Session Y19: Physics and Effects on Transport of Ion-Ion Correlation in Electrolyte Materials

Live

Dielectric relaxation spectroscopy (DRS) constitutes an excellent experimental tool to characterize liquid electrolytes, through the identification of solvation processes from the frequency-dependent generalized dielectric constant, and through the description of collective motion and couplings between different species. We adopt an atomic-scale computational approach to DRS, based on classical molecular dynamics simulations. With this method, we investigate structural and dynamical features of tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) mixtures, using a newly developed interatomic potential with unprecedented accuracy. We cover a wide range of salt concentrations, which allows us to investigate the transition from a dilute solution to a solvate ionic liquid of such a mixture. Our computational approach allows to partition the frequency-dependent generalized dielectric constant according to the chemical constituents composing the electrolytes. We show how specific solvated species contribute to the total dielectric spectrum, and extract insights regarding charge transport mechanisms.   

Live

The correlated motion of ions plays a critical role in determining the ionic conductivity. In the present study we explored the finite size effect on the transport properties under periodic boundary conditions. While the ionic conductivity computed from the Nernst-Einstein relation contains a finite-size error, that from the Green-Kubo relation is size-independent. This allows us to estimate the ion-ion correlations through the box size at which they crossover. Our results suggest viscosity as a dominating factor for the observed deviations from the Nernst-Einstein relation in both molecular dynamics simulations and a number of experiments.   

Live

Quantifying ion association and collective dynamics in electrolytes is essential for fundamental property interpretation and optimization for electrochemical applications. The extent of ion correlation depends on both the ion concentration and dielectric strength of the solvent; ions may be largely uncorrelated in high-dielectric solvents at low concentration, but properties of concentrated electrolytes are dictated by correlated and collective ion processes. In this work, we utilize molecular dynamics simulations to characterize ion association and collective ion dynamics in electrolytes composed of binary mixtures of BMIM⁺BF₄⁻ and various organic solvents, water, and LiBF₄ and LiTFSI salts. We illustrate different physical regimes of distinct ion correlation character. Electronic polarization and solvent dielectric controls the extent of ion pairng and clustering, changing the dominant ion correlation mechanism characterized by quantifying the fractional self and distinct contributions to the net ionic conductivity. The analysis also shed light on understanding the negative transference number observed in ionic liquid mixtures and concentrated polymer electrolytes.   

Live

We propose a new general method for analyzing and calculating diffusivity and ionic conductivity in media with strong ionic correlations. Previously, the only two options were the dilute uncorrelated approximation that is rapid but inaccurate, and the exact Green-Kubo method that is prohibitively expensive for complex systems. Our method automatically extracts and utilizes the collective diffusion eigenmodes of the displacement correlation matrix. The proposed approach is universally applicable, simple, and provably superior to previously available methods, exhibiting speed ups of several orders of magnitude. It opens wide opportunities to study correlated diffusion in previously inaccessible complex electrolytes.   

Live

Room-temperature ionic liquids (RTILs) are promising electrolytes that are composed entirely of ions but are liquid at room temperature. Their remarkable properties such as wide electrochemical window make them ideal electrolytes in many electrochemical systems. Here, using molecular dynamics simulations, we studied transport of RTILs in conical nanopores under external electric fields. The current through the pores was found to increase nonlinearly as applied voltage increases. The nonlinearity was traced back to that the RTILs’ electrical conductivity increases as the magnitude of the electric field increases and as the ion concentration decreases. At the same applied voltage, the current through the nonuniformly charged pore was weaker than that through the neutral pore, while the current through the uniformly charged pore was stronger. This is because the hindered transport due to the higher total ion concentration and the lower degree of ion depletion dominates in the nonuniform case, while the enhanced transport due to the formation of electrical double layers dominates in the uniform case. Physical insights provided in this study demonstrate the importance of solvent-free nature and strong ion–ion correlations in RTILs on their nonequilibrium transport in nanopores.   

Live

Solid (solvent-free) polymer electrolytes are promising alternatives to conventional liquid counterparts in Li-ion batteries. By separating the effect of T_g and the effect of solvent polarity in our molecular dynamics simulations, we show that the maximum in the diffusion coefficient of Li+ with respect to the dielectric constant of polymer-solvent ε_p is due to transitions in the transport mechanism. The balanced interactions between Li-ion with TSFI and PEO at an intermediate value of ε_p leads to the fast ion conduction.   

Live

Advances in the fabrication of nanofluidic devices have enabled the study of transport processes on small scales, where novel phenomena emerge from the interplay between confinement, fluctuations and molecular granularity. Some of the most striking recent observations have been in electrokinetic transport of electrolyte solutions confined to nanometer dimensions, where nonlinear effects are often recorded. [1-5]
We develop a formalism and a numerical approach to efficiently compute the electric field dependent conductivity of electrolytes. [6] Specifically, we derive a relationship for the latter in terms of molecular fluctuations that is valid arbitrarily far from equilibrium. Moreover, we devise a novel nonequilibrium reweighting approach that provides access to rare dynamical fluctuations within nonequilibrium molecular dynamics simulations. We investigate bulk solutions of differing ionic strengths, differing implicit solvent conditions and the effect of water as a solvent.

