Anomalous Sodium Storage Behavior in Al/F Dual-Doped P2-Type Sodium Manganese Oxide Cathode for Sodium-Ion Batteries

Various types of sodium manganese oxides are promising cathode materials for sodium storage systems. One of the most considerable advantages of this family of materials is their widespread natural abundance. So far, only a few host candidates have been reported and there is a need to develop new materials with improved practical electrochemical performance. Here, P2-type Al/F-doped sodium manganese oxide as well as its unique sodium storage mechanism is demonstrated by a combination of electrochemical characterization, structural analyses from powder X-ray diffraction (XRD) data, and 3D bond valence energy level calculations for the sodium diffusion pathways. The material exhibits a high reversible capacity of 164.3 mAh g⁻¹ (0.3C rate) and capacity retention of 89.1% after 500 cycles (5C rate). The study clearly unravels the beneficial effect of the doping and the unique sodium intercalation mechanism devoid of the low diffusion O3 transformation.