Free energy landscape of sodium solvation into graphite

TL;DR

This study uses quantum mechanics-based metadynamics to explore the free energy landscape of sodium ion intercalation into graphite from DMSO solvent, revealing the role of solvent in stabilizing sodium during intercalation.

Contribution

It provides the first detailed free energy landscape for sodium intercalation into graphite, highlighting solvent effects and proposing new solvent designs to enhance sodium mobility.

Findings

Sodium prefers to be solvated by 3 or 4 DMSO molecules.

Free energy of solvation is 0.17 eV, indicating thermodynamic stability.

Weak interactions observed between sodium and graphite sheets.

Abstract

Na is known to deliver very low energy capacity for sodium intercalation compared to Lithium. In this study, we use quantum mechanics based metadynamics simulations to obtain the free energy landscape for sodium ion intercalation from Dimethyl sulfoxide (DMSO) solvent into graphite. We find that the lowest free energy minima from the metadynamics are associated with sodium solvated by 3 or 4 DMSO. The free energy minima of these states are activated by a free energy of solvation computed to be 0.17 eV, which in turn are the most thermodynamically stable. We observe weak interactions of sodium with graphite sheets during the unbiased and biased molecular dynamics simulations. Our simulations results indicate that solvent plays an important role in stabilizing the sodium intercalation into graphite through shielding of the sodium while modulating the interaction of the solvent with the graphite sheets. In order to facilitate this intercalation, we propose solvents with negatively charged groups and aromatic cores (e.g., cyclic ethers) that could allow a greater rate of anion exchange to increase Na+ mobility.