Unravelling K intercalation on graphene and its variants through two-dimensional cluster expansion modeling

Abstract

In this study, we investigate K ordering and its impact on K intercalation and storage capacity in graphite, bilayer and single-layer graphene through two-dimensional cluster expansion implemented in first-principles calculations with van der Waals corrections. For graphite, similar to the Li counterpart, K intercalation exhibits a multistage feature as the K concentration increases from C2 to KC8. However, beyond KC8, K atoms become crowded in graphite, resulting in a negative discharge voltage. For bilayer and single-layer graphene, a noticeable displacement of K from original hexagonal sites occurs at higher K concentrations compared to graphite. K intercalation in graphite after the initial segment (from C2 to KC8) leads to an increase in interlayer distance, while in bilayer graphene causes much larger elongation in interlayer distance. The calculated overpotential for K intercalation on bilayer and single-layer graphene is found to be 0.17 and 0.13 V, respectively, which is lower than the overpotential (0.86 V) observed on graphite. The charge redistribution between K and carbon layers indicates that bilayer and single-layer graphene offer pseudo-capacitance, facilitating faster charge transfer and higher K capacity. The above observation suggests the graphene structures have a superior K-storage capacity compared to graphite.