Effect of water molecules on the diffusions of alkali metal ions in two-dimensional transition metal sulfides

Two-dimensional sulfide materials are promising cathode materials for alkali-ion batteries. However, the presence of water can significantly impact their performance. Using density functional theory, this study reveals how water molecules influence alkali metal ion diffusions in MS₂ (M = Mo, V, Ni) cathodes. In a pristine MS₂ layer, the diffusion barrier for a single ion along the sulfur path is high. When water is introduced into the interlayer, it forms hydrated alkali metal ions. We found that among the three two-dimensional transition metal sulphides (MoS₂, WS₂, NiS₂), Li exhibits the highest capacity for accommodating hydrated ions. The diffusion of these hydrated ions results in a significantly lower energy barrier. This reduction in the diffusion barrier becomes more substantial as the number of water molecules increases. These findings indicate that strategically increasing interlayer water concentration could be essential for enhancing the performance of two-dimensional sulfide-based batteries.