Plasma Modified Polypropylene Membranes as the Lithium-Ion Battery Separators

To reduce the thermal shrinkage of the polymeric separators and improve the safety of the Li-ion batteries, plasma treatment and plasma enhanced vapor chemical deposition (PECVD) of SiOx-like are carried out on polypropylene (PP) separators, respectively. Critical parameters for separator properties, such as the thermal shrinkage rate, porosity, wettability, and mechanical strength, are evaluated on the plasma treated PP membranes. O₂ plasma treatment is found to remarkably improve the wettability, porosity and electrolyte uptake. PECVD SiO_x-like coatings are found to be able to effectively reduce the thermal shrinkage rate of the membranes and increase the ionic conductivity. The electrolyte-philicity of the SiO_x-like coating surface can be tuned by the varying O₂ content in the gas mixture during the deposition. Though still acceptable, the mechanical strength is reduced after PECVD, which is due to the plasma etching.