In this paper, a novel material-3D potassium ion preintercalated graphene was designed and synthesized via one step using a new reaction between K and CO. Furthermore, this material exhibited excellent performance as electrodes for aqueous symmetrical supercapacitors. When the electrode was scaled up from 3.0 to 8.0 mg/cm(2), negligible capacitance degradation was observed, leading to a very high areal capacitance of 1.50 F/cm(2) at 1 A/g. Furthermore, even if a large operating temperature of -15 or 55 degrees C was employed, its excellent electrochemical performance remained with specific capacitances of 208 F/g at 55 degrees C, 184 F/g at 25 degrees C, and 98 F/g at -15 degrees C. This could be attributed to 3D structure and K preintercalation of the material, which provides rich active sites for electric double-layer formation, lower ion transport resistance, and shorter diffusion distance.