoflowers for ultralong-life and high-rate Li+/Na+ storage

Anatase TiO2 is a promising safe and high-rate anode for Li- and Na-ion batteries owing to its moderate redox potential and multi-dimensional ion diffusion paths. However, the capacity, rate and cycle life of anatase TiO2 are severely hindered by the low Li+/Na+ diffusion coefficients. Ti vacancies have been predicted to significantly improve Li+ diffusion kinetics by previous theoretical calculations. However, experimental evidence is still lacking because the existing methods to create Ti vacancies commonly rely on aliovalent doping, i.e., the coexistence of Ti vacancies and foreign anions precludes revelation of the true role and contribution of Ti vacancies alone. The current work reports the synthesis of mesoporous flower-like titanium-deficient anatase TiO2 (TDAT). The formation mechanisms of the Ti vacancies and the micro-architectures are tentatively discussed. Its undoped nature allows elucidation of the unambiguous roles of Ti vacancies on Li+/Na+ storage. Electrochemical results show high capacity, high rate and ultra-long cycle stability for both Li+/Na+ storage in TDAT. DFT calculations reveal that the presence of Ti vacancies results in reduced energy barrier for Li+/Na+ intercalation, enhanced diffusion kinetics, additional Li+/Na+ storage sites and diffusion pathways. For Na+ storage, it achieves a high capacity of 219.9 mAh g+ 1 at 50 mA g+ 1, and superior stability over ultra-long 15,000 cycle test at 2000 mA g+ 1. This work complements with the prevailing view of anion vacancy for improved Li+/Na+ storage.