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

The severe shuttle effect and slow kinetics of lithium polysulfide (LiPS) conversion are two major obstacles to the practical application 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) using 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.