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

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 NaFeTiS and NaFeTiO as model cathodes to explore the covalency effects on metal-ligand interactions during anionic redox process. NaFeTiS with strengthened covalent interaction of metal-ligand bonds exhibits smaller voltage hysteresis and faster kinetics than NaFeTiO during (de)sodiation process. Theoretical calculations suggest that Fe is the dominant redox-active center in NaFeTiS, whereas the redox-active center moves from Fe to O with the removal of Na in NaFeTiO. We attribute the above different redox behaviors between NaFeTiS and NaFeTiO to the charge transfer kinetics from ligand to metal. Moreover, the structural stability of NaFeTiS 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.