Ball-milling-enhanced capacitive charge storage of activated graphene in aqueous, organic and ionic liquid electrolytes

Abstract Ball-milling under either air or argon is explored as a facile way to adjust the textural properties, surface chemistry and morphology of activated reduced graphene oxide to the requirements for optimum electrode materials in electrical double layer capacitors operating in aqueous (KOH), organic (tetraethylammonium tetrafluoroborate in acetonitrile) and ionic liquid (1-Ethyl-3-methylimidazolium bis(trifluoromethyls​ulfonyl)​imide) electrolytes. Ball-milling is evidenced to specifically remove excessively large pores through the collapse of mesoporosity with no negative effect on in-pore resistance in any electrolyte, a concomitant improvement in volume-based storage and no loss of gravimetric performance. Ball-milling under air results in high oxygen content in the materials, which brings about performance deterioration in tetraethylammonium tetrafluoroborate in acetonitrile, but not significantly in aqueous and ionic liquid electrolytes. The best performance is achieved using activated graphene ball-milling under argon, with a volumetric capacitance of 90, 60, 70 F.cm−3 and a characteristic cell response time of 0.28, 1.3 and 8 s for aqueous, organic and ionic liquid electrolyte. While the highest energy density of 25 Wh.L−1 is reached in ionic liquid electrolyte at a cell potential of 3 V, the highest practical power density of 15 kW.L−1 is measured in tetraethylammonium tetrafluoroborate in acetonitrile at the energy density of 10 Wh.L−1. Our study underscores that simple ball-milling can provide the best trade-off among multiple performance parameters, resulting in sufficiently high volumetric capacitance with no detriment in high-rate response and cycle stability.