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

• Ti-deficient undoped anatase TiO 2 (TDAT) is prepared by solvothermal self-assembly. • TDAT shows superior capacity, rate and cycle performances for Li + and Na + storage. • TDAT achieves unprecedented cycle stability of 15,000 cycles for Na + storage. • Ti vacancy creates lower energy barrier, higher kinetics, new storage sites and diffusion pathways. Anatase TiO 2 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 TiO 2 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 co-existence 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 TiO 2 (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.