Exploring the Role of Compositional Space of Disordered Rock Salt Na-Ion Battery Cathodes

Sodium-ion batteries (SIBs) provide a broader chemical design space for cathodes in terms of composition, crystal structure, and redox chemistry, yet current research largely remains confined to ordered layered frameworks. Recently, Li-excess cation-disordered rock-salt (DRX) cathodes have provided an alternative design space, enabled by their exceptional tunability in composition, stoichiometry, and stability/metastability. Inspired by these advances, we significantly expand the compositional space of Na-based cathodes by developing a series of metastable Na-based DRX cathodes based on Na–Ti–Mn–O systems spanning both stoichiometric and over-stoichiometric regimes. We reveal rich structural and chemical complexity arising from Na content and over-stoichiometry and demonstrate that unique local cation ordering motifs are closely correlated with enhanced anionic redox activity and Na-ion utilization. By systematically comparing the electrochemical and structural evolution of stoichiometric and over-stoichiometric compositions, we elucidate distinct metastability-driven mechanisms governing their electrochemical behavior. Our findings fill an important knowledge gap in metastable Na-based DRX chemistry.