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

Abstract

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 Na1−x[Li2/6Mn4/6]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 Li2−xIrO3 reversibly for 0.0 ≤ x ≤ 1.5, structural variations reveal that O3-type MP Na[Li2/6Mn4/6]O2 is transformed into O1-type OP Na0Li2/6[□2/6Mn3/6M1/6]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 Na1−x[Li2/6Mn3/6M1/6]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.