Structural and dynamical heterogeneities at the nanoscale in alkali/earth alkaline ionic liquid electrolytes: experiment and molecular simulation

Abstract

Ionic liquids (ILs) are versatile solvents for a wide range of substances, including alkali and alkaline earth salts. Mixtures of ILs and these salts are promising electrolytes for Li and post-Li batteries, offering unique properties such as stability and viscosity. These characteristics stem from the multiscale coupling between structural and dynamical aspects of ILs. We focus in this study on a family of electrolytes based on the iconic IL 1-butyl-3-methylimidazolium trifluoromethylsulfonimide (BMImTFSI), in which alkali (Li⁺, Na⁺, K⁺, Cs⁺) and alkaline earth (Mg²⁺) TFSI salts are dissolved. First, we demonstrate how classical molecular dynamics (MD) allows reproducing structural scattering features and self-diffusion coefficients, as measured by wide angle X-ray scattering and pulsed field gradient NMR. With this approach, we show that MD also helps to decipher the subtle differences between electrolytes in which alkali or earth alkaline ions are dissolved. Second, molecular dynamics analyses based on a single particle tracking strategy allows unravelling the correlations between structural and dynamical heterogeneities in a multiscale approach (from coordination spheres/fs to nanosegregation/hundreds of ns), therefore evidencing a large distribution of dynamics and the presence of clusters with long lifetime. All these results sort out time/temperature equivalence, which appears as a key to understand the behavior of nanostructured fluids.