Solvation and Transport of Lithium Ions in Deep Eutectic Solvents

TL;DR

This study combines molecular dynamics simulations and neutron scattering to elucidate the mechanisms of lithium ion solvation and transport in deep eutectic solvents, revealing the dominant vehicular diffusion process and solvation structure.

Contribution

It provides the first detailed analysis of lithium ion solvation states and their dynamics in DES, advancing understanding of ion transport mechanisms.

Findings

Approximately 36% of acetamide molecules H-bond to lithium ions.

Lithium ion diffusion is mainly vehicular, involving the first solvation shell.

Lithium ions are H-bonded to about 3.2 acetamide molecules on average.

Abstract

Lithium based deep eutectic solvents (DESs) are excellent candidates for eco-friendly electrolytes in lithium ion batteries. While some of these DES have shown promising results, a clear mechanism of lithium ion transport in DESs is not yet established. This work reports the study on the solvation and transport of lithium in a DES made from lithium perchlorate and acetamide using Molecular Dynamics (MD) simulation and neutron scattering techniques. Based on hydrogen bonding (H-bonding) of acetamide with neighbouring molecules/ions, two states are largely prevalent: 1) acetamide molecules which are H-bonded to lithium ions (~ 36 %) and 2) acetamide molecules that are entirely free (~ 58%). Analysing their stochastic dynamics independently, it is observed that the long-range diffusion of the former is significantly slower than the latter one. This is also validated from the neutron scattering experiment on the same DES system. Further, the analysis the lithium dynamics shows that the diffusion of acetamide molecules in the first category is strongly coupled to that of lithium ions. On an average the lithium ions are H-bonded to ~ 3.2 acetamide molecules in their first solvation. These observations are further bolstered through the analysis of the H-bond correlation function between acetamide and lithium ions, which show that ~ 90% of lithium ionic transport is achieved by vehicular motion where the ions diffuse along with its first solvation shell. The findings of this work are an important advancement in understanding solvation and transport of lithium ion in DES.