# Transition from Vehicular to Structural Ionic Transport in Electrified Alkali Aqueous Solutions

## TL;DR

This study reveals how different alkali ions conduct electricity in water under electric fields, showing unique transport mechanisms for lithium, sodium, and cesium.

## Contribution

The paper introduces a novel simulation method to uncover distinct ionic transport mechanisms under electric fields, revealing a field-induced transition for sodium ions.

## Key findings

- Lithium ions conduct through stable, 4-fold coordinated structures at all field strengths.
- Cesium ions rely on structural diffusion with transient water coordination bonds.
- Sodium ions show a field-induced transition from vehicular to structural transport, increasing current density.

## Abstract

A molecular understanding of the solvation and dynamics
of ions
under static electric fields is crucial for modeling a wide range
of natural and technological processes. Yet, traditional simulation
methods suffer from a trade-off that has to be made between accuracy
and statistical convergence. To bridge this gap, herein, we extend
our recently introduced perturbed neural network potential molecular
dynamics (PNNP MD) approach to investigate the solvation structures
and ionic transport mechanisms of electrified alkali cationic solutions.
We obtain ionic conductivities for Li+, Na+,
and Cs+ from the field dependence of the ionic current
density in good agreement with experiment. Surprisingly, the migration
mechanism is found to be strikingly different for the three ions,
despite their similar ionic conductivities. While Li+ conducts
predominantly through vehicular migration of a stable
4-fold coordinated ion at all field strengths, Cs+ conducts
strictly through a structural diffusion mechanism,
where 9–12 transient first shell water coordination bonds are
continuously broken and reformed. Notably, aqueous Na+ emerges
as a “Goldilocks” ion: its ion–water interactions
are strong enough to maintain distinct 5–6-fold coordination
shells at zero field (unlike Cs+) yet labile enough to
be strongly perturbed by electric fields (unlike Li+).
As a consequence, we observe an electric-field-induced transition
from vehicular to structural ionic transport for Na+ that
is accompanied by a marked increase in ionic current density. Our
results imply that the conductance mechanism of ions with moderate
ion-solvent interactions can be effectively tuned by external electric
fields.