For lithium ion batteries operating at elevated temperature, i.e., 333 K, an imidazolium based ionic liquid electrolyte, without any organic additives, was prepared by combining 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide [C(1)C(6)ImNTf(2)] and [LiNTf2] (1.0 mol.L-1). This binary electrolyte [Li][C(1)C(6)Im][NTf2], thermally stable up to 573 K, is ionically conductive (7.423 mS.cm(-1) at 333 K). This electrolyte, in a full system configuration with Li4Ti5O12 anode and LiFePO4 cathode, displays a discharge capacity of 130-135 mA.h.g(-1) at C/10 with a columbic efficiency of 98% at 333 K. (C) 2013 The Electrochemical Society. All rights reserved.