The AHA Moment: Assessment of the Redox Stability of Ionic Liquids Based on Aromatic Heterocyclic Anions (AHAs) for Nuclear Separations and Electric Energy Storage

Abstract

Because of their extended conjugated bond network, aromatic compounds generally have higher redox stability than less saturated compounds. We conjectured that ionic liquids (ILs) consisting of aromatic heterocyclic anions (AHAs) may exhibit improved radiation and electrochemical stability. Such properties are important in applications of these ILs as diluents in radionuclide separations and electrolytes in the electric energy storage devices. In this study, we systematically examine the redox chemistry of the AHAs. Three classes of these anions have been studied: (i) simple 5-atom ring AHAs, such as the pyrazolide and triazolides, (ii) AHAs containing an adjacent benzene ring, and (iii) AHAs containing electron-withdrawing groups that were introduced to reduce their basicity and interaction with metal ions. It is shown that fragmentation in the reduced and oxidized states of these AHAs does not generally occur, and the two main products, respectively, are the H atom adduct and the imidyl radical. The latter species occurs either as an N σ-radical or as an N π-radical, depending on the length of the N-N bond, and the state that is stabilized in the solid matrix is frequently different from that having the lowest energy in the gas phase. In some instances, the formation of the sandwich π-stack dimer radical anions has been observed. For trifluoromethylated anions, H adduct formation did not occur; instead, there was facile loss of fluoride from their fluorinated groups. The latter can be problematic in nuclear separations, but beneficial in batteries. Overall, our study suggests that AHA-based ILs are viable candidates for use as radiation-exposed diluents and electrolytes.

Interfacial Structures in Ionic Liquid-Based Ternary Electrolytes for Lithium-Metal Batteries: A Molecular Dynamics Study

“…The development of new electrolytes is key to stabilize and make LMBs feasible . In this context, ionic liquid (IL)-based electrolytes can form a stable and passive electrolyte interface that decreases dendrite growth in the battery. − The use of IL electrolytes can also increase the battery’s Coulombic efficiency and enables a better control of the side reactions in, for example, Li–S batteries . Beyond ILs, solid polymer electrolytes (SPEs) − also show great applicability for the LMBs.…”

Chemical Stability of Lithium 2-Trifluoromethyl-4,5-dicyanoimidazolide, an Electrolyte Salt for Li-Ion Cells

“…We remind the reader that (due to the electron-withdrawing groups) TDI − (unlike the unsubstituted triazoles and imidazoles) is a weakly metal-coordinating anion that can be readily replaced by carbonate molecules in the coordination sphere of the metal ion. Combining LiTDI with Al(TfO) 3 and Al(TFSI) 3 in the carbonate solvents also did not yield insoluble products, and 27 Al NMR spectra 3 suggest that Al 3+ ions in these solutions are complexed by solvent molecules.…”

“…The latter would rationalize the formation of (i) thick, unstable SEI electrolyte observed in ref using scanning electron microscopy and (ii) large voltage polarization apparent for cell B in Figure . Below we suggest a mechanistic explanation for the detrimental effect of LiTDI on SEI formation in the carbonate solvents using insights from recent X-ray photoelectron spectroscopy studies of LiTDI-based electrolytes in graphite and silicon anode cells, , radiolysis studies of aromatic heterocyclic anions, and computational chemistry methods.…”

“…22−26 Imidazole is an N-heterocyclic base; electron-withdrawing cyanide and trifluoromethyl groups are included to turn it into a strong acid, so the anion cannot be easily protonated. 24,27,28 produced conveniently and in high yield in a one-pot synthesis, and it is thermally stable to 250 °C. 22,29 The potential of 4.6 V vs Li/Li + for the onset of Al anodic currents was estimated for LiTDI in carbonate electrolytes using cyclic voltammetry, 22 and this compound began to attract increasing attention, especially for high-energy/high-voltage batteries containing silicon (Li−Si) and graphite (Li−Gr) based negative electrodes.…”

Multidimensional materials and device architectures for future hybrid energy storage

“…Similarly, substituted organic molecules containing N, O and S heterocycles can be reversibly oxidized and reduced. These molecules are generally more stable and can show stable and reversible redox activity 76 .…”