Covalency modulation enables stable Na-rich layered oxide cathodes for Na-ion batteries

As the analogs of Li-rich materials, Na-rich transition metal layered oxides are promising cathode materials for Na-ion batteries owing to their high theoretical capacity and energy density through cumulative cationic and anionic redox. However, most of the reported Na-rich cathode materials are mainly Ru- and Ir-based layered oxides, which limits the practical application. Herein, we report a Na-rich and Ru-doped O3-type Na1.1Ni0.35Mn0.55O2 cathode to mitigate this issue. By partially substituting Mn4+ with high-electronegativity Ru4+, the structural stability and electrochemical performance of the cathode are both greatly improved. It is validated that the high covalency of Ru–O bonds could harden the structural integrity with rigid oxygen framework upon cycling, leading to enhanced O3-P3 phase transition reversibility. Ru doping also induces an enlarged interlayer spacing to boost the Na+ diffusion kinetics for improved rate capability. In addition, benefiting from the large energetic overlap between Ru 4d and O 2p states, the reinforced Ru–O covalency enables highly reversible Ru4+/Ru5+ redox accompanied with more stable oxygen redox, leading to improved specific capacity and stability over cycling. Our present study provides a promising strategy for designing high-performance Na-rich layered oxide cathode materials through covalency modulation toward practical applications.