Routes to Electrochemically Stable Sulfur Cathodes for Practical Li-S Batteries

Lithium-sulfur (Li-S) batteries have been investigated intensively as a post-Li-ion technology in the past decade; however, their realizable energy density and cycling performance are still far from satisfaction for commercial development. Although many extremely high-capacity and cycle-stable S cathodes and Li anodes are reported in literature, their use for practical Li-S batteries remains challenging due to the huge gap between the laboratory research and industrial applications. The laboratory research is usually conducted by use of a thin-film electrode with a low sulfur loading and high electrolyte/sulfur (E/S) ratios, while the practical batteries require a thick/high sulfur loading cathode and a low E/S ratio to achieve a desired energy density. To make this clear, the inherent problems of dissolution/deposition mechanism of conventional sulfur cathodes are analyzed from the viewpoint of polarization theory of porous electrode after a brief overview of the recent research progress on sulfur cathodes of Li-S batteries, and the possible strategies for building an electrochemically stable sulfur cathode are discussed for practically viable Li-S batteries from the authors' own understandings. The "quasi-solid-state reaction" mechanism can enable the redox reaction of sulfur to proceed in a solid-phase conversion manner in liquid electrolytes without the generation and dissolution of polysulfide intermediates at cycling, and therefore is a promising approach to develop high-capacity and cycle-stable sulfur cathodes for practical lithium-sulfur (Li-S) batteries.image