Potassium-pre-intercalated delta-phase MnO2 is uniformly grown on carbon nanofibers for the positive electrode of asymmetric supercapacitors. An electrospun CNF is chemically activated with KOH at 800 degrees C for the negative electrode, showing ideal capacitive behavior. The crystallinity of MnO2 is significantly reduced by the pre intercalation of K ions into its layered structure. This textural characteristic is beneficial to the K+ diffusion into/out the interlayer structure, leading to effective utilization of the electroactive material of KxMnO2. This unique composite electrode provides both ideal pseudo-capacitive behavior from KxMnO2 and excellent electric conductivity from the CNF network, exhibiting a fairly high specific capacitance value of 279 F g(-1) at 1 A g(-1) with ca. 82.3% capacitance retention from 1 to 32 A g(-1). A flexible ASC consisting of the positive KxMnO2@CNF electrode, a paper separator, and the negative ACNF electrode is successfully assembled. This cell shows superior ASC performances: a high cell voltage between 0 and 2 V, excellent capacitance retention (10,000 cycles with 10% decay), and simultaneously reaching high specific energy and power of 21.1 Wh kg(-1) and 9.5 kW kg(-1). The charge storage behavior of this cell without bending and with a bending angle of 90 degrees shows no apparent difference, demonstrating its potential in the next-generation flexible energy storage devices.