Net-C18: A Predicted Two-Dimensional Planar Carbon Allotrope and Potential for an Anode in Lithium-Ion Battery

Net-C18, a predicted two-dimensional (2D) graphene-like carbon allotrope, is investigated via first-principles calculations. Its space group is Pmmm. There are 18 carbon atoms per cell. Net-C18 has five-, six-, and eight-membered rings. Net-C18 may be formed by adding even pairs of carbon atoms on the top of hexagons to reconstruct new five- and eight-membered rings, extending the strategy of Haeckelite. Compared to that of graphene (-9.28 eV atom(-1)), the total energy of net-C18 (-9.15 eV atom(-1)) is only 0.13 eV atom(-1) higher, revealing that net-C18 is energetically metastable. The calculations of phonon and ab initio molecular dynamics (AIMD) demonstrate dynamical and thermal stability of net-C18. The independent elastic constants of net-C18 meet the criterial for the mechanical stability of 2D structure. Its in-plane stiffness along x or y axis is comparably large. The AIMD results reveal that net-C18 has good thermal stability at 1500 K. The band structure also demonstrates that it is metallic. Furthermore, the diffusion of Li atoms on net-C18 has a low energy barrier (0.32 eV), and net-C18 has a low open-circuit voltage (0.024 V) and a high theoretical specific capacity (403 mAh g(-1)). Thus, net-C18 may provide high-temperature resistant, flexible electrode in electronics and a promising metallic anode in lithium-ion battery. The proposed formation of net-C18 may open a new pattern defect for the designs of new carbon allotropes.