The electrical double layer (EDL) structure formed by ions at a charged surface, which is the key to determining the performance of supercapacitors, has been extensively studied for many monocationic ionic liquids (MILs). However, it is not known what effect replacing MILs with dicationic ionic liquids (DILs) will have on the EDL structure. In this work, the interfacial structure and electrochemical performance of DILs [C-n(mim)(2)](BF4)(2) (n = 3, 6, 9) and [C-6(mim)(2)](Tf2N)(2) were investigated using classical molecular dynamics (MD) simulation for comparison with their monocationic counterparts. Different EDL structures formed by DILs and MILs near an onion-like carbon (OLC) electrode were observed. The interfacial orientation of the imidazolium plane in dications was verified to be similar to that of monocations. Moreover, the dissimilar sizes of the cation/anion and the specific ion adsorption on OLC were found to contribute to the distinctive shape of the differential capacitance-electric potential (C-V) curves, which were also dependent on the type of anions. Increased capacitance in BF4-containing DILs was not observed in comparison with their counterpart MILs, whereas dicationic [C-6(mim)(2)](Tf2N)(2) yielded higher differential capacitance in contrast to monocationic [C(6)mim][Tf2N], which was substantiated by cyclic voltammetry measurements as well. This work provides molecular insights into the EDL structure and C-V curves of imidazolium-based DILs in OLC-based supercapacitors.