Enhancement of long stability of sulfur cathode by encapsulating sulfur into micropores of carbon spheres

TLDR

Understanding how micropores affect electrochemical cycle stability is essential for designing efficient electrode materials. The study aims to enhance the long stability of sulfur cathodes in high‑energy lithium–sulfur batteries by preparing a sulfur–carbon sphere composite. The composite is fabricated by encapsulating sulfur into the micropores of carbon spheres through thermal treatment of a mixture of sublimed sulfur and carbon spheres. The sulfur–carbon sphere composite exhibits highly dispersed sulfur, delivers large reversible capacity and excellent high‑rate discharge, and retains stable performance for 500 cycles at 42 wt % sulfur, demonstrating that confinement within micropores is the key to its long‑term stability.

Abstract

To enhance the long stability of sulfur cathode for a high energy lithium–sulfur battery system, a sulfur–carbon sphere composite was prepared by encapsulating sulfur into micropores of carbon spheres by thermal treatment of a mixture of sublimed sulfur and carbon spheres. The elemental sulfur exists as a highly dispersed state inside the micropores of carbon spheres with a large surface area and a narrow pore distribution, based on the analyses of the X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), thermogravimetry (TG) and local element line-scanning. It is demonstrated from galvanostatic discharge–charge process, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) that the sulfur–carbon sphere composite has a large reversible capacity and an excellent high rate discharge capability as cathode materials. In particular, the sulfur–carbon sphere composite with 42 wt% sulfur presents a long electrochemical stability up to 500 cycles, based on the constrained electrochemical reaction inside the narrow micropores of carbon spheres due to strong adsorption. Therefore, the electrochemical reaction constrained inside the micropores proposed here would be the dominant factor for the enhanced long stability of the sulfur cathode. The knowledge acquired in this study is important not only for the design of efficient new electrode materials, but also for understanding the effect of the micropores on the electrochemical cycle stability.