Two ionic liquids (ILs), which are functionalized with bis(oxalato)borate (BOB) and diﬂuoro(oxalato)borate (DFOB) as the anion, have been synthesized and applied as additives for the battery grade electrolyte 1 M LiPF6 in an equivolume mixture of EC-DEC-DMC (LP71). When 0.3 M IL was added to LP71, peaks attributed to the formation of solid electrolyte interface (SEI) were observed during cyclic voltammetry for both cathodic and anodic sides. The presence of peaks during anodic scans suggests that the ILs possess the ability to form SEI on cathode surfaces. The high voltage Li | LiNi0.5Mn1.5O4 (LNMO) cells, employing the integrated electrolytes, exhibited a discharge capacity of around 120 mAh g−1. The retention rates of discharge capacity after 100 cycles were found to be 99.2% and 98.1% for the solution with 0.3 M of BOB-IL and DFOB-IL, respectively. By means of impedance analysis during the galvanostatic cycling, the growth of SEI was confirmed when BOB-IL was used as the additive. In this case, in the infrared spectra of the cycled LNMO electrodes, peaks related to borate compounds and polycarbonate were observed. This confirms that the BOB-IL promotes the formation of stable interfaces between the electrolyte and LNMO.