With the continuous development of lithium-ion batteries (LIBs), the electrochemical performance and safety of LIBs have become increasingly challenged recently. Therefore, research on high-performance LIB separators has gradually attracted widespread attention. As an important component of lithium batteries, the diaphragm plays an important role in battery safety. Improving battery separator performance is an important way to improve batteries’ safety and electrochemical performance. Herein, to prepare a new type of LIB separator, a preparation strategy in which polyimide (PI) nanofibers can be coaxially encapsulated by the inorganic boehmite (AlOOH) ceramics through an adsorption and complexation process is developed. This strategy involves immersing partially imidized PI nanofiber membranes into anhydrous aluminum chloride ethanol solution and dilute ammonia, where the adsorption−complexation and hydrolysis processes are completed. During the hydrolysis process, the PI nanofibers are evenly coated with AlOOH. The amount of AlOOH can be controlled easily by adjusting the concentration of the anhydrous aluminum chloride ethanol solution. After being encapsulated with AlOOH, the separator exhibits better mechanical properties, wettability, thermal stability, cycling durability, and a high capacity, along with faster ion transport speed. All excellent features prove that the PI@AlOOH composite nanofiber membrane is an advanced high-performance LIB separator.