We report here a carbonate-free electrolyte for lithium-ion batteries with low flammability which consist of lithium bis(fluorosulfonyl)imide (LiFSI) and 2-methylglutaronitrile (MGN) (LiFSI/MGN). LiFSI/MGN enables reversible lithium ion intercalation into graphite without any additives or co-solvents due to LiFSI and MGN-derived SEI formed on the surface of graphite negative electrode. LiCoO2/graphite full-cells with LiFSI/MGN show good cycle stability even at 1C despite its low ionic conductivity and high viscosity while those with LiFSI/succinonitrile (SN) and lithium bis(trifluorosulfonyl)imide (LiTFSI)/SN exhibit capacity fade. Moreover, LiFSI/MGN shows stable liquid phase at low temperature owing to its branched structure. Thus, LiFSI/MGN opens the possibility of lithium-ion batteries with enhanced safety owing to the high thermal stability and low flammability of MGN.