Unexpected Energy Applications of Ionic Liquids.

Ionic liquids and their various analogues are without doubt the scientific sensation of the last few decades, paving the way to a more sustainable society. Their versatile suite of properties, originating from an almost inconceivably large number of possible cation and anion combinations, allows tuning of the structure to serve a desired purpose. Ionic liquids hence offer a myriad of useful applications from solvents to catalysts, through to lubricants, gas absorbers, and azeotrope breakers. The purpose of this review is to explore the more unexpected of these applications, particularly in the energy space. It guides the reader through the application of ionic liquids and their analogues as i) phase change materials for thermal energy storage, ii) organic ionic plastic crystals, which have been studied as battery electrolytes and in gas separation, iii) key components in the nitrogen reduction reaction for sustainable ammonia generation, iv) as electrolytes in aluminum-ion batteries, and v) in other emerging technologies. It is concluded that there is tremendous scope for further optimizing and tuning of the ionic liquid in its task, subject to sustainability imperatives in line with current global priorities, assisted by artificial intelligence.

Insights into the Carbon Dioxide Separation Performance of Bis(trifluoromethylsulfonyl)imide-based Plastic Crystal Composite Membranes with Fluorinated Polar Polymers.

Membrane-based gas separation technologies are one solution towards mitigating global emissions of CO2. New membrane materials with improved separation performance are still highly sought after. Composite membranes based on organic ionic plastic crystals (OIPCs) have shown preferential interaction for CO2 over N2, leading in some cases to competitive CO2/N2 selectivities. However, these ionic materials have been scarcely studied in the field of gas separation. Here, OIPCs based on the bis(trifluoromethylsulfonyl)imide ([TFSI]-) anion were investigated for use as gas separation membranes for the first time. The effect of the polymer type was also investigated, through the comparison of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) OIPC membranes. A strong temperature dependence of the gas separation performance was found, particularly in the N-methyl-N-ethylpyrrolidinium-based composites where the material undergoes a solid-solid phase transition within the testing temperature range. The polymer type was noted to induce a strong effect on the structure of the composites, as well as affecting the gas and ionic transport. Thus, this research provides insights on the influence of the [TFSI]- anion on the structure and separation properties of OIPC-based composites, and new information towards the development of novel OIPC-based membranes with enhanced gas separation performance.

New organic ionic plastic crystals utilizing the morpholinium cation.

Organic ionic plastic crystals (OIPCs) are emerging candidates as safer, quasi solid-state ion conductors for various applications, especially for next-generation batteries. However, a fundamental understanding of these OIPC materials is required, particularly concerning how the choice of cation and anion can affect the electrolyte properties. Here, we report the synthesis and characterisation of a range of new morpholinium-based OIPCs and demonstrate the benefit of the ether functional group in the cation ring. Specifically, we investigate the 4-ethyl-4-methylmorpholinium [C2mmor]+ and 4-isopropyl-4-methylmorpholinium [C(i3)mmor]+ cations paired with bis(fluorosulfonyl)imide [FSI]- and bis(trifluoromethanesulfonyl)imide [TFSI]- anions. A fundamental study of the thermal behaviour and transport properties was performed using differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The free volume within the salts has been investigated by positron annihilation lifetime spectroscopy (PALS) and the ion dynamics using solid-state nuclear magnetic resonance (NMR) analysis. Finally, the electrochemical stability window was studied using cyclic voltammetry (CV). Out of the four morpholinium salts, [C2mmor][FSI] exhibits the widest phase I range from 11 to 129 °C, which is advantageous for their application. [C(i3)mmor][FSI] displayed the highest conductivity of 1 × 10-6 S cm-1 at 30 °C, whereas the largest vacancy volume of 132 Å3 was found for [C2mmor][TFSI]. These insights into the properties of new morpholinium-based OIPCs will be important for developing new electrolytes with optimised thermal and transport properties for a range of clean energy applications.

