Ultrathin Carbon-Shell-Encapsulated Cobalt Nanoparticles with Balanced Activity and Stability for Lithium-Sulfur Batteries.

High-performance metal-based catalysts are pursued to improve the sluggish reaction kinetics in lithium-sulfur batteries. However, it is challenging to achieve high catalytic activity and stability simultaneously due to the inevitable passivation of the highly active metal nanoparticles by lithium polysulfides (LiPSs). Herein, we show a design with well-balanced activity and stability to solve the above problem, that is, the cobalt (Co) nanoparticles (NPs) encapsulated with ultrathin carbon shells prepared by the one-step pyrolysis of ZIF-67. With an ultrathin carbon coating (∼1 nm), the direct exposure of Co NPs to LiPSs is avoided, but it allows the fast electron transfer from the highly active Co NPs to LiPSs for their conversion to the solid products, ensuring the efficient suppression of shuttling in long cycling. As a result, the sulfur cathode with such a catalyst exhibited good cycling stability (0.073% capacity fading over 500 cycles) and high sulfur utilization (638 mAh g -1 after 180 cycles under a high sulfur mass loading of 7.37 mg cm -2 and a low electrolyte/sulfur ratio of 5 μL mg -1 ). This work provides insights into the rational design of a protection layer on a metal-based catalyst to engineer both high catalytic activity and stability toward high-energy and long-life Li-S batteries.