An EC/DEC [40:60% (v/v)] solvent mixture has been added in various amounts to the ionic liquid (IL) hexyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N1116-NTf2) in the presence of LiNTf2 (lithium bis(trifluoromethylsulfonyl)imide) as lithium salt for possible use as electrolytes in lithium-ion batteries. These electrolytes exhibit a larger thermal stability than the reference electrolyte EC/DEC [40:60]+LiNTf2 1M when the percentage of the IL exceeds 30% (v/v). All studied electrolytes are glass forming ones with an ideal glass transition temperature of ca. −85°C(±5°C), which has been determined by application of the VTF theory to conductivity and viscosity measurements and confirmed by DSC (T g =−90±5°C). An electrochemical window of about 5V versus Li/Li+ was measured at a glassy carbon electrode. The cycling ability of the optimized electrolyte N1116-NTf2/EC:DEC (40/60% (v/v))+1M LiNTf2 has been investigated at a titanate oxide (Li4Ti5O12) and a cobalt oxide (Li x CoO2) electrodes. Cycling the positive and the negative electrodes was conducted successfully with a high capacity and without any significant fading.