Constructing Oxygen Vacancy to Stable Anionic Redox Reaction for High Energy Sodium Battery

Oxygen vacancies can be generated during sintering drive phase transitions in Na─Mn─O composites, controllable by sintering atmosphere and duration. The L_O/V_O (lattice oxygen/ oxygen vacancy) ratio is a key descriptor for phase content. The NMO-Air (Na₂Mn₃O₇, P2, O'3) shows a higher specific capacity (245 mA g⁻¹) and energy density (596 Wh kg⁻¹) than single-phase counterparts.

Abstract

Constructing heterostructure for synergistic effect plays an indispensable role in enhancing the energy density and cycling stability of layered oxide for sodium-ion batteries. However, the mechanisms of heterostructure formation and synergistic effects remain inadequately understood. In this study, the strategy of controlling oxygen vacancies is carried out based on Na₂Mn₃O₇ cathode material. The formation of oxygen vacancy can change the coordination environment of Mn and Na⁺ occupancy between MnO₂ layers, which is a significant driving force for structure transitions. Furthermore, the ratio of lattice oxygen to vacancy oxygen (L_O/V_O) demonstrates a distinct nonlinear relationship with the structural proportion in heterostructure materials, which can be used as a critical descriptor for evaluating the structural proportion. The obtained heterostructure with Na₂Mn₃O₇ (P1¯${\rm{P\bar 1}}$, 55 wt.%), P2-Na_0.67MnO₂ (P6₃/mmc, 40 wt.%) and O′3-NaMnO₂ (C/2 m, 5 wt.%) retains anionic redox characteristics, exhibits a high specific capacity of 245 mAh g⁻¹ with an energy density of 596 Wh kg⁻¹. The formation of heterogeneous interfaces provides numerous Na⁺ insertion/extraction sites and the presence of a minor amount of O′3-NaMnO₂ effectively mitigates the Jahn-Teller effect at low voltages, enhancing structural stability. This work offers new insights into the rational design and application of heterostructure layered oxide cathodes.