LiC2, the super-dense high pressure phase of lithium intercalated graphite retains a high Li density upon pressure release; it stabilizes at LiC2.18if 0.5 at.% boron-substituted graphite is used for synthesis. We investigate it's in-plane structure and thermal decomposition using high resolution X-ray powder diffraction. This yields a 2⨿3 hexagonal cell with a = 8.54 Å and c = 3.71 Å. LiC2.18is stable indefinitely at ambient T and P. Above 100°C, a decomposition product LiC3.4–3.8is observed, corresponding to the stable phase at ambient conditionswithoutboron. In both cases, we attribute the decomposition to Li diffusion with formation of (meta)stable Li7clusters; we speculate that the boron impurity impedes this process and thereby ‘pins’ the denser of the two superlattices.