Synergistic Effect of Bimetallic MOF Modified Separator for Long Cycle Life Lithium-Sulfur Batteries

Severe polysulfide dissolution and shuttling are the main challenges that plague the long cycle life and capacity retention of lithium-sulfur (Li-S) batteries. To address these challenges, efficient separators are designed and modified with a dual functional bimetallic metal-organic framework (MOF). Flower-shaped bimetallic MOFs (i.e., Fe-ZIF-8) with nanostructured pores are synthesized at 35 degrees C in water by introducing dopant metal sites (Fe), which are then coated on a polypropylene (PP) separator to provide selective channels, thereby effectively inhibiting the migration of lithium polysulfides while allowing homogeneous transport of Li-ions. The active sites of the Fe-ZIF-8 enable electrocatalytic conversion, facilitating the conversion of lithium polysulfides. Moreover, the developed separator can prevent dendrite formation due to the uniform pore size and hence the even Li-ion transport and deposition. A coin cell using a Fe-ZIF-8/PP separator with S-loaded carbon cathode displayed a high cycle life of 1000 cycles with a high initial discharge capacity of 863 mAh g-1 at 0.5 C and a discharge capacity of 746 mAh g-1 at a high rate of 3 C. Promising specific capacity has been documented even under high sulfur loading of 5.0 mg cm-2 and electrolyte to the sulfur ratio (E/S) of 5 mu L mg-1. A multifunctional bimetallic MOF-based separator is designed specifically for lithium-sulfur batteries that can selectively block and convert polysulfides while providing even transport of lithium ions. Remarkably higher catalytic activity is found for the conversion of polysulfides by the Fe-doped MOF (ZIF-8) compared to the parent ZIF-8. Meanwhile, the incorporation of Fe (II) centers into the ZIF framework dramatically improves the specific capacity and rate capability.image