Jin‐Wei KangHsu‐Chen ChengHsiang‐Jung ChenShao‐Chu HuangChih‐Heng LeeCHIN-LUNG KUOSheng‐Yu YuChia‐Ching LinHeng‐Liang WuChun‐Han KuoHao‐Hsiang ChangChih‐Wei HuShu‐Chih HawHsin‐Yi Tiffany ChenHan‐Yi Chen2025-05-222025-05-222025https://www.scopus.com/record/display.uri?eid=2-s2.0-105002596458&origin=resultslisthttps://scholars.lib.ntu.edu.tw/handle/123456789/729667Oxygen redox-based cathode materials offer higher capacity than conventional Na-based layered transition metal oxides in Na-ion batteries (NIBs). Still, their performance is impeded by voltage hysteresis and structural instability. Herein, a novel P2-Na0.61Ca0.03[Mg2/9Cu1/9Mn2/3]O2 cathode material is developed with Li/Co-free composition for cost-effectiveness and environmental friendliness. Cu substitution in transition-metal layers stabilizes O ions during oxygen redox, while Ca doping in alkaline-metal layers acts as structural “pillars” to suppress phase transformation. The charge storage mechanism is analyzed via operando X-ray absorption spectroscopy, operando X-ray diffraction analysis, on-line gas chromatography, and density functional theory computation. Na0.61Ca0.03[Mg2/9Cu1/9Mn2/3]O2 exhibits a high specific capacity (205 mAh g−1 at 0.1 C), good cyclic stability, and impressive rate capability (142 mAh g−1 at 2.5 C). A Na0.61Ca0.03[Mg2/9Cu1/9Mn2/3]O2//hard carbon full cell with a high energy density (250.7 Wh kg−1) is achieved, demonstrating its potential for high-energy NIBs. This work provides new insights into oxygen-redox-dominated cathodes through a facile sol-gel synthesis and advanced characterization techniques.cathode materialsoperando X-ray absorption spectroscopyoperando X-ray diffractionoxygen redoxsodium-ion batteriestransition metal layered oxidesjournal article10.1002/adfm.202504642