Electronic paddle-wheels in a solid-state electrolyte.

Solid-state superionic conductors (SSICs) are promising alternatives to liquid electrolytes in batteries and other energy storage technologies. The rational design of SSICs and ultimately their deployment in battery technologies is hindered by the lack of a thorough understanding of their ion conduction mechanisms. In SSICs containing molecular ions, rotational dynamics couple with translational diffusion to create a paddle-wheel effect that facilitates conduction. The paddle-wheel mechanism explains many important features of molecular SSICs, but an explanation for ion conduction and anharmonic lattice dynamics in SSICs composed of monatomic ions is still needed. We predict that ion conduction in the classic SSIC AgI involves electronic paddle-wheels, rotational motion of localized electron pairs that couples to and facilitates ion diffusion. The electronic paddle-wheel mechanism creates a universal perspective for understanding ion conductivity in both monatomic and molecular SSICs that will create design principles for engineering solid-state electrolytes from the electronic level up to the macroscale.

Dielectric Saturation in Water from a Long-Range Machine Learning Model.

Machine learning-based neural network potentials have the ability to provide ab initio-level predictions while reaching large length and time scales often limited to empirical force fields. Traditionally, neural network potentials rely on a local description of atomic environments to achieve this scalability. These local descriptions result in short-range models that neglect long-range interactions necessary for processes like dielectric screening in polar liquids. Several approaches to including long-range electrostatic interactions within neural network models have appeared recently, and here we investigate the transferability of one such model, the self-consistent field neural network (SCFNN), which focuses on learning the physics associated with long-range response. By learning the essential physics, one can expect that such a neural network model should exhibit at least partial transferability. We illustrate this transferability by modeling dielectric saturation in a SCFNN model of water. We show that the SCFNN model can predict nonlinear response at high electric fields, including saturation of the dielectric constant, without training the model on these high field strengths and the resulting liquid configurations. We then use these simulations to examine the nuclear and electronic structure changes underlying dielectric saturation. Our results suggest that neural network models can exhibit transferability beyond the linear response regime and make genuine predictions when the relevant physics is properly learned.

Microstructures of Choline Amino Acid based Biocompatible Ionic Liquids.

Bio-compatible ionic liquids (Bio-ILs) represent a class of solvents with peculiar properties and exhibit huge potential for their applications in different fields of chemistry. Ever since they were discovered, researchers have used bio-ILs in diverse fields such as biomass dissolution, CO2 sequestration, and biodegradation of pesticides. This review highlights the ongoing research studies focused on elucidating the microscopic structure of bio-ILs based on cholinium cation ([Ch]+ ) and amino acid ([AA]- ) anions using the state-of-the-art a b i n i t i o ${ab\hskip0.25eminitio}$ and classical molecular dynamics (MD) simulations. The microscopic structure associated with these green ILs guides their suitability for specific applications. ILs of this class differ in the side chain of the amino acid anions, and varying the side chain significantly affects the structure of these ILs and thus helps in tuning the efficiency of biomass dissolution. This review demonstrates the central role of the side chain on the morphology of choline amino acid ([Ch][AA]) bio-ILs. The seemingly matured field of bio-ILs and their employment in various applications still holds significant potential, and the insights on their microscopic structure would steer the field of target specific application of these green ILs.

How NaFTA salt affects the structural landscape and transport properties of Pyrr1,3FTA ionic liquid.

Recently, it has been demonstrated that ionic liquids (ILs) with an asymmetric anion render a wider operational temperature range and can be used as a solvent in sodium ion batteries. In the present study, we examine the microscopic structure and dynamics of pure 1-methyl-1-propylpyrrolidinium fluorosulfonyl(trifluoromethylsulfonyl)amide (Pyrr1,3FTA) IL using atomistic molecular dynamics simulations. How the addition of the sodium salt (NaFTA) having the same anion changes the structural landscape and transport properties of the pure IL has also been explored. The simulated x-ray scattering structure functions reveal that the gradual addition of NaFTA salt (up to 1.2 molal) suppresses the charge alternating feature of the pure IL because of the replacement of the Pyrr+ cations with the Na+ ions. The Na+ ions are majorly found near the oxygen atoms of the anions, but the probability of finding the Na+ ions near these atoms slightly decreases with increasing salt concentration. As expected, the Na+ ions stay away from the Pyrr+ cations. However, the probability of finding the anions around anions increases with increasing salt concentration. The simulated self-diffusion coefficients of the ions in the pure IL reveal slightly faster diffusion of the Pyrr+ cations as compared to the FTA- anions. Interestingly, in the salt solution, despite having smaller size, the diffusion of the Na+ ions is found to be lesser than the Pyrr+ cations and the FTA- anions. The analysis of the ionic conductivity and transport numbers reveals that the fractional contribution of the FTA- anion to the overall conductivity remains nearly constant with increasing salt concentration, but the contribution of Pyrr+ cation decreases and Na+ ion increases.

