Multifunctional separators for lithium secondary batteries via in-situ surface modification of hydrophobic separator using aqueous binders

Advanced batