P2‐Na_0.61Ca_0.03[Mg_2/9Cu_1/9Mn_2/3]O₂ as a High‐Energy Oxygen Redox Cathode for Na‐Ion Batteries: Investigation of Cu Substitution and Ca Doping to Enhance Cycling Stability

Abstract Oxygen 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‐Na_0.61Ca_0.03[Mg_2/9Cu_1/9Mn_2/3]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. Na_0.61Ca_0.03[Mg_2/9Cu_1/9Mn_2/3]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 Na_0.61Ca_0.03[Mg_2/9Cu_1/9Mn_2/3]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.