Ultra-lightweight Ti3C2T MXene modified separator for Li–S batteries: Thickness regulation enabled polysulfide inhibition and lithium ion transportation

The practical application of lithium–sulfur (Li–S) batteries is limited by the easy dissolution of polysulfides in the electrolyte, resulting in the lithium polysulfide (LPS) shuttle effect. Several two-dimensional (2D) materials with abundant active binding sites and high surface-to-volume ratios have been developed to prepare functional separators that suppress the diffusion of polysulfides. However, the influence of modified layer thickness on Li+ transport has not been considered. Herein, we synthesized individual and multilayered 2D Ti3C2Tx MXene nanosheets and used them to fabricate a series of Ti3C2Tx-PP modified separators. The separators had mass loadings ranging from 0.16 to 0.016 mg cm−2, which is the lowest value reported for 2D materials to the best of our knowledge. The corresponding reductions in thickness ranged from 1.2 µm to 100 nm. LPS shuttling was effectively suppressed, even at the lowest mass loading of 0.016 mg cm−2. Suppression was due to the strong interaction between LPS intermediates and Ti atoms and hydroxyl functional groups on the separator surface. The lithium-ion diffusion coefficient increased with the reduction of Ti3C2Tx layers on the separator. Superior cycling stability and rate performance were attained when the separator with a Ti3C2Tx-PP mass loading of 0.016 mg cm−2 was incorporated into a Li–S battery. Carbon nanotubes (CNTs) were introduced into the separators to further improve the electrical and Li+ ionic conductivity in the cross-plane direction of the 2D Ti3C2Tx layers. With the ultra-lightweight Ti3C2Tx/CNTs modified PP separator, the cell maintained a capacity of 640 mAh g−1 after 200 cycles at 1 C with a capacity decay of only 0.079% per cycle.