Influence of ion size on structure and redox chemistry in Na-rich and Li-rich disordered rocksalt battery cathodes

Li-rich disordered rocksalts are promising next generation cathode materials for Li-ion batteries. Recent reports have shown it is also possible to obtain Na-rich disordered rocksalts, however, it is currently poorly understood how our knowledge of the structural and redox chemistry translates from the Li-rich to the Na-rich analogues. Here, we compare the properties of Li2MnO2F and Na2MnO2F which have different ion sizes (Li+ = 0.76 Å versus Na+ = 1.02 Å) but the same disordered rocksalt structure and stoichiometry. We find that Na2MnO2F exhibits lower voltage Mn- and O-redox couples, opening access to a wider compositional range within the same voltage limits. Furthermore, the intercalation mechanism switches from predominantly single-phase solid solution behaviour in Li2MnO2F to a two-phase transition in Na2MnO2F, accompanied by a greater decrease in the average Mn-O/F bond length. Li2MnO2F retains its long-range disordered rocksalt structure throughout the first cycle. In contrast, Na2MnO2F becomes completely amorphous during charge and develops a local structure characteristic of a post-spinel. This amorphisation is partially reversible on discharge. Our results show how the ion intercalation behaviour of disordered rocksalts differs dramatically when changing from Li- to Na-ions and offer routes to control the electrochemical properties of these high energy density cathodes.