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

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_spH = -28.2 kJ mol_S8 ^-1. With an estimated reaction entropy of Δ_spS = 273.78 J mol_S8 ^-1 K^-1, the Gibbs Free Energy of the spillover process is estimated as Δ_spG = -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.