Composition–structure mapping of fluorine–graphite intercalation compounds

Abstract

Fluorine–graphite intercalation compounds (FGICs) have been extensively studied for their enhanced conductivity, yet their reported composition–structure relationships remain scattered. Here, we present an empirical analysis of FGICs focusing on interlayer spacing dᵢ, carbon-to-fluorine ratio C/F, and stage number s, compiling literature data spanning multiple decades. Variations to graphite host (including highly oriented pyrolytic graphite, natural graphite, powder, fiber, mesophase pitch, microbead, and specialized carbon sources) and synthetic methods are also considered. Despite substantial variability in experimental protocols, the data (184 entries) collapse into three regimes across stages 1–4 with C/F ∼0.63–16.7 (71.5−8.6 wt%F). The dᵢ ∼4.4, ∼6.0, and ∼11.4 Å regimes have been interpreted in the literature as corresponding to fluorine nesting, semi-ionic fluorine intercalation, and graphite bi-intercalation structures, respectively. Other smaller subsets are also identified at dᵢ ∼5.5 Å (coexistence of planar sp2 C=C/puckered sp3 C−F), 6.0 Å (covalent C−F), and 7.8 Å (F2 perpendicular to graphene sheets). This diversity reflects the anisotropic mechanical response of graphite: stiffness along the stacking direction constrains expansion, while in-plane softness accommodates compositional variation. We introduce Vgal representing the available gallery volume (i.e., at the interlayer space) per carbon atom which does not require assumptions about fluorine size. This descriptor preserves such three regimes, whereas the traditional packing fraction varies widely and may exceed physically meaningful limits. Taken together, this work provides an interpretative classification of FGICs, consolidating insights into fluorine intercalation that are not evident from individual reports while helping place future FGICs within, and potentially extend, the known composition–structure landscape.