Lithium-sulfur (Li-S) batteries are increasingly pursued as an alternative to Li-ion batteries due to their potential for reduced environmental impact. The combination of abundant, low-cost sulfur and its high theoretical capacity positions it as a promising candidate. Typically, Li-S batteries employ a highly porous carbon-sulfur cathode with an organic electrolyte and a lithium metal anode. This setup facilitates the reversible cycling of sulfur to lithium sulfide (S/Li₂S) via intermediate polysulfides, Li₂S_x (where 2 < x < 8). However, several challenges hinder the practical realization of Li-S cells, including poor S/Li₂S mass loadings, rapid capacity fading, low rate capabilities, and the irreversible reactions of polysulfides at the anode. These issues largely stem from an incomplete understanding of the conversion mechanism.

While the formation and dissolution mechanisms of solid Li₂S are subjects of ongoing debate, numerous studies suggest that Li₂S forms via direct electroreduction of Li₂S₂ or longer-chain polysulfides at the carbon-electrolyte interface. The observation that Li₂S deposits are tens to hundreds of nanometers in size and porous indicates a solution-mediated formation process, potentially through the disproportionation of dissolved polysulfides or direct electroreduction of molecular Li₂S₂. Achieving a comprehensive understanding of Li₂S formation mechanisms necessitates a detailed chemical and structural analysis at both atomic and nanometer scales [1], [2].

To extend the practical cycle life of Li-S batteries, integrating sulfur-infused microporous carbon with carbonate-based electrolytes [3] has shown promise. This approach, featuring the formation of a protective cathode-electrolyte interphase (CEI), helps mitigate polysulfide dissolution and reduce capacity fading. However, the mechanism, the factors limiting capacity, rate, and sulfur loadings are not fully understood.

In this context, we present an investigation aimed at deepening the understanding of solid-state sulfur conversion in classical Li-S cell and those with confined spaces. A variety of structure-sensitive and electrochemical techniques have been employed. These include electrochemical impedance spectroscopy (EIS), galvanostatic charge/discharge testing, operando X-ray diffraction, spectroscopy, electron microscopy, and small-angle scattering. Each technique has its limitations, but recent advancements in operando small and wide-angle X-ray scattering (SAXS/WAXS) and small-angle neutron scattering (SANS) have enabled simultaneous structural and chemical insights from atomic to sub-micrometer scales, with time resolutions as short as several seconds.