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 the knowledge of the structural and redox chemistry translates from the Li-rich to the Na-rich analogs. Here, the properties of Li₂MnO₂F and Na₂MnO₂F 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 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 behavior 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 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.