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.

Solvent-in-salt electrolytes (SISEs) are a promising alternative to the electrolytes currently used in commercial devices. Despite the SISEs' advantages, their utilization is not yet realized due to the poor mobility of their chemical species. We explore this problem by adding chloroform to a SISE formed by acetonitrile and a Li-salt. First, we performed illustrative cycling experiments to highlight the potential of this approach. Then, we focused on the description of the microscopic dynamics of the electrolytes and exposed the relevant aspects to be considered for their optimal performance. While the conductivity at low temperatures may be enhanced by the addition of chloroform, only subtle changes occur at room temperature. As revealed by molecular dynamics simulations and quasielastic neutron scattering (QENS) experiments, this effect is related to the preservation of the structure expected for a highly concentrated solution and promotion of the formation of ionic aggregates. These outcomes occur despite the increase in the overall mobility of the chemical species. The dynamics of the electrolytes in porous carbon was also investigated using QENS. In these circumstances, low concentrations of chloroform lead to diffusivities of the molecular species higher than those observed for the bulk electrolytes. As chloroform's concentration increases, no further changes in the diffusivities are observed. Nonetheless, chloroform is mostly immobilized on the carbon surfaces and this behavior may be intensified at compositions closer to the eutectic mixture.