Layered tetragonal zinc chalcogenides for energy-related applications: From photocatalysts for water splitting to cathode materials for Li-ion batteries

Two-dimensional (2D) materials with tunable direct bandgaps are attractive for energy-related applications such as visible-light optical devices and cathode materials in metal-ion batteries. Here we perform first-principles calculations to investigate the structural, electrical, optical, and electrochemical properties of 2D tetragonal (t-) ZnX (X = S, Se) with layered structures and we also explore their applications in photocatalysts and Li-ion batteries. We find that t-ZnX layers prefer the AA stacking pattern when forming multi-layer (ML) structures. We also show that t-ZnX MLs and 3D bulks are all stable according to phonon calculations and ab initio molecular dynamics (MD) simulations. The band edge positions of these layered materials can be tuned by modifying the number of layers to transform them into being more suitable for photocatalysis. We further show that the t-ZnX layered structures, in particular t-ZnS single-layer (SL), are promising cathode materials for Li-ion batteries exhibiting a strong adsorption of Li atoms without reducing the Li mobility. Finally, we find that the most favorable adsorption configuration of Li atoms on t-ZnX SL strongly depends on the Li concentration. It is worth pointing out that the almost barrierless feature of Li diffusion on t-ZnS SLs makes t-ZnS SL a good candidate for a fast-charging device. Our work opens a promising avenue for the modulation of novel t-ZnX layered structures for a wealth of potential applications in energy conversion and storage.