Influence of experimental conditions on the electrochemical window. Case study on bis(trifluoromethylsulfonyl)imide-based ionic liquids

Synthesis and physical properties of tunable aryl alkyl ionic liquids based on 1-aryl-4,5-dimethylimidazolium cations

We present a new class of tunable aryl alkyl ionic liquids (TAAILs) based on 1-aryl-4,5-dimethylimidazolium cations with electron-withdrawing and -donating substituents in different positions of the phenyl ring and the bis(trifluoromethylsulfonyl)imide (NTf2) anion. We investigated the effect of additional methyl groups in the backbone of the imidazolium core on the physical properties regarding viscosity, conductivity and electrochemical window. With an electrochemical window of up to 6.3 V, which is unprecedented for TAAILs with an NTf2 anion, this new class of TAAILs demonstrates the opportunities that arise from modifications in the backbone of the imidazolium cation.

Tailoring the Properties of Gel Polymer Electrolytes for Sodium-Ion Batteries Using Ionic Liquids: A Review

Ionic liquids are an extraordinary group of compounds, fully ionic in structure like inorganic salts but with low melting points, that resemble organic molecular solvents. Their chemical, electrochemical, and thermal stability is what draws the attention and enables their use in many applications, including electrochemical power sources. Even though they are no longer considered eco-friendly because of nonnegligible toxicity and long bioaccumulation, they can still be efficiently recovered, purified, and reused. These attributes can be harvested to enhance the properties of gel polymer electrolytes for the emerging sodium-ion batteries. The variety of anions and cations for ILs and their influence on the final properties of the compound opens the road to tuning the properties of gel polymer electrolytes. Ionic liquids as plasticizers constitute a major part of gel polymer electrolytes (average of 70 wt%) and hence, they affect the fundamental properties of gel electrolytes like ionic conductivity and electrochemical window. They also improve the safety features of sodium-ion batteries, which is relevant for their anticipated applications in stationary energy storage and electric vehicles. The presented review paper aims to explain the relationship between the cation and anion in ionic liquid and the properties of gel electrolytes for sodium-ion batteries.

Alkylcyanoborate Anions: Building Blocks for Fluorine-Free Low-Viscosity, Electrochemically and Thermally Stable Ionic Liquids

A set of mixed-substituted potassium alkylcyano- and alkylcyanofluoroborates has been synthesized using easily accessible starting compounds and characterized by elemental analysis, NMR and vibrational spectroscopy, and mass spectrometry. In addition, single-crystal structures of salts of the cyanoborate anions have been derived from X-ray diffraction experiments. The 1-ethyl-3-methylimidazolium room temperature ionic liquids ([EMIm]+ -RTILs) with the new borate anions have been synthesized and their physicochemical properties, that is, high thermal and electrochemical stability, low viscosity, and high conductivity, have been compared to the properties of related [EMIm]+ -RTILs. The influence of the different alkyl substituents at boron has been assessed. The exemplary study on the properties with the [EMIm]+ -ILs with the mixed water stable alkylcyanoborate anions points towards the potential of these fluorine-free borate anions, in general.

Ionic Liquids Facilitate the Dispersion of Branched Polyethylenimine Grafted ZIF-8 for Reinforced Epoxy Composites

Metal-organic frameworks (MOFs) have been previously shown as an emerging modified class of epoxy resin. In this work, we report a simple strategy for preventing zeolitic imidazolate framework (ZIF-8) nanoparticles from agglomerating in epoxy resin (EP). Branched polyethylenimine grafted ZIF-8 in ionic liquid (BPEI-ZIF-8) nanofluid with good dispersion was prepared successfully using an ionic liquid as both the dispersant and curing agent. Results indicated that the thermogravimetric curve of the composite material had no noticeable change with increasing BPEI-ZIF-8/IL content. The glass transition temperature (T_g) of the epoxy composite was reduced with the addition of BPEI-ZIF-8/IL. The addition of 2 wt% BPEI-ZIF-8/IL into EP effectively improved the flexural strength to about 21.7%, and the inclusion of 0.5 wt% of BPEI-ZIF-8/IL EP composites increased the impact strength by about 83% compared to pure EP. The effect of adding BPEI-ZIF-8/IL on the T_g of epoxy resin was explored, and its toughening mechanism was analyzed in combination with SEM images showing fractures in the EP composites. Moreover, the damping and dielectric properties of the composites were improved by adding BPEI-ZIF-8/IL.

Cyano(fluoro)borate and cyano(hydrido)borate ionic liquids: low-viscosity ionic media for electrochemical applications

The synthesis and detailed characterization of low-viscosity room-temperature ionic liquids (RTILs) and [BnPh₃P]⁺ salts with the cyano(fluoro)borate anions [BF(CN)₃]^- (MFB), [BF₂(CN)₂]^- (DFB), and [BF₃(CN)]^- as well as the new mixed-substituted anion [BFH(CN)₂]^- (FHB) is described. The RTILs with [EMIm]⁺ or [BMPL]⁺ as countercations were obtained in yields of up to 98% from readily available alkali metal salts and in high purities that allow application in electrochemical devices. Trends in thermal stability, melting and freezing behavior, density, electrochemical stability, dynamic viscosity, specific conductivity and ion diffusivity have been assessed and compared to those of the related tetracyanoborate- and cyano(hydrido)borate-RTILs. The crystal structure analysis of the [BnPh₃P]⁺ salts of [BF_n(CN)_4-n]^- (n = 0-4), [BH_n(CN)_4-n]^- (n = 1-3) and [BFH(CN)₂]^- provided experimental access to anion volumina that together with ion molecular mass, electrostatic potential, shape and chemical stability have been correlated to physicochemical properties. In addition, the cytotoxicity of the [EMIm]⁺-ILs and potassium or sodium salts was studied.

