Formulating room-temperature ionic liquid (RTIL) mixed electrolytes was recently proposed as an effective and convenient strategy to increase the capacitive performance of electrochemical capacitors. Here we investigate the electrical double-layer (EDL) structure and the capacitance of two RTILs, 1-ethyl-3-methyl­imidazolium bis­(tri­fluoro­methyl­sulfonyl)­imide (EMI-TFSI) and 1-ethyl-3-methyl­imidazolium tetra­fluoro­borate (EMI-BF4), and their mixtures with onion-like carbon electrodes using experiment and classical density functional theory. The principal difference between these ionic liquids is the smaller diameter of the BF4 – anion relative to the TFSI– anion and the EMI+ cation. A volcano-shaped trend is identified for the capacitance versus the composition of the RTIL mixture. The mixture effect, which makes more counterions pack on and more co-ions leave from the electrode surface, leads to an increase of the counterion density within the EDL and thus a larger capacitance. These theoretical predictions are in good agreement with our experimental observations and offer guidance for designing RTIL mixtures for EDL supercapacitors.