Amorphous carbonaceous material with specific surface area of 422 m(2) g(-1) has been prepared from natural flake graphite by high-energy ball-milling under air atmosphere. Milling treatment introduces abundant oxygen functional groups (carboxyl and alkoxy) to the graphite surface, and oxygen content reaches as high as 11.5 atom% after 75 h milling. The ball-milled graphite exhibits a three electrode stable potential window of 1.8 V in 1 mol L-1 Na2SO4 aqueous electrolyte. Symmetric supercapacitor based on ball-milled graphite shows a stable charge/discharge cycle behavior when cell voltage is set as high as 1.6 V with a specific capacitance of 81 F g(-1). The high utilization ratio (89%) of potential window for this system is attributed to the more negative value of E-0V (electrodes potential when the cell voltage is set to 0 V). (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.071205jes] All rights reserved.