Enhanced Dissolution of Metal Oxides in Hydroxylated Solvents - Towards Application in Lithium-Ion Battery Leaching.

The recovery of critical metals from spent lithium-ion batteries (LIBs) is rapidly growing. Current methods are energy-intensive and hazardous, while alternative solvent-based strategies require more studies on their 'green' character, metal dissolution mechanism and industrial applicability. Herein, we bridged this gap by studying the effect of dilute HCl solutions in hydroxylated solvents to dissolve Co, Ni and Mn oxides. Ethylene glycol emerged consistently as the most effective solvent, dissolving up to four times more Co and Ni oxides than using aqueous acidic media, attributed to improved chloro-complex formation and solvent effects. These effects had a significant contribution compared to acid type and concentration. The highest Co dissolution (0.27 M) was achieved in 0.5 M HCl in 25 % (v/v) glycerol in water, using less acid and a significant amount of water compared to other solvent systems, as well as mild temperatures (40 °C). This solvent was applied to dissolve battery cathode material, achieving 100 % dissolution of Co and Mn and 94 % dissolution of Ni, following what was concluded to be a mixed mechanism. These results offer a simple alternative to current leaching processes, reducing acid consumption, enhancing atomic efficiency, and paving the way for optimized industrial hydrometallurgical processes leaning to 'greener' strategies.

Development of New Plastic-Crystal Based Electrolytes using Pyrrolidinium- Bis(fluorosulfonyl)imide Dicationic Salts.

Dicationic organic salts are an interesting class of solid-state electrolyte materials due to their unique structure. Here we present, for the first time, the synthesis and characterization of three dicationic-FSI salts, 1,2-bis(N-methylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C2 -Pyrr1][FSI]2 ), 1,2-bis(N-ethylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C2 -Pyrr2][FSI]2 ) and 1,2-bis(N-n-propylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C2 -Pyrr3][FSI]2 ). The structure and dynamics of the organic salts were probed using variable temperature solid-state NMR and were compared with the thermal and transport properties. The investigation revealed that [C2 -Pyrr1][FSI]2 , with shorter alkyl-side chains on the dication, displayed increased transport properties compared to [C2 -Pyrr2][FSI]2 and [C2 -Pyrr3][FSI]2 . To determine the proficiency of these dicationic-FSI salts as electrolyte materials for battery applications, 10 mol% and 50 mol% lithium bis(fluorosulfonyl)imide (LiFSI) was mixed with [C2 -Pyrr1][FSI]2 and [C2 -Pyrr2][FSI]2 . Increased transport properties were observed for [C2 -Pyrr1][FSI]2 /10 mol % LiFSI in comparison to [C2 -Pyrr2][FSI]2 /10 % LiFSI, while pulse field gradient NMR analysis revealed the highest Li+ self-diffusion ratio for [C2 -Pyrr1][FSI]2 /50 % LiFSI out of the four Li-salt-containing mixtures.

Phase Change Materials for Renewable Energy Storage at Intermediate Temperatures.

Thermal energy storage technologies utilizing phase change materials (PCMs) that melt in the intermediate temperature range, between 100 and 220 °C, have the potential to mitigate the intermittency issues of wind and solar energy. This technology can take thermal or electrical energy from renewable sources and store it in the form of heat. This is of particular utility when the end use of the energy is also as heat. For this purpose, the material should have a phase change between 100 and 220 °C with a high latent heat of fusion. Although a range of PCMs are known for this temperature range, many of these materials are not practically viable for stability and safety reasons, a perspective not often clear in the primary literature. This review examines the recent development of thermal energy storage materials for application with renewables, the different material classes, their physicochemical properties, and the chemical structural origins of their advantageous thermal properties. Perspectives on further research directions needed to reach the goal of large scale, highly efficient, inexpensive, and reliable intermediate temperature thermal energy storage technologies are also presented.

Probing the molecular interactions and physicochemical properties of a cobalt-based redox electrolyte system for thermo-electrochemical cells.

