Insights into the Chemo-Mechanics of Sulfur Conversion Electrodes via Operando Acoustic Transmission for Lithium Batteries

The chemo-mechanics of the lithium-sulfur battery are unique in Li-ion batteries due to the sulfur electrode undergoing two first-order phase changes during discharge and charge. However, dissolution of sulfurous species in liquid electrolytes is a primary degradation mode in Li-S systems. Here, we use acoustic transmission to track the mechanical phasing of the sulfur electrode. We show that acoustic time of flight (or sound speed) is directly correlated to sulfur's physio-chemical phase dynamics. This is because the acoustic sound speed is a composite property dependent on the density and elastic modulus of the medium. By accounting for cell dilation, we show that inter-cycle and intra-cycle sound speed changes in time of flight are due to the sulfur dynamics alone. This understanding is linked with ex-situ techniques like XRD and TGA to track the effect of polysulfide shuttling.