In Situ Stress Measurements of Li-S Half-Cells during Electrochemical Cycling

Lithium polysulfide flow and semi-flow batteries are a candidate system for flow battery applications with the potential to achieve high energy and low cost. The insulating nature of lithium sulfide and the polysulfide shuttle process are challenges to feasibility. The formation of polysulfides and sulfides in Li-S half-cells are studied using thin-film graphitic carbon electrodes as a model surface. Raman spectroscopy is used to investigate the effect of carbon growth parameters and surface treatment on electrochemical performance. Stress evolution in a lithium polysulfide battery electrode is measured during electrochemical cycling using a Multi-beam Optical Stress Sensor (MOSS) through measurement of wafer curvature as an in situ technique to measure film growth. A significant reversible and irreversible stress response is observed during cycling. An irreversible stress is observed at lower potentials, which are associated with the formation of sulfides; a reversible stress response is seen during voltage holds at 2.6V during sulfur formation. Kinetic processes of the sulfur, polysulfide, and sulfide regimes are explored using potentiostatic holds. Multiple mechanisms can be distinguished in the voltage regimes explored as stress signatures and are shown to be influenced by prior voltage holds. Post-mortem samples are analyzed using electron microscopy and energy dispersive x-ray spectroscopy to provide insight into sulfide formation and stability.