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

Abstract

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.

Insights into Multiphase Reactions during Self-Discharge of Li-S Batteries

“…Although there was evidence to suggest that charge was not the exact reversal of discharge, the assumption can be made in this case for numerical simplicity. ,,− Figure c shows the precipitation/dissolution reactions forming solid PSs that were considered to take place. Depending on the local concentrations of ionic species, one or more of the following precipitates may be formed: Li 2 S 8 , Li 2 S 6 , Li 2 S 4 , Li 2 S 2 , and Li 2 S. However, previous experimental and simulation studies indicate that all lithium sulfides except Li 2 S 2 and Li 2 S are substantially soluble in the electrolyte. ,− In addition to precipitation of Li 2 S 2 and Li 2 S, the model also includes the precipitation of Li 2 S 8 and Li 2 S 4 for illustration purposes.…”

Elevated electrochemical performances enabled by a core–shell titanium hydride coated separator in lithium–sulphur batteries

“…So, the lower plateau is more sensitive to the discharge rates and degraded faster at high rates. If the initial potential of the lower plateau, also called the voltage dip or the oversaturation point by some researchers, 47,48 dropped below 1.7 V, the lower plateau would disappear, leading to the sharply dropped capacity, as shown in Fig. 7c.…”

Research Progress on Improving the Sulfur Conversion Efficiency on the Sulfur Cathode Side in Lithium–Sulfur Batteries

“…The dispersion of deposited Li 2 S can be observed with the assistance of a surface morphology analysis, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), or theoretical calculations . The sites that have much higher electrochemical selectivity and activity prefer the precipitation of the end product with the discharge process, such as the defected site or the edge plane on the 2D materials …”

“…232,268 Polar materials with a conductive substrate can synergistically promote the deposition of Li 2 S as well as provide close contact and a delicate construction among the interfaces (Figure 9a). The dispersion of deposited Li 2 S can be observed with the assistance of a surface morphology analysis, such as scanning electron microscopy (SEM), 269 atomic force microscopy (AFM), or theoretical calculations. 270 The sites that have much higher electrochemical selectivity and activity prefer the precipitation of the end product with the discharge process, such as the defected site 159 or the edge plane on the 2D materials.…”