Application of the N-propyl-N-methyl-pyrrolidinium Bis(fluorosulfonyl)imide RTIL Containing Lithium Bis(fluorosulfonyl)imide in Ionic Liquid Based Lithium Batteries

In seeking to develop ionic liquid based electrolytes for use in lithium metal batteries, we present an investigation of the electrochemical properties of N-propyl-N-methyl-pyrrolidinium bis(fluorosulfonyl)imide and lithium bis( fluorosulfonyl) imide at Ni, Pt, and Li electrodes by cyclic voltammetry, chronoamperometry, and impedance spectroscopy. While lithium electrodeposition and stripping are chemically reversible, the magnitude of peak currents during successive cycles is strongly dependent on the substrate. Severe decreases are observed at Ni, only moderate falls at Pt, while Li electrodes support modest increases in current, consistent with roughening of the electrode with each deposition cycle. We discuss this behavior on the basis of competition between (i) formation of a solid electrolyte interphase at the deposited lithium surface and (ii) strength of interaction between deposited lithium and substrate. Chronoamperometric data indicate that lithium deposition proceeds via instantaneous nucleation and growth, which favors smooth rather than nodular deposit morphology. Symmetrical (Li vertical bar electrolyte vertical bar Li) cells display excellent cycling behavior (>470 cycles), at current densities up to 10 mA cm(-2), with only transient evidence of dendrite formation. Initially high impedance is reduced by increasing the concentration (similar to 0.5 mol kg(-1)) of lithium salt, although all cells eventually reach relatively low values of <10 Omega cm(2). The properties of this electrolyte system make it a strong candidate for future application in lithium metal batteries. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3257978] All rights reserved.