Structure-defined viologen-polyoxometalate modified separator dominating endurable Li-S batteries by a synergistic adsorption-electrocatalysis mechanism

The crystalline materials can provide atomically accurate model for comprehending the structure–property-function relationships, which would be an innovative strategy to address the inherent issues in Li-S batteries. Herein, we synthesize a new crystalline compound of (HAV)[PMo12O40]·6H2O (marked as {AV-PMo12}) using Keggin-type H3PMo12O40·3H2O (PMo12) and aminopropyl viologen (AV) as precursors, which are connected by intermolecular hydrogen bonding and electrostatic attraction. When {AV-PMo12} is utilized as a modified material for separator, PMo12captures lithium polysulfides (LiPSs) by forming Li-O bonds and AV interacts with polysulfides via electrostatic attraction. More significantly, based on the precise and stable structure of {AV-PMo12}, this work innovatively utilizes in-situ Raman spectra and ex-situ XPS to intuitively reveal the catalytic mechanism of {AV-PMo12}. Specifically, {AV-PMo12} possesses bidirectional catalytic activity during charge–discharge cycles, accompanied by the stable conversion between reduced and oxidized states of PMo12. AV preferentially captures electrons and transfers them to PMo12 through hydrogen bond, which improves electron-obtaining ability of PMo12, thus promoting the catalytic activity of PMo12 for LiPSs conversion. PMo12 facilitates the desolvation process of Li+, which accelerates high-flux lithium ion diffusion and achieves uniform lithium deposition. As expected, Li-S cell using {AV-PMo12} modified separator achieves decent reversibility of 469 mAh g−1 at 2.0C after 1000cycles and the degradation rate is only 0.034 % per cycle at 5.0C upon 1000cycles. Meanwhile, it achieves a high capacity of 550 mAh g−1 after 200cycles at 0.2C under a sulfur loading of 5.1 mg cm−2 with a low electrolyte/sulfur ratio of 5 μL mg−1. This work offers a comprehensive analysis of catalytic mechanism of viologen-polyoxometalate based functional material at the molecular level to boost the high-performance Li-S batteries.