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 sp 2 -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 ∼C 6 Na 2 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 sp 2 -bonded carbon atoms, which effectively bind multiple Na atoms. We propose that sp-sp 2 carbon sheets can be promising candidates for high-capacity Na-ion battery anodes.