Chemo-Mechanical Hysteresis of Sulfur Conversion Electrodes via Operando Acoustic Transmission

The chemo-mechanics of lithium-sulfur (Li-S) batteries are unique in lithium-based batteries because sulfur undergoes a solid-liquid-solid transition during each half-cycle. The dissolution of sulfurous species in liquid electrolytes is a primary degradation mode in Li-S systems. While this challenge is well known, tracking and measuring sulfur liquefaction requires ex-situ experiments or hard-to-parallelize X-ray techniques. Here, we show that operando acoustic analyses can track both physicochemical phase changes and the mechanical dynamics of sulfur lithiation. We show time-of-flight can monitor sulfur phase changes during density and effective elastic moduli dynamics. Acoustic wave damping is highly sensitive to the state-of-matter transitions of the sulfur electrode. By accounting for cell dilation from Li plating and stripping, we show sulfur’s chemo-mechanical phase changes dominate time-of-flight’s nonlinear, non-monotonic signatures. By utilizing inter-cycle and intra-cycle time-of-flight trends, we develop a semi-quantitative method that can be calibrated to measure the dissolution of sulfur into the electrolyte and verify this with ex-situ TGA and XRD. Lastly, we pair acoustics with voltammetry to observe slow chemo-mechanical dynamics alongside the sluggish kinetics of sulfur utilization. Operando acoustic analyses can elucidate the chemo-mechanical dynamics of the sulfur electrode noninvasively and aid development efforts to slow and mitigate S migration.