Chemical Modulation of Local Transition Metal Environment Enables Reversible Oxygen Redox in Mn-Based Layered Cathodes

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_xLi_yMn_1-yO₂ in Na cells. We find that Na_xLi­_yMn_1-yO₂ with Jahn-Teller distorted MnO₆ 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 MnO₆ 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_xLi_yMn_1-yO₂. 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.