Guideline of Dynamic Tunnel Structural Evolution for Durable Sodium‐Ion Oxide Cathodes

Abstract Mn‐based oxide cathodes hold great promise for sodium‐ion batteries (SIBs) due to their cost‐effectiveness and environmental compatibility. In this study, using tunnel‐type Na 0.44 MnO 2 as a prototype, a systematic investigation is conducted to examine how different element substitutions affect structural evolution and found that these element substitutions alter the total energy of the pristine system, driving the structure to evolve gradually from a tunnel to a different crystal configuration. Notably, using advanced scanning transmission electron microscopy (STEM), the transition zone is captured from tunnel to layered structure for the first time, providing direct evidence of phase evolution. Density functional theory (DFT) calculations reveal that Mg substitution uniquely facilitates the formation of layered/spinel heterostructures, enabling intimate interfacial integration that reduces Na⁺ transport barriers and enhances structural integrity. COMSOL simulations further demonstrate that the layered/spinel configuration effectively mitigates stress accumulation, achieving high rate and long cycle performance. These findings provide comprehensive design principles of dynamic tunnel structural evolution of Mn‐based oxide cathodes, thereby advancing the design of high‐performance SIBs.