Lithium-sulfur (Li-S) batteries, owing to their exceedingly high theoretical energy density (2600 Whkg-1), abundant sulfur reserves in the Earth's crust, cost-effectiveness, and environmental friendliness, stand out as one of the most promising next-generation high-performance energy storage devices. However, commercial development is significantly impeded by challenges such as sluggish kinetics of sulfur redox reactions, the shuttle effect due to the dissolution of polysulfides (LiPSs) in the electrolyte, and complications like corrosion of the lithium metal anode and dendrite growth.
In recent years, our research group made a noteworthy discovery. By integrating Keggin-type polyoxometalate (POM) molecular clusters, renowned for their "electron sponge" properties, as catalysts in lithium-sulfur battery systems, we achieved bidirectional catalytic conversion of S-Li2S species (ACS Nano 2021, 15, 12222-12236. DOI:10.1021/acsnano.1c03852). However, during lithiation, the reduced "heteropoly blue" species formed is soluble in the electrolyte, resulting in the leaching of the POM catalyst from the sulfur cathode. Hence, the main challenge in applying POM catalyst to lithium-sulfur batteries lies in finding a suitable POM structure and effectively anchoring it within the battery system. The construction of supramolecular/POM organic-inorganic hybrid self-assembled frameworks not only effectively immobilizes the POM catalyst but also captures polysulfides and electrolyte molecules through host-guest interactions, thereby suppressing the shuttle effect and enhancing lithium ion transport. Prior to this, our research group successfully utilized cyclodextrins (CD) as functional supramolecular host materials in Li-S battery systems (Energy Storage Mater., 2019, 21, 378-389. DOI: 10.1016/j.ensm.2018.12.002). However, the amalgamation of these two distinct properties - polyoxometalate inorganic molecular clusters (POM) and cyclodextrin (CD) organic supramolecular hosts to construct novel polyoxometalate-cyclodextrin clusters-organic supramolecular framework crystalline materials (POM-CD-COSF) remains an extremely formidable task.
To tackle this challenge, Prof. Guowang Diao and Prof. Lubin Ni from Yangzhou University, in collaboration with Prof. Yongge Wei from Tsinghua University, devised a novel multi-metal-oxide cyclodextrin cluster organic supramolecular framework crystalline material {Zn2W2@2CD} (POM-CD-COSF) based on the Keggin-type polyoxometalate [Zn2(WO2)2(SbW9O33)2]10- and β-CD building units. When the POM-CD-COSF framework crystalline material was prepared as a multifunctionally modified lithium-sulfur battery separator, it displayed exceptional electrochemical performance: high sulfur utilization, prolonged cycle life, and outstanding rate capability. The main experimental work for this article was conducted jointly by three master's students from Yangzhou University: Jie Gu (Master's student, Class of 2018), Hongjie Xu (Master's student, Class of 2017), and Xinyuan Jiang (Master's student, Class of 2020, PhD candidate as of 2022).
By uniformly depositing a {Zn2W2@2CD}-CNT-Super P slurry layer onto the surface of a polypropylene (PP) separator, a super-light modified lithium-sulfur battery separator was fabricated (approximately 0.3 mg cm−2). This innovative strategy of constructing the POM-CD-COSF framework in the lithium-sulfur battery system effectively capitalizes on the dynamic capture of LiPSs and electrolyte molecules by the β-CD supramolecular unit within the framework structure, providing a Li+ transport channel with excellent affinity for the electrolyte. Simultaneously, the bidirectional catalytic action of the POM cluster further accelerates the kinetics of sulfur redox and suppresses LiPS shuttle and lithium dendrite growth. By combining DFT theoretical calculations and XPS characterization, additional studies were carried out on the reactive sites of the {Zn2W2@2CD} catalyst in the adsorption and catalytic conversion of polysulfides. Subsequently, various experimental methods such as in-situ X-ray diffraction (XRD), in-situ UV-Vis absorption spectroscopy in conjunction with electrochemical Li2S nucleation experiments, unveiled the sulfur solid-liquid-solid transformation mechanism of the {Zn2W2@2CD} catalyst in the battery reaction.
In summary, this work not only diversifies the array of POM-CD-COSF framework materials and broadens their application domains, but also presents novel research avenues for the advancement of high-performance commercial Li-S batteries.
The details of the paper： Lubin Ni,* Jie Gu, Xinyuan Jiang, Hongjie Xu, Zhen Wu, Yuchao Wu, Yi Liu, Ju Xie, Yongge Wei* and Guowang Diao*. Polyoxometalate-Cyclodextrin-Based Cluster-Organic Supramolecular Framework for Polysulfide Conversion and Guest–Host Recognition in Lithium-sulfur Batteries. Angew. Chem. Int. Ed. 2023, e202306528.
The link to the paper： https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202306528