Redox-active materials play a primary role in the high-performance electrochemical device research field. Their bulk ion dynamics and performances can be studied using different electrochemical analysis methods, but their molecular level interactions and dynamics on which these depend are often not well understood. Here, nuclear magnetic resonance (NMR) relaxation and double-stimulated echo pulsed field gradient (PFG) techniques have been used to gain insights into the molecular level interactions, exchange dynamics and self-diffusivity of the various species present in a cobalt-based redox active electrolyte system used for thermo-electrochemical applications, including how these factors depend on the oxidation state and concentration of the redox species. A series of liquid electrolyte samples consisting of a Co2+/3+(bpy)3(NTf2)2/3 redox couple (where bpy = bipyridyl and NTf2 = bis(trifluoromethanesulfonyl)imide) in 3-methoxypropionitrile (MPN) have been investigated using NMR as well as viscosity and conductivity measurements carried out over a temperature range 293 to 353 K. The results provide insights into the mobilities and interactions between the various species present, including the exchange of the NTf2- anions between the solvation shells of the Co(bpy)3 species. Such information will be useful in understanding the behaviour of these electrolytes in devices such as thermo-electrochemical cells.

Exploration of phase diagram, structural and dynamic behavior of [HMG][FSI] mixtures with NaFSI across an extended composition range.

Hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI]) has recently been shown to be a promising solid state organic ionic plastic crystal with potential application in advanced alkali metal batteries. This study provides a detailed exploration of the structural and dynamic behavior of [HMG][FSI] mixtures with the sodium salt NaFSI across the whole composition range from 0 to 100 mol%. All mixtures are solids at room temperature. A combination of differential scanning calorimetry (DSC), synchrotron X-ray diffraction (SXRD) and multinuclear solid state NMR spectroscopy is employed to identify a partial phase diagram. The 25 mol% NaFSI/75 mol% [HMG][FSI] composition presents as the eutectic composition with the eutectic transition temperature at 44 °C. Both DSC and SXRD strongly support the formation of a new compound near 50 mol% NaFSI. Interestingly, the 53 mol% NaFSI [HMG][FSI] composition was consistently found to display features of a pure compound whereas the 50 mol% materials always showed a second phase. Many of the compositions examined showed unusual metastable behaviour. Moreover, the ion dynamics as determined by NMR, indicate that the Na+ and FSI- anions are signifcantly more mobile than the HMG cation in the liquid state (including the metastable state) for these materials.

Halide-Free Synthesis of New Difluoro(oxalato)borate [DFOB]- -Based Ionic Liquids and Organic Ionic Plastic Crystals.

The implementation of next-generation batteries requires the development of safe, compatible electrolytes that are stable and do not cause safety problems. The difluoro(oxalato)borate ([DFOB]- ) anion has been used as an electrolyte additive to aid with stability, but such an approach has most commonly been carried out using flammable solvent electrolytes. As an alternative approach, utilisation of the [DFOB]- anion to make ionic liquids (ILs) or Organic Ionic Plastic Crystals (OIPCs) allows the advantageous properties of ILs or OIPCs, such as higher thermal stability and non-volatility, combined with the benefits of the [DFOB]- anion. Here, we report the synthesis of new [DFOB]- -based ILs paired with triethylmethylphosphonium [P1222 ]+ , and diethylisobutylmethylphosphonium [P122i4 ]+ . We also report the first OIPCs containing the [DFOB]- anion, formed by combination with the 1-ethyl-1-methylpyrrolidinium [C2 mpyr]+ cation, and the triethylmethylammonium [N1222 ]+ cation. The traditional synthetic route using halide starting materials has been successfully replaced by a halide-free tosylate-based synthetic route that is advantageous for a purer, halide free product. The synthesised [DFOB]- -based salts exhibit good thermal stability, while the ILs display relatively high ionic conductivity. Thus, the new [DFOB]- -based electrolytes show promise for further investigation as battery electrolytes both in liquid and solid-state form.

