Enhanced wettability and thermal stability of nano-SiO2/poly(vinyl alcohol)-coated polypropylene composite separators for lithium-ion batteries

To improve the electrolyte wettability and thermal stability of polypropylene (PP) separators, nano-SiO2/poly(vinyl alcohol)-coated PP composite separators were prepared using a simple but efficient sol–gel and dip-coating method. The effects of the tetraethoxysilane (TEOS) dosage on the morphology, wettability, and thermal stability of the composite separators were investigated using Fourier-transform infrared spectroscopy, scanning electron microscopy, and contact-angle measurements. All the composite separators gave a smaller contact angle, higher electrolyte uptake, and lower thermal shrinkage compared with the PP separator, indicating enhanced wettability and thermal stability. Unlike the case for a traditional physical mixture, SiOC covalent bonds were formed in the coating layer. The composite separator with a TEOS dosage of 7.5 wt% had a unique porous structure combining hierarchical pores with interstitial voids, and gave the best wettability and thermal stability. The ionic conductivity of the composite separator containing 7.5 wt% TEOS was 1.26 mS/cm, which is much higher than that of the PP separator (0.74 mS/cm). The C-rate and cycling performances of batteries assembled with the composite separator containing 7.5 wt% TEOS were better than those of batteries containing PP separators.