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.