<p>Oxygen
redox plays a prominent role in enhancing the energy density of Mn-based
layered cathodes. However, understanding the factors affecting the reversibility
of oxygen redox is nontrivial due to the complicated concurrent structural and
chemical transformations. Herein, we show that local Mn‒O symmetry induced structural
and chemical evolutions majorly dictate the reversibility of oxygen redox of Na<sub>x</sub>Li<sub>y</sub>Mn<sub>1-y</sub>O<sub>2</sub>
in Na cells. We find that Na<sub>x</sub>Li<sub>y</sub>Mn<sub>1-y</sub>O<sub>2</sub>
with Jahn-Teller distorted MnO<sub>6</sub> octahedra undergoes severe Mn
dissolution during cycling, which destabilizes the transition metal layer resulting
in poor Li retention and irreversible oxygen redox. Jahn-Teller distortion of
MnO<sub>6</sub> octahedra can be suppressed by modulating the local charge of
Mn and Mn‒O distance through Mg/Ti dual doping. This leads to reduced Mn
dissolution resulting in more reversible oxygen redox. Such stabilization
significantly improves the electrochemical performance of Mg/Ti dual doped Na<sub>x</sub>Li<sub>y</sub>Mn<sub>1-y</sub>O<sub>2</sub>.
Through this work, we show that promoting
reversible oxygen redox can benefit from structural stabilization at local
length scale, and that modifying the chemical environment through doping
chemistry is an efficient strategy to promote local structural stability and
thus, oxygen redox.</p>