Atomistic simulation of structure and dynamics of the plastic crystal diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate

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.

Surface and Conductivity Characterization of Layered Organic Ionic Plastic Crystal (OIPC)-Polymer Films

Organic ionic plastic crystals (OIPCs) are attractive solid electrolyte materials for advanced energy storage systems owing to their inherent advantages (e.g., high plasticity, thermal stability, and moderate ionic conductivity), which can be further improved/deteriorated by the addition of polymer or metal oxide nanoparticles. The role of the nanoparticle/OIPC combinations on the resultant interphase structure and transport properties, however, is still unclear due to the complexity within the composite structures. Herein, we demonstrate a systematic approach to specifically interrogating the interphase region by fabricating layered OIPC/polymer thin films via spin coating and correlating variation in the ionic conductivity of the OIPC with their microscopic structures. In-plane interdigitated electrodes have been employed to obtain electrochemical impedance spectroscopy (EIS) spectra on both OIPC and layered OIPC/polymer thin films. The thin-film EIS measurements were evaluated with conventional bulk EIS measurements on the OIPC pressed pellets and compared with EIS obtained from the OIPC-polymer composites. Interactions between the OIPC and polymer films as well as the morphology of the film surfaces have been characterized through multiple microscopic analysis tools, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, and optical profilometry. The combination of EIS analysis with the microscopic visualization of these unique layered OIPC/polymer thin films has confirmed the impact of the OIPC-polymer interphase region on the overall ionic conductivity of bulk OIPC-polymer composites. By changing the chemistry of the polymer substrate (i.e., PMMA, PVDF, and PVDF-HFP), the importance of compatibility between the components in the interphase region is clearly observed. The methods developed here can be used to screen and further understand the interactions among composite components for enhanced compatibility and conductivity.

The effects of vacancies and their mobility on the dynamic heterogeneity in 1,3-dimethylimidazolium hexafluorophosphate organic ionic plastic crystals

Organic ionic plastic crystals (OIPCs) are the crystals of electrolytes with a long-range translational order. The rotational modes of ions in OIPCs are, however, activated even in solid phases such that the diffusion of dopants such as lithium ions may be facilitated. OIPCs have been, therefore, considered as good candidates for solid electrolytes. Recent experiments and theoretical studies have suggested that both the translational and the rotational diffusion of ions are quite heterogeneous: the diffusion of some ions are quite fast while other ions of the same kind hardly diffuse, either rotationally or translationally. Such dynamic heterogeneity would be a key to the transport mechanism of dopants in solid state electrolytes. In this work, we investigate the effects of defects on the dynamic heterogeneity of OIPCs. We perform atomistic molecular dynamics simulation of 1,3-dimethylimidazolium hexafluorophosphate ([MMIM][PF6]) with a pair of cation and anion vacancies. At low temperature, vacancies undergo hopping motions toward each other and form a charge-neutral cluster. At high temperature, two vacancies act like a loosely bonded molecule and diffuse together via hopping motions. We find that the translational diffusion of ions is correlated strongly with the vacancy diffusion and becomes heterogeneous when the vacancies hop. The rotation of ions also becomes active when the ions are close to vacancies such that the rotational dynamic heterogeneity strengthens.

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 ([P_122i4][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.

The free energy calculation shows the different free energy changes of the adsorption and absorption of gas molecules into an organic ionic plastic crystal, successfully predicting the gas selectivity of this new type of gas separation material.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

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.

Ion Vacancies and Transport in 1-Methylimidazolium Triflate Organic Ionic Plastic Crystal

Organic ionic plastic crystals (OIPCs) are an important family of materials that have shown exciting possibilities as solid electrolytes for lithium ion batteries and other electrochemical devices. In this study we demonstrate for the first time that, although the X-ray shows sharp diffraction peaks, both cation and anion clearly exhibit significant ion diffusion in solid phase I. Two phases with ion diffusivities differing by 2 orders of magnitude can be identified. The populations of the cation and anion in both phases are found to be unequal, hinting at the existence of (negatively charged) cation vacancies in the plastic crystal phase and a positively charged grain boundary phase. These interesting properties of ion vacancies and unequal populations of cation and anion are likely to be ubiquitous in other OIPCs, and it is of paramount importance to be aware of these features to correctly understand the structure-property relationships of this important material family.

N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide-electrospun polyvinylidene fluoride composite electrolytes: characterization and lithium cell studies

LiFSI doped [C₂mpyr][FSI]–PVdF composites were developed as solid-state, self-standing electrolyte membranes.

Thermal phase behavior and ion hopping in a 1,2,4-triazolium perfluorobutanesulfonate protic organic ionic plastic crystal

Critical aspects of thermal behavior and the electrolytic properties of solid-state Protic Organic Ionic Plastic Crystals (POIPCs) are unknown. We present molecular dynamics (MD) simulations on a perfect crystal and a vacancy model to probe such physical phenomena in POIPCs using 1,2,4-triazolium perfluorobutanesulfonate ([TAZ][pfBu]) as an example. The results show the existence of a rotator phase wherein the cations, although translationally ordered are disordered rotationally and exhibit a tumbling motion which significantly affects hydrogen bond lifetimes. van Hove correlation functions characterize the concerted hopping of ions (cation or anion) at 500 K. These results are substantiated by calculated free energy barriers (cation = 2.5 kcal mol(-1) and anion = 6 kcal mol(-1)) and suggest that proton and ion transport influenced by facile hydrogen bond dynamics in the rotator phase contribute to the solid-state conductivity of POIPCs.

A novel class of gas separation membrane based on organic ionic plastic crystals

The first demonstration of organic ionic plastic crystal-based membranes for CO₂ separation.

Elucidation of transport mechanism and enhanced alkali ion transference numbers in mixed alkali metal–organic ionic molten salts

This work reveals how structure facilitates diffusion of the Li/Na ion in ionic liquids with the high Li/Na concentration.

New insights into the thermal behaviour of organic ionic plastic crystals: magnetic resonance imaging of polycrystalline morphology alterations induced by solid–solid phase transitions

Morphology alterations induced by solid–solid phase transitions in Organic Ionic Plastic Crystals (OIPC) elucidate molecular dynamics, micro-structural behaviour and conductive properties of OIPCs.

Computational approaches to understanding reaction outcomes of organic processes in ionic liquids

The utility of using a combined experimental and computational approach for understanding ionic liquid media, and their effect on reaction outcome, is highlighted through a number of case studies.