Enhancing the β Phase of Poly(vinylidene fluoride) Nanofibrous Membranes for Thermostable Separators in Lithium-Ion Batteries

It is challenging to simultaneously control the phase and the finer nanofiber shape to prepare a thinner poly(vinylidene fluoride) (PVDF)-based separator. Here, an ultrafine β-PVDF nanofibrous membrane was fabricated by combining the melt electrospinning and phase-changing techniques. Temperature and electrostatic fields were introduced simultaneously to boost the β phase of PVDF. Meanwhile, phase-sacrificial polylactic acid (PLA) was employed to further reduce the diameter of the melt-electrospun fibers, which can significantly reduce the separator thickness and increase the specific surface area, addressing the main limitation of the application of nanofiber membranes in commercial separators. Furthermore, the strong interactions between the −C–F bonds in PVDF and the −C═O bonds in PLA further enhanced the regular TTTT structural β phase. Besides, SiO2 and Al2O3 ceramic nanoparticles were codeposited on the β-PVDF nanofiber membrane by magnetron sputtering, which not only improves the thermostability but also prevents coating layer depowdering and thickness increase. The lithium-ion battery (LIB) with such a composite separator showed an ion conductivity of 2.242 mS/cm and a high thermostability of 150 °C. This work elucidates the controlling mechanism of the crystalline phase of PVDF-based nanofibrous membrane and provides encouraging guidance for constructing a functional layer of battery separators.