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

Abstract

Room temperature ionic liquids ͑RTILs͒ with the bis͑fluorosulfonyl͒imide ͑FSI͒ anion exhibit higher conductivities than the corresponding bis͑trifluoromethanesulfonyl͒imide ͑TFSI͒ 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 ͑CV͒ and electrochemical impedance spectroscopy. Addition of LiBF 4 , LiPF 6 , or LiTFSI extends cathodic limits to significantly more negative values and allows reversible lithium electrodeposition. Variable-current cycling of symmetrical Li ͉ 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.Increasing consumer demand for power and energy density is driving the development of the next generation of lithium-ion batteries. 1 To date, however, these developments have been incremental, with the active materials employed in today's batteries being little different from those in Sony's first release back in 1991. It can be argued that this lack of progress is largely due to a failure to develop improved electrolytes. New "high voltage" cathode materials have been described regularly over the years; yet any benefits are seldom realized due to the limited electrochemical stability of organic carbonate-based electrolytes. 2 These systems are also compromised by poor thermal stability, appreciable volatility ͑and flammability͒, and significant toxicity.Certain members of the vast family of compounds known as room temperature ionic liquids ͑RTILs͒ have properties that address many of the concerns with classic organic electrolytes. 3 Holzapfel et al. were one of the first groups to report sustained cycling of a lithium-ion battery with an ionic-liquid electrolyte medium. 4 They used 1-ethyl-3-methyl-imidazolium bis͑trifluoromethanesulfo-nyl͒imide ͑C 2 mimTFSI͒ and LiTFSI doped with vinylidene carbonate ͑VC͒ in an effort to form a stable solid electrolyte interphase ͑SEI͒ at the graphite anode. 4 While these cells displayed a reasonably constant discharge capacity, VC is progressively consumed during long-term charge-discharge cycling, 5 with the result that intercalation of the imidazolium cation into the gra...

Effect of LiNO3 additive and pyrrolidinium ionic liquid on the solid electrolyte interphase in the lithium–sulfur battery

“…The impedance behaviour of IL electrolyte based cells is more complex as reported previously [17,20]. Nyquist plots for the DOL:DME electrolytes show that the electrode/electrolyte interface resistance is mostly due to Li ion transfer through the SEI film rather than charge transfer resistance.…”

“…Prior to investigating the effects of LiNO 3 and ionic liquid on the full LieS cells, it is desirable to decouple the effects of anode and cathode on the total impedance spectrum. Previously, EIS has been used to study SEI properties of lithium surfaces by use of LijelectrolytejLi symmetrical cells [17,20,21]. By utilizing this methodology, effects of the additives on the lithium electrode can be first ascertained.…”

“…In the case of the DOL:DME electrolyte cell, after sufficient cycling, the cell appears to somewhat recover when the current density is lower. This could be attributed to the dendrites creating localised heating through high resistance effects causing melting and subsequent liberation of the dendrite into the electrolyte solution with the overall effect of smoothing out the lithium electrode [17,23].…”

Electrolytes and Interphases in Li-Ion Batteries and Beyond

“…Unsaturated substituent (allyl) on pyrrolidinum assisted in forming a more protective film on LiCoO 2 , suggesting an oxidative polymerization mechanism of pyrrolidinum cation. 356 The combination of pyrrolidinum with FSI anion yielded RTIL of high ion conductivities and low viscosities, although at the expense of thermal stability due to the labile F in FSI; 341,350,358,359 however, the high ion conductivities and low viscosities failed to translate into higher rate capability when tested in LiCoO 2 -based cells. 360 The introduction of FSI also decreased the cathodic stability toward lithium, raising the suspicion that impurities that came with LiFSI may be responsible for the parasitic reactions on cathode surfaces.…”

Physical and Electrochemical Properties of Some Phosphonium-Based Ionic Liquids and the Performance of Their Electrolytes in Lithium-Ion Batteries

“…15,16 In this study, we present the physical and electrochemical properties of three ionic liquids based on phosphonium cations with different alkyl chain sizes and two different anions, trimethyl propyl phosphonium bisfluorosulfonyl imide (P 1113 FSI), trimethyl isobutyl phosphonium bisfluorosulfonyl imide (P 111i4 FSI) and trimethyl isobutyl phosphonium bis-trifluoromethylsulfonyl imide (P 111i4 TFSI) (Scheme 1).…”