(28d) Design and Applications of Ionic Liquids for Electrochemical Systems

Ionic liquids are promising solvents for new chemical processes and electrochemical applications. Ionic liquids are conducting and non-volatile at ordinary temperatures, show a large liquid range and, in most cases, are thermally stable. These properties make them appealing materials for lithium ion batteries and capacitors. The physicochemical properties of an ionic liquid vary greatly depending on the molecular structure, providing the possibility to design task-specific applications. For rechargeable lithium ion batteries, we require that any electrolyte, including ionic liquids, provide adequate durability, high conductivity, good transport properties and, very important, chemical and electrochemical stability with safety. The ionic liquids studied in this work include various neat ionic liquids and lithium-salt solutions. The ionic liquid cations studied were mostly imidazolium, pyridinium and pyrrolidinium, with PF6-, AsF6-, BF4-, trifluoromethansulfonate, and bis(trifluoromethylsulfonyl)imide anions. Phase-, thermal-, and transport properties of ionic liquid systems have been measured in this work. In addition, battery cycling tests have been carried out for several ionic-liquid lithium-salt systems with different cathode- and anode materials. For electrochemical applications using lithium batteries, we must be sure that the ionic liquid is stable over a sufficient range of electric potential and, when lithium salt is added, be stable in a wide temperature range. In this case, the non volatility and non flammability of an ionic liquid provide better safety. Ionic-liquid cations are known to reduce in the presence of lithium metal. By choosing saturated organic cations like pyrrolidinium we anticipate fewer undesired side reactions compared to those observed using aromatic imidazolium or pyridinium cations. Electrochemical stability of an ionic liquid in a battery system is observed by measuring the interfacial impedance between electrodes and electrolyte.