<p>Reduced graphene oxide (rGO) was synthesized by thermal reduction of graphene oxide prepared using a modified Hummers method and subsequently evaluated as a conductive additive in sulfur composite cathodes for sulfide-based all-solid-state lithium–sulfur batteries (ASSLSBs). Structural and surface characterization confirmed that the synthesized rGO exhibited a highly exfoliated and porous morphology with restored sp² carbon domains while retaining residual oxygen-containing functional groups. Although the electronic conductivity of rGO was comparable to that of acetylene black (AB), full-cell electrochemical tests revealed markedly different cathode behaviors depending on the composition of the conductive additive. Specifically, the cathode employing rGO as the sole conductive additive displayed severe initial irreversibility and low conversion efficiency, accompanied by lithium trapping, as evidenced by the formation of carbide-like species. In contrast, the hybrid conductive network comprising rGO and AB (50:50 wt%) delivered near-unity conversion efficiency and stable cycling performance, without detectable lithium carbide formation. These findings demonstrate that cathode reversibility in ASSLSBs is governed not only by electronic conductivity but also by the surface chemistry and microstructural characteristics of the conductive additives. This underscores the importance of hybrid conductive architectures for enabling reversible sulfur conversion.</p>