Ionic Liquids with the Bis(fluorosulfonyl) imide Anion: Electrochemical Properties and Applications in Battery Technology

Room temperature ionic liquids (RTILs) with the bis(fluorosulfonyl) imide (FSI) anion exhibit higher conductivities than the corresponding bis(trifluoromethanesulfonyl) imide (TFSI) compounds, thereby generating interest as novel electrolytes for lithium batteries. The electrochemical properties of a series of FSI RTILs, at inert metal and lithium electrodes, have been investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy. Addition of LiBF4, LiPF6, or LiTFSI extends cathodic limits to significantly more negative values and allows reversible lithium electrodeposition. Variable-current cycling of symmetrical Li vertical bar Li coin cells reveals significant changes in electrode-electrolyte interphasial impedance, which depends on the identity of the lithium salt anion, the concentration of the salt, and the RTIL cation. For most cells, voltage-time curves become unsteady early in duty, which is consistent with the formation of dendrites on the lithium surface. A stable voltage behavior returns within around 20 cycles, at notably a lower current density presumably because detachment/reattachment of dendrites eventually re-establishes a contiguous lithium electrode with a higher surface area. Importantly, the combination of the kinetics of lithium deposition and morphology of the deposit in FSI anion-based RTIL media does not result in lithium penetration of the separator. Therefore, FSI-based electrolytes can play a key role in the development of a viable lithium-metal battery technology. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3429886] All rights reserved.