Capacity Decay Mechanism of Lithium-Sulfur Batteries Using a Microporous Activated Carbon-Sulfur Composite as the Cathode Material

Lithium-sulfur batteries,which are expected tofunctionas next-generation secondary batteries, have great advantages in termsof cost and resource abundance but suffer from performance issuesowing to their cycle stability. We investigated the electrochemicalproperties of a microporous activated carbon-sulfur (AZC-S)composite as an active material for a lithium-sulfur batteryelectrode. AZC-S exhibits a near-theoretical capacity of sulfur(1672 mAh g(-1)) during the first discharge process.The cell capacity decreased as the polarization increased, and thedischarge capacity was approximately 550 mAh g(-1) after 50 cycles. According to the thermal analysis of the AZC-Selectrode after 50 cycles, a portion of sulfur was found to be convertedinto electrochemically inactive species that are thermally stableup to 300 degrees C. A cross-sectional scanning electron microscopyimage of the electrode showed that the aluminum foil, as a currentcollector, was eroded into a hemispherical shape after 50 cycles.The energy-dispersive X-ray spectroscopy analysis revealed that themajor elements of the hemisphere were sulfur and oxygen. These resultssuggest that the accumulation of inactive sulfur species within theelectrode leads to cell degradation.