TiO2 nanoshell@polyimide nanofiber membrane prepared via a surface-alkaline-etching and in-situ complexation-hydrolysis strategy for advanced and safe LIB separator

As the third electrode, separators with fabulous properties are in urgent need at present since the safety problem is increasingly standing out with the development of high-power lithium ion batteries (LIBs). However, it is extremely hard to improve simultaneously the thermal stability, flame-resistance, wettability and ion-transport property of the polymeric separators. Considering this great challenge, herein, an innovative and green technique is reported for building up titania (TiO2)-nanolayer-encapsulated polyimide (PI) nanofibers, i.e., TiO2 nanoshell@PI core nanofibers, by employing titanium oxysulfate (TiOSO4) as the source of TiO2. Distinct from the traditional ceramic-coating process, such unique TiO2@PI shell-core configuration is fabricated via a surface-alkaline-etching and in-situ complexation-hydrolysis strategy, and the thickness of the TiO2 nanoshell could be readily regulated by varying the complexing time in TiOSO4 solution. With the encapsulation of the compact TiO2 nanoshell, the TiO2@PI shell-core nanofiber membrane displays great flame-resistance, excellent wettability, outstanding thermal dimensional stability at 300 °C and higher ionic conductivity when being utilized as LIB separator. The lithium iron phosphate (LiFePO4) half-cell using this TiO2@PI nanofabric separator manifests a high capacity of 132.8 mAh g−1 @ 5C, which is much higher than the cells using pristine PI separator (122.3 mAh g−1 @ 5C) and Celgard PP (95.3 mAh g−1 @ 5C). Besides, the cell with this novel separator shows excellent cycling stability at the high temperature of 120 °C with no capacity decay after 100 cycles. All these characteristics indicate that the TiO2@PI shell-core nanofiber membranes are advanced and safe separator materials for LIBs, especially for high-power LIBs.