Plastic crystal-based electrolytes using novel dicationic salts.

The unique structures of dications increase the number of possible combinations of cations and anions that can be used to obtain new materials with a wide range of physicochemical properties. However, structure-property relationships related to dicationic organic salts are seldom explored. Here, we report the synthesis and characterization of two new dicationic salts, 1,2-bis(N-ethylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C2-Pyrr2][TFSI]2) and 1,2-bis(N-n-propylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C2-Pyrr3][TFSI]2). To investigate the physicochemical properties of the organic salts, local structure and dynamics were investigated by variable temperature solid-state NMR and correlated with the thermal analysis and ionic conductivity. These studies revealed that [C2-Pyrr3][TFSI]2, with the longer alkyl-side chain on the dication, showed improved transport properties compared to [C2-Pyrr2][TFSI]2. Further exploration of the organic salts as potential electrolyte materials was conducted by mixing with 10 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). This study demonstrates the effect that lithium salt addition has on thermal and ionic conductivity properties, where the largest increase in conductivity was found for [C2-Pyrr3][TFSI]2/LiTFSI (10 mol% LiTFSI). Solid-state NMR analysis revealed that Li+ and [TFSI]- ions acted as the major contributors to ionic conductivity while the dications in the bulk structure showed lower mobility.

Zwitterionic materials with disorder and plasticity and their application as non-volatile solid or liquid electrolytes.

Zwitterionic materials can exhibit unique characteristics and are highly tunable by variation to the covalently bound cationic and anionic moieties. Despite the breadth of properties and potential uses reported to date, for electrolyte applications they have thus far primarily been used as additives or for making polymer gels. However, zwitterions offer intriguing promise as electrolyte matrix materials that are non-volatile and charged but non-migrating. Here we report a family of zwitterions that exhibit molecular disorder and plasticity, which allows their use as a solid-state conductive matrix. We have characterized the thermal, morphological and structural properties of these materials using techniques including differential scanning calorimetry, scanning electron microscopy, solid-state NMR and X-ray crystallography. We report the physical and transport properties of zwitterions combined with lithium salts and a lithium-functionalized polymer to form solid or high-salt-content liquid electrolytes. We demonstrate that the zwitterion-based electrolytes can allow high target ion transport and support stable lithium metal cell cycling. The ability to use disordered zwitterionic materials as electrolyte matrices for high target ion conduction, coupled with an extensive scope for varying the chemical and physical properties, has important implications for the future design of non-volatile materials that bridge the choice between traditional molecular and ionic solvent systems.

Operando magnetic resonance imaging for mapping of temperature and redox species in thermo-electrochemical cells.

Low-grade waste heat is an abundant and underutilised energy source. In this context, thermo-electrochemical cells (i.e., systems able to harvest heat to generate electricity) are being intensively studied to deliver the promises of efficient and cost-effective energy harvesting and electricity generation. However, despite the advances in performance disclosed in recent years, understanding the internal processes occurring within these devices is challenging. In order to shed light on these mechanisms, here we report an operando magnetic resonance imaging approach that can provide quantitative spatial maps of the electrolyte temperature and redox ion concentrations in functioning thermo-electrochemical cells. Time-resolved images are obtained from liquid and gel electrolytes, allowing the observation of the effects of redox reactions and competing mass transfer processes such as thermophoresis and diffusion. We also correlate the physicochemical properties of the system with the device performance via simultaneous electrochemical measurements.

Strategies for Integrated Capture and Conversion of CO2 from Dilute Flue Gases and the Atmosphere.

