Intermediate‐Phase Buffering Unlocks Reversible High‐Voltage Cycling in O3‐Type Sodium Oxide Cathodes

The irreversible O3‐P3 phase transition in layered sodium oxide cathodes leads to severe volumetric strain and capacity fading. While recent progress via high‐entropy design or dual‐phase structuring has improved structural stability, these strategies primarily focus on suppressing rather than guiding the phase transition pathway. Herein, we propose an “intermediate phase engineering” concept in a high‐entropy O3‐type cathode, Na 0.9 Ni 0.32 Zn 0.08 Co 0.1 Fe 0.1 Mn 0.3 Ti 0.1 O 2 (Na9NZCFMT), which enables the spontaneous formation of a strain‐buffering OP2 intermediate phase at 3.8 V. Unlike conventional approaches that suppress phase transitions, the OP2 phase acts as a structural buffer that actively guides oxygen‐layer gliding along an ordered, low‐strain pathway, reducing the volumetric strain from 6.86% to 3.12%. Through integrated in situ XRD, XAS, STEM, and DFT calculations, we unravel the formation condition and buffering mechanism of this OP2 phase: The local coordination environment modulated by specific elements lowers the energy barrier for its formation, leading to a thermodynamically favored and kinetically accessible intermediate state. The cathode exhibits highly reversible structural evolution and anionic redox, delivering 91.5% capacity retention after 100 cycles. This work offers a generalizable strategy for designing stable high‐voltage layered cathodes.