All-solid-state sulfur-based rechargeable lithium batteries have been expected to have superior energy density and high reliability so far. In general, the solid-solid interface between electrode and electrolyte particles has strong influence on the cell performance. Recently it is realized that all-solid-state lithium–sulfur batteries exhibit good cycling performance by reducing the particle size down to submicron scale. However, the origin of excellent reversibility has not been understood. Here we clearly demonstrate Li2S nanocomposites underlying high-capacity and cycling stability in all-solid-state lithium–sulfur batteries. Through high-resolution transmission electron microscopy (TEM) and energy-dispersed X-ray (EDX) spectroscopy experiments, reversible structural and morphological changes at the nanoscale during the full-electrochemical cycles in next-generation all-solid-state lithium–sulfur batteries have been revealed for the first time. Reversible variations during cycles between crystallization and amorphization of sulfur-based active nanoparticles are responsible for the feasibility of the high capacity and cycling stability. The smooth and adhesive interface between them is truly realized at the nanoscale, which is fabricated by mechanical milling technique. Our experimental findings will lead to new route to generate the sulfur-based rechargeable batteries with high-capacity and cycling stability.