Microwave-regulated Bi nanoparticles on carbon nanotube networks as a freestanding electrode for flexible sodium-ion capacitors

Abstract

High capacity, long cycle life, and fast kinetics are highly desired for alloying anodes in sodium ion capacitors (SICs). However, the huge repeatedly volume changes during the alloying/dealloying process cause electrode pulverization, seriously degrading the capacity and cycling stability. To address this issue, we developed a microwave irradiation technology for the in-situ growth of nano-sized Bi uniformly anchored on the surface of carbon nanotubes (CNTs). The as-synthesized freestanding electrode film effectively retards the pulverization of Bi nanoparticles, enabling fast sodium storage kinetics for high-power performance (278.1 mAh g−1 @ 30 A g−1), as well as high-capacity retention of 94% for over 3,500 cycles. The coin-cell type SICs of a Bi/CNTs anode paired with an activated carbon (AC)/CNTs cathode can deliver a maximum energy density of 128.5 Wh kg−1 and a high power density of 12.3 kW kg−1 with a remaining energy density of 85 Wh kg−1. Additionally, the flexible quasi-solid SICs using a gel electrolyte demonstrated a high volumetric energy density of 21 mWh cm−3 with good cycling stability (90%) for over 1500 cycles. These results show great promise for our developed SICs as the next-generation energy storage to bridge the performance gap between batteries and supercapacitors, as well as for flexible energy storage applications.