An EC/DEC [40:60% (v/v)] solvent mixture has been added in various amounts to the ionic liquid (IL) hexyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N-1116-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 1 M 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 degrees C(+/- 5 degrees C), which has been determined by application of the VTF theory to conductivity and viscosity measurements and confirmed by DSC (T-g = -90 +/- 5 degrees C). An electrochemical window of about 5 V versus Li/Li+ was measured at a glassy carbon electrode. The cycling ability of the optimized electrolyte N-1116-NTf2/EC:DEC (40/60% (v/v)) + 1 M LiNTf2 has been investigated at a titanate oxide (Li4Ti5O12) and a cobalt oxide (LixCoO2) electrodes. Cycling the positive and the negative electrodes was conducted successfully with a high capacity and without any significant fading. (C) 2008 Elsevier Ltd. All rights reserved.