The effects of defects on the transport mechanisms of lithium ions in organic ionic plastic crystals

Organic ionic plastic crystals (OIPCs) consist of molecular ions of which interactions are strong enough to maintain crystalline order but are weak enough to allow the rotations of the molecular ions at sufficiently high temperatures. When defects such as Schottky vacancies and grain boundaries are introduced into OIPCs, the defects facilitate the transport of dopants such as Li+ ions, for which OIPCs are considered as strong candidates for solid electrolytes. The transport mechanism of dopant ions in OIPCs with defects, however, remains elusive at a molecular level partly because it is hard in experiments to track the dopant ions and control the types of defects systematically. In this work, we perform molecular dynamics simulations for 1,3-dimethylimidazolium hexafluorophosphate ([MMIM][PF6]) OIPCs with Li+ ions doped and show that the transport mechanism of Li+ ions depends on the types and concentrations of defects. A high concentration of Schottky vacancies enhance the overall ion conduction, but decrease the transference number. The transference numbers of Li+ ions in [MMIM][PF6] with grain boundaries are similar to that in [MMIM][PF6] with 0.78 mol% point vacancies. We also find that the transport of ions in OIPCs is strongly heterogeneous and the time scales of the dynamic heterogeneity of the ions are sensitive to the types of defects.

Molecular Dynamics Simulation of the Influence of External Electric Fields on the Glass Transition Temperature of the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide

We present the results of molecular dynamics simulations of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁im][NTf₂] in the presence of external electric fields (EEFs) of varying strengths to understand the effects of EEFs on the glass transition temperature T_g. We compute T_g with an automated and objective method and observe a depression in T_g when cooling the IL within an EEF above a critical strength. The effect is reversible, and glasses prepared with EEFs recover their original zero-field T_g when heated. By examining the dynamics and structure of the liquid phase, we find that the EEF lowers the activation energy for diffusion, reducing the energetic barrier for movement and consequently T_g. We show that the effect can be leveraged to drive an electrified nonvapor compression refrigeration cycle.

Does Explicit Polarizability Improve Molecular Dynamics Predictions of Glass Transition Temperatures of Ionic Liquids?

Molecular dynamics simulations are used for predictions of the glass transition temperatures for a test set of five aprotic ionic liquids. Glass transitions are localized with the trend-shift method, analyzing volumetric and transport properties of bulk amorphous phases. A classical nonpolarizable all-atom OPLS force-field model developed by Canongia Lopes and Pádua (CL&P) is employed as a starting level of theory for all calculations. Alternative approaches of charge scaling and the Drude oscillator model, accounting for atomic polarizability either implicitly or explicitly, respectively, are used to investigate the sensitivity of the glass transition temperatures to induction effects. The former nonpolarizable model overestimates the glass transition temperature by tens of Kelvins (37 K on average). The charge-scaling technique yields a significant improvement, and the best estimations were achieved using polarizable simulations with the Drude model, which yielded an average deviation of 11 K. Although the volumetric data usually exhibit a lesser trend shift upon vitrification, their lower statistical uncertainty enables to predict the glass transition temperature with lower uncertainty than the ionic self-diffusivities, the temperature dependence of which is usually more scattered. Additional analyses of the simulated data were also performed, revealing that the Drude model predicts lower densities for most subcooled liquids but higher densities for the glasses than the original CL&P, and that the Drude model also invokes some longer-range organization of the subcooled liquid, greatly impacting the temperature trend of ionic self-diffusivities in the low-temperature region.