Correlating Li+-Solvation Structure and its Electrochemical Reaction Kinetics with Sulfur in Subnano Confinement.

Combining theoretical and experimental approaches, we investigate the solvation properties of Li+ ions in a series of ether solvents (dimethoxyethane, diglyme, triglyme, tetraglyme, and 15-crown-5) and their subsequent effects on the solid-state lithium-sulfur reactions in subnano confinement. The ab initio and classical molecular dynamics (MD) simulations predict Li+ ion solvation structures within ether solvents in excellent agreement with experimental evidence from electrospray ionization-mass spectroscopy. An excellent correlation is also established between the Li+-solvation binding energies from the ab initio MD simulations and the lithiation overpotentials obtained from galvanostatic intermittent titration techniques (GITT). These findings convincingly indicate that a stronger solvation binding energy imposes a higher lithiation overpotential of sulfur in subnano confinement. The mechanistic understanding achieved at the electronic and atomistic level of how Li+-solvation dictates its electrochemical reactions with sulfur in subnano confinement provides invaluable guidance in designing future electrolytes and electrodes for Li-sulfur chemistry.

In situ and quantitative characterization of solid electrolyte interphases.

Despite its importance in dictating electrochemical reversibility and cell chemistry kinetics, the solid electrolyte interphase (SEI) on graphitic anodes remains the least understood component in Li ion batteries due to its trace presence, delicate chemical nature, heterogeneity in morphology, elusive formation mechanism, and lack of reliable in situ quantitative tools to characterize it. This work summarizes our systematic approach to understand SEI live formation, via in situ electrochemical atomic force microscopy, which provides topographic images and quantitative information about the structure, hierarchy, and thickness of interphases as function of electrolyte composition. Complemented by an ex situ chemical analysis, a comprehensive and dynamic picture of interphase formation during the first lithiation cycle of the graphitic anode is described. This combined approach provides an in situ and quantitative tool to conduct quality control of formed interphases.