Ultra-thin 2D MoO₂ nanosheets coupled with CNTs as efficient separator coating materials to promote the catalytic conversion of lithium polysulfides in advanced lithium-sulfur batteries

A severe shuttle effect and the slow kinetics of lithium polysulfide (LiPS) conversion are two major obstacles to the practical use of lithium sulfur batteries. Ultra-thin 2D MoO₂ nanosheets (MoO₂ NSs) have been synthesized by chemical vapor deposition and then mixed with carbon nanotubes (CNTs) for use as coating materials of the Celgard 2400 polypropylene separator to solve these problems. The 2D character of MoO₂ NSs produced high surface/volume ratios and abundant active binding sites for anchoring LiPSs. In addition, the partial reduction of MoO₂ NSs in a H₂/Ar mixture introduced oxygen vacancies in their surface, which acted as catalytic sites for LiPS conversion, while the CNT network ensured rapid electron transfer for LiPS conversion reactions. Symmetric dummy cell tests showed that a 30wt%MoO₂/CNT coated separator reduced the energy barrier for Li₂S nucleation, and first-principles calculations verified its strong binding energy to entrap LiPSs and increase Li₂S precipitation. Because of these features, a cell with a 30wt%MoO₂/CNT coated separator had an improved specific capacity of 738 mAh·g⁻¹ at 1 C with a slow decay rate of 0.053% for 800 cycles.