This Account covers research dating from the early 1960s in the field of low-melting molten salts and hydrates,which has recently become popular under the rubric of "ionic liquids". It covers understanding gained in the principal author's laboratories (initially in Australia, but mostly in the U.S.A.) from spectroscopic, dynamic, and thermodynamic studies and includes recent applications of this understanding in the fields of energy conversion and biopreservation. Both protic and aprotic varieties of ionic liquids are included, but recent studies have focused on the protic class because of the special applications made possible by the highly variable proton activities available in these liquids.In this Account, we take a broad view of ionic liquids (using the term in its current sense of liquids comprised of ions and melting below 100°C). We include, along with the general cases of organic cation systems, liquids in which common inorganic cations (like Mg 2+ and Ca 2+ ) strongly bind a hydration or solvation shell and then behave like large cation systems and melt below 100°C.We will try to organize a wide variety of measurements on low-melting ionic liquid systems into a coherent body of knowledge that is relevant to the ionic liquid field as it is currently developing.Under the expanded concept of the field, ionic liquid studies commenced in the Angell laboratory in 1962 with a study of the transport properties of the hydrates of Mg(NO 3 ) 2 and Ca(NO 3 ) 2 . These were described as melts of "large weak-field cations" 1 and their properties correlated with those of normal anhydrous molten salts via their effective cation radii, the sum of the normal radius plus one water molecule diameter. This notion was followed in 1966 with a full study under the title "A new class of molten salt mixtures" in which the hydrated cations were treated as independent cation species. The large cations mixed ideally with ordinary inorganic cations such as Na + and K + , to give solutions in which such cohesion indicators as the glass transition temperature, T g , changed linearly with composition. The concept proved quite fruitful, and a field ("hydrate melts" or "solvate melts") developed in its wake in which asymmetric anions like SCN -and NO 2 -were used with solvated cations to create a very wide range of systems that were liquid at room temperature, while remaining ionic in their general properties.The validity of the molten salt analogy was given additional conviction by the observation that transition metal ions such as Ni(II) and Co(II) could be found in the complex anion states NiCl 4 2-and CoCl 4 2-when added to the "hydrate melt" chlorides. 2 Of special interest here was the observation 3 of extreme downfield NMR proton chemical shifts of hydrate protons when the hydrated cation was Al 3+ . These lay much further downfield than did the protons in strong mineral acids at the same concentration, leading to the design of solutions of highly acidic character from salts that would usually be considered neutral. Inde...

We report the reversible thermal unfolding/refolding, and long period stabilization against aggregation and hydrolysis, of >200 mg ml(-1) solutions of lysozyme in ionic liquid-rich, ice-avoiding, solvents.

The future of lithium metal batteries as a widespread, safe and reliable form of high-energy-density rechargeable battery depends on a significant advancement in the electrolyte material used in these devices. Molecular solvent-based electrolytes have been superceded by polymer electrolytes in some prototype devices, primarily in a drive to overcome leakage and flammability problems, but these often exhibit low ionic conductivity and prohibitively poor lithium-ion transport. To overcome this, it is necessary to encourage dissociation of the lithium ion from the anionic polymer backbone, ideally without the introduction of competing, mobile ionic species. Here we demonstrate the effect of zwitterionic compounds, where the cationic and anionic charges are immobilized on the same molecule, as extremely effective lithium ion 'dissociation enhancers'. The zwitterion produces electrolyte materials with conductivities up to seven times larger than the pure polyelectrolyte gels, a phenomenon that appears to be common to a number of different copolymer and solvent systems.

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The quest for the development of rechargeable lithium-metal batteries has attracted vigorous worldwide research efforts. The advantage of lithium-metal-based batteries over traditional nickel-cadmium, nickel-metal-hydride, and lithium-ion batteries is the higher theoretical energy density of metallic lithium.[1] The key limitation of the lithium-metal battery is the growth of dendrites during cycling. [2] This is potentially hazardous and leads to reduced cycle lifetime. A number of well-documented accidents have occurred with experimental cells due to this problem, and as a result research had largely stopped until recently. [3] Hence, to realize the ultimate potential of the lithium-metal cell, a means to suppress dendrite growth is required.Alternative electrolytes have been investigated, such as the use of a solid electrolyte to act as a mechanical barrier, [1] or electrolytes which control the properties of the solid-electrolyte interphase (SEI), i.e., the passivation layer, which is now well accepted as forming on the lithium-metal surface.[4] The SEI layer controls the performance of the battery by protecting the lithium-metal surface, while still allowing Li-ion transport. At the extremely negative potential of the lithium electrode the presence of an SEI appears to be ubiquitous. [5] It is also well known that the composition and morphology of the SEI layer can be controlled through the use of additives, which can improve plating behavior and cycle lifetime. [6] Nonetheless, the limited cycle life and safety problems associated with dendrite formation remain in devices incorporating these volatile electrolytes. It has recently been shown by Howlett et al., Passerini and co-workers, Katayama et al., and Sakaebe et al. [7] that a new class of electrolytes-room-temperature ionic liquids (RTILs)-have properties that can support lithium electrochemistry. In addition, ionic-liquid electrolytes facilitate enhanced cycling efficiency and favorable plating morphology of lithium.[7a] These electrolytes, in particular the N-methyl-N-alkylpyrrolidinium bis(trifluoromethanesulfonyl)amide [8] family of ionic liquids, are finding wide use in many electrochemical devices, such as electrochromic windows, [9] dye-sensitized solar cells, [10] and other devices. [9] Ionic liquids have many properties which make them highly suitable for use in rechargeable lithium batteries: these properties include excellent thermal stability, effectively zero volatility and flammability, high conductivity, and a wide electrochemical window. Nevertheless, despite improved cycling performance when compared to most traditional solvent-based electrolytes, dendrite formation still occurs when practical current densities are used. [7a] This dendrite formation may be due to mass-transport limitations either in the electrolyte, or in the SEI layer itself. Ultimately this is what restricts the application of ionicliquid electrolytes in rechargeable lithium-metal and lithiumion batteries. Zwitterionic compounds (Scheme 1) are compounds that ar...