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

Abstract

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 1000 cycles and the degradation rate is only 0.034 % per cycle at 5.0C upon 1000 cycles. Meanwhile, it achieves a high capacity of 550 mAh g−1 after 200 cycles 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.