Heterogeneity and Nanostructure of Superconcentrated LiTFSI-EmimTFSI Hybrid Aqueous Electrolytes: Beyond the 21 m Limit of Water-in-Salt Electrolyte.

Ionic liquids such as EmimTFSI (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) have been found to improve the solubility of LiTFSI salt in water-in-salt electrolyte (WiSE) from 21 to 60 m. However, the molecular origin of such enhancement in the solubility is still unknown. In the present work, we elucidate the microscopic structures of LiTFSI-EmimTFSI-based hybrid aqueous electrolytes and compare them with the structure of LiTFSI-based WiSE using molecular dynamics simulations. Our analysis reveals the presence of alternating water-rich clusters and TFSI-rich extended domains in the WiSE. In these clusters and domains, the Li+ ions reside such that the total number of oxygen atoms around them is conserved to four, where water contributes about three oxygen atoms. The addition of EmimTFSI in the WiSE results in removal of water from the nearest-neighbor solvation shell of TFSI- ions but not from the Li+ ions. Significant structural changes are observed when LiTFSI salt is further added to LiTFSI-EmimTFSI aqueous solution. In both the hybrid electrolytes, water and Emim+ cations are found to avoid each other. The simulated X-ray scattering structure factor reveals the presence of larger length-scale heterogeneity in the most concentrated solution of the hybrid aqueous electrolyte. We observe that this nanoscale heterogeneity originates from a water-TFSI-Emim-TFSI-water-TFSI-Emim-TFSI-like arrangement in which Li+ ions are dispersed such that the coordination number of oxygen atoms around them is enhanced to five, wherein the major contribution comes from the TFSI- ions. We envision that the enhanced LiTFSI solubility originates from the replacement of water molecules with TFSI- ions in the first solvation shell of Li+ ions.

An Overview of Structure and Dynamics Associated with Hydrophobic Deep Eutectic Solvents and Their Applications in Extraction Processes.

Recent development of novel water-immiscible green solvents known as hydrophobic deep eutectic solvents (HDESs) has opened the gates for applications requiring media where the presence of water is undesirable. Ever since they were prepared, researchers have used HDESs in diverse fields such as extraction processes, CO2 sequestration, membrane formation, and catalysis. The structure and dynamics associated with the species comprising HDESs guide their suitability for specific applications. For example, varying the alkyl tail length of the HDES components significantly affects the dynamics of the components and thus helps in tuning the efficiency of extraction processes. However, the development of HDESs is still in infancy, and very few theoretical studies are available in the literature that help in understanding the structure and dynamics of HDESs. This review highlights the recent studies focused on the microscopic structure and dynamics of HDESs and their potential applications, particularly in extraction processes. We have also provided a glimpse of how the integration of experiments and computational techniques can help delineate the mechanism of extraction processes.

Structure of cholinium glycinate biocompatible ionic liquid at graphite electrode interface.

We use constant potential molecular dynamics simulations to investigate the interfacial structure of the cholinium glycinate biocompatible ionic liquid (bio-IL) sandwiched between graphite electrodes with varying potential differences. Through number density profiles, we observe that the cation and anion densities oscillate up to ∼1.5 nm from the nearest electrode. The range of these oscillations does not change significantly with increasing electrode potential. However, the amplitudes of the cation (anion) density oscillations show a notable increase with increasing potential at the negative (positive) electrode. At higher potential differences, the bulkier N(CH3)3CH2 group of cholinium cations ([Ch]+) overcomes the steric barrier and comes closer to the negative electrode as compared to oxygen atom (O[Ch]+ ). We observe an increase in the interaction between O[Ch]+ and the positive electrode with a decrease in the distance between them on increasing the potential difference. We also observe hydrogen bonding between the hydroxyl group of [Ch]+ cations and oxygens of glycinate anions through the simulated tangential radial distribution function. Orientational order parameter analysis shows that the cation (anion) prefers to align parallel to the negative (positive) electrode at higher applied potential differences. Charge density profiles show a positive charge density peak near the positive electrode at all the potential differences because of the presence of partially positive charged hydrogen atoms of cations and anions. The differential capacitance (Cd) of the bio-IL shows two constant regimes, one for each electrode. The magnitude of these Cd values clearly suggests potential application of such bio-ILs as promising battery electrolytes.

