Room-Temperature Molten Salts: Protic Ionic Liquids and Deep Eutectic Solvents as Media for Electrochemical Application

Abstract

Ionic liquids (ILs) are room-temperature molten salts (RTMS) composed mostly of organic ions that may undergo almost unlimited structural variations. ILs and deep eutectic solvents (DESs) have been applied in various fields, such as electrolytes for lithium ion batteries, redox flow batteries (RFBs), electrodeposition, and electropolishing, and even in fuel cells. This chapter covers the newest aspects of H-donor ILs in applications where their transport properties (ion conductivity, diffusion, and viscosity) are exploited, for example, as electrochemical solvents for energy storage where conventional media, organic solvents (in batteries), or water (in supercapacitors) fail. Metal nanoparticles' (NPs) formulation and stabilization in H-donor RTMS with some unique size, shape, and properties are also discussed. In these decidedly different materials, a mixture with a molecular solvent shows electroactivity that is not exhibited in more traditional systems, creating huge potential for energy storage and electrocatalysis. The remarkable recent increase in the level of interest in RTMS (protic ionic liquids, PILs, and DESs) in academics and industry is illustrated by the rapid growth in the number of publications on the topic (Fig. 7.1), with more than a 100-fold increase observed over the period 2004 to 2014.The first main objective of this chapter is to summarize the synthesis and physical properties of PILs and DESs and discuss their suitability as green solvents for different applications. The second main objective is to review the electrochemical applications of RTMS and discuss their advantages and disadvantages as electrolytes in comparison with an aprotic homologue for energy storage applications. I hope to generate interest in the wider community and encourage others to make use of proton-donor RTMS-PILs and DESs-in tackling scientific challenges.

Deep Eutectic Solvents: A Review of Fundamentals and Applications

“…Because current electrolytes used in lithium-ion batteries (LIBs) have resulted in several highly publicized battery failures, the possibility of a flame-resistant substitute which does not compromise in other areas is certainly appealing. As such, DESs are being considered as potential electrolytes for LIBs owing to their suppressed flammability, wide liquidus range, and high conductivity. ,,,− Work by Boisset et al , for example, demonstrated that for the various DESs they tested, wide electrochemical stability windows and favorable characteristics for LIBs were achieved . A test cell they constructed (utilizing a LiNO 3 -based DES as an electrolyte and LiFePO 4 (LFP) cathode) demonstrated a capacity of up to 160 mA h g –1 with an efficiency of 99%, even with residual water present.…”

Nonstoichiometric Triazolium Protic Ionic Liquids for All-Organic Batteries

“…Room temperature protic ionic liquids (PILs) have increasingly attracted attention as proton conducting electrolytes for fuel cells and batteries due to their low cost, ease of synthesis, and high electrochemical and thermal stability. − PILs are generally produced by protonating a base with a strong acid to achieve a low melting salt . While aprotic ionic liquids have been extensively studied as electrolytes, the same multitude of cation/anion combinations into PILs has so far not been reported .…”

Deep eutectics and analogues as electrolytes in batteries

“…220 A more recent development in the field comes from the incorporation of controlled amounts of water (~6 wt%, ~30 mol.%) in a ternary type IV DES composed of LiTFSI:Zn(TFSI)2:U to prepare a stable aqueous electrolyte for zinc batteries. 214 As detected by FTIR and 17 O NMR spectroscopy and supported by DFT-MD simulations, in this concentration (dubbed "water-in-DES" regime), DES's nature is retained and all water molecules participate in DES's internal interaction (H-bonding and coordinating) network, leading to a suppressed reactivity with Zn anode. The addition of water increases conductivity and decreases viscosity of the pristine DES, with the electrolyte LiTFSI: Zn(TFSI)2:U:H2O at mole ratio 1:0.05:3.8:2 the best performing one (Table 6 and Fig.…”

“…93 As alternative to AA, NMAA has been considered as a good and green solvent for the formulation of new DES-based electrolytes when combined to several Li salts (LiTFSI, LiPF6, and LiNO3). 17,37,141 This is due to its "water-like" physical properties, namely very high dielectric constant (ε = 178.9), high dipolar moment (6.8 D), and very low vapor pressure (0.050 kPa at 40 °C). Like U-and AA-based mixtures, NMAA acts as a complex agent for both Li + cation and the anion in solution, weakening and even breaking down the bonding between the Li + cation and the anion.…”

“…On the other hand, a number of works report on systems which correspond to the general formula of a DES but are referred to with a different name. A likely non-exhaustive list of terms -used sometimes for the same system -includes Deep Eutectic Solvents (DESs), 5 Deep Eutectic Systems (DESs), 9 Deep Eutectic Mixtures (DEMs), 10 Deep Eutectics (DEs), 11 Deep Eutectic Electrolytes (DEEs), 12 Ionic Liquid Analogs (ILAs), 13 quasi-ILs, 14 eutectic-based ILs, 15,16 Room Temperature Molten Salts (RTMSs), 17 Low-Transition-Temperature Mixtures (LTTMs), 18 Liquid Coordination Complexes (LCCs). 19 Also, it occurs that the same system has been labelled in the literature as IL or DES according to the authors, and even the same authors did use different definitions (one for all the overlap between the classifications of eutectic based ILs, 16 and DESs, 20 by Abbott's group).…”