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

Abstract

Lithium–sulfur batteries, which are expected to function as next-generation secondary batteries, have great advantages in terms of cost and resource abundance but suffer from performance issues owing to their cycle stability. We investigated the electrochemical properties of a microporous activated carbon–sulfur (AZC–S) composite as an active material for a lithium–sulfur battery electrode. 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 the discharge capacity was approximately 550 mAh g–1 after 50 cycles. According to the thermal analysis of the AZC–S electrode after 50 cycles, a portion of sulfur was found to be converted into electrochemically inactive species that are thermally stable up to 300 °C. A cross-sectional scanning electron microscopy image of the electrode showed that the aluminum foil, as a current collector, was eroded into a hemispherical shape after 50 cycles. The energy-dispersive X-ray spectroscopy analysis revealed that the major elements of the hemisphere were sulfur and oxygen. These results suggest that the accumulation of inactive sulfur species within the electrode leads to cell degradation.