PNNL

Abstract

Molecular clustering and associated dynamic processes of lithium polysulfide species were unraveled using classical molecular dynamics and ab initio metadynamics calculations. The spectroscopic signatures of polysulfide clusters were analyzed using a multimodal analysis including experimental and computational NMR and XAS spectroscopies. Lith-ium polysulfide solutes (Li2S4, Li2S6, and Li2S8) and their mixtures in 1,3-dioxolane and 1,2-dimethoxyethane (DOL/DME) solvent undergo aggregation driven by intramolecular Li-S interactions, leading to distributions of cluster sizes which could critically influence the functioning of lithium-sulfur batteries. Representative polysulfide clusters with systematic increases in molecular size were extracted from the classical MD trajectories for subsequent structural and spectroscopic property calculations using DFT analysis. Structural analysis of these clusters reveal progressively decreasing solvent involvement in Li+ coordination varying from Li2S4 to Li2S8, with more pronounced variation and changes in DME compared with that of DOL. These observations are reflected in the analysis of the experimental and theoretical 7Li and 17O NMR chemical shifts and PFG-NMR diffusion measurements. A comparison of experimental and theoretical S K-edge XANES spectra show that relatively large lithium sulfide chain clusters are likely to occur in the DOL/DME-solvated lithium sulfide systems. Ab initio metadynamics simulations and NMR analysis indicates that Li+ solvated by only the solvent can occur through Li+ dissoci-ation from sulfide chains. However, the occurrence of “sulfide-free” Li+ is a minor mechanism compared with the dynamic aggregation and shuttling processes of polysulfide solvates in DOL/DME based electrolytes of Li-S battery. Overall, atom-istic insights gained about clustering and lithium exchange dynamics will be critical for predictive understanding of poly-sulfide shuttling and nucleation process that dictates the Li-S battery performance.