Cobalt Surface Treatment Induces Tailored Reconstructed Layer to Enhance High‐Voltage Cycling Stability of O3‐Type Layered Oxide Cathodes for Sodium‐Ion Batteries

ABSTRACT O3‐type layered oxides are promising sodium‐ion battery cathodes due to high specific capacity and feasible synthesis, but phase transformation‐induced stress and lattice oxygen evolution cause long‐cycle capacity degradation, limiting their applications. In this study, an oxygen reduction functional Na x CoO 2 coating integrated with a subsurface P2/O3 composite phase through Co surface treatment is developed to construct a tailored reconstructed layer, thereby enhancing the high‐voltage cycling stability of O3‐type NaNi 0.2 Fe 0.4 Mn 0.4 O 2 (NFM). The P2/O3 interlocking effect mitigates Na + de/intercalation lattice strain to prevent structural collapse, while the Na x CoO 2 coating captures and reduces oxidized oxygen anions, suppressing lattice oxygen evolution at deep charge state. As a result, the Co surface treated NFM exhibits excellent long‐cycle performance, with a capacity retention of up to 80.78% after 500 cycles at 1 C (2.0–4.0 V), significantly higher than the 49.76% of pristine samples. Notably, even after 200 cycles at 1 C (2.0–4.3 V), as Co surface treated sample retains 61.15% of its initial capacity and even maintains a high‐capacity retention of 53% after 300 cycles, which is higher than that of the untreated sample (38%). This work provides crucial insights into enhancing the electrochemical performance of O3‐type layered oxide cathode materials for sodium‐ion batteries.