Selective Anionic Redox and Suppressed Structural Disordering Enabling High‐Energy and Long‐Life Li‐Rich Layered‐Oxide Cathode

Abstract Despite their high energy densities, Li‐rich layered oxides suffer from low capacity retention and continuous voltage decay caused by the migration of transition‐metal cations into the Li layers. The cation migration stabilizes oxidized oxygen anions through the decoordination of oxygen from the metal once the anions participate in the redox reaction. Structural disordering is thus considered inevitable in most Li‐rich layered oxides. However, herein, a Mg‐substituted Li‐rich layered oxide, Li 1.2 Mg 0.2 Ru 0.6 O 2 , with high structural and electrochemical stability is presented. Although using both cationic and anionic redox reactions, Ru migration in Li 1.2− x Mg 0.2 Ru 0.6 O 2 is thermodynamically unfavored as a result of selectively oxidized O ions, suppressed structural disordering, and the formation of short (1.75 Å) Ru=O bonds enabled within the layered framework, which effectively decoordinate the oxidized O ions. The unprecedentedly high structural stability of Li 1.2 Mg 0.2 Ru 0.6 O 2 leads to not only a high energy density of 964 Wh kg −1 but also outstanding rate capability and cycle performance. These findings demonstrate the potential of this practical strategy for the stabilization of Li‐rich layered oxides even with prolonged cycling.