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 (Li2Sx) 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 Li2Sx. 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. (C) 2018 Elsevier B.V. All rights reserved.