A viologen auxiliary strategy to accelerate the catalytic efficiency of polyoxometalates for high-rate lithium–sulfur batteries

For lithium–sulfur (Li–S) batteries, the serious obstacles for sulfur cathode are the polysulfides shuttle effect and slow redox kinetics during cycles. The functionalizing separator modified by catalytic material can not only effectively prevent polysulfides from shuttling between the cathode and anode but also extremely facilitate polysulfides conversion reaction. Herein, we propose an auxiliary catalytic strategy by aminopropyl viologen-K3[PW12O40] (AV-PW12)composite as the separator modification material in Li–S batteries. AV as a good electron receiver can quickly obtain electrons and transfer them to PW12 through hydrogen bonding (N–H⋯O), which is conducive to enhancing the conductivity of PW12 and further promoting the catalytic reaction kinetics of PW12. The high-efficiency catalytic activity of AV-PW12 plays a pivotal role in improving the rate performance of Li–S batteries. Moreover, AV with certain capture capacity for polysulfides by electrostatic attraction assists the adsorption of PW12, improving the probability of lithium polysulfides being catalyzed by PW12. Therefore, when applied to modified separator material, the AV-PW12-based cell achieves the high initial discharge capacity of 1596 mAh/g at 0.5C and runs for 1000 cycles at 10 C within a low capacity decay of 0.041%. Even with a low electrolyte/sulfur ratio of 5 μL/mg, the reversible capacity can still retain 561.6 mAh/g at 0.5 C upon 200 cycles under a high sulfur loading of 4.5 mg/cm2. This work demonstrates an innovative method for the design of polyoxometalates-based composite at the molecular level, which gives more possibilities for the application of polyoxometalates as catalytic materials in Li–S batteries.