sp-sp² Carbon Sheets as Promising Anode Materials for Na-Ion Batteries

We explore the applicability of graphynes, two-dimensional carbon sheets with sp- and sp²-bonds, as sodium (Na)-ion battery anodes using first-principles density functional theory. We found that voltages attainable from the charging-discharging of Na into multilayer graphyne are proper for use as anodes. The composite is ∼C₆Na₂ at the maximum Na concentration, corresponding to gravimetric and volumetric capacities of ∼837 mAh g^-1 and ∼1056 mAh cm^-3, respectively. These are significantly greater than the corresponding values (372 mAh g^-1 and 818 mAh cm^-3) of graphite for lithium. We ascribe the enhancement of the capacities to their nanoporous structures with sp- and sp²-bonded carbon atoms, which effectively bind multiple Na atoms. We propose that sp-sp² carbon sheets can be promising candidates for high-capacity Na-ion battery anodes.