The Preparation Of Novel Lithium Ion Battery Separator By Plasma Technology

The microporous polypropylene separator of Li-ion battery was modified by SiO_x coating via plasma enhanced vapor chemical deposition (PECVD) technology. For the purpose of improvement the thermal stability of separator and safety of the battery, the investigation of the sort of plasmas (Ar, O₂) and SiO_x coating on thermal shrinkage, wetting property, mechanical property, anti-corrosion of electrolyte and electrical performance of membrane were performed in this thesis.In here the radio frequency (13.56MHz) capacitively coupled plasma was employed to deposited SiO_x coating, in which oxygen gas and hexamethyldisiloxane (HMDSO) were used as oxidant and monomer, respectively, and argon was utilized as a diluted gas. The chemical structure of as-deposited SiO_x coatings was characterized by Fourier transform infrared spectroscope (FTIR). Scanning electron microscope (SEM) and atomic force microscope (AFM) were utilized to scan the morphology of coatings. The wettabilty and the deposition rate of SiO_x coating were measured by the water contact angle and step profiler, respectively. The influence of SiO_x coating on the membrane's mechanical property, porosity, electrolyte adsorption rate was emphasized in this work. Finally, button-type batteries used the treated and coated membranes as separators were assembled to evaluate the possibility by the measurement of C rates charge/discharge, cycle performance and AC impedance. The following results were concluded:1. The water contact angles of the SiO_x matrix were increased along with the decrease of O₂ concentration. The maximum deposition rate of SiO_x was attained at O₂: HMDSO ratio of 1:2;2. The O₂ plasma treatment demonstrated a remarkable improvement of the wettability, porosity and electrolyte adsorption rate. Both Ar and O₂ plasma treatments damaged the membrane mechanical property;3. When the ratio of O₂: HMDSO at 8:1 the water contact angle of SiO_x coated membrane could achieve about 36°. The thermal shrinkage of the membranes after coated SiO_x was significantly improved.4. The electrochemical tests showed that the SiO_x coated separator didn't decrease the battery charge/discharge capacity and cycle ability rather than the impendence of the battery system. There were some abnormal performances when the SiO_x coating was excess thickness.