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_0.9Ni_0.32Zn_0.08Co_0.1Fe_0.1Mn_0.3Ti_0.1O₂ (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.