A critical problem of electric double-layer capacitors (EDLCs) is that their actual operational voltages can hardly
reach the theoretical values. This condition results from the unsuitable potential ranges of electrodes. Designing
methods to optimize the potential ranges of electrodes is important to entirely utilize the theoretical operational
voltages, thereby maximizing the energy densities of EDLCs. Here, we report a silica-grafted ionic liquid as an
electrolyte additive for a model EDLC to adjust the potential ranges of electrodes. The operational voltage of the
EDLC increased from 2.8 V to a theoretical value of 3.2 V, thereby resulting in an approximately 39% increase in
energy density and considerably enhanced cycling stability.