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

Abstract 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 (Li 2 S/Li 2 S 2 ) 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.