Functionalized bacterial cellulose as a separator to address polysulfides shuttling in lithium–sulfur batteries

In lithium–sulfur batteries (LSBs), soluble long-chain polysulfide intermediates can easily shuttle between the cathode and the anode, causing rapid performance degradation. Although significant progress, through rational cathode structure and composition design, has been made to solve the polysulfide shuttling problem, this challenging issue still exists. Considering the function of a separator in a cell is to isolate the cathode and anode materials, the transport properties of species across the separator should be investigated. Using bacterial cellulose (BC) as an example of a functional separator, we hypothesize that grafting anionic function groups on the cellulose chains could create an energy barrier that will block the diffusion of polysulfides across the separator. In our study, BC is functionalized by oxidizing hydroxyl groups on cellulose chains into carboxylate groups. Physicochemical and electrochemical studies confirm polysulfide shuttling is effectively suppressed. As a result, functionalized BC separator equipped LSB cells with a sulfur load of 4 mg/cm² delivers ~1,300 mAh/g of specific capacity at 0.1°C, which can be maintained after 100 cycles above 1,000 mAh/g at 0.3°C, demonstrating its superior performance over commercial polyolefin-based separators.