Probing the “Universal” Amorphization of Crystalline Sulfur in its Mixture with Ultrahigh Surface Area Porous Carbon

We discuss here a fundamental observation regarding the time-dependent amorphization of sulfur in contact with ultrahigh surface area porous carbon. The simple sulfur–carbon mixture discussed here effectively resembles a sulfur-cathode in metal–sulfur batteries. Systematic time dependent powder X-ray diffraction and Raman spectroscopy clearly reveal a crystalline to amorphous phase transition, which has been recently referred as a liquid spillover effect. The occurrence of the structural phase transformation only above a certain carbon surface area (≥2000–3000 m²/g) in the binary sulfur–carbon mixture is equivalent to a “universal” phenomenon, having deep implications in metal–sulfur battery electrochemistry. As revealed by thermal and electron paramagnetic resonance measurements, the (nonpolar) interactions between sulfur and carbon induces a transformation from crystalline orthorhombic to a highly dispersed amorphous phase comprising of small chain sulfur radicals. Amorphization leads to higher degree of sulfur-mass utilization and effective polysulfide management resulting in superior specific capacity of the sulfur-cathode in a Li–S battery.