Preparation And Characterization Of SiO₂/PVdF-HFP/PP Non-woven Composite Membrane For Lithium-ion Battery

The rapid development of various kinds of electronic products put forward higherdemands for lithium ion battery. Studying and improving performance of the separator,which is one of the core components of lithium-ion batteries, will be very importantsignificance for performance improvement of lithium-ion battery. In order to fabricatelithium ion battery separater with high ionic conductivity, stabile electrochemicalperformance and high safety performance, a kind of nano-SiO2/PVdF-HFP/PPnon-woven composite separater had been studied in this paper.(1) SiO2nanoparticles were modified with (heptadecafluoro-1,1,2,2-tetradecyl)trimethoxysilane (TSL-8233) to improve its dispersibility and compatibilityin the polymer matrix. The structure and property of SiO2nanoparticles before and aftermodification were characterized by FT-IR, TGA and TEM analysis. Together withpoly(vinylidene fluoride-co-hexafluoropropylene)(PVdF-HFP) and polypropylene (PP)nonwoven fabric, these SiO2nanoparticles were then used to construct two kind ofcomposite separators (abbreviated as PHS for pristine SiO2and PHS-8233for modifiedTSL-8233@SiO2). The morphology, electrolyte uptake, ionic conductivity andelectrochemical 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-8233composite separator exhibited an improved poredistribution, electrolyte uptake (280wt%) and ionic conductivity (1.90×10-3S cm-1).Even more importantly, LiFePO4/Li cells assembled with PHS-8233compositeseparator displayed remarkable C-rate performance, which showed an enhancement inthe chemical stability and discharge capacity. The capacity kept above144mAh g-1after100charge-discharge cycles.(2)2,2,3,3,4,4,5,5-Octafluoropentyl methacrylate (OFPMA) was grafted onthe surface of PP nonwoven by plasma irradiation to improve its adhesion andcompatibility with PVdF-HFP. The chemical composition on the surface of PP nonwoven before and after plasma treatment was characterized by FT-IR and XPSanalysis. Together with PVdF-HFP and SiO2nanoparticles, different PP nonwovenswere used to construct two kind of composite separators (abbreviated as PHS forpristine PP nonwoven and PHS-n for OFPMA-modified PP nonwoven, n means plasmatreatment time). The morphology, electrolyte uptake, ionic conductivity andelectrochemical properties of the composite separators were analyzed by SEM analysis,AC impedance measurements, charge-discharge cycle and C-rate tests, respectively. Theresults indicated that PHS-10composite separator using the modified PP nonwoventreated by plasma for10min exhibited much better properties than PHS separator, forexample, an improved mechanical property, electrolyte uptake (290wt%) and ionicconductivity (1.76×10-3S cm-1). More importantly, LiFePO4/Li cells assembled withPHS-10composite separator displayed good C-rate performance, which presented anenhancement in the chemical stability and discharge capacity. The capacity could keepabove150mAh g-1after100charge-discharge cycles.(3) To reduce the cost of the composite separator acrylic acid was grafted on PPnonwoven fabric by plasma treatment, the contact angle of PP non-woven reduced from114°to15°, PP non-woven changed from hydrophobic to hydrophilic. The membranemodified PP non-woven with nano-SiO2/PVdF-HFP, showed excellent electrochemicalstability and high ionic conductivity. The ionic conductivity was1.69×10-3S cm-1. Thecell assembled with the composite membrane exhibited high discharge specific of152mAh g-1after100charge–discharge cycles showing excellent properties.