Neoteric Polyimide Nanofiber Encapsulated by the TiO2 Armor As the Tough, Highly Wettable, and Flame-Retardant Separator for Advanced Lithium-Ion Batteries

Nowadays, separators with superior properties have drawn widespread attention for the development of advanced and safe large-scale lithium-ion batteries (LIBs). Yet it is still a great challenge for improving overall the thermostability, flame endurance, wetting property, and ion-transport resistance of the polymer-based separators. Herein, a novel and green strategy is reported to address the aforementioned issue by means of the advanced nanostructured surface configuration design in which polyimide (PI) nanofibers are encapsulated by titania (TiO2) nanolayer via dipping in the titanium oxysulfate (TiOSO4) solution, which serves as the source of TiO2. Unlike conventional ceramics-coating methods, this distinctive TiO2@PI core–shell nanostructure is fabricated by the in situ hydrolysis deposition process, and the TiO2 nanoshell thickness can be controlled via simply changing the soaking time in TiOSO4 solution. After being encapsulated by the uniform TiO2 nanolayer, the PI-TiO2 core–shell separator manifests superior flame resistance, outstanding wettability for electrolyte, great thermal dimensional stability at 300 °C, higher glass transition temperature at 400 °C, and better ionic conductivity. Moreover, the cell installed in the PI-TiO2 core–shell separator displays brilliant cycling durability at 120 °C and high-rate property with as high as 82% capacity retention under 5 C (135 mAh g–1), which is superior to the cell using Celgard PP (60%, 90 mAh g–1) and PI nonwoven (75%, 123 mAh g–1). All the admirable features make PI-TiO2 nanofibrous membrane advanced and secure separator for large-scale power LIBs.