Bis(oxlate)borate anion (BOB-) has high reduction potential for 1-ethyl-3-methyl-imdazolium tetrafluoroborate (EMIBF4) and forms solid electrolyte interface (SEI) on lithium (Li) metal surface. The addition of BOB- may make possible the use of EMIBF4 for Li metal battery. The electrochemical properties of ionic liquids containing EMIBF4 were analyzed. We investigated the stability of LiBOB/EMIBF4 and LiBF4/EMIBF4 electrolyte for Li metal and the performance of Li/MnO2 batteries with the electrolytes by H-1 NMR, ac impedance spectra and the electrochemical method. From the H-1 NMR spectra, LiBOB/EMIBF4 was stable for Li metal and LiBF4/EMIBF4 was reduced on the surface of Li metal. The discharge capacity of MnO2 cathode with LiBOB/EMIBF4 was 235 mAh g(-1). Li metal plating and stripping and Li-Al alloying reaction were reversible in LiBOB/EMIBF4. Charge-discharge reaction of Li-Al alloy in LiBOB/EMIBF4 had a high coulomb efficiency. The Coulomb efficiency at 1st cycle was 86% and the efficiency at 5th cycle was up to 94%. (C) 2009 Elsevier B.V. All rights reserved.