To obtain symmetric supercapacitors (SCs) with high energy density,it is critical to fabricate an electrode with wide potential window and excellent capacitive performance.Herein,by using the strong double hydrolysis reaction between anions and cations,the FeOOH nanosheets on the surface of activated carbon cloth (FeOOH@AC) are prepared through a simple hydrothermal process.The FeOOH@AC electrode exhibits maximum capacitance of 4,090 m F·cm-2 at wider potential window-1-0 V and 3,250 m F·cm-2 at 0-1 V versus SCE in 2 M LiNO3 electrolyte.With two pieces of FeOOH@AC electrodes the obtained symmetric SC can operate at the voltage window of 2 V.This FeOOH symmetric SC shows high energy density of 13.261 mWh·cm-3 at a power density of 14.824 mW·cm-3 and maintains 4.175 mWh·cm-3 at a maximum power density of 118.564 mW·cm-3,as well as excellent charge storage capacity and cyclic stability.Li ion adsorption and diffusion mechanism on the (200) facets of FeOOH are explained by the density functional theory (DFT) calculations.The simple synthesis process and excellent capacitance performance of the FeOOH@AC composite make it a very promising candidate for high performance symmetric SC electrodes.