An ultrafast rechargeable hybrid potassium dual‐ion capacitor based on carbon quantum dot@ultrathin carbon film cathode

Ultrafast rechargeable hybrid potassium dual‐ion capacitors (HPDICs) were designed by employing carbon quantum dot@ultrathin carbon film (CQD@CF) as the cathode material. The designed CQD@CF is self‐assembled by a simple catalytic graphitization route followed by an acid leaching process. The special composite features a large adsorption interface, a remarkable anion hybrid storage capability and outstanding structure stability. The electrochemical performance of CQD@CF composite is fully tapped in a half‐cell system at the operating voltage between 1.4 and 4.5 V, achieving an admirable specific discharge capacity of 128.5 mAh·g −1 at 50 mA·g −1 , an ultra‐high capacity retention ratio of 97.97% after cycling 5000 times at 1000 mA·g −1 and a 96.10% high capacity retention ratio after cycling 40,000 times at 5000 mA·g −1 , respectively. Besides, with the support of ex situ TEM and XPS, the structural stability principle and anionic hybrid storage mechanism of CQD@CF electrode are investigated deeply. In the full‐cell system, HPDICs with the CQD@CF as cathode and nano‐graphite powder as anode also present a 141.5 Wh·kg −1 high energy density, a 5850 W·kg −1 power density and a super‐long cycle stability (90.2% capacity retention ratio after cycling 30,000 times at 5000 mA·g −1 ).