Bulk O2 formation and Mg displacement explain O-redox in Na0.67Mn0.72Mg0.28O2

O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O-2 on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na-0.(67)[Mn0.72Mg0.28]O-2, which contains Mg2+ on the TML. The molecular O-2 is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O-2 is trapped in voids that are formed by Mg2+ out-ofplane displacement and Mn4+ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na-0.(67)[Mn0.72Mg0.28]O-2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering.

Automated summary

Redox processes involving O-2 formation and reduction were observed in compounds containing Mg2+ on the transition-metal layers (TML), similar to what occurs in compounds with Li+. However, there are differences in the behavior of the compounds during cycling. The re-appearance of the high-voltage plateau in P2-Na-0.(67)[Mn0.72Mg0.28]O-2 during the second charge is attributed to the partial reversibility of Mn and Mg migration and the local reformation of the honeycomb ordering.