Mixing Ionic Liquids Affects the Kinetics and Thermodynamics of the Oxygen/Superoxide Redox Couple in the Context of Oxygen Sensing

The electrochemical oxygen reduction reaction is vital for applications such as fuel cells, metal air batteries and for oxygen gas sensing. Oxygen undergoes a 1-electron reduction process in dry ionic liquids (ILs) to form the electrogenerated superoxide ion that is solvated and stabilized by IL cations. In this work, the oxygen/superoxide (O₂/O₂ ^•-) redox couple has been used to understand the effect of mixing ILs with different cations in the context of developing designer electrolytes for oxygen sensing, by employing cyclic voltammetry at both gold and platinum electrodes. Different cations with a range of sizes, geometries and aromatic/aliphatic character were studied with a common bis(trifluoromethylsulfonyl)imide ([NTf₂]^-) anion. Diethylmethylsulfonium ([S_2,2,1]⁺), N-butyl-N-methylpyrrolidinum ([C₄mpyrr]⁺) and tetradecyltrihexylphosphonium ([P_14,6,6,6]⁺) cations were mixed with a common 1-butyl-3-methylimidazolium ([C₄mim]⁺) cation at mole fractions (x) of [C₄mim]⁺ of 0, 0.2, 0.4, 0.6, 0.8, and 1. Both the redox kinetics and thermodynamics were found to be highly dependent on the cation structure and the electrode material used. Large deviations from "ideal" mixtures were observed for mixtures of [C₄mim][NTf₂] with [C₄mpyrr][NTf₂] on gold electrodes, suggesting a much higher amount of [C₄mim]⁺ ions near the electrode surface despite the large excess of [C₄mpyrr]⁺ in the bulk. The electrical double layer structure was probed for a mixture of [C₄mim]_0.2[C₄mpyrr]_0.8[NTf₂] using atomic force microscopy measurements on Au, revealing that the first layer was more like [C₄mim][NTf₂] than [C₄mpyrr][NTf₂]. Unusually fast kinetics for O₂/O₂ ^•- in mixtures of [C₄mim]⁺ with [P_14,6,6,6]⁺ were also observed in the electrochemistry results, which warrants further follow-up studies to elucidate this promising behavior. Overall, it is important to understand the effect on the kinetic and thermodynamic properties of electrochemical reactions when mixing solvents, to aid in the creation of designer electrolytes with favorable properties for their intended application.

Screening of the Role of the Chemical Structure in the Electrochemical Stability Window of Ionic Liquids: DFT Calculations Combined with Data Mining

Ionic liquids have attracted the attention of researchers as possible electrolytes for electrochemical energy storage devices. However, their properties, such as the electrochemical stability window (ESW), ionic conductivity, and diffusivity, are influenced both by the chemical structures of cations and anions and by their combinations. Most studies in the literature focus on the understanding of common ionic liquids, and little effort has been made to find ways to improve our atomistic understanding of those systems. The goal of this paper is to explore the structural characteristics of cations and anions that form ionic liquids that can expand the HOMO/LUMO gap, a property directly linked to the ESW of the electrolyte. For that, we design a framework for randomly generating new ions by combining their fragments. Within this framework, we generate about 10⁴ cations and 10⁴ anions and fully optimize their structures using density functional theory. Our calculations show that aromatic cations are less stable ionic liquids than aliphatic ones, an expected result if chemical rationale is used. More importantly, we can improve the gap by adding electron-donating and electron-withdrawing functional groups to the cations and anions, respectively. The increase can be about 2 V, depending on the case. This improvement is reflected in a wider ESW.

Temperature-Dependent Electrochemical Stability Window of Bis(trifluoromethanesulfonyl)imide and Bis(fluorosulfonyl)imide Anion Based Ionic Liquids

The electrochemical stability of 22 commercially available hydrophobic ionic liquids was measured at different temperatures (288.15, 298.15, 313.15, 333.15 and 358.15 K), to systematically investigate ionic liquids towards electrolytes for supercapacitors in harsh weather conditions. Bis(trifluoromethanesulfonyl)imide and bis(fluorosulfonyl)imide anions in combination with 1-Butyl-1-methylpyrrolidinium, 1-Ethyl-3-methylimidazolium, N-Ethyl-N, N-dimethyl-N(2methoxyethyl)ammonium, 1-Methyl-1-(2-methoxyethyl)pyrrolidinium, N-Pentyl-N-methylpyrrolidinium, N, N-Diethyl-N-methyl-N-propylammonium, N, N-Dimethyl-N-ethyl-N-benzyl ammonium, N, N-Dimethyl-N-Ethyl-N-phenylethylammonium, N-Butyl-N-methylpiperidinium, 1-Methyl-1-propylpiperidinium, N-Tributyl-N-methylammonium, N-Trimethyl-N-butylammonium, N-Trimethyl-N-butylammonium, N-Trimethyl-N-propylammonium, N-Propyl-N-methylpyrrolidinium cations were selected for the study. Linear regression with a numerical model was used in combination with voltammetry experiments to deduce the temperature sensitivity of both anodic and cathodic potential limits (defining the electrochemical stability window), in addition to extrapolating results to 283.15 and 363.15 K. We evaluated the influence of the cations, anions, and the presence of functional groups on the observed electrochemical stability window which ranged from 4.1 to 6.1 V.