Unraveling the Li₂S Deposition Process on a Polished Graphite Cathode for Enhancing Discharge Capacity of Lithium-Sulfur Batteries

Solid deposition accounts for three-quarters of the theoretical capacity in lithium-sulfur (Li-S) batteries with liquid electrolyte. Despite extensive research efforts on cathode material synthesis, little knowledge has been gained so far in understanding and controlling the growth of solid discharge product in Li-S batteries. In this work, a polished graphite was used as a cathode to understand the growth mechanism of Li₂S. The SEM/EDS analysis of the discharged cathodes indicates that the Li₂S precipitate can grow over a micrometer in size and its morphology strongly depends on the depth of discharge (DODs) and discharge rate of the cell. In addition, the morphology evolution and the in situ electrochemical impedance spectra (EIS) show that the Li₂S follows a dissolution-precipitation mechanism during its deposition on the graphite surface. Finally, a mathematical model based on the multicomponent transport theory is developed and used to describe the nucleation and precipitation phenomena on the 2D surface and the EIS spectra at different DODs. The model confirms that the surface passivation of the cathode plays a major role during the discharge of the battery and offers a simple way to measure experimentally the surface coverage as a function of the DOD in Li-S batteries. This work highlights the importance of deferring cathode surface passivation in Li-S batteries and indicates the potential utilization of nonporous carbons as alternative sulfur hosts.