Nature of FeSe₂/N‐C Anode for High Performance Potassium Ion Hybrid Capacitor

Abstract Potassium ion hybrid capacitors have great potential for large‐scale energy devices, because of the high power density and low cost. However, their practical applications are hindered by their low energy density, as well as electrolyte decomposition and collector corrosion at high potential in potassium bis(fluoro‐sulfonyl)imide‐based electrolyte. Therefore, anode materials with high capacity, a suitable voltage platform, and stability become a key factor. Here, N‐doping carbon‐coated FeSe 2 clusters are demonstrated as the anode material for a hybrid capacitor, delivering a reversible capacity of 295 mAh g −1 at 100 mA g −1 over 100 cycles and a high rate capability of 158 mAh g −1 at 2000 mA g −1 over 2000 cycles. Meanwhile, through density functional theory calculations, in situ X‐ray diffraction, and ex situ transmission electron microscopy, the evolution of FeSe 2 to Fe 3 Se 4 for the electrochemical reaction mechanism is successfully revealed. The battery‐supercapacitor hybrid using commercial activated carbon as the cathode and FeSe 2 /N‐C as the anode is obtained. It delivers a high energy density of 230 Wh kg −1 and a power density of 920 W kg −1 (the energy density and power density are calculated based on the total mass of active materials in the anode and cathode).