Further Evidences for the Sulfur Spillover on Carbon: Thermodynamic Aspects and Effects on Li‐S Solid‐State Batteries

ABSTRACT

This study investigates sulfur spillover, the spontaneous and rapid spread of sulfur over carbon surfaces at ambient temperature. The process is studied by small‐angle X‐ray scattering (SAXS), X‐ray radiography, and microcalorimetry from which a number of physicochemical properties are derived. The results confirm that sulfur immediately interacts with porous carbon upon mixing, with most signal changes occurring within a few hours. For the first time, thermodynamic data of the process are determined. Using microcalorimetry, the lower bound of the reaction enthalpy is determined to be Δ _sp H = −28.2 kJ mol _S8 ⁻¹ . With an estimated reaction entropy of Δ _sp S = 273.78 J mol _S8 ⁻¹  K ⁻¹ , the Gibbs Free Energy of the spillover process is estimated as Δ _sp G = −100.23 kJ mol _S8 ⁻¹ , which reduces the theoretical cell voltage of Li‐S batteries by about 64 mV. Furthermore, depending on the configuration, DFT calculations have revealed repulsive and attractive interactions; the latter are caused by defects that eventually result in ring opening and the formation of S─C bonds. This suggests that spillover includes various processes that occur simultaneously. Considering metal‐sulfur batteries, sulfur spillover may be critical for preparing and cycling sulfur‐carbon composite cathodes, which is demonstrated for a Li–S solid‐state battery.