P2-Na0.61Ca0.03[Mg2/9Cu1/9Mn2/3]O2 as a High-Energy Oxygen Redox Cathode for Na-Ion Batteries: Investigation of Cu Substitution and Ca Doping to Enhance Cycling Stability

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-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.