Mechanisms of adsorption and diffusion of Na on a VSe monolayer with engineering-induced vacancies

Although research on vacancy engineering of anode materials has sufficiently advanced to obtain heightened battery capacity, the effect on the diffusion barrier underlying the mechanism remains to be elucidated. Herein, we investigated the effect of vacancy engineering on Na adsorption and diffusion on a vanadium diselenide (VSe) monolayer using first-principles calculations to reveal the underlying physics behind the performance optimization of anode materials in a sodium-ion battery. The results demonstrate that the structure of the substrate is responsible for the difference between the adsorption energy and diffusion barrier that resulted from cation and anion vacancies. As there is an absent Se atom (V) on the surface layer of the substrate, diffusion of Na on the surface could become pressurized with a high diffusion barrier up to 0.33 eV and a high adsorption energy (−1.92 eV) to capture additional Na atoms. However, because the V layer is sandwiched between two Se layers, there is less interaction with Na, and the adsorption energy and diffusion barrier are −1.58 and 0.13 eV, respectively, when a V atom is nonexistent (V). Moreover, the defective VSe increased the battery capacity, with little impact on open-circuit voltage. In this work, we analyzed the effect of vacancy engineering on VSe monolayer material, which provides theoretical clues for the design of efficient sodium-ion batteries with heightened capacity.