Effects of fluorinated SiO2 nanoparticles on the thermal and electrochemical properties of PP nonwoven/PVdF-HFP composite separator for Li-ion batteries

In this paper, SiO2 nanoparticles were first modified with (heptadecafluoro-1, 1, 2, 2-tetradecyl)trimethoxysilane (TSL-8233) to improve its dispersibility and compatibility in the polymer matrix. The structure and property of SiO2 nanoparticles before and after modification were characterized by FT-IR, TGA and TEM analysis. Together with poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and polypropylene (PP) nonwoven fabric, these SiO2 nanoparticles were then used to construct two kinds of composite separators (abbreviated as PHS for pristine SiO2 and PHS-8233 for modified TSL-8233@SiO2). The morphology, electrolyte uptake, ionic conductivity and electrochemical properties of the composite separators were analyzed by SEM analysis, AC impedance measurements, charge–discharge cycle and C-rate tests, respectively. These results indicated that PHS-8233 composite separator exhibited an improved pore distribution, electrolyte uptake (280 wt%) and ionic conductivity (1.90 mS cm−1). Even more importantly, LiFePO4/Li cells assembled with PHS-8233 composite separator displayed remarkable C-rate performance, which showed an enhancement in the chemical stability and discharge capacity. The capacity kept above 144 mA h g−1 after 100 charge–discharge cycles.