In this work, we demonstrate cerium (Ce) based metal−organic frameworks (MOFs) combined with carbon nanotubes (CNTs) to form Ce-MOF/CNT composites as separator coating material in the Li−S battery system, which showed excellent electrochemical performance even under high sulfur loading and much better capacity retention. At the sulfur loading of 2.5 mg/ cm 2 , initial specific capacity of 1021.8 mAh/g at 1C was achieved in the Li−S cell with the Ce-MOF-2/CNT coated separator, which was slowly reduced to 838.8 mAh/g after 800 cycles with a decay rate of only 0.022% and the Coulombic efficiency of nearly 100%. Even at a higher sulfur loading of 6 mg/cm 2 , the cell based on Ce-MOF-2/CNT separator coating still exhibited excellent performance with initial specific capacity of 993.5 mAh/g at 0.1 C. After 200 cycles, the specific capacity of 886.4 mAh/g was still retained. The excellent performance is ascribed to the efficient adsorption of the Ce-MOF-2 to Li 2 S 6 species and its catalytic effect toward conversion of polysulfides, resulting in suppressed shuttle effect of polysulfides in the Li−S batteries.

A lithium-sulfur (Li-S) battery is regarded as the most promising candidate for next generation energy storage systems, because of its high theoretical specific capacity (1675 mA h g) and specific energy (2500 W h kg), as well as the abundance, low cost and environmental benignity of sulfur. However, the soluble polysulfides LiS (4 ≤ x ≤ 8) produced during the discharge process can cause the so-called "shuttle effect" and lead to low coulombic efficiency and rapid capacity fading of the batteries, which seriously restrict their practical application. Using porous materials as hosts to immobilize the polysulfides is proved to be an effective strategy. In this article, a dual functional cage-like metal-organic framework (Cu-MOF), Cu-TDPAT, combining the Lewis basic sites from the nitrogen atoms of the ligand HTDPAT with the Lewis acidic sites from Cu(ii) open metal sites (OMSs), was employed as the sulfur host in a Li-S battery for lithium ions and polysulfide anions (S). In addition, the size of nano-Cu-TDPAT was also optimized by microwave synthesis to reduce the internal resistance of the batteries. The electrochemical test results showed that the optimized Cu-TDPAT material can efficiently confine the polysulfides within the MOF, and the resultant porous S@Cu-TDPAT composite cathode material with the size of 100 nm shows good cycling performance with a reversible capacity of about 745 mA h g at 1C (1C = 1675 mA g) after 500 cycles, to the best of our knowledge, which is higher than those of all reported S@MOF cathode materials. The DFT calculation and XPS data indicate that the good cycling performance mainly results from the dual functional binding sites (that is, Lewis acid and base sites) in nanoporous Cu-TDPAT, providing the comprehensive and robust interaction with the polysulfides to overcome their dissolution and diffusion into the electrolyte. Clearly, our work provides a good example of designing MOFs with suitable interaction sites for the polysulfides to achieve S@MOF cathode materials with excellent cycling performance by multiple synergistic effects between nanoporous host MOFs and the polysulfides.

Covalent organic frameworks (COFs) have been proven to be an efficient host material for trapping sulfur in lithium–sulfur batteries. However, the potential application as the coating layer of the separator has not been well-addressed yet. Here, we synthesized an imine-based COF, DMTA-COF, which exhibited an AB-stacking mode and had a pore size of 0.56 nm. For the first time, we applied this nanoporous COF as the coating layer of the ceramic separator; the corresponding cell gave an initial discharge capacity up to 1415 mA h/g, and 1000 mA h/g remained after 100 cycles at 0.5 C. The performance is much better than that of the pristine ceramic separator and the super-P coated ceramic separator, demonstrating that the nanopores in the composite separator can effectively block the polysulfide across the separator, thus reducing the “shuttle” effect and the loss of active materials. This study provides a new design strategy for separators in lithium–sulfur batteries.

The two key problems for the industrialization of Li−S batteries are the dendrite growth of lithium anode and the shuttle effect of lithium polysulfides (LiPSs). Herein, we report the Janus separator prepared by coating anionic Bio-MOF-100 and its derived single-atom zinc catalyst on each side of the Celgard separator. The anionic metal−organic framework (MOF) coating induces the uniform and rapid deposition of lithium ions, while its derived single-atom zinc catalyzes the rapid transformation of LiPSs, thus inhibiting the lithium dendrite and shuttle effect simultaneously. Consequently, compared with other reported Li−S batteries assembled with single-atomic catalysts as separator coatings, our SAZ-AF Janus separator showed stable cyclic performance (0.05% capacity decay rate at 2 C with 1000 cycles), outstanding performance in protecting lithium anode (steady cycle 2800 h at 10 mAh cm −2 ), and equally excellent cycling performance in Li−SeS 2 or Li−Se batteries. Our work provides an effective separator coating design to inhibit shuttle effect and lithium dendrite.