Unlocking the origin of triggering hysteretic oxygen capacity in divalent species incorporated O-type sodium layered-oxide cathodes

A novel electrochemistry featuring anion redox via oxygen ions beyond conventional cation redox via transition metal (TM) ions has been investigated intensively to achieve high-energy-density cathodes for A-ion batteries, where A refers to alkali metals. Unlike lithium-ion batteries (LIBs), this novel paradigm is crucial and realizable for sodium-ion batteries (SIBs) because Li+-excess layered oxides showed highly reversible oxygen capacities during the first cycle. However, the hysteretic oxygen redox induced by Li+-migration and cyclic degradation derived from oxygen loss remain unsolved in monovalent-species-containing cathodes. In this study, the Mg2+-containing oxygen redox picture was investigated in detail to determine the critical factors that trigger the hysteretic oxygen capacities in the Mn- and Ti-based cathode models (i.e., Na[Mg1/3Mn2/3]O2 and Na[Mg1/3Ti2/3]O2) and the chemical features of Mn(3d)−O(2p) and Ti(3d)−O(2p) were generalized to exploit the full potential of oxygen redox in SIBs and LIBs. First, the Mg2+-site stability is critically determined using the Na+ coordination number, and it is located in the thermodynamically most stable Mn or Ti layer, thereby mitigating the Mg2+ migration during desodiation. Second, the hysteretic oxygen capacity for Na[MgxTM1−x]O2 is attributed to the phase transition induced by the interlayer O−O dimerization. Finally, the level of O−O dimerization, which is directly correlated with the hysteretic oxygen capacity, is governed by the TM−O bonding properties, such as ionic or covalent characteristics. Therefore, our findings provide insights into the utilization of the oxygen redox reaction of O-type Mg2+-incorporated oxide cathodes for advanced SIBs.