Two hydrophobic ionic liquids (room temperature molten salts) based on 1-butyl-3-methylimidazolium cation (BMI+), BMI+PF6- and BMI+Tf2N-, were used in developing a highly efficient lithium anode system for lithium/seawater batteries. The lithium anode system was composed of lithium meta/ionic liquid/Celgard membrane. Both BMI+PF6- and BMI+Tf2N-maintained high apparent anodic efficiency (up to 100%) under potentiostatic polarization (at +0.5 V versus open-circuit potential (OCP)) in a 3% NaCl solution. Eventually, traces of water contaminated the ionic liquid and a bilayer film (LiH and LiOH) on the lithium surface was formed, decreasing the rate of lithium anodic reaction and hence the discharge current density. BMI+Tf2N- prevented traces of water from reaching the lithium metal surface longer than BMI+PF6- (60 h versus 7 h). However, BMI+PF6- was better than BMI+Tf2N- in keeping a constant current density (similar to 0.2 mA cm(-2)) before the traces of water contaminated the lithium surface due to the non-reactivity of BMI+PF6- with the lithium metal that kept the bare lithium surface. During the discharge process, BMI+PF6- and BMI+Tf2N- acted as ion transport media of Li+, Cl-, OH- and H2O, but did not react with them because of the excellent chemical stability, high conductivity, and high hydrophobicity of these two ionic liquids. Both BMI+PF6- and BMI+Tf2N- gels were tentative approaches used to delay the traces of water coming in contact with the lithium surface. (c) 2005 Elsevier B.V. All rights reserved.