Electrospun nanofiber Surface-Modified polyethylene separator for enhanced cycling stability and Low-Temperature performance of Sodium-Ion batteries

Sodium-ion batteries (SIBs) are emerging as promising low-cost and long-cycle energy storage systems. However, the poor wettability of the conventional polyolefin separators with polar electrolytes leads to low ionic conductivity and high battery resistance, causing rapid capacity decay. Herein, we propose using a polyethylene (PE) separator coated with a nanofiber composited of poly(vinylidene fluoride) (PVDF) and Al₂O₃ filler via electrospinning. Compared to the standard PE separators, this composite separator offers much improved electrolyte wettability, mechanical strength, and electrochemical stability. Electrochemical tests demonstrate that the Na[Ni_1/3Fe_1/3Mn_1/3]O₂||hard carbon pouch cells based on the PVDF-Al₂O₃/PE composite separator exhibit a capacity retention of 95.1% after 800 cycles at 1C. Additionally, the separator significantly enhances low-temperature discharge performance and cycling stability. Characterizations based on Fourier transform infrared spectroscopy, scanning electron microscopy, and X-ray diffraction confirm the successful integration of Al₂O₃ nanoparticles into the PVDF matrix, resulting in a homogeneously dispersed and well-connected structure, which improves ion transport efficiency and stability, thereby effectively boosting battery performance. This research highlights the potential of PVDF-Al₂O₃ nanofiber composite separators for advanced SIBs with high reversibility, a wide operating temperature range, and long cycling life.