Unraveling vacancy-induced oxygen redox reaction and structural stability in Na-based layered oxides

The promising P2-layered sodium manganese oxide (Na2/3MnO2) cathode often suffers from multiple structural transformations due to the Jahn–Teller distortion induced by Mn3+, hence the rapid capacity falloff. Other than many previous studies on partial substitution of Mn by low valence ions, here we have introduced vacancies into Mn sites in P2-Na2/3MnO2 (e.g., Na2/3[□1/6Mn5/6]O2, where □=vacancy) to oxidize Mn close to Mn4+. Combined experimental and theoretical studies reveal that such vacancy introduction overcomes the detrimental structural distortion due to irreversible phase transitions to O2 and OP4 and triggers oxygen redox activated by an orphaned O 2p state around Mn vacancies in addition to the conventional cationic redox. P2-Na2/3[□1/6Mn5/6]O2 shows smooth and continuous charge/discharge curves indicative of restrained phase transitions, leading to much superior cycling stability than the Jahn-Teller distorted pristine counterpart. This study demonstrates the critical role of transition-metal vacancies in triggering the anionic redox reaction and manipulating the phase transitions in P2-type layered cathodes.