Inconsistency between superstructure stability and long-term cyclability of oxygen redox in Na layered oxides

Abstract

Reversibility of oxygen anionic redox (OAR) in 3d transition-metal layered oxides holds the key to its practical utilization in sustainable batteries. However, the influence of cationic superstructure ordering on the reversibility of OAR during long-term battery operation remains unclarified. Herein we explore an inconsistency between the superstructure stability and long-term cyclability of OAR by comparing two contrasting systems, i.e., ribbon superstructured P2- and P3-Na_0.6Li_0.2Mn_0.8O₂. The “less Li and more Na” feature of the O-type domain formed in desodiated P3-Na_0.6Li_0.2Mn_0.8O₂ provides the driving force for the reconstruction of the ribbon superstructure, while the “more Li and less Na” feature formed in desodiated P2-Na_0.6Li_0.2Mn_0.8O₂ does not, thereby bringing about a better stability of the superstructure and a decent reversibility of local Mn–O coordination for P3-Na_0.6Li_0.2Mn_0.8O₂. More importantly, we reveal that the progressive loss of the ribbon superstructure and the accompanying sluggish local structural rearrangements occurring in P3-Na_0.6Li_0.2Mn_0.8O₂ result in a worse cyclability of OAR relative to P2-Na_0.6Li_0.2Mn_0.8O₂. It is thus reasonable to conclude that it is the reversible out-of-plane displacement of Li⁺ that intrinsically governs the cyclability of OAR rather than the stability of the superstructure. These findings represent a conceptual breakthrough towards the impact of the superstructure on the behavior of OAR.