Intermediate-Phase Buffering Unlocks Reversible High-Voltage Cycling in O3-Type Sodium Oxide Cathodes.
basic_science · Level V
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- Record sourced from PubMed, PMID 42638537.
- Also identified by DOI 10.1002/adma.74755.
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Abstract
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<sub>0.9</sub>Ni<sub>0.32</sub>Zn<sub>0.08</sub>Co<sub>0.1</sub>Fe<sub>0.1</sub>Mn<sub>0.3</sub>Ti<sub>0.1</sub>O<sub>2</sub> (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.