This study introduces a physical heat treatment method to address the increased interfacial resistance and reduced electrochemical performance of Ni-rich NCM811 (LiNi₀.₈Co₀.₁Mn₀.₁O₂) cathode material, which degrades when exposed to ambient conditions. The formation of surface impurities from reactions with moisture and carbon dioxide elevates interfacial resistance with the solid electrolyte, leading to performance degradation in all-solid-state battery (ASSB) systems. The heat treatment in an O₂ atmosphere effectively removes these impurities and improves conductivity, resulting in a significant reduction in interfacial resistance. Analyzing the heat-treated NCM811 using scanning electron microscopy (SEM), X-ray diffraction (XRD), and inductively coupled plasma (ICP) revealed that its electrochemical performance could be restored to levels similar to commercially available materials. Overall, the O₂ heat treatment proves effective in suppressing degradation and enhancing interfacial contact, offering a promising approach to improve the electrochemical performance of sulfide-based ASSB systems and advancing the commercialization of high-energy-density all-solid-state batteries.