What Triggers Oxygen Loss in Oxygen Redox Cathode Materials?

It is possible to increase the charge capacity of transition-metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na<sub>0.67</sub>Mg<sub>0.28</sub>Mn<sub>0.72</sub>O<sub>2</sub> and P2-Na<sub>0.78</sub>Li<sub>0.25</sub>Mn<sub>0.75</sub>O<sub>2</sub>. On charging to 4.5 V, >0.4<em>e</em><sup>–</sup> are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na<sub>0.78</sub>Li<sub>0.25</sub>Mn<sub>0.75</sub>O<sub>2</sub> but displaced from the TM to the alkali metal layers, showing that vacancies in the TM layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li<sub>0.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>]O<sub>2</sub>, are not a trigger for oxygen loss. On charging at 5 V, P2-Na<sub>0.78</sub>Li<sub>0.25</sub>Mn<sub>0.75</sub>O<sub>2</sub> exhibits oxygen loss, whereas P2-Na<sub>0.67</sub>Mg<sub>0.28</sub>Mn<sub>0.72</sub>O<sub>2</sub> does not. Under these conditions, both Na<sup>+</sup> and Li<sup>+</sup> are removed from P2-Na<sub>0.78</sub>Li<sub>0.25</sub>Mn<sub>0.75</sub>O<sub>2</sub>, resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface-localized O loss. In contrast, for P2-Na<sub>0.67</sub>Mg<sub>0.28</sub>Mn<sub>0.72</sub>O<sub>2</sub>, oxygen remains coordinated by at least 2 Mn<sup>4+</sup> and 1 Mg<sup>2+</sup> ions, stabilizing the oxygen and avoiding oxygen loss.