  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.
