Anomalous Redox Features Induced by Strong Covalency in Layered NaTi_1-y V_y S₂ Cathodes for Na-Ion Batteries

The rising demand for energy density of cathodes means the need to raise the voltage or capacity of cathodes. Transition metal (TM) doping has been employed to enhance the electrochemical properties in multiple aspects. The redox voltage of doped cathodes usually falls in between the voltage of undoped layered cathodes. However, we found anomalous redox features in NaTi_1-y V_y S₂ . The first discharge platform potential (2.4 V) is significantly higher than that of undoped NaTiS₂ and NaVS₂ (both around 2.2 V), and the energy density is raised by 15 %. We speculate that the anomalous voltage is mainly attributed to the strong hybridization in the Ti-V-S system. Ti³⁺ and V³⁺ undergo charge transfer and form a more stable Ti (t_2g ⁰ e_g ⁰ ) and V (t_2g ³ e_g ⁰ ) electronic configuration. Our results indicate that higher voltage of cathode materials could be achieved by strong TM-ligand covalency, and this conclusion provides possible opportunities to explore high voltage materials for future layered cathodes.