Lithium-sulfur (Li-S) batteries promise extraordinary theoretical capacities, combined with the low cost, and sustainability of sulfur. They have one of the highest potentials amongst post-lithium-ion batteries. Solid-state S/Li₂S conversion using carbonate electrolytes and microporous carbons could solve the cycle life issue of Li-S batteries by forming a cathode-electrolyte interphase (CEI) and circumventing the dissolution of polysulfides. However, major progress in improving energy densities, sulfur mass loadings and rate performance is necessary.

In this study, we aim to identify the capacity and rate-limiting processes during solid-state S/Li₂S conversion in the confined geometry of microporous carbons. We use a broad range of experimental techniques, including operando small angle neutron scattering (SANS), operando X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and galvanostatic cycling. Operando SANS and XRD confirm the CEI formation in the nanopores during the first discharge and solid-state conversion during further cycles. EIS during galvanostatic intermittent titration technique (GITT) measurements monitors the CEI formation, Li ion transport and interfacial charge transfer during galvanostatic cycling. Systematic parameter and materials variations during galvanostatic cycling further demonstrates that next to Li ion diffusion in and out of the carbon particles, charge transfer across the S/Li₂S-C, is one of the deciding factors. Our results indicate that the poor interfacial charge transfer during charging is the rate/capacity limiting factor in Li-S batteries with microporous carbons and carbonate electrolytes.