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

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.