Lithium-ion batteries are essential for modern energy storage, yet achieving simultaneous high-temperature and high-voltage operation remains challenging due to interfacial compatibility. In this study, we introduce a polyetherimide (PEI)–polyimide (PI) functional coating on the separator that enhances wettability, thermal stability, and mechanical strength, while markedly improving cathode stability under harsh conditions. By integrating theoretical calculations with experimental validation, we demonstrate that the PEI/PI coating modulates the solvation structure of lithium-ions, thereby facilitating the interfacial desolvation process. More importantly, the PEI/PI layer regulates electrolyte decomposition at the interface, promoting the formation of a uniform and thermally stable cathode–electrolyte interphase. Consequently, LiCoO₂ cathodes exhibit improved cycling performance at 60°C. Overall, this work underscores the pivotal role of separator coatings in governing interfacial chemistry and provides a viable strategy for designing high-performance lithium-ion batteries capable of enduring both high temperatures and high