Graphullerite: A Thermally Conductive and Remarkably Ductile Allotrope of Polymerized Carbon

The understanding of the fundamental relationships between chemical bonding and material properties, especially for carbon allotropes with diverse orbital hybridizations, is significant from both scientific and applicative standpoints. Here, we elucidate the influence of the intermolecular covalent bond configuration on the mechanical and thermal properties of polymerized fullerenes by performing systematic atomistic simulations on graphullerite, a newly synthesized crystalline polymer of C₆₀ with a hexagonal lattice similar to that of graphene. Specifically, we show that the polymerization of C₆₀ molecules into two-dimensional sheets (and three-dimensional layered structures) offers tunable control over their mechanical and thermal properties via the replacement of weak intermolecular van der Waals interactions between the fullerene molecules with strong sp³ covalent bonds. More specifically, we show that graphullerite possesses highly anisotropic mechanical as well as thermal properties resulting from the variation in the chemical bonding configuration along the different directions. In terms of their mechanical properties, we find that graphullerite can be remarkably ductile if strained along a certain direction with oriented double bonds connecting the fullerenes. Combined with their drastically reduced Young's modulus and bulk modulus as compared to graphite, these materials have the potential to be utilized in flexible electronics and advanced battery electrode applications. In terms of their thermal properties, we show that the bonding orientation dictates the intrinsic phonon scattering mechanisms, which ultimately dictates their anisotropic temperature-dependent thermal conductivities. Taken together, their flexible nature combined with their remarkably high thermal conductivities as polymeric materials positions them as ideal candidates for a plethora of applications such as for the next generation of battery electrodes.