First-principles based theoretical investigation of impact of polyolefin structure on photooxidation behavior.

Despite their mass production and large applications, polyolefins' stability and durability toward the air, moisture, and weather resistance is a challenge for the ecosystem. After long-term exposure to ultraviolet (UV) radiation or high-temperature or erosion, polyolefins undergo degradation generating microplastics (MPs). The MPs generated after the degradation of these polyolefins are hazardous for the ecosystem. In the present work, we have carried out density functional theory (DFT) studies to investigate the photodegradation of six different polyolefins ranging from polyethylene to polydecene, differing in side-chain. Herein, we have investigated photooxidized derivatives of different polyolefins and analyzed their relative stability, conformations, UV-visible spectral behavior, and carbonyl index. The photooxidized derivatives of various polyolefins formed during degradation are examined. The time-dependent density functional theory analysis confirms that the carbonyl groups of photooxidized products show absorption peak in Infrared (IR) and visible region, acting as light-absorbing species. The relative stabilities of hydroperoxide formed during photo/thermal oxidation of different polyolefins have been evaluated to explain the degradation behavior. The oligomerization and stabilization energies of their corresponding hydroperoxide's were computed and analyzed to explain the degradation behavior of the polyolefins. The computed results suggest that polyolefins in their pristine state are stable toward photooxidation, but chemical impurities like carbonyl, unsaturated carbonyl, carboxylic acid, and hydroperoxide derivatives make them prone to undergo degradation, a fundamental process leading to generation of MPs. The comparative results confirmed that the side-chain length affects the stability and degradation of different polyolefins toward photooxidation.

Unique and generic structural features of cholinium amino acid-based biocompatible ionic liquids.

Cholinium amino acid-based (Ch-AA) biocompatible ionic liquids (bio-ILs) are synthesized from renewable components and are efficiently used for biomass processing. However, their microscopic structural features that lead to their application as biomass solvents remain undetermined. Herein, we use atomistic simulations to investigate the structures of six different Ch-AA bio-ILs up to the nanometer length scale and demonstrate that, depending on the anion side chain structure, the respective IL exhibits structural ordering at different length scales. All the six Ch-AA bio-ILs investigated here show a generic feature of having a strong hydrogen bonding network between the hydroxyl group of the cholinium cation and the carboxyl group of the amino acid anions. We illustrate that each of these bio-ILs also displays a unique feature. Distinctive intermediate range structural ordering leads to heterogeneity in methioninate- and phenylalaninate-based ILs caused by the anion side chain segregation. Intermediate range ordering is not observed in glutaminate- and glutamate-based ILs because significant anion side chain and backbone interactions hinder the formation of side chain clusters. Interestingly, for the cysteinate-based IL, the side chains do not interact with the backbones and the intermediate range ordering is not observed because of a shorter anionic side chain.

Intercalation-deintercalation of water-in-salt electrolytes in nanoscale hydrophobic confinement.

Intercalation-deintercalation of water-in-salt (WIS) electrolytes in nanoscale confinement is an important phenomenon relevant to energy storage and self-assembly applications. In this article, we use molecular simulations to investigate the effects of intersurface separation on the structure and free energy underlying the intercalation-deintercalation of the Li bis(trifluoromethane)sulfonimide ([Li][TFSI]) water-in-salt (WIS) electrolyte confined between nanoscale hydrophobic surfaces. We employ enhanced sampling to estimate the free energy profiles for the intercalation behaviour of WIS in confining sheets at several intersurface separations. We observe that the relative stability of the condensed and vapour phases of WIS in the confinement depends on the separation between the confining surfaces and the WIS concentration. We find that the critical separation at which the condensed and vapour phases are equally stable in confinement depends on the concentration of WIS. The relative height of the free energy barrier also strongly depends on the concentration of [Li][TFSI] inside the confined space, and we find that this concentration dependence can be attributed to changes in line tension. The process of deintercalation passes through vapour tube formation inside the confined space, and this process is initiated by vapour bubble formation. The size of the critical vapour tube required for spontaneous evaporation of WIS from the confinement is also found to depend on the intersurface separation and WIS concentration.

