Entrapment of polysulfides by Al2O3 modified separator for high energy Li–S redox flow batteries

Although the lithium–sulfur redox flow batteries (Li–S RFBs) are promising candidates for large-scale energy storage application because of outstanding solubility of long-chain polysulfides and low cost of sulfur. Their implementation has been impeded by multiple challenges, especially the dissolution of intermediate lithium polysulfide (Li₂S_X) species into the electrolyte. Here, Al₂O₃ particles decorated polypropylene separator (PP-Al₂O₃) was investigated. The results show that the thermal stability and the electrolyte wettability of the PP-Al₂O₃ separator are improved obviously. When the PP-Al₂O₃ separator is used for Li–S RFB, the cyclic stability and rate capability of the battery are enhanced. The PP-Al₂O₃ separators are effective in improving the initial discharge capacity of Li–S RFBs from 27.5 to 91.5 mAh g⁻¹ at 6.25 mA cm⁻². The reason could be ascribed to that the polar Al₂O₃ coating not only alleviates the shuttle effect by chemical interaction and physical barrier, but also facilitates Li-ion migration by favorable electrolyte wettability. In addition, theoretical calculations reveal that chemical bonds are formed between Al₂O₃ and Li₂S_X. This work provides the rational design strategy for functional separators at cell scale to effective utilizing of active sulfur and retarding of polysulfides, which offers the possibility of high energy density Li–S RFBs with long cycling life.