QPHO-graphene: a two-dimensional hexagon-free carbon allotrope as a high-performance anode material for sodium-ion batteries

Two-dimensional honeycomb carbon sheets (<i>e.g.</i>, graphene) cannot be directly applied as sodium-ion battery (SIB) anodes due to their chemical inertness. To disrupt the delocalized π bonding network, we theoretically predict a hexagon-free carbon allotrope (called QPHO-graphene), which consists of quadrilateral, pentagonal, heptagonal, and octagonal rings. It possesses intrinsic metallicity with robust thermodynamic, mechanical and dynamic stability. As an anode material with excellent performance for SIBs, QPHO-graphene exhibits a low Na ion migration barrier (0.16 eV), a high theoretical specific capacity (1595 mA h g^-1), a moderate average open-circuit voltage (0.34 V), and a minor lattice alteration (<2%). The existence of solvents having high dielectric constants can facilitate the adsorption and diffusion of Na. In addition, the appearance of a monovacancy defect enhances the binding strength of Na on QPHO-graphene but decreases the ion mobility. These findings make QPHO-graphene a potential anode material for high-performance SIBs.