Flexible all-solid-state supercapacitors are fabricated with liquid-exfoliated black-phosphorus (BP) nanoflakes as an electrode material. These devices deliver high specific volumetric capacitance, power density, and energy density, up to 13.75 F cm(-3), 8.83 W cm(-3), and 2.47 mW h cm(-3), respectively, and an outstanding long life span of over 30 000 cycles, demonstrating the excellent performance of the BP nano flakes as a flexible electrode material in electrochemical energy-storage devices.