Non-hexagonal symmetry-induced QH-graphene as a promising anode material for sodium-ion batteries: Effects of solvent, vacancy defect, and interlayer coupling

Developing two-dimensional (2D) carbon electrode materials with high performance has become an increasingly fascinating pursuit. However, the most popular carbon allotrope, graphene, possesses chemical inertness arising from its delocalized π-electron network. Breaking of the hexagonal symmetry in graphene can disrupt its π-conjugated system, thus increasing the surface reactivity. Here, by employing first-principles calculations, we predict a 2D carbon allotrope (called QH-graphene), which exhibits remarkable stability across the dynamic, thermal, and mechanical aspects. It has several advantages as an anode material for sodium-ion batteries (SIBs), including a high theoretical capacity (1116.7 mA h g−1), a moderate Na migration barrier (0.35–0.60 eV), a suitable average open-circuit voltage (0.55 V), and a small change in lattice parameters (∼3%). When contacted with electrolyte solvents, the Na adsorption and diffusion capabilities are enhanced. Moreover, introducing a monovacancy defect in QH-graphene improves the adsorption strength of Na but reduces its mobility. Compared with single-layer QH-graphene, QH-graphene bilayer has a stronger binding affinity for Na while maintaining rapid ion diffusion on its exterior surface.