Identifying cycling current-dependent degradation pathways in high-voltage layered sodium–ion cathodes

High-voltage operation boosts the energy density of sodium layered oxides by activating oxygen redox, but the underlying degradation mechanisms remain controversial, hindering rational materials design. Here we demonstrate that the applied cycling current determines which degradation pathway dominates, thereby resolving this long-standing inconsistency. Using synchrotron-based X-ray probes, advanced microscopic characterizations and first-principle calculation on a compositionally controlled set of Sc-, Sc–Ti-, and Sc–Ti–B-doped O3-type oxides, it is revealed that low current preserves oxygen-redox participation and drives oxygen-loss-induced degradation, whereas high current suppresses oxygen redox and induces a transition to a structure-governed evolution pathway along simplified P3-type routes. This compositional set makes the current-dependent degradation behaviour experimentally distinguishable, allowing the oxygen-redox-driven and structure-governed modes to be clearly disentangled. These findings establish a unified current-governed mechanistic framework and provide guidance for designing durable high-voltage sodium-ion cathodes.