Succinonitrile (SN) has been used as functional additive to improve the thermal stability and broaden the oxidation electrochemical window of commercial electrolyte 1 M LiPF6/EC/DEC (1:1, by volume) for high-voltage LIBs (cathode: Li1.2Ni0.2Mn0.6O2, anode: Li). 1 wt % SN-based electrolyte showed a wide electrochemical oxidation window of 5.4 V vs Li+/Li and excellent thermal stability demonstrated by thermogravimetry (TG) and X-ray photoelectron spectroscopy (XPS), as well as theoretical analysis according to molecular orbital theory. The LNMO (Li1.2Ni0.2Mn0.6O2) battery with 1 wt % SN-based electrolyte showed better cyclability and capacity retention when charged to higher cut-off voltage. The improved battery performance is mainly attributed to the formation of uniform cathode electrolyte interface (CEI) formed by interfacial reactions between the LNMO cathode and electrolyte. The outcome of this work and the continuous research on this subject can generate critical knowledge for designing thermal stability electrolytes for large format lithium-ion batteries.