Rational design of exfoliated 1T MoS2@CNT-based bifunctional separators for lithium sulfur batteries

Abstract

Lithium-sulfur (Li-S) batteries are experiencing a design shift from a closed structure to an open structure to further improve their performance, expanding the design realm from the development of nanostructured materials for the cathode to the production of functional separators. Rational guidelines for preparing a bifunctional separator with exfoliated MoS₂ and CNTs are suggested to deal with two conflicting issues: guaranteeing the electron pathway while strongly trapping polysulfide species. In addition, various exfoliation methods ranging from mechanical to chemical were investigated to identify an adequate method for preparing exfoliated MoS₂ based-bifunctional separators. The electrochemical exfoliation method was found to be effective in not only exfoliating high quality MoS₂ in terms of the lateral size and number of layers, but also providing a favorable MoS₂ phase, 1T metallic MoS₂. A bifunctional separator of 1T exfoliated MoS₂@CNT in a tandem configuration (layer-by-layer structure)-coated Celgard rather than a hetero-configuration delivered an excellent electrochemical performance of ∼670 mA h g⁻¹ after 500 cycles at a high current density of 1C. In addition, the separator was highly effective in trapping polysulfide species and facilitating electron transfer to the irreversible discharge products. The rational guidelines suggested in this study will be extended to other two-dimensional transition-metal dichalcogenides, and applied to the development of other functional membranes.