(482d) Molecular Characterization of High Ionic Conductivity in Fluoroether Lithium Metal Battery Electrolytes

In hopes of designing and engineering next generation electrolytes for dendrite-free lithium metal batteries with sufficiently high conductivity, we studied the underlying Li-ion transport mechanisms in several novel fluoroether solvents. Here, we use all-atom molecular dynamics (MD) simulations to elucidate the properties of ion transport and solvation for several fluoroether chains with varying lengths of ether segments and the number of fluorinated terminal groups. We show that ionic conductivity can be tuned by changing the fraction of ether and fluorine contents. More specifically, we find that solvents with one fluorinated terminal group generally outperforms those with two fluorinated terminal groups with higher ionic conductivity and more stable solvation environment. We find that the solvent size determines the lithium-ion transport kinetics, solvation structures, and solvation energies. As the length of the fluoroether chain increases, lithium ions bind more strongly to the solvent. Consequently, the lithium-ion transport mechanism shifts from ion hopping between solvation sites in different fluoroether chains in short-chain solvents to the ion-solvent co-diffusion in long-chain solvents. Free energy calculations reveal that longer solvent molecules have lower solvation free energies, stabilizing lithium ions. Experimental measurements predict a surprising non-monotonic behavior of solvent size effect on the lithium-ion conductivity, where the maximum ionic conductivity occurs at the medium-length solvent chains, which agrees with our analysis. Achieving high ionic conductivity, therefore, requires a trade-off between increasing the solvent self-diffusivity by using shorter-length fluoroether chains and increasing the stability of local solvation shells by using longer-length chains. Our study demonstrates that this critical balance between high solvent self-diffusivity and low solvation energy is a key design principle for achieving lithium battery electrolyte solutions with high lithium-ion conductivity.