Achieving reversible Mn2+/Mn4+ double redox couple through anionic substitution in a P2-type layered oxide cathode

P2-type Mn-based layered oxides have emerged as promising cathode materials for sodium-ion batteries (SIBs) due to their high theoretical capacity. However, the incorporation of Mn3+ cations induce severe Jahn-Teller (J-T) effect, which seriously distorts the localized lattice structure and results in insufficient battery performance. Herein, we successfully introduce reversible Mn2+/Mn4+ double redox couple into P2-type Na0.6Mg0.3Mn0.7O2 cathode with suppressed J-T effect through F anion doping strategy. The comprehensive results clearly suggest that F-substitution with an appropriate amount enlarges the interlayer spacing and reduces the band gap, resulting in accelerated charge transfer kinetics. In addition, the strong interaction between Mn and F suppresses the Mn2+ dissolution at discharged states and promotes the reversible conversion from Mn2+ to Mn4+ at charged states, leading to ameliorated J-T effect and enhanced electrochemical performance. Moreover, the oxygen redox reaction is also enhanced through F anion doping strategy. Our present study provides an effective avenue to address the J-T effect by introducing reversible Mn2+/Mn4+ double redox couple in Mn-based oxides, which is expected to revive this cathode family toward practical applications of SIBs.