Li-rich Mn-based oxide (LRMO) cathodes represent promising candidates for high-energy-density all-solid-state lithium batteries (ASSLBs). Nonetheless, irreversible oxygen release and sluggish transport kinetics result in faded voltage and degraded cycling stability, severely impeding their practical applications in ASSLBs. Herein, a high-quality artificial interface layer was constructed on the LRMO surface via a facile sol–gel method followed by thermal treatment, yielding a Li₃ScF₆ protective layer comprising a Li₃ScF₆ surface coating region and a subsurface Sc doping region. Specifically, Li₃ScF₆ surface coating effectively suppresses continuous interfacial side reactions between the cathode and solid electrolyte, thereby improving interfacial transport kinetics; the strong Sc–O bond stabilizes the lattice oxygen framework and inhibits oxygen release, thereby enhancing the reversibility of the oxygen redox reaction. Consequently, the ASSLBs with the modified LRMO cathode exhibit remarkable fast-charging capability (136.8 mAh g⁻¹ at 1.0 C) and excellent capacity retention (83.9% after 500 cycles at 0.3 C). In addition, the ASSLBs achieve outstanding long-term cycling stability at a high areal capacity of 4.17 mAh cm⁻², retaining 81.8% of its capacity after 300 cycles at 60 °C. This study offers new insights into the rational design of high-capacity and high-voltage LRMO cathode materials for high-energy-density ASSLBs.