Ultra-microporous carbons encapsulate small sulfur molecules for high performance lithium-sulfur battery
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 small sulfur molecules (S 2–4 ) as cathodes can avoid the shuttle problem, but the preparation of ultra-microporous carbon to encapsulate S 2–4 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 small S 2–4 molecules and eliminate large S 8 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-S 8 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.
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