Na2[Mn3Vac0.1Ti0.4]O7: A new layered negative electrode material for aqueous Na-ion batteries

Layered oxides have been investigated as electrode materials for Na-ion battery owing to its abundant species and easy preparation. Most of the layered oxides are sensitive to water and can’t be applied to aqueous Na-ion battery. Layered anode Na2[Mn3Vac0.5]O7 with vacancies distributed uniformly in the transition-metal layer is moisture-stable, and the working potential is suitable for aqueous system. However, no effort has been devoted on the development aqueous Na-ion battery based on layered Na2[Mn3Vac0.5]O7. Moreover, the cycling and rate performance are poor. In this paper, we first proposed to change the local crystal structure by doping the vacancy with Ti4+ ions. Na2[Mn3Vac0.1Ti0.4]O7 was screened out with enhanced rate capability and cycling stability. As demonstrated by in-situ X-ray diffraction patterns and electrochemical impedance spectroscopy, the enhanced performance is ascribed to the improved kinetics after Ti doping at the vacancy, which facilitates fast transportation of electron and Na+ ion. Soft X-ray absorption results suggest that Mn4+/Mn3+ redox is involved in the charge transfer during the cycling process. Base on this, aqueous Na-ion cell was successfully fabricated using layered Na2[Mn3Vac0.1Ti0.4]O7 as anode for the first time, which exhibited excellent rate and cycle performance.