Role of Ligand-to-Metal Charge Transfer in Anionic Redox Activity of the Layered Oxide Cathode for Na-Ion Batteries

Abstract

Developing high-performance cathode materials is crucial for advancing grid-scale sodium-ion batteries (SIBs). Here, we introduce a P2-type layered oxide cathode material, Na0·67Ni0·33Mn0·51Ru0·16O2, which demonstrates exceptional electrochemical performance through a robust dual cationic–anionic redox mechanism. This material delivers a high discharge capacity of 175.6 mAh g−1 at an average voltage of 3.6 V and maintains remarkable cycling stability with ∼93% capacity retention after 1000 cycles. A key to this performance is the strategic Ru doping, which plays a pivotal role in electronically stabilizing the oxidized oxygen states. By mediating the reversible formation of oxygen holes and preventing oxygen undercoordination, the Ru dopant stabilizes the anionic redox process. Unlike Na-rich layered oxides, this ligand-to-metal charge-transfer mechanism effectively suppresses detrimental oxygen evolution and preserves the structural integrity of the layered Na-deficient P2-type framework. This design strategy enables a synergistic redox landscape, confirmed by ex situ and operando advanced characterizations and theoretical calculations such as Fukui functions, offering critical insights into the rational design of high-capacity cathode for next-generation SIBs.