Theoretical Investigation of Ca2+ Intercalation in WS2 as a Negative Electrode Material for Calcium-Ion Batteries: Supported by Experimental Evaluation

Tungsten disulfide (WS2), a two-dimensional layered material with favorable electronic properties, has been explored as a promising negative electrode material for calcium-ion batteries (CIBs). Despite its use in monovalent systems, its performance in divalent Ca2+ intercalation remains poorly understood. Herein, a combined theoretical and experimental framework is used to elucidate the electronic mechanisms underlying Ca2+ intercalation. Theoretical insights were obtained through density functional theory calculations, incorporating periodic simulations using the Vienna Ab initio Simulation Package, and localized orbital-level analysis using the discrete variational Xα method. These approaches reveal that Ca2+ insertion induces significant interlayer expansion, lowers diffusion barriers, and narrows the bandgap compared to Li+. Orbital analysis revealed strengthened W–S bonding and diminished antibonding interactions, which may contribute to the improved structural resilience. Electrochemical tests validated these predictions; the CaWS2 electrode delivered an initial discharge capacity of 208 mAh·g−1 at 0.1C, with 61% retention after 50 cycles at 1C. The voltage profile exhibits a distinct plateau near 0.7 V, consistent with a two-phase-like intercalation mechanism, contrasting with the gradual slope observed for Li+. These findings suggest that Ca2+ intercalation facilitates both rapid ion transport and enhanced structural robustness. This study offers mechanistic insights into multivalent-ion storage and supports the design of high-performance CIB electrodes.