Rational Design of Na(Li 1/3 Mn 2/3 )O 2 Operated by Anionic Redox Reactions for Advanced Sodium‐Ion Batteries

In an effort to develop high‐energy‐density cathodes for sodium‐ion batteries (SIBs), low‐cost, high capacity Na(Li 1/3 Mn 2/3 )O 2 is discovered, which utilizes the labile O 2p‐electron for charge compensation during the intercalation process, inspired by Li 2 MnO 3 redox reactions. Na(Li 1/3 Mn 2/3 )O 2 is systematically designed by first‐principles calculations considering the Li/Na mixing enthalpy based on the site preference of Na in the Li sites of Li 2 MnO 3 . Using the anionic redox reaction (O 2− /O − ), this Mn‐oxide is predicted to show high redox potentials (≈4.2 V vs Na/Na + ) with high charge capacity (190 mAh g −1 ). Predicted cathode performance is validated by experimental synthesis, characterization, and cyclic performance studies. Through a fundamental understanding of the redox reaction mechanism in Li 2 MnO 3 , Na(Li 1/3 Mn 2/3 )O 2 is designed as an example of a new class of promising cathode materials, Na(Li 1/3 M 2/3 )O 2 (M: transition metals featuring stabilized M 4+ ), for further advances in SIBs.