Recent Advances in Ionic Liquids for Lithium Secondary Batteries

Abstract

Ionic liquids (ILs) composed only of ionic species without any organic molecules possess various unique properties such as reduced flammability, reduced volatility, and a relatively high ionic conductivity at ambient temperature. The existence of ILs has become especially popular in various fields during the past two decades due to the discovery of moisture-insensitive ILs based on perfluoroanions—such as tetrafluoroborate [BF4]−, and bis(trifluoromethylsulfonyl)amide [(CF3SO2)2N]−, denoted as Tf2N−—which are easy to prepare and handle under atmospheric conditions [1]. At the same time, these perfluoroanions have attracted attention as a nonflammable electrolyte for various electrochemical energy devices, especially for lithium secondary batteries, because these perfluoroanions have already been examined as a counter anion for lithium salts [2]. The number of papers about battery applications using these ILs has increased during the past decade since early reports indicated their compatibility with conventional composite electrodes such as lithium cobalt dioxide, LiCoO2 [3–5]. Although various cathodes and anodes, and almost all composite electrodes, have been investigated, except for one carbon-based anode found to be compatible with these ILs, their performance, especially their rate performance, was found to be quite deficient compared to a conventional organic electrolyte. During the past decade, only a few anionic species have been developed to solve various defects derived from the conventional ILs. In particular, one of these new anions, bis(fluorosulfonyl)amide, denoted as [(FSO2)2N]−, dramatically reverses the poor impression of ILs as a lithium battery electrolyte [6–9]. Recent research on the thermal stability of ILs, which have been seriously investigated since the appearance of new ILs composed of [(FSO2)2N]−, revealed that the ILs are indeed harder to ignite; however, once ignited by an external source of energy, significantly high heat release occurs when the ILs burn under forced conditions [10]. Furthermore, not all ILs exhibited an expected safety feature as a nonflammable electrolyte under severe conditions such as the charged state [11, 12]. However, such facts have not reduced motivation for studying ILs as a battery electrolyte because various new results on next-generation batteries, which do not operate well in a conventional organic electrolyte, are beginning to be published, for example, Li-Air [13, 14], Li-S [15], and sodium secondary batteries [16]. In this chapter, the recent progress regarding the study of ILs for a lithium battery system will be briefly described. Especially, the course of development of new anionic species during the past decade will be the focus in order to demonstrate the difference between these new anions and a conventional anion, such as [(CF3SO2)2N]− or [BF4]−. Since there are many reports on the ILs for use as a battery electrolyte and also their basic electrochemical properties, all the reports could not be referred in this chapter, but several books and reviews are recommended [17–21].