Abstract Lithium‐ion batteries operating at high voltage generally endure drastic capacity fading and serious safety issues. Working on the electrolytes’ stability can be a solution to mitigate these problems related to high voltage. Herein, the beneficial impact of functional electrolyte additives in a state‐of‐the‐art carbonate‐based electrolyte is demonstrated. The combination of fluoroethylene carbonate (FEC) with succinonitrile (SN) as additives was used to enhance the thermal stability of the electrolyte reference 1 M LiPF 6 in EC:DMC (1 : 1, by weight) and cycling stability of a high voltage lithium‐ion device, consisting of a LiMn 1.5 Ni 0.5 O 4 cathode and a metallic lithium anode. The electrolyte using the FEC/SN mixture displayed a wider electrochemical stability window (ESW) exhibited by linear sweep voltammetry (LSV). Furthermore, this electrolyte allowed the device to exhibit better rate capability and a capacity retention of 75 % after 100 cycles. Interestingly, the FEC+SN‐based electrolyte exhibited better thermal stability using operando accelerating rate calorimetry (ARC) by virtue of the lower heat quantity generated by the battery device. The remarkable improvements can be ascribed to the formation of a protective cathode‐electrolyte interface (CEI) produced by interfacial reactions between the cathode surface and electrolyte compounds.