Confinement of polysulfides within bi-functional metal–organic frameworks for high performance lithium–sulfur batteries

Abstract

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.

A Comprehensive Understanding of Lithium–Sulfur Battery Technology

“…MOFs, known as porous coordination polymers (PCPs), are a kind of crystalline porous materials composed of metal ions/clusters nodes and organic ligands linkers in an infinite array configuration . By simply adjusting the metal species, geometry, size, and functionality of the modules, various kinds of MOFs could be realized, nowadays counting more than 20000 type of MOFs .…”

Co−Ni Binary‐Metal Oxide Coated with Porous Carbon Derived from Metal‐Organic Framework as Host of Nano‐Sulfur for Lithium‐Sulfur Batteries

“…[28][29][30][31][32] For example, a Ni-based NiÀ MOF can effectively fix the polysulfide into the cathode structure with physical and chemical interactions synergistic effects, thereby enhancing the cycle performance of NiÀ MOF/S composite. [28][29][30][31][32] For example, a Ni-based NiÀ MOF can effectively fix the polysulfide into the cathode structure with physical and chemical interactions synergistic effects, thereby enhancing the cycle performance of NiÀ MOF/S composite.…”

Metal-Organic Frameworks for Batteries

“…MOFs constructed by open metal sites (OMSs) and/or organic ligands with Lewis basic functional groups are promising host materials. 62,63,[110][111][112] Previous studies have shown that the coordinatively unsaturated OMSs in MOFs, such as HKUST-1, 111 Ni 6 (BTB) 4 (BP) 3 (BTB = benzene-1,3,5-tribenzoate and BP = 4,4 0 -bipyridyl), 62 MOF-525, 63 and Cu-TDPAT (H 6 TDPAT = 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine), 112 can serve as Lewis acidic sites and provide chemical affinities to interact with sulfur and polysulfides. Based on the calculations by Siegel and Park, OMSs are revealed as the dominant adsorption sites for S 8 , Li 2 S 4 , and Li 2 S. 113 In the Li 2 S 4 chain, the sulfur atoms contact the OMSs, while the terminal lithium atoms tend to be adjacent to the organic moieties (e.g., oxygen anions or nitrogen groups in ligands).…”

“…Based on the calculations by Siegel and Park, OMSs are revealed as the dominant adsorption sites for S 8 , Li 2 S 4 , and Li 2 S. 113 In the Li 2 S 4 chain, the sulfur atoms contact the OMSs, while the terminal lithium atoms tend to be adjacent to the organic moieties (e.g., oxygen anions or nitrogen groups in ligands). 112,113 Despite these improvements, the practical utilization of MOFs as sulfur host materials for LSBs is greatly restricted by their poor electrical conductivity, which leads to some severe issues, including slow redox kinetics and limited sulfur utilization, and, thus, a poor rate capability and cycling stability. Combining MOF/sulfur composites with conductive additives, such as carbon materials (e.g., reduced graphene oxide and carbon nanotubes) and polymers (e.g., polypyrrole and poly(3,4-ethylenedioxythiophene)), has shown some possibility in alleviating this issue.…”