Design picture in enabling reversible oxygen capacity for O-type Na 3d layered oxides

An intriguing design concept that mimics the highly reversible electrochemistry of 5d-Li₂IrO₃ for lithium-ion batteries (LIBs) is proposed here to harness the full potential of oxygen redox (OR) in O3-type Na_1−x[Li_2/6Mn_4/6]O₂ for sodium-ion batteries (SIBs). 3d transition metals (M) were systematically investigated as doping candidates for layered oxides for the formation of crystal structures similar to delithiated Ir-based cathodes through a stepwise screening process consisting of physicochemical layers. Considering that the phase transition of O3 (monoclinic phase, MP)−O1 (orthorhombic phase, OP) occurs in Li_2−xIrO₃ reversibly for 0.0 ≤ x ≤ 1.5, structural variations reveal that O3-type MP Na[Li_2/6Mn_4/6]O₂ is transformed into O1-type OP Na₀Li_2/6[□_2/6Mn_3/6M_1/6]O₂ in the fully desodiated state during Li migration into the NaO₂ layer. Considering the structural similarity between Li oxide and Na oxide, the thermodynamics of Li migration and the physicochemistry of the formation of inter- and intralayer O−O bonds are incorporated into the dopant screening step based on Na_1−x[Li_2/6Mn_3/6M_1/6]O₂ 3d layered oxide models. Using this process, V- and Cr-doped O3-type oxide cathodes are rationally selected from among the doping candidates, and their fully desodiated structures were found to exhibit O1-type OP in the target Li configuration. In addition, the O−O distances below 1.5 Å were not discovered for both materials. The detailed electronic structures in the screened oxides clearly show cationic and anionic redox reactions without the formation of O−O dimers upon charging. In addition, delocalized lattice O(2p) electron densities over the entire crystal framework were observed for V- and Cr-doped Na cathodes, featuring a reversible oxygen capacity. Combining the stepwise screening process, this unified design concept of mimicking the ideal redox reaction of a Li cathode not only provides an intriguing guidance for sustainable OR in SIBs, but also an interchangeable design approach between LIBs and SIBs.