Different contents of fluoroethylene carbonate (FEC) as cosolvent is added into succinonitrile (SN) solution to form a novel electrolyte for lithium batteries. The SN-based electrolyte with 20 wt % FEC exhibits the most favorable properties involving the good thermal stability, wide electrochemical window and high ionic conductivity. Comparing with the commercial electrolyte, the 20% FEC-SN electrolyte demonstrates the advantage of high safety and excellent interfacial compatibility with lithium due to the form of compact and smooth solid electrolyte interphase layer on the anode. LiCoO/Li cells using the SN-based electrolyte behave a high reversible discharge capacity of 122.4 mAh g and keep an outstanding capacity retention of 91% (122.1 mAh g) at 0.5 C after 100 cycles at 25 °C, 50 °C, respectively. More importantly, the soft-package cells with the SN-based electrolyte can withstand harsh surroundings at 120 °C for 30 min without gas emitted, and can still keep the capacity retention of 77% compared to that before heat treatment, significantly higher than traditional commercial electrolyte (0%). All above results indicate the novel SN-based electrolyte can be an excellent alternative electrolyte in a practical lithium battery.