Lithium bis(fluorosulfonyl)imide (LiFSI) is one of the most promising lithium salts for use in lithium-ion batteries owing to its high ionic conductivity, high solubility in organic solvents, and excellent solid electrolyte interphase (SEI)-forming ability. In this study, we developed an ethylene carbonate (EC)-free electrolyte comprising LiFSI and sulfolane (SL) (LiFSI/SL). LiFSI/SL exhibits ideal Walden electrolyte behavior and crystallinity gap phenomena as well as reversible lithium intercalation into a graphite electrode. LiCoO2/graphite full-cells containing the new electrolyte show superior cycle stabilities at room and high temperature compared to those with conventional LiPF6/EC-based electrolytes. For example, after 200 cycles at 60 °C followed by 200 cycles at 70 °C, a cell with 1.5 mol dm−3 LiFSI/SL delivers a discharge capacity of 1.12 mAh, while that with 1.0 mol dm−3 LiPF6/EC/EMC delivers a discharge capacity of only 0.84 mAh under the same conditions. The enhanced performance of batteries with LiFSI/SL can be attributed to the effective SEI derived from LiFSI.