P2-layered sodium-ion battery (NIB) cathodes are a promising class of Na-ion electrode materials with high Na + mobility and relatively high capacities. In this work, we report the structural changes that take place in P2− Na 0.67 [Mg 0.28 Mn 0.72 ]O 2 . Using ex situ X-ray diffraction, Mn K-edge extended X-ray absorption fine structure, and 23 Na NMR spectroscopy, we identify the bulk phase changes along the first electrochemical charge−discharge cycleincluding the formation of a high-voltage "Z phase", an intergrowth of the OP4 and O2 phases. Our ab initio transition state searches reveal that reversible Mg 2+ migration in the Z phase is both kinetically and thermodynamically favorable at high voltages. We propose that Mg 2+ migration is a significant contributor to the observed voltage hysteresis in Na 0.67 [Mg 0.28 Mn 0.72 ]O 2 and identify qualitative changes in the Na + ion mobility.

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mixtures thereof) [1,2] represent a major family of positive electrode materials for sodium-ion batteries (SIBs). 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. [3] 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. [4,5] In general, substitution of spectator elements, such as Li, [6] Mg, [7,8] and Zn [9] for Mn provides a rigid crystal structure during cycling and suppresses Jahn-Teller distortions, at the expense of Mn-derived capacity. 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. [10][11][12][13][14][15][16] 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 Activation of oxygen redox represents a promising strategy to enhance the energy density of positive electrode materials in both lithium and sodium-ion batteries. However, the large voltage hysteresis associated with oxidation of oxygen anions during the first charge represents a significant challenge. Here, P3-type Na 0.67 Li 0.2 Mn 0.8 O 2 is reinvestigated and a ribbon superlattice is identified for the first time in P3-type materials. 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. 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. By preparing materials under three different synthetic conditions, the degree of ordering between Li and Mn is varied. The sample with the maximum cation ordering delivers the largest capacity regardless of the voltage windows applied. 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 ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202102325.