sp–sp(2) Carbon Sheets as Promising Anode Materials for Na-Ion Batteries

[Image: see text] 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.