Reversible Oxygen Redox With Enhanced Structural Stability Through Covalency Modulation for Layered Oxide Cathodes.

P2-type Mn-based layered oxides have emerged as one of the most promising cathode materials for sodium-ion batteries owing to their advantages of facile preparation and high theoretical capacity. However, challenges such as phase transition and irreversible oxygen release during cycling often lead to rapid structural distortion and the formation of oxygen vacancies, ultimately resulting in rapid capacity decay. Herein, a covalency modulation strategy is adopted to address these challenges and successfully achieved a stable P2-type Mn-based layered oxide by introducing strong covalent Ni─O bonds. The robust Ni─O motif plays a crucial role in maintaining the rigidity of transition metal (TM) layered frameworks, which efficiently alleviates the structural distortion and degradation of the coordination environments of local TM sites, thereby achieving durable structural stiffness over extended cycles. In addition, the strong covalent Ni─O bonds can also stabilize the local oxygen environment, effectively suppressing the irreversible oxygen release. Benefiting from these advancements, the as-designed Na 0.6 Mg 0.15 Mn 0.7 Ni 0.15 O 2 cathode displays a full solid-solution behavior with a low volume change of only 0.9% and an enhanced reversibility of lattice oxygen redox (OR) reaction. This investigation emphasizes the crucial role of covalency modulation in regulating OR chemistry and structural integrity to achieve high-energy-density Mn-based layered oxides.