Strong Covalent Metal-Ligand Interaction Enables a Fast Kinetic and Structurally Stable Na-Ion Layered Cathode

Abstract

Anionic redox chemistry has attracted increasing attention for the improvement in the reversible capacity and energy density of cathode materials in Li/Na-ion batteries. However, adverse electrochemical behaviors, such as voltage hysteresis and sluggish kinetics resulting from weak metal-ligand interactions, commonly occur with anionic redox reactions. Currently, the mechanistic investigation driving these issues still remains foggy. Here, we chemically designed Na_0.8Fe_0.4Ti_0.6S₂ and Na_0.8Fe_0.4Ti_0.6O₂ as model cathodes to explore the covalency effects on metal-ligand interactions during anionic redox process. Na_0.8Fe_0.4Ti_0.6S₂ with strengthened covalent interaction of metal-ligand bonds exhibits smaller voltage hysteresis and faster kinetics than Na_0.8Fe_0.4Ti_0.6O₂ during (de)sodiation process. Theoretical calculations suggest that Fe is the dominant redox-active center in Na_0.8Fe_0.4Ti_0.6S₂, whereas the redox-active center moves from Fe to O with the removal of Na⁺ in Na_0.8Fe_0.4Ti_0.6O₂. We attribute the above different redox behaviors between Na_0.8Fe_0.4Ti_0.6S₂ and Na_0.8Fe_0.4Ti_0.6O₂ to the charge transfer kinetics from ligand to metal. Moreover, the structural stability of Na_0.8Fe_0.4Ti_0.6S₂ is enhanced by increasing the cation migration barriers through strong metal-ligand bonds during desodiation. These insights into the originality of metal-ligand interactions provide guidance for the design of high-capacity and structurally stable cathode materials for Li/Na-ion batteries.