context: To meet the ever-increasing energy storage demand in electric vehicles and stationary storage systems, a substantial increase in the energy densities of the current Li-ion batteries (LIBs) is essential. This motivates researchers to develop alternate chemistries superior to those based on conventional LiFePO4 and LiCoO2 cathode materials. In this regard, Li–S batteries stand out among their peers because of their high energy capacity. Li–S batteries operate via a conversion based reduction reaction of sulfur (S) with Li (16Li+ + S8 + 16e− ↔ 8Li2S), leading to an exceptionally high theoretical energy density of ∼2500 W h kg−1—five times more than that of conventional LIBs (387 W h kg−1 for LiCoO2/graphite batteries). Additionally, the abundant availability and environment friendly nature of S makes it an attractive cathode material. Despite enormous potential, the practical application of Li–S batteries with liquid electrolyte systems is hindered by issues such as large volumetric changes of S and Li2S (∼80%), and the insulating nature of S and Li2Sx. The reduction of S to Li2S during the discharge cycle follows a multistep reduction reaction process, including the formation of highly soluble long-chain polysulfides (Li2Sn, 4 ≤ n ≤ 8), which are produced as intermediates of this process. Such polysulfides shuttle towards the Li anode which leads to chemical short-circuits and an unstable SEI formation on the Li anode. These polysulfides dry out the liquid electrolyte during long-term cycling which severely affects the performance of the cell. The situation is further complicated by the high loading of S on the cathode (>70% S loading, >3 mgsulfur cm−2) during cycling, which intensifies polysulfide dissolution and volume changes, and enforces structural and morphological variations. Overall, such characteristics of the Li–S system result in self-discharge phenomena, poor capacity retention and low coulombic efficiency. On the other hand, the instability of Li anodes with organic liquid electrolytes is also well-known. Li forms mossy or needle like dendritic structures upon cycling which can potentially pierce the conventional polypropylene separators, resulting in internal short circuits of the cells. To solve these problems, various strategies have been explored for both the S and the Li electrodes. For example, synthesizing S composites with carbonaceous materials and/or surface coating the cathodes with conducting polymers have helped in trapping polysulfides. Tuning the properties of organic electrolytes using different additives has been beneficial too. On the Li side, efforts include ex situ coating of Li with polymer- and carbon-based materials or formation of artificial solid–electrolyte interface (SEI) layers to protect the Li electrode from reacting with the organic liquid electrolyte.
answer: Li 