Investigation of Various Ionic Liquids and Catalyst Materials for Lithium-Oxygen Batteries

Abstract

We report about the use of different ionic liquids and catalyst materials in lithium-oxygen batteries. Different types of oxygen electrodes such as self-supporting oxygen electrodes and catalyst-coated separators were prepared by hot-pressing and spray-coating procedures, respectively. Porous carbon material such as Ketjen Black, binder material and different metal oxides were used to prepare the cathodes. Self-supporting oxygen electrodes and catalyst-coated separators consist of a carbon material loading of 10 and 1.5 mg/cm², respectively. Electrolyte systems based on lithium bis-(trifluoromethylsulfonyl) imide (LiTFSI) in ionic liquids and lithium hexafluorophosphate (LiPF₆) in carbonate solvents were investigated in lithium-oxygen batteries. 1-butyl-1-methyl-pyrrolidinium bis-(trifluoromethylsulfonyl) imide and 1-butyl-1-methyl-pyrrolidinium tetracyanoborate as well as 1-ethyl-3-methyl-imidazolium bis-(trifluoromethylsulfonyl) imide, thi oisocyanat and dicyanamide were tested. The cell potential recorded for ionic liquids ranged from 2 to 2.5 V during the discharge process. At a current density of 0.1 mA/cm²_geom discharge capacities were found to be higher for lithium-oxygen cells using imidazolium-based ionic liquids compared to pyrrolidinium-based ionic liquids. Discharge voltages between 2.7 and 2.8 V were observed for the carbonate-based solvents. The discharge potentials observed were independent of the metal oxide used, but the charge potentials were highly dependent on the catalyst materials employed. Good reversibility was obtained when the corresponding lithium-oxygen cells were cycled at 20–40% of their maximum capacity (mA h/g C).