New family of layered N-based cathode materials for sodium-ion batteries

The cathode materials of sodium-ion batteries (SIBs) have received considerable attention not only because of their abundant natural reserves and chemical properties similar to those of lithium-ion batteries but also their great potential in energy storage and conversion technologies. However, their low capacity and high diffusion barrier remain unsolved problems. In this work, we systematically studied the theoretical capacity and sodium ion diffusion barrier in a new family of layered transition metal compounds, named MX₂ (M = Ti, V, Cr, Mn, and Fe; X = C, N, and O), as the cathode materials of SIBs. The results indicate that all 2H-phase MX₂ materials possess a high theoretical capacity of over 300 mA h g^-1. Moreover, it is found that the 2H-phase CrN₂ exhibits a desirable sodium ion diffusion barrier, indicating high mobility of sodium ions. In addition, the layered CrN₂ has a remarkable voltage window (3.1-3.8 V) and outstanding electrochemical performance arising from the charge transfer between Na and N atoms, which is induced by the large electronegativity of nitrogen. Our research provides a promising candidate for application in SIB cathode materials in the future.