Efficient Encapsulation of Small S2-4 Molecules in MOF-Derived Flowerlike Nitrogen-Doped Microporous Carbon Nanosheets for High-Performance Li-S Batteries.

Lithium-sulfur (Li-S) battery is regarded as one of the most promising next-generation efficient energy storage systems because of its ultrahigh theoretical capacity of 1675 mAh/g and energy density of 2600 Wh/kg accompanied by the environmental benignity and abundance from natural sulfur. However, the insulating nature of sulfur and the dissolution of the polysulfides Li2S n (4 ≤ n ≤ 8) seriously restrict its practical application. The metastable small sulfur molecules (S2-4) stored in microporous carbon (pore size of <0.6 nm) as the active materials can avoid the production of the soluble polysulfide and solve the shuttle effect thoroughly. In addition, the conductivity of sulfur can be also improved. However, the preparation of microporous carbon materials with reasonable pore size and unique morphology for efficiently encapsulating S2-4 is still challenging. Herein, three flowerlike microporous nitrogen-doped carbon nanosheets with the pore size of <0.6 nm (namely, FMNCN-800, -900, and -1000) as the cathode materials in Li-S batteries were obtained from temperature-dependent carbonization of the metal-organic framework (MOF), Zn-TDPAT, which was from the simply reflux reaction of N-rich ligand H6TDPAT with Zn(II) salt. Our study showed that the FMNCN-900 from carbonization of Zn-TDPAT at 900 °C has suitable pore volume and nitrogen content, accommodating small S2-4 molecules in its micropores with the mass uptake of about 45%. Meanwhile, the appropriate amount of the nitrogen doping and the unique nanostructure of the flowerlike carbon nanosheet in the FMNCN-900 can effectively support its fast electronic transmission and lithium-ion conduction. The resulting S@FMNCN-900 composite cathode material presents the excellent electrochemical property in the Li-S battery (here the carbonate as electrolytes) with a reversible capacity of about 1220 mAh/g at 0.1C after 200 cycles and even 727 mAh/g at 2C after the long-term cycle of 1000 with only around 0.02% capacity loss per cycle. Obviously, the results indicate that the delicate construction of MOF-derived nitrogen-doped microporous carbon nanosheet is a promising strategy to develop novel electrode material for high-performing Li-S batteries.

Confinement of polysulfides within bi-functional metal-organic frameworks for high performance lithium-sulfur 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-1) and specific energy (2500 W h kg-1), as well as the abundance, low cost and environmental benignity of sulfur. However, the soluble polysulfides Li2Sx (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 H6TDPAT 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 (Sx2-). 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-1 at 1C (1C = 1675 mA g-1) 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.

Sulfate ion (SO4(2-)) release from old and new cation exchange resins used in condensate polishing systems for power plants.

In this study, a dynamic cycle test, a static immersion method and a pyrolysis experiment were combined to examine the characteristics of SO4(2-) released from several new and old cation exchange resins used in condensate polishing systems for power plants. The results show that the quantity and velocity of SO4(2-) released from new and old resins tend to balance in a short time during the dynamic cycle experiment. SO4(2-) is released by 1500H (monosphere super gel type cation exchange resins) and 001 × 7 (gel type cation exchange resins) new and old cation exchange resins, the quantity of which increases according to immersion time. In the pyrolysis experiment, the quantity of SO4(2-) released from resins increases and the pH of the pyrolysis solution transforms from alkaline to acidic with an increase in temperature.