Sandwich-structured electrospun all-fluoropolymer membranes with thermal shut-down function and enhanced electrochemical performance

High safety and rate capability of lithium-ion batteries (LIBs) remain challenging. In this study, sandwich-structured poly(vinylidene fluoride)/poly(vinylidene fluoride-co-hexafluoropropylene)/poly(vinylidene fluoride) (PVDF/PVDF-HFP/PVDF) membranes with thermal shut-down function were successfully prepared through electrospinning. The effects of different weight ratios of PVDF and PVDF-HFP in a composite membrane on the physical and electrochemical properties of the membrane were explored. It was found that the composite membrane with 36 wt% PVDF-HFP (P/H2/P) showed excellent electrolyte absorption (367%) and ionic conductivity (2.5 × 10−3 S/cm). Half-cell with P/H2/P as separator exhibited higher discharge capacity and better cycle performance than commercial PP membrane. More importantly, thermally stable high melting-temperature PVDF was chosen as outer layer, while low melting-temperature PVDF-HFP was used as inner layer. Self-shutdown function of this separator was achieved when heated at 140 °C, providing a safety measure for LIBs. These results indicate that PVDF/PVDF-HFP/PVDF composite membrane is a promising separator candidate in high performance LIBs applications.