Ultra-microporous carbons encapsulate **all sulfur molecules for high performance lithium-sulfur battery

Qizhen Zhu, Qian Zhao, Yabin An, Babak Anasori, Haoran Wang, Bin Xu*

Lithium-sulfur (Li-S) batteries are one of the most promising energy storage systems with high energy density. However, they suffer from fast capacity fading due to the shuttle of the dissolved polysulfides. Using **all sulfur molecules as cathodes can avoid the shuttle problem, but the preparation of ultra-microporous carbon to encapsulate is challenging. Here, we report a Li-S battery with an excellent cycling performance by using a unique ultra-microporous carbon (UMC) with a uniform pore size of 0.55 nm. The UMC was synthesized from PVDF via a facile pyrolysis process to only accommodate **all molecules and eliminate large molecules. This S/UMC composite electrode is compatible with the carbonate-based electrolyte used in Li-S batteries. The resulting battery shows a typical discharging profile with only one long potential plateau because the reductions from cyclo to dissolved polysulfides and consequence shuttle problem are avoided. The S/UMC composite retains a capacity of about 852 mAh g^-1 and Coulombic efficiency approaching 100% after 150 cycles at 0.1 C and delivers a long-term cycling at 1 C for 1000 cycles with only around 0.03% capacity loss per cycle. We also tested this S/UMC composite electrode in Na-S systems and observed similar electrochemical behaviour. These results indicate that the S/UMC composite is a promising candidate as an electrode material in Li-S and Na-S batteries and related systems.