Unraveling the Sodium Storage Properties and Mechanism of Cation-Deficient Ti0.84□0.16O1.42F0.37(OH)0.21

Na-ion batteries (NIBs) are potential candidates for electrical energy storage systems owing to the naturally abundant sodium resources. In contrast to Li-ion batteries (LIBs), the anodes of NIBs suffer from the slow kinetics of sodiation because of larger ion radius, which deteriorates their performance. In this work, the cationic vacancies are introduced by a solvothermal method to produce cation-deficient Ti0.84□0.16O1.42F0.37(OH)0.21 with improved sodium storage performance, showing a charge capacity (215 mAh g-1, 0.1C) and remarkable long-term cyclic stability (the capacity retention rate is 94.6% under 1C over 700 cycles). The insertion of Na ion into the vacancies and lattice gap without dramatic structure change is verified by in situ synchrotron-based X-ray diffraction (XRD) and ex situ high-resolution transmission electron microscopy (HRTEM). The introduced cationic vacancies are beneficial to accelerate sodium storage kinetics and to improve electrochemical performance. This work highlights the injecting cationic vacancies into metal oxides is a positive strategy to achieve new anode materials for NIBs.