The integrated capture and conversion of CO2 has the potential to make valorization of the greenhouse gas more economically competitive, by eliminating energy-intensive regeneration processes. However, integration is hindered by the extremely low concentrations of CO2 present in the atmosphere (0.04 vol.%), and the presence of acidic gas contaminants, such as SOx and NOx , in flue gas streams. This Review summarizes the latest technological progress in the integrated capture and conversion of CO2 from dilute flue gases and atmospheric air. In particular, the Review analyzes the correlation between material properties and their capture and conversion efficiency through hydrogenation, cycloaddition, and solar thermal-mediated electrochemical processes, with a focus on the types and quantities of product generated, in addition to their energy requirements. Prospects for commercialization are also highlighted and suggestions are made for future research.

Predicting gas selectivity in organic ionic plastic crystals by free energy calculations.

Organic ionic plastic crystals (OIPCs) are molecularly disordered solids, and their potential for the development of gas separation membranes has recently been demonstrated. Here, the gas absorption capability of the OIPC, diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P122i4][PF6]), for four gases is predicted through potential of mean force (PMF) calculations based on two methods - average force method and adaptive biasing force method. Both methods correctly predicted the different trends of adsorption and absorption of these gases across the OIPC-gas interface. The distinct energy barriers of the PMF profiles of CO2 and N2 near the interface directly reflect the good selectivity of OIPC to these two gases. However, the selectivity of CH4 and O2 cannot be accurately reflected by the PMF curve near the interface, because the relative energy varies greatly at different positions inside the OIPC. Thus the average free energy change should be calculated over the entire OIPC box to evaluate the difference in selectivity between the two gases. This also suggests that gas absorption in OIPCs is greatly affected by the structural order and chemical environment. The adaptive biasing force method overall outperforms the average force method. The method should be able to provide a prediction of gas selectivity for a wider range of organic ionic plastic crystals and other solid materials.

Organic Ionic Plastic Crystal-Based Composite Membranes for Light Gas Separation: The Impact of Varying Ion Type and Casting Method.

The promise of organic ionic plastic crystals (OIPCs) for development of a novel type of gas separation membrane with competitive gas selectivity for CO2 /N2 was recently demonstrated. This work aimed to design more selective membranes by investigating a different type of OIPC and a new membrane preparation method. Two different OIPCs were solvent-cast or co-cast with poly(vinylidene difluoride) (PVDF), and their gas transport properties were compared. The first OIPC, methyl(diethyl)isobutylphosphonium hexafluorophosphate ([P122i4 ][PF6 ]), was previously studied using the co-cast method, and this was used as a benchmark. The second, N-methyl-N-ethylpyrrolidinium bis(fluorosulfonyl)imide ([C2 mpyr][FSI]), was investigated for the first time for gas separation applications, achieving high selectivities (α CO 2 / N 2 >40). The thermophysical properties of the composites indicated that the co-casting method is a good way to fabricate solid, mechanically stable and durable membranes. Additionally, the enhanced molecular interactions indicated in OIPC/PVDF co-cast composites point to a new approach for synthesis of other highly selective OIPC-based membranes.

The influence of alkyl chain branching on the properties of pyrrolidinium-based ionic electrolytes.

Ionic liquids and plastic crystals based on pyrrolidinium cations are recognised for their advantageous properties such as high conductivity, low viscosity, and good electrochemical and thermal stability. The pyrrolidinium ring can be substituted with symmetric or asymmetric alkyl chain substituents to form a range of ionic liquids or plastic crystals depending on the anion. However, reports into the use of branched alkyl chains and how this influences the material properties are limited. Here, we report the synthesis of six salts - ionic liquids and organic ionic plastic crystals - where the typically used linear propyl chain substituent is replaced by the branched alternative, isopropyl, to form the cation [C(i3)mpyr]+, in combination with six different anions: dicyanamide, (fluorosulfonyl)(trifluoromethanesulfonyl)imide, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, tetrafluoroborate and hexafluorophosphate. The thermal and transport properties of these salts are compared to those of the analogous N-propyl-N-methylpyrrolidinium and N,N-diethylpyrrolidinium-based salts. Finally, a high lithium salt content ionic liquid electrolyte based on the bis(fluorosulfonyl)imide salt was developed. This electrolyte showed high coulombic efficiencies of lithium plating/stripping and high lithium ion transference number, making it a strong candidate for use in lithium metal batteries.

