Li–S cells typically employ ether-based electrolytes, in which lithium salts (LiTFSI) are dissolved in a combination of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) solvents
[27]. Lithium nitride (LiNO
3) is a widely adopted additive for these liquid electrolytes (
Figure 3) owing to its ability to effectively suppress the decomposition of the electrolytes and induce the polymerization of DOL to form a polymeric layer to passivate the lithium metal anode
[28]. LiNO
3 also aids in hindering the negative effects of dissolved polysulfide species on the Li metal anode by oxidizing the polysulfides to Li
xSO
y. SEI layers formed in the presence of LiNO
3 have been shown to have a more optimal composition for Li–S pouch cell performance. However, the exact composition of this SEI is yet to be identified, it is known to be a complex mixture of organic (e.g., ROLi, ROCOLi, RCOOLi, where R is the organic group) and inorganic compounds (e.g., LiF, Li
2O, Li
3N, Li
2S, LiS
xO
y, Li
xNO
y)
[21][29][30][31].
Table 1 presents a performance comparison of Li–S pouch cells with and without LiNO
3 additives in their electrolytes using various cathode architectures. The SEI layer formed with a LiNO
3 additive effectively suppresses parasitic reactions of the polysulfides with lithium metal to prevent the formation of Li dendrites in the Li–S pouch cells during operation. Long-term galvanostatic cycling tests showed that pouch cells with a LiNO
3 electrolyte additive maintained a stable capacity for up to 100 cycles with a sulfur loading of 6 mg cm
−2 [32]. Other functional additives with a high donor number, such as salts containing the NO
3− anion, have also been explored in Li–S cells to suppress electrolyte decomposition on the Li metal by regulating the Li
+ solvation shell. This regulation enhances the cycling performance of the lean electrolyte Li–S pouch cells. Fluorinated solvents with lower viscosity can form a LiF-rich SEI on the lithium metal. LiF is regarded as a good SEI component for stabilizing the lithium metal surface and allowing Li-ion transfer while suppressing dendrite nucleation. This enhanced LiF-rich SEI is effective in yielding a safe and practical Li–S cell with improved cycle life
[33].
However, these functional additives are constantly consumed at both the anode and the cathode during cycling and cannot maintain a long cycle-life at the pouch cell level. Thus, exploring new electrolyte additives that can decrease parasitic side reactions and enhance the stability of the SEI layer in Li–S cells is a worthwhile endeavor for realizing industrially relevant Li–S pouch cells.