Extending nonhysteretic oxygen capacity in P2-type Ni-Mn binary Na oxides

Environmental degradation and resource depletion have brought a lot of attention to sustainable and eco-friendly energy sources. Developing high-energy-density cathode material for alternating lithium-ion batteries (LIBs) is crucial to fulfill the demands of a reasonable price. In this regard, sodium-ion batteries (SIBs) are considered as promising alternatives for large-scale applications. To improve the energy and power densities, a considerable amount of spotlight is placed on activating the oxygen redox (OR) process of Na layered-oxide cathodes because its reaction occurs in a high voltage region (∼4.2 V) upon charging. Herein, we compare P2-type Ni-Mn binary oxide, Na_2/3[Mn_3/4Ni_1/4]O₂ (NMNO) and a V-doped Na_2/3[Mn_3/4Ni_1/4]O₂ (V-NMNO) based on a combined study of experiments and first-principles calculations. It is found that V-NMNO, compared with the bare material, extends the nonhysteretic oxygen capacity and exhibits an OR activity with prolonged and flatter plateau upon charging. This oxide provides a minimal voltage hysteresis that indicates high reversibility of OR reaction. Its remarkably well-defined plateau is attributed to the stabilization of the phase transition upon cycling to 4.3 V, which can be theoretically understood by the comparison of the formation energies of NMNO and V-NMNO. In addition, our experimental and computational results demonstrate that the nonhysteretic oxygen capacity combined with the partial Ni redox is extended under the host Ni-Mn binary oxide via V-substitution. From the systematic understandings on the two oxide models, our findings provide an intriguing direction for harnessing the full potential of OR reactions for high-energy-density SIBs.