Implications of Anion Structure on Physicochemical Properties of DBU-Based Protic Ionic Liquids.

Protic ionic liquids (PILs) are potential candidates as electrolyte components in energy storage devices. When replacing flammable and volatile organic solvents, PILs are expected to improve the safety and performance of electrochemical devices. Considering their technical application, a challenging task is the understanding of the key factors governing their intermolecular interactions and physicochemical properties. The present work intends to investigate the effects of the structural features on the properties of a promising PIL based on the 1,8-diazabicyclo[5.4.0]undec-7-ene (DBUH+) cation and the (trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide (IM14-) anion, the latter being a remarkably large anion with an uneven distribution of the C-F pool between the two sides of the sulfonylimide moieties. For comparison purposes, the experimental investigations were extended to PILs composed of the same DBU-based cation and the trifluoromethanesulfonate (TFO-) or bis(trifluoromethanesulfonyl)imide (TFSI-) anion. The combined use of multiple NMR methods, thermal analyses, density, viscosity, and conductivity measurements provides a deep characterization of the PILs, unveiling peculiar behaviors in DBUH-IM14, which cannot be predicted solely on the basis of differences between aqueous pKa values of the protonated base and the acid (ΔpKa). Interestingly, the thermal and electrochemical properties of DBUH-IM14 turn out to be markedly governed by the size and asymmetric nature of the anion. This observation highlights that the structural features of the precursors are an important tool to tailor the PIL's properties according to the specific application.

Enhanced Li+ Transport in Ionic Liquid-Based Electrolytes Aided by Fluorinated Ethers for Highly Efficient Lithium Metal Batteries with Improved Rate Capability.

FSI- -based ionic liquids (ILs) are promising electrolyte candidates for long-life and safe lithium metal batteries (LMBs). However, their practical application is hindered by sluggish Li+ transport at room temperature. Herein, it is shown that additions of bis(2,2,2-trifluoroethyl) ether (BTFE) to LiFSI-Pyr14 FSI ILs can effectively mitigate this shortcoming, while maintaining ILs' high compatibility with lithium metal. Raman spectroscopy and small-angle X-ray scattering indicate that the promoted Li+ transport in the optimized electrolyte, [LiFSI]3 [Pyr14 FSI]4 [BTFE]4 (Li3 Py4 BT4 ), originates from the reduced solution viscosity and increased formation of Li+ -FSI- complexes, which are associated with the low viscosity and non-coordinating character of BTFE. As a result, Li/LiFePO4 (LFP) cells using Li3 Py4 BT4 electrolyte reach 150 mAh g-1 at 1 C rate (1 mA cm-2 ) and a capacity retention of 94.6% after 400 cycles, revealing better characteristics with respect to the cells employing the LiFSI-Pyr14 FSI (operate only a few cycles) and commercial carbonate (80% retention after only 218 cycles) electrolytes. A wide operating temperature (from -10 to 40 °C) of the Li/Li3 Py4 BT4 /LFP cells and a good compatibility of Li3 Py4 BT4 with LiNi0.5 Mn0.3 Co0.2 O2 (NMC532) are demonstrated also. The insight into the enhanced Li+ transport and solid electrolyte interphase characteristics suggests valuable information to develop IL-based electrolytes for LMBs.

Acidic Ionic Liquids Enabling Intermediate Temperature Operation Fuel Cells.

Herein we show that protic ionic liquids (PILs) are promising electrolytes for fuel cells operating in the temperature range 100-120 °C. N,N-Diethyl-N-methyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DEMSPA][HSA]), N,N-diethyl-N-methyl-3-sulfopropan-1-ammonium triflate ([DEMSPA][TfO]), N,N-diethyl-3-sulfopropan-1-ammonium hydrogen sulfate ([DESPA][HSA]), and N,N-diethyl-3-sulfopropan-1-ammonium triflate ([DESPA][TfO]) are investigated in this study with regard to their specific conductivity, thermal stability, viscosity, and electrochemical properties. The [DEMSPA][TfO] and [DESPA][TfO] electrolytes offer high limiting current densities for the oxygen reduction reaction (ORR) on platinum electrodes, that is, about 1 order of magnitude larger than 98% H3PO4. This is explained by the minor poisoning of the Pt catalyst and the significantly larger product of the oxygen self-diffusion coefficient and concentration in these two PILs.