Distinct Solvation Structures of CO2 and SO2 in Reline and Ethaline Deep Eutectic Solvents Revealed by AIMD Simulations.

Deep eutectic solvents (DESs) are emerging as an alternative media for the sequestration of greenhouse gases such as CO2 and SO2. Herein, we performed ab initio molecular dynamics (AIMD) simulations to elucidate the solvation structure around CO2 and SO2 in choline chloride-based DESs, namely, reline and ethaline. We show that in all four systems the structures of the nearest neighbor shells around these molecules are distinct. We observe that because of the electrophilic character, the carbon atom of CO2 and the sulfur atom of SO2 are preferentially solvated by the chloride anions. The strength of the correlation between the chloride anions and the sulfur atom is much stronger because of charge transfer, which is more profound in ethaline DES. In both DESs, the choline cations are found to be closer to the oxygen atoms of CO2 and SO2. We observe that upon changing the solute from CO2 to SO2, the nearest neighbor solvation structure changes drastically; while the chloride anions prefer to stay in a circular shell around the carbon atom of CO2, they are found to be much more localized near the sulfur atom of SO2. The solvation shells formed by the urea molecules in reline and EG molecules in ethaline also overlap with that of the chloride anion around CO2. In ethaline, the hydroxyl group of the choline cation is found to be closer to the solute molecules as compared to its ammonium headgroup.

Molecular dynamics investigation of wetting-dewetting behavior of reline DES nanodroplet at model carbon material.

Deep eutectic solvents (DESs) have emerged as a promising class of solvents for application in nanotechnology, particularly for designing new functional nanomaterials based on carbon. Here, we have employed molecular dynamics simulations to understand the structuring of choline chloride and urea-based DES, reline, nanodroplets on carbon sheets with varying strength of the DES-sheet interaction potentials. The wetting-dewetting nature of reline has been investigated by analyzing simulated contact angles formed by its nanodroplets on the carbon sheets. Through this investigation, we find that at the lowest DES-sheet interaction strength, the contact angle formed by the reline nanodroplet on the carbon surface exceeds 150°, indicating that the surface is supersolvophobic. On the other hand, at the higher interaction potentials, reline DES wets the surface of the sheets, forming an adlayer primarily consisting of urea molecules. The choline cation and urea molecules are observed to exhibit stronger interactions with the carbon surface as compared to that of chloride anions. At the supersolvophobic carbon surface, the urea molecules have relatively higher density in the bulk of the nanodroplet, whereas the choline cation and chloride have major contributions to the outer layers of the droplets. Moreover, at the solvophilic surfaces, urea molecules are present in the adlayer, as well as in the bulk of the droplets, whereas the reline-vapor interface majorly consists of choline and chloride ions.

Molecular dynamics investigation of wetting-dewetting behavior of model carbon material by 1-butyl-3-methylimidazolium acetate ionic liquid nanodroplet.

In order to comprehend the wetting-dewetting behavior of a solid surface by a liquid, it is crucial to contemplate both the surface flexibility and the interactions involved. Herein, by employing molecular dynamics simulations, we aim to understand the structural changes in 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) ionic liquid (IL) nanodroplets on model carbon sheets with varying IL-sheet interaction potentials along with the flexibility of the carbon sheet. The extent of the wetting is estimated by computing the average contact angle formed by [bmim][Ac] nanodroplets on the sheet surface. We observe that the wetting-dewetting behavior of the sheet and its affinity toward [bmim]+ and [Ac]- depend not only on the IL-sheet interaction but also on its flexibility or rigidity. The extent of wetting is observed to be consistently greater for the rigid surface in the entire range of IL-sheet interaction potentials studied herein. Although in the adlayer, [bmim]+ rings and [Ac]- anions prefer to be parallel to the carbon surface, the ordering of the [bmim]+ rings is highly affected by the introduction of flexibility in the carbon surface. Enhanced structural and orientational ordering of imidazolium rings of [bmim]+ cations in the adlayer of the rigid surface is observed, supporting the comprehension of greater wetting extent of the rigid surface by the IL droplet.