Room-temperature ionic liquids (RTILs) are a class of organic salts that are liquid at room temperature. Their physiochemical properties, including low vapor pressure and wide electrochemical stability window, have driven their use as electrolytes in many electrochemical applications; however, the slow transport properties of many RTILs have limited their utility in some applications. This issue is often mitigated by solvating ionic liquids in neutral organic solvents. To date, however, solvent interactions have only been explored for a small number of solvents, particularly acetonitrile and propylene carbonate, at only a few compositions. In this work, we use molecular dynamics simulations in the context of a computational screening approach to study mixtures of ionic liquids in many different solvents at a range of concentrations. Building on prior work, we again find that ionic liquid diffusivity increases monotonically with greater solvent concentration. In contrast to prior work, we find that pure solvent diffusivity, not polarity, is the most influential solvent property on mixture behavior. We also explore the concentration dependence of ionic conductivity and find maxima at intermediate concentrations. Experimental conductivity measurements, inspired by the computational screening study, support this observation with qualitatively consistent results. These results can further guide the selection of solvents for electrochemical applications of RTILs.

Most polar solvent molecules are unstable toward electrode materials used in Li-based batteries. Solid electrolytes and ionic liquids are far more stable; however, they have relatively low conductivity, and therefore electrical energy storage devices based on them would suffer from low power. Solvent-in-salt (SIS) systems combine chemical stability with relatively high conductivity. Here, we show how the nature of the employed anion affects the structure and dynamics of SIS systems. The transport of ions in lithium bis(fluorosulfonyl)imide (Li-FSI) systems was determined to be always faster than that in lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) systems. Moreover, we found that viscosity does not solely control conductivity and that the lower conductivity of TFSI − solutions is related to their stronger interaction with the solvent. This restricts solvent dynamics and slows down ion motions compared to that of FSI − . Interestingly, the TFSI−solvent interaction also leads to better charge separation (weaker ion−ion correlations) and a higher transference number for Li. Our results suggest that the ability to tune the solvent network formed around the anions may further improve electrolyte conductivity and Li transference number for safer and more efficient energy storage devices.

MXenes exhibit excellent capacitance at high scan rates in sulfuric acid aqueous electrolytes, but the narrow potential window of aqueous electrolytes limits the energy density. Organic electrolytes and room‐temperature ionic liquids (RTILs) can provide higher potential windows, leading to higher energy density. The large cation size of RTIL hinders its intercalation in‐between the layers of MXene limiting the specific capacitance in comparison to aqueous electrolytes. In this work, different chain lengths alkylammonium (AA) cations are intercalated into Ti3C2Tx, producing variation of MXene interlayer spacings (d‐spacing). AA‐cation‐intercalated Ti3C2Tx (AA‐Ti3C2), exhibits higher specific capacitances, and cycling stabilities than pristine Ti3C2Tx in 1 m 1‐ethly‐3‐methylimidazolium bis‐(trifluoromethylsulfonyl)‐imide (EMIMTFSI) in acetonitrile and neat EMIMTFSI RTIL electrolytes. Pre‐intercalated MXene with an interlayer spacing of ≈2.2 nm, can deliver a large specific capacitance of 257 F g−1 (1428 mF cm−2 and 492 F cm−3) in neat EMIMTFSI electrolyte leading to high energy density. Quasi elastic neutron scattering and electrochemical impedance spectroscopy are used to study the dynamics of confined RTIL in pre‐intercalated MXene. Molecular dynamics simulations suggest significant differences in the structures of RTIL ions and AA cations inside the Ti3C2Tx interlayer, providing insights into the differences in the observed electrochemical behavior.

Layered titanium carbide (Ti 3 C 2 T x ) MXene is a promising electrode material for use in next-generation electrochemical capacitors. However, the atomic-level information needed to correlate the distribution of intercalated cations with surface redox reactions, has not been investigated in detail. Herein we report on sodium preintercalated MXene with high sodium content (up to 2Na per Ti 3 C 2 T x formula) using a solution of Na-biphenyl radical anion complex (E 0 ≈ −2.6 SHE). Multiple sodiation sites and formation of a twodimensional sodium domain structure at interfaces/surfaces is identified through combined computational simulations with neutron pair distribution function analysis. The induced layer charges and the redox process characterized by the densityfunctional tight-binding method on a local scale are found to greatly depend on the location of sodium ions. Electrochemical testing of the pre-sodiated MXene as an electrode material in a sodium-ion capacitor shows excellent reversibility and promising performance, indicating the feasibility of chemical preintercalation as an approach to prepare MXene electrodes for ion capacitors.

Systems composed of soft matter (e.g., liquids, polymers, foams, gels, colloids, and most biological materials) are ubiquitous in science and engineering, but molecular simulations of such systems pose particular computational challenges, requiring time and/or ensemble-averaged data to be collected over long simulation trajectories for property evaluation. Performing a molecular simulation of a soft matter system involves multiple steps, which have traditionally been performed by researchers in a "bespoke" fashion, resulting in many published soft matter simulations not being reproducible based on the information provided in the publications. To address the issue of reproducibility and to provide tools for computational screening, we have been developing the open-source Molecular Simulation and Design Framework (MoSDeF) software suite.In this paper, we propose a set of principles to create Transparent, Reproducible, Usable by others, and Extensible (TRUE) molecular simulations. MoSDeF facilitates the publication and dissemination of TRUE simulations by automating many of the critical steps in molecular simulation, thus enhancing their reproducibility. We provide several examples of TRUE molecular simulations: All of the steps involved in creating, running and extracting properties from the simulations are distributed on open-source platforms (within MoSDeF and on GitHub), thus meeting the definition of TRUE simulations.