A fast bismuth-carbon composite anode for achieving kinetic matching between the anode and cathode of sodium-ion capacitors

Sodium-ion capacitors (SICs) typically feature a hybrid design, incorporating a battery-type anode that operates by faradaic redox reactions and an activated carbon cathode that functions through electrical double-layer (EDL) adsorption/desorption. However, the kinetics of faradaic processes are inherently slower than those of EDL processes, leading to a fundamental problem known as kinetic imbalance between the electrodes, which hinders the development of high-performance SICs. To address this, we synthesized composites of bismuth nanoparticles in N-doped carbon (Bi@NC) by a high-temperature sintering method. The resulting Bi@NC anode has a specific capacity of 300 mAh g−1 at 0.5 A g−1, an exceptional rate capability (maintaining performance at currents exceeding 75 A g−1), and outstanding cycling stability over 12 000 cycles. Three-electrode Swagelok cell tests revealed that this high-rate Bi@NC composite effectively decreases the kinetic gap with the activated carbon cathode, as shown by an analysis of their respective potential swing windows (vs. Na/Na+). This enables the fabricated SIC to achieve a maximum energy density of 115 Wh kg−1, a peak power density of 45 535 W kg−1, and a long cycle life exceeding 8 000 cycles.