The development of electrolytes capable of performing at a high voltage (>5 V) is essential for the advancement of lithium-ion batteries. In the present work, we have investigated a dinitrile–mononitrile-based electrolyte system that can offer electrochemical stability up to 5.5 V at room temperature. The electrolytes consist of 1.0 M lithium bis­(trifluoromethane)­sulfonamide in various volume proportions of glutaronitrile, a dinitrile, and butyronitrile, a mononitrile (10/0; 8/2; 6/4; 4/6; 2/8; 10/0). The ionic conductivity of the electrolytes was found to be 3.1 × 10–3–10.6 × 10–3 S cm–1 at 30 °C, comparable with commercially used carbonate-based electrolytes. However, butyronitrile reacts with Li metal to give 3-amino-2-ethylhex-2-ene-nitrile, 2,6-dipropyl-5-ethylpyrimidin-4-amine, and oligomers/polymers. These compounds have been characterized by nuclear magnetic resonance techniques, and based on these findings, a plausible mechanism of reactivity of mononitriles toward Li metal has been proposed. Finally, 5 wt % of vinylene carbonate is added to the glutaronitrile/butyronitrile (6/4 ratio) system to inhibit the reductive decomposition of butyronitrile. The resultant electrolyte system is used in the assembly of several coin cells consisting of a LiFePO4 composite cathode and a Li metal anode. The cells perform up to 3 C charge/discharge rate with reasonably good discharge capacity and also display a cycle life of more than 100 cycles at a 0.5 C rate with capacity retention above 95% at room temperature.