This work presents an efficient sol–gel method to coat SiO2 nanopheres onto tri-layered polymeric separators for Li-ion batteries, consisting of Li4Ti5O12 (LTO) anode/Li cathode. The surface density of silica nanospheres with diameter of 300–500 nm is chosen as controlling factor in affecting the electrochemical performance and thermal stability of. ••This work adopts a sol–gel method to coat SiO2 spheres onto polymeric separators.••The loading of silica serves as a factor in affecting thermal stability of battery.••The presence of silica improves rate capability and dimensional/thermal stability.••An appropriate amount of SiO2 features wettability and mass uptake of electrolyte.••Polymeric separatorSilica coatingLithium-ion batteriesSol–gel methodHigh energy density and power density of Li-ion batteries have served as a popular and reliable power source for portable electronic devices, and more recently, hybrid electric vehicles (EVs) and plug-in hybrid electric vehicles,,,. It is well known that Li-ion battery is fabricated by four crucial components: cathode, anode, polymeric separator, and electrolyte. Among the components, the selection of polymeric separator is crucial in influencing the performance and the safety issue of Li-ion batteries. Traditionally, the selection of separators is based on three criteria: chemical resistance, mechanical robustness, and porosity of dielectric membranes, so that the selected separators are capable of isolating the cathode and anode to prevent electric short circuit and allowing the transport of ionic charged carriers,. Thus far, many scientists and researchers have devoted themselves to modifying the commercial separators, taking into account the safety issues for applications in EVs.So far, commercial separators are structured by a sandwich-type polymeric composites, i.e., polypropylene (PP)/polyethylene (PE)/PP films. The tri-layered separator has been extensively used in the fabrication of Li-ion batteries due to its good reliability and compatibility. However, tri-layered separator still faces two major drawbacks including high thermal shrinkage and poor wettability to. 2.1. Sol–gel synthesis of silica nanospheresThe sol–gel method for synthesizing SiO2 nanospheres onto tri-layered PP/PE/PP membrane (Celgard 2320) could be described as follows. First, silica spheres were synthesized in colloidal solution by using sol–gel method that consists of hydrolysis of tetraethoxysilane (TEOS, molecular formula: C8H20O4Si, reagent grade, Aldrich) in an alcoholic medium followed by the base-catalyzed polycondensation of silicic acid groups leading to the formation of giant silica “macromolecules”. A mixture of 42 ml of ethanol (purity: 99.8%), 7.5 ml of ammonium solution (0.1 N), and 3.0 ml of TEOS (purity: 99.9999%) was mixed well in a sealed glass flask in an ultrasonic bath. The ammonia solution was used to adjust the pH value of the Si-containing sol at pH 12. Subsequently, the mixture was magnetically stirred at room temperature for 0.5 h. The pH of colloidal suspension was adjusted using NH3 solution to stabilize silica nanospheres with the desired particle size. Three surface densities of silica nanospheres onto the PP/PE/PP membranes were prepared, based on the amount of TEOS in the colloidal suspension, i.e., 1.5, 3.0, and 6.0 ml TEOS. A binder, poly(vinylidene fluoride-hexafluoro propylene) (PVdF-HFP) was added to the silica suspension with a ratio of 4/1, SiO2/PVdF-HFP (w/w). Each membrane was slowly impregnated in the colloidal suspension, and it was ma.
