<p>Understanding and manipulating the interplay between lattice oxygen redox and interfacial stability is critical for realizing the full potential of lithium-rich manganese-based oxide positive electrodes in all-solid-state lithium batteries. Herein, we comprehensively investigate interfacial evolution of Li-rich manganese-based positive electrodes in sulfide electrolyte (Li₆PS₅Cl) systems and report that a potential mechanism where irreversible lattice oxygen redox at high potential induces severe electrolyte decomposition, forming porous oxygenated by-products that dominate interfacial impedance and hinder anionic redox reactions. Discharge-driven reduction of these by-products—particularly between 2.6-2.0 V—could facilitates interphases reconstruction via lithium incorporation, enhancing ionic transport kinetics and enabling reversible oxygen redox. Building on this identified mechanism, we propose a pre-activation strategy (3.9-2.0 V pre-cycling) with oxygen-modified Li₆PS₅Cl, which accelerates interphases stabilization and enables high oxygen redox utilization during the first cycle. It delivers a specific capacity of 318 mAh g⁻¹ at 10 mA g⁻¹, and 226 mAh g⁻¹ at 200 mA g⁻¹ with 83.2% retention over 500 cycles at 60 °C. This work has the potential to provide an effective interfacial reconstruction strategy to enhance anionic redox reversibility, providing a design framework for high-energy lithium-rich positive electrodes in all-solid-state lithium batteries.</p>