Synergistic Zn2+-doping and in situ alkali conversion toward high-voltage and air-stable O3-type sodium-ion battery cathodes

O3-type layered transition metal oxides are considered promising cathodes for sodium-ion batteries due to their high capacity and production scalability; however, their practical deployment is impeded by severe structural degradation at high voltages and inherent moisture sensitivity stemming from surface residual alkalis. Herein, a synergistic dual-modification strategy combining bulk Zn²⁺ doping and in situ sodium acetate surface coating is proposed to address these challenges simultaneously. In situ X-ray diffraction and kinetic analyses reveal that Zn²⁺ doping plays an important role in stabilizing the layered framework and effectively modulates the phase evolution pathway from O3–P3 to a highly reversible O3–P3–OP2 process. This induced OP2 intermediate phase acts as a buffer to significantly mitigate the lattice strain and prevent severe structural distortion above 4.0 V. Simultaneously, an in situ surface neutralization strategy converts the detrimental residual alkalis into a uniform, ionically conductive sodium acetate protective layer, achieving a “waste-to-treasure” surface reconstruction. Consequently, the Zn²⁺-doped and acetate-coated NFMZ@AC cathode delivers a high specific capacity of 146 mAh g⁻¹ and exhibits superior capacity retention of 81.4% after 100 cycles at a high cutoff voltage of 4.2 V. Moreover, the material demonstrates exceptional air stability, retaining 90.3% of its capacity after 7 days of exposure to moist air. A full cell assembled with a hard carbon anode delivers a high energy density of 470.8 Wh kg⁻¹ (based on cathode mass) and stable cycling performance. This work provides a facile and effective bulk-surface dual-regulation strategy for developing high-energy and durable cathodes for sodium-ion batteries.