Understanding the synergic roles of MgO coating on the cycling and rate performance of Na0.67Mn0.5Fe0.5O2 cathode

Fe and Mn-based P2-Na0.67Mn0.5Fe0.5O2 has attracted significant interest as cathode material for sodium ion batteries (SIBs). However, their poor cycling and rate performance are the main problems. In this study, we report an integrated strategy of MgO coating and Mg2+ doping to improve the electrochemical properties of Na0.67Mn0.5Fe0.5O2. The synergistic roles of MgO coating and Mg2+ doping in the cycling and rate performance were identified. Firstly, the MgO layer suppresses the dissolution of transition metal ions and the side reactions between the electrode and electrolyte. Secondly, the MgO coating layer reduces the interface resistance and enhances e−/Na+ migration. Thirdly, the slab thickness of the TMO2 layer was also expanded by a small fraction of Mg doping, which decreases Na+ migration barrier. Finally, operando X-ray diffraction indicates that the MgO coating alleviates the change in interlay spacing (d-spacing) induced by Na+ intercalation/extraction, and inhibits irreversible phase transformations in the charging-discharging cycles, which enhances the layered structural stability of the cathode materials. The capacity retention ratio of Na0.67Mn0.5Fe0.5O2 at 1 C increased from 20% to 72% by MgO coating after 100 cycles. Insights on the roles of MgO coating are also instructive for developing cathode materials with high performance.