Field-driven Ion Pairing Dynamics in Concentrated Electrolytes

TL;DR

This study uses molecular simulations and nonequilibrium rate theory to analyze ion pairing dynamics in concentrated electrolytes under electric fields, revealing nonlinear conductivity enhancements and limitations of classical theories.

Contribution

It introduces a dynamical proxy for free-ion population and demonstrates the importance of solvent effects, advancing understanding of nonequilibrium ion transport in electrolytes.

Findings

Field-induced conductivity increases by 40% in acetonitrile at 50 mV/Å.

Onsager's classical theory overestimates ion dissociation enhancement.

Solvent-mediated pathways and dielectric effects suppress ion pair dissociation.

Abstract

We investigate ion pairing dynamics in electrolytes driven far from equilibrium using molecular simulations and nonequilibrium rate theory. Focusing on 0.5 M $\mathrm{LiPF_6}$ in water and acetonitrile under uniform electric fields, we compute transition path theory observables including reactive fluxes and mean first-passage times of ion pairing. Moreover, we introduce a dynamical proxy of free-ion population, where its field-induced change is strongly correlated with the nonlinear enhancement of conductivity, yielding an increase of $40 \ \%$ at 50 mV/{\AA} in acetonitrile, compared to less than $10 \ \%$ in aqueous electrolytes. Further kinetic analysis elucidates that Onsager's classical theory substantially overestimates field-induced enhancement of ion pair dissociation in molecular electrolytes. This discrepancy arises from solvent-mediated dynamical pathways and field-induced dielectric decrement that suppress ion pair dissociation within explicit solvents, highlighting that a faithful description of molecular details is essential. Our results provide a molecular interpretation of nonlinear electrolyte transport beyond continuum theories and establish a general framework for quantifying nonequilibrium reaction kinetics in condensed phase systems.