Na₂S in‐situ infiltrated in actived carbon as high‐efficiency presodiation additives for sodium ion hybrid capacitors

Sodium ion hybrid capacitors (SIHC) are emerging as promising next‐generation energy storage devices with high energy/power density. Presodiation is an essential part of SIHC production due to the lack of sodium sources in the cathode and anode. However, in the current presodiation methods, electrochemical presodiation by galvanostatic current charging and discharging requires a temporary half‐cell or a complex reassembling process, which severely hinders the commercialization of SIHC. Herein, in situ synthesized Na 2 S infiltrated in activated carbon was used as a sodium salt additive for supplying Na + in SIHC. Due to a low ratio of Na 2 S additive attributed to high theoretical specific capacity, the fabricated Na 2 S/activated carbon composite//HC SIHC can show a higher energy density of 129.71 Wh kg −1 than previously reported SIHC on presodiation of cathode additives. Moreover, the designed SIHC shows an excellent cycling performance of 10,000 cycles, which is attributed to the Na 2 S additive with the advantages of low decomposition potential and no gas generation. This work provides a novel approach for the fabrication of highly efficient Na 2 S additive composite cathodes for SIHC.