Insight into the Fading Mechanism of the Solid-Conversion Sulfur Cathodes and Designing Long Cycle Lithium–Sulfur Batteries

The cathode electrolyte interface (CEI) formed on the surface of the sulfur cathode plays a vital role in determining whether lithium–sulfur batteries can function through a solid-phase conversion reaction, which can effectively hinder the dissolution of polysulfides. However, there is still a lack of systematical research on the evolution and failure mechanism of the CEI for Li–S batteries. Here, it is found that the integrity of the as-formed CEI is strongly related to the content of sulfur and the amount of electrolyte. When the volume of reduction product (Li₂S/Li₂S₂) exceeds the maximal volume of the carbon host, the as-formed CEI is unable to withstand the volume variation upon repeated lithiation/delithiation. The repeated fracture and repair of CEI unceasingly consumes electrolyte and active materials. Thus, to achieve prolonged cycle stability via solid-phase conversion, the content of sulfur and the interior space of the host should be well matched. Based on the above understanding, the designed sulfur-graphite full cell shows an excellent cyclability over 2000 cycles. This work reveals the failure mechanism of solid-phase conversion reactions in Li–S batteries, and provides some inspiration for designing long-life and high-sulfur-content cathode materials.