(Invited) Ionic Liquids for Rechargeable Metal Batteries

The emergence of global warming driven by greenhouse gases has put tremendous emphasis on the need to develop improved large scale energy storage systems including low-cost, and high energy density batteries such as rechargeable metal batteries. Batteries in which metals such as lithium (Li), magnesium (Mg), sodium (Na) and zinc (Zn) act as the electroactive species offer great potential in these applications. Room temperature ionic liquids (RTILs) that are, generally, non-volatile, non-flammable and thermally stable have recently been shown to enhance the electrochemical characteristics and long-term cycling of some of these metals[ 1 ]. For example, Figure 1 below compares the charging behaviour of a Li|LiCoO2 cell in conventional organic electrolyte (solid bold line) to that of RTIL (dashed line)[ 2 ]. The high rate charging behaviour is substantially improved in the latter electrolyte due to the high solubility of Li ions in the RTIL.  The thermal stability of RTILs is also becoming very significant in offering improved safety with these large-scale batteries. Recent work has demonstrated the importance of functional groups in the RTIL cations and anions [ 3 ] and the role of additives[ 4 ] in influencing speciation and electrochemical behaviour.  These functionalised ILs are providing a pathway towards cheaper, rechargeable batteries such as those based on zinc[ 5 ] and magnesium[ 6 ].  This talk will discuss recent work from our group, and others, in these areas. Figure 1:  1 C to 5 C charging profiles of organic liquid electrolyte [1M LiPF6, EC:DMC = 50:50 vol.%] (Solid bold line) and 3.2 mol.kg-1 LiFSI in C3mpyrFSI (Dashed line). (Reproduced from ref [ 2 ]). [1]          a) M. Kar, T. J. Simons, M. Forsyth, D. MacFarlane, Phys. Chem. Chem. Phys. 2014, 16, 18658-18674; b) D. R. MacFarlane, M. Forsyth, P. Howlett, M. Kar, S. Passerini, J. Pringle, H. Ohno, M. Watanabe, F. Yan, W. Zheng, S. Zhang, J. Zhang, Nature Review Materials 2016 - in press. [2]          H. Yoon, P. C. Howlett, A. S. Best, M. Forsyth, D. R. MacFarlane, J. Electrochem. Soc. 2013, 160, A1629-A1637. [3]          M. Kar, B. Winther-Jensen, M. Forsyth, D. R. MacFarlane, Phys. Chem. Chem. Phys. 2013, 15, 7191-7197. [4]          T. J. Simons, A. A. J. Torriero, P. C. Howlett, D. R. MacFarlane, M. Forsyth, Electrochem. Commun. 2012, 18, 119-122. [5]          a) M. Kar, T. J. Simons, B. Winther-Jensen, M. Forsyth, M. Armand, D. MacFarlane, Electrochim. Acta 2014; b) T. J. Simons, M. Salsamendi, P. C. Howlett, M. Forsyth, D. R. Macfarlane, C. Pozo-Gonzalo, ChemElectroChem 2015. [6]          M. Kar, Ma, Z., Chen, K, Azofra, L. M,  Forysth, M, MacFarlane, D.R., Chem Comm - submmitted 2015. Figure 1