Two Birds with One Stone: Ammonium-Induced Carbon Nanotube Structure and Low-Crystalline Cobalt Nanoparticles Enabling High Performance of Lithium-Sulfur Batteries

Abstract

The electrochemical performance of lithium–sulfur batteries (LiSBs) has been hampered by the slow redox kinetics and shuttle effect of lithium polysulfides (LiPSs), which require the rational design and synthesis of highly active electrocatalysts towards this reaction. Herein, worm-like N-doped porous carbon nanotube-supported low-crystalline Co nanoparticles (a-Co-NC@C) were derived from binary Zn–Co ZIF via a two-step thermal annealing method. Initial thermal annealing 950 °C in Ar + H2 atmosphere results in the carbonization of binary Zn–Co ZIF and the formation of high crystalline Co nanoparticles. Thermal annealing in ammonia atmosphere at 350 °C not only results in the reduced crystallinity of cobalt nanoparticles; it also promotes the growth of highly graphitized and heavily N-doped intertwined carbon nanotubes. The enlarged porous carbon nanotube structure offers accommodation for sulfur content, while the doped carbon and Co nanoparticles with reduced crystallinity facilitate the redox kinetics of LiPSs, improving the cycling stability, rate performance and capacity of LiSBs batteries. As a result, the a-Co-NC@C cathode displays a specific capacity of 559 mAh g−1 after 500 cycles at 1 C, and a specific capacity of 572 mAh g−1 at 3 C. It delivers a specific capacity of 579 mAh g−1 at high sulfur loading of a 2.55 mg cm−2 at 1 C after 400 cycles. This work highlights the importance of phase engineering of carbon matrix and transition metal nanoparticles in electrochemical performance of Li-S batteries.