Hydrophobic ionic liquids based on the 1-butyl-3-methylimidazolium cation for lithium/seawater batteries

Abstract

Two hydrophobic ionic liquids (room temperature molten salts) based on 1-butyl-3-methylimidazolium cation (BMI +), BMI +PF 6 − and BMI +Tf 2N −, were used in developing a highly efficient lithium anode system for lithium/seawater batteries. The lithium anode system was composed of lithium metal/ionic liquid/Celgard membrane. Both BMI +PF 6 −and BMI +Tf 2N − 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 +Tf 2N − prevented traces of water from reaching the lithium metal surface longer than BMI +PF 6 − (60 h versus 7 h). However, BMI +PF 6 − was better than BMI +Tf 2N − in keeping a constant current density (∼0.2 mA cm −2) before the traces of water contaminated the lithium surface due to the non-reactivity of BMI +PF 6 − with the lithium metal that kept the bare lithium surface. During the discharge process, BMI +PF 6 − and BMI +Tf 2N − acted as ion transport media of Li +, Cl −, OH − and H 2O, but did not react with them because of the excellent chemical stability, high conductivity, and high hydrophobicity of these two ionic liquids. Both BMI +PF 6 − and BMI +Tf 2N − gels were tentative approaches used to delay the traces of water coming in contact with the lithium surface.