Influence of Ion Size on Structure and Redox Chemistry in Na‐Rich and Li‐Rich Disordered Rocksalt Battery Cathodes

Abstract 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 the knowledge of the structural and redox chemistry translates from the Li‐rich to the Na‐rich analogs. Here, the properties of Li 2 MnO 2 F and Na 2 MnO 2 F are compared, which have different ion sizes (Li + = 0.76 vs Na + = 1.02 Å) but the same disordered rocksalt structure and stoichiometry. It is found that Na 2 MnO 2 F 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 behavior in Li 2 MnO 2 F to a two‐phase transition in Na 2 MnO 2 F, accompanied by a greater decrease in the average Mn─O/F bond length. Li 2 MnO 2 F retains its long‐range disordered rocksalt structure throughout the first cycle. In contrast, Na 2 MnO 2 F becomes completely amorphous during charge and develops a local structure characteristic of a post‐spinel. This amorphization is partially reversible on discharge. The results show how the ion intercalation behavior of disordered rocksalts differs dramatically when changing from Li‐ to Na‐ions and offers routes to control the electrochemical properties of these high‐energy‐density cathodes.