Improved capacitive energy storage via surface functionalization of activated carbon as cathodes for lithium ion capacitors

Lithium ion capacitors (LICs) have the potential to combine the high energy density of lithium ion batteries and the high power density of supercapacitors into one device. In this study, we have investigated surface functionalization of activated carbon (AC) powder as the cathode for LICs with non-aqueous electrolytes. It is found that solution chemistry treatment is an effective way to impart pseudocapacitance and thus increase the specific capacitance of the AC powder. The surface functionalization has led to increases in the specific capacitance from 18 - 35 F/g to 80-140 F/g, while the areal specific capacitance per BET surface area has increased from 3.6 μF/cm2 to 74.8 μF/cm2. The latter is 3.5 times the theorectical electrical double layer value for graphene, indicating the existence of redox reactions and their great potential in enhancing the capacitance for LICs. The mechanism of capacitance improvement has been diagnosed and attributed mainly to the pseudocapacitive redox reaction on the CO sites. In addition, the enhancement in the specific capacitance is found to vary with the composition of electrolytes, likely due to the change in wetting behavior and the size of solvated ions. This work has opened up a new route to increase the specific capacitance of low cost and widely used AC powder for LICs.