Improving electronegativity of N-doped carbon by encapsulating CoFe alloy clusters with a chainmail-like structure for high-energy sodium-ion capacitors

Abstract

Metal-nitrogen-carbon has attracted great research interests for sodium storage. However, the limited and unstable M-Nx active sites result in poor rate performance and inferior cycle life. Herein, CoFe nanoalloy clusters embedded into N-doped carbon nanofibers (denoted as CoFe-N-C) with hollow porous structures are designed. 3D conductive carbon networks increase electronic conductivity, and the interconnected hollow structures effectively suppress volume expansion during cycling. According to the theoretical calculations, the CoFe alloy nanoclusters serve as active sites that improve electronegativity of N-doped carbon, thus providing suitable bonding energy for Na+ and facilitating Na+ diffusion. When applied for sodium-ion capacitors, such a CoFe-N-C anode can deliver a high capacity of 298 mAh g−1 at 1 A g−1 over 3000 cycles with nearly 100% capacity retention. The assembled sodium-ion capacitors with CoFe-N-C as anode gives a broad working voltage window (0-4.0 V), an outstanding energy density of 211.6 Wh kg−1 at 200.0 W kg−1 and a superior power density of 20.0 kW kg−1 at 127.8 Wh kg−1, together with a low capacity decay of 0.012% per cycle after 900 cycles.