[1] S. J. Kim, Y.-C. Wang, J. H. Lee, H. Jang, and J. Han, Physical review letters 99, 044501 (2007).
[2] R. Karnik, C. Duan, K. Castelino, H. Daiguji, and A. Majumdar, Nano letters 7, 547 (2007).
[3] W. Guan, R. Fan, and M. A. Reed, Nature communications 2, 506 (2011).
[4] U. Vermesh, J. W. Choi, O. Vermesh, R. Fan, J. Nagarah, and J. R. Heath, Nano letters 9, 1315 (2009).
[5] J. Feng, K. Liu, M. Graf, D. Dumcenco, A. Kis, M. Di Ventra, and A. Radenovic, Nature materials 15, 850 (2016).
[6] D. Lesnicki, C. Y. Gao, B. Rotenberg and D. Limmer, Physical review letters 124, 206001 (2020).   

Live

The study of ionic transport is an important tool for the optimization of the performance of Lithium ion batteries. The diffusion of cations across the electrolyte system often exhibits complex correlated motion, which remains poorly understood. Building on previous studies showing the existence of cluster motion in different electrolytes (solid polymers, ionic salts et cetera), we study the fundamentals of the clustering behavior, to provide insights into cluster formation. In particular, we develop algorithms to describe conditions for clustering to appear, the nature and composition of these clusters, the distribution in size, composition and diffusion coefficient of clusters during transport. We also investigate the lifetime of cluster and relate it to the transport properties of the electrolyte, particularly with regard to the recently reported negative transference number in a variety of systems. This study is also linked back to our work on the voltage stability of organic electrolytes, that shows how the stability of anions in these systems is weakened by the presence of the solvent, and increased by the presence of cations. Therefore, clustering behavior would impact not only the diffusion properties but also the voltage window of the electrolyte.   

Live

A fundamental understanding of the of mixtures of polymer electrolytes and lithium salts is important for the rational design of solid-state lithium-ion batteries. We report the molecular dynamics studies of the mixtures of Poly(ethylene oxide) (PEO) and lithium bis(trifluoromethane) imidate (LiTFSI) in experimentally relevant salt concentration regime. Near the critical salt concentration, rc=1/6 [Li+]/[EO], the simulation results show the emergence of a small plateau of salt activity coefficient. At the same concentration, the dependence of polymer chain size on salt concentration exhibits a reversal from contraction to expansion. These unusual thermodynamic and chain conformational behaviors are attributed to qualitative changes of the solvation structure near rc.The scattering profile shows two peaks as the salt concentration increases. The high-q peak is due to neighboring correlation between PEO segments, whereas the low-q peak is entirely due to the local correlation between TFSI anions. These simulation results are in good agreement with experimental observations.   

Live

Electrolytes control battery recharge time and efficiency, anode/cathode stability, and ultimately safety, consequently electrolyte optimization is crucial for the design of modern energy storage devices. We adopt theoretical and molecular modeling techniques to shine light on transport properties and correlation effects in the electrolyte system. Here we focus on ionic liquid-based electrolytes.

Electrolytes containing ionic liquids (ILs) possess superior chemical stability, however, poor transport properties are hindering their applicability. Given the ionic nature, these systems possess high degrees of ion-ion correlation, therefore posing a non-trivial yet crucial and interesting challenge to understanding their transport properties. We show that, first, the strong ionic interactions result in significant correlations and deviations from ideal solution behavior. Second, we computationally confirm the recently measured negative Li transference number in Li-containing IL-based electrolytes, and extend this surprising result to a vast range of different chemistries, suggesting a universal behavior of this class of electrolytes. Third, we leverage our microscopic understanding to suggest and test modifications to increase the cation transference number.   

Live

We present a combined experimental and theoretical investigation of atomic dynamics in the superionic compound Cu₇PSe₆, rationalizing the atomistic diffusion mechanism and the impact of host lattice dynamics. Inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS) were performed as a function of temperature, and were complemented with ab-initio molecular dynamics extended to long time scales with the use of machine-learned potentials (MLMD). INS data reveal characteristic changes at the onset of superionic behavior, providing insights into the role of the host lattice dynamics, while QENS probes the superionic Cu diffusion via the jump length, residence time and diffusion constant. The MLMD simulations reveal that the long-range Cu diffusion is limited by an inter-cluster hopping step, which is strongly coupled to the host phonon dynamics. Further, MLMD simulations enable us to capture the ultralow lattice thermal conductivity within the Green-Kubo framework. These results supersede the traditional quasiharmonic phonon picture to capture the strong anharmonicity in this superionic system.