The Zwitterion Effect in Ionic Liquids: Towards Practical Rechargeable Lithium‐Metal Batteries

Abstract

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...

Ionic Liquids in Microemulsions—A Concept To Extend the Conventional Thermal Stability Range of Microemulsions

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“…11 Other studies concern the use of ionic liquids as electrolytes in batteries, 12 double layer capacitors or solar cells. 13,14211,212,213,214,215,216,217,218 In recent years, progress has been made in the determination of thermodynamic and transport properties of RTILs, with imidazolium-based compounds being in the focus of many studies.…”

Ion transport and storage of ionic liquids in ionic polymer conductor network composites

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Ionic Liquids Gelation with Polymeric Materials: The Ion Jelly Approach

“…This set of properties together with the fact that most of ILs are nonvolatile and nonflammable, has driven their application as electrolytes for different electrochemical devices, such as dye synthesized solar cells, double layer capacitors, fuel cells, electrochemical windows and of course lithium secondary batteries. (Byrne et al, 2005;Fernicola et al, 2006;Galinski et al, 2006;Lu et al, 2002;Stephan, 2006) In fact the actual trend in electrochemical devices points to ILs as the most promising approach to develop safety and highly conductive electrolytes.…”