Multifunctional Electrocatalytic Cathodes Derived from Metal-Organic Frameworks for Advanced Lithium-Sulfur Batteries.

The rechargeable lithium-sulfur (Li-S) battery is a promising candidate for the next generation of energy storage technology, owing to the high theoretical capacity, high specific energy density, and low cost of electrode materials. The main drawbacks in the development of long-life Li-S batteries are capacity fading and the sluggish kinetics at the cathode caused by the polysulfides shuttle. These limitations are addressed through the design of novel nanocages containing cobalt phosphide (CoP) nanoparticles embedded in highly porous nitrogen-doped carbon (CoP-N-GC) by thermal annealing of ZIF-67 in a reductive atmosphere followed by a phosphidation step using sodium hypophosphite. The CoP nanoparticles, with large surface area and uniform homogeneous distribution within the N-doped nanocage graphitic carbon, act as electrocatalysts to suppress the shuttle of soluble polysulfides through strong chemical interactions and catalyze the sulfur redox. As a result, the S@CoP-N-GC electrode delivers an extremely high specific capacity of 1410 mA h g-1 at 0.1 C (1 C=1675 mA g-1 ) with an excellent coulombic efficiency of 99.7 %. Moreover, capacity retention from 864 to 678 mA h g-1 is obtained after 460 cycles with a very low decay rate of 0.046 % per cycle at 0.5 C. Therefore, the combination of the CoP catalyst and polar conductive porous carbon effectively stabilizes the sulfur cathode, enhancing the electrochemical performance and stability of the battery.

Substituent Effects and the Energetics of Noncatalyzed Aryl Halide Aminations: A Theoretical Investigation.

We report the influence of substituents and physical conditions on activation energies for the noncatalyzed amination (C-N cross-coupling reactions) of aryl halides. We uncover a significant correlation between the barrier heights of the C-N bond formation and Hammett σ parameters-a formal measure of the electron-withdrawing or -donating ability of substituents on the aryl halides. Our results indicate that such correlations are useful predictive tools for the amination of aryl halides over a wide range of substituent types. From 54 cases studied (six substituents occupying specific positions relative to halogen atoms), the 2-COOHPhI + NH2 n Pr amination reaction is predicted to possess the lowest noncatalyzed activation free energy (135.6 kJ mol-1) using the B3LYP method. The lower barriers for the 2-COOHPhX (for X = Cl, Br, and I) compounds are shown to originate from collusion between steric and electronic effects-specifically, the momentary formation of a hydrogen bond between an oxygen site on the ortho-COOH and the lone pair of the entering amine. Internal reaction coordinate (IRC) path calculations afforded us these and other key insights into the nature of the reactions. The control exerted by substituents on the arrangement of the transition state structure, as well as the sensitivity of the reaction barriers to temperature and solvent polarity, are discussed. These results offer new perspectives from which to assess the nature of the C-N bond formation and suggest new avenues for future exploration, especially in progress toward the metal-free amination of aryl compounds.