Toward High-Energy-Density Lithium Metal Batteries: Opportunities and Challenges for Solid Organic Electrolytes.

With increasing demands for safe, high capacity energy storage to support personal electronics, newer devices such as unmanned aerial vehicles, as well as the commercialization of electric vehicles, current energy storage technologies are facing increased challenges. Although alternative batteries have been intensively investigated, lithium (Li) batteries are still recognized as the preferred energy storage solution for the consumer electronics markets and next generation automobiles. However, the commercialized Li batteries still have disadvantages, such as low capacities, potential safety issues, and unfavorable cycling life. Therefore, the design and development of electromaterials toward high-energy-density, long-life-span Li batteries with improved safety is a focus for researchers in the field of energy materials. Herein, recent advances in the development of novel organic electrolytes are summarized toward solid-state Li batteries with higher energy density and improved safety. On the basis of new insights into ionic conduction and design principles of organic-based solid-state electrolytes, specific strategies toward developing these electrolytes for Li metal anodes, high-energy-density cathode materials (e.g., high voltage materials), as well as the optimization of cathode formulations are outlined. Finally, prospects for next generation solid-state electrolytes are also proposed.

Organic salts utilising the hexamethylguanidinium cation: the influence of the anion on the structural, physical and thermal properties.

The synthesis and characterisation of new solid-state electrolytes is a key step in advancing the development of safer and more reliable electrochemical energy storage technologies. Organic ionic plastic crystals (OIPCs) are an increasingly promising class of material for application in devices such as lithium or sodium metal batteries as they can support high ionic conductivity, with good electrochemical and thermal stability. However, the choice of OIPC-forming ions is still relatively limited. Furthermore, understanding of the influence of different cations and anions on the thermal, structural and transport properties of these materials is still in its infancy. Here we report the synthesis and in-depth characterisation of a range of new OIPCs utilising the hexamethylguanidinium cation ([HMG]) with five different anions. The thermal, structural, transport properties and free volume in the different salts have been investigated. The free volume within the salts has been investigated by positron annihilation lifetime spectroscopy, and the single crystal and powder X-ray diffraction analysis of [HMG] bis(trifluoromethanesulfonyl)imide ([TFSI]) in phase I and II, [HMG] hexafluorophosphate ([PF6]) and [HMG] tetrafluoroborate ([HMG][BF4]) are reported. The HMG cation can exhibit significant disorder, which is advantageous for plasticity and future use of these materials as high ionic conductivity matrices. The bis(fluorosulfonyl)imide salt, [HMG][FSI], is identified as particularly promising for use as an electrolyte, with good electrochemical stability and soft mechanical properties. The findings introduce a range of new materials to the solid-state electrolyte arena, while the insights into the physico-chemical relationships in these materials will be of importance for the future development and understanding of other ionic electrolytes.

Water-Facilitated Electrodeposition of Neodymium in a Phosphonium-Based Ionic Liquid.

Rare-earth metals are considered critical metals due to their extensive use in energy-related applications such as wind turbines and nickel-metal hybrid batteries found in hybrid electrical vehicles. A key drawback of the current processing methods includes the generation of large amounts of toxic and radioactive waste. Thus the efficient recovery of these valuable metals as well as cleaner processing methods are becoming increasingly important. Here we report on a clean electrochemical route for neodymium (Nd) recovery from [P6,6,6,14][TFSI], trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide which is amplified three times by the presence of water, as evidenced by the cathodic current density and thicker deposits. The role of Nd salt concentrations and water content as an additive in the electrochemistry of Nd3+ in [P6,6,6,14][TFSI] has been studied. The presence of metallic neodymium in the deposits has been confirmed by X-ray photoelectron spectroscopy.