Transition metal vacancies in P2- and P3-type compounds have been shown to trigger oxidation of oxygen anions independent of the identity of the electrochemically inactive M species, while reversible oxygen redox, namely reduction of oxidized O species, is only observed in the P3-type compounds with a characteristic narrow polarization stemming from the vacancies in the transition metal layers. It has also been reported that the design of Mn-deficient compounds is an effective way to activate reversible oxygen redox, by moving O 2p-based states nearer the Fermi level, as shown in Na 0.653Mn 0.929O 2. In this material, the presence of transition metal vacancies not only generates the orphaned O 2p states but also maintains the oxygen stacking sequence without irreversible structural change and cationic migration from the transition metal layers to the Na layers. Negligible voltage hysteresis is observed in Na 4/7O 2 (empty square represents vacancies in Mn sites) possessing inherent vacancies with a unique ordering between vacancies and Mn. In contrast, the honeycomb superstructure, shown in the majority of oxygen active compounds adopting P2, P3, and O3 phases, lowers the reduction voltage of oxygen redox due to the changed local coordination around oxygen as a result of in-plane Mn migration. ![]() Recently, it has been reported that the ribbon superstructure of P2-type Na 0.6Li 0.2Mn 0.8O 2 is responsible for the suppression of the voltage hysteresis as a result of the absence of in-plane migration of Mn. However, the oxidation of oxygen at high voltages often leads to large voltage hysteresis due to cationic migration from the transition metal layers to the alkali metal layers with concomitant structural evolution. Additionally, these electrochemically inactive dopants enable the activation of oxygen redox by creating nonbonding O 2p states at the top of the valence band upon desodiation, which represents an effective way to raise the energy density of positive electrode materials. ![]() In general, substitution of spectator elements, such as Li, Mg, and Zn for Mn provides a rigid crystal structure during cycling and suppresses Jahn–Teller distortions, at the expense of Mn-derived capacity. The compositional and structural phase spaces available to these materials are vast, which enables properties such as capacity, rate capability, operating voltage, and cyclability to be carefully tuned. They adopt one of the polymorphs O3, P3, and P2, depending on the coordination environment of the Na ions and the number of MnO 2 slabs in the unit cell. Manganese-based sodium layered oxides, Na xM yMn 1− yO 2 (0.4 ≤ x ≤ 1.0, 0.05 ≤ y ≤ 0.5, M = Li, Mg, Ti, Fe, Co, Ni, Zn, and mixtures thereof) represent a major family of positive electrode materials for sodium-ion batteries (SIBs). These findings highlight the importance of cationic ordering in the transition metal layers, which can be tuned by synthetic control to enhance anionic redox and hence energy density in rechargeable batteries. ![]() The sample with the maximum cation ordering delivers the largest capacity regardless of the voltage windows applied. ![]() By preparing materials under three different synthetic conditions, the degree of ordering between Li and Mn is varied. In addition, a range of spectroscopic techniques reveal that a strongly hybridized Mn 3d–O 2p favors ligand-to-metal charge transfer, also described as a reductive coupling mechanism, to stabilize reversible oxygen redox. The ribbon superstructure is maintained over cycling with very minor unit cell volume changes in the bulk while Li ions migrate reversibly between the transition metal and Na layers at the atomic scale. Here, P3-type Na 0.67Li 0.2Mn 0.8O 2 is reinvestigated and a ribbon superlattice is identified for the first time in P3-type materials. However, the large voltage hysteresis associated with oxidation of oxygen anions during the first charge represents a significant challenge. Activation of oxygen redox represents a promising strategy to enhance the energy density of positive electrode materials in both lithium and sodium-ion batteries.
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