Correlating Mg Displacement with Topologically Regulated Lattice Oxygen Redox in Na-Ion Layered Oxide Cathodes

Lattice oxygen redox (LOR) has been explored in transition-metal oxides with a variety of topological structures. However, a clear-cut correlation between topological structures and the behavior of LOR has not yet been definitively established. Here, we discover the close interplay between Mg displacement and the LOR stability during long-term battery operation for two closely related model materials, i.e., P2- and P3-Na2/3Mg1/3Mn(IV)2/3O2 (NMMO). The substantially distinct LOR stability for P2- and P3-NMMO is shown to stem from the different evolution mechanisms between P-type and O-type layers at local scale, accompanied with different degrees of Mg displacement. The theoretical calculations corroborate that out-of-plane Mg displacement is kinetically favorable for desodiated P3-NMMO, which brings about a labile electronic structure and noticeable O2 loss during LOR. By contrast, a stable electronic structure and a limited O2 loss during LOR is revealed for P2-NMMO with confined Mg displacement. This study builds a cornerstone in elucidating the tight correlation between the displacement of "nonredox active"cations and the stability of LOR.