Descriptor driven engineering of Nb2CuS4 as a cathode material for rechargeable batteries: a first principles study

Rechargeable magnesium and sodium-ion batteries are attractive candidates for next generation energy storage because of their elemental abundance, low-cost, high-energy densities and eco-friendly. However, the development of suitable cathodes remains challenging due to sluggish diffusion kinetics, structural instability, and low operating potential. In this study, we evaluated Nb 2 CuS 4 as a promising cathode for Mg and Na storage using density functional theory (DFT). We compared bulk and interlayer ions accommodation first and found that bulk insertion is dynamically unfavorable, whereas interlayer intercalation is structurally stable. Therefore, the electrochemical behavior of Nb 2 CuS 4 interlayer-storage was systematically investigated. Pristine and ion-intercalated Nb 2 CuS 4 exhibit metallic character, indicating favorable electronic conductivity. The CI-NEB calculated diffusion barriers are 1.03 eV for Mg and 0.64 eV for Na, while theoretical capacities are 142 and 71 mAhg −1 with corresponding high operating voltages of 3.24 and 2.11 V, respectively. Charge-transfer and bonding analyses further reveal that sufficiently moderate ion-host interactions support reversibility, and the volume expansion remains limited upon increasing ion concentration. These results identify Nb 2 CuS 4 as a promising cathode host for Mg- and Na-based rechargeable batteries and provide insight into descriptor-guided design of ternary sulfide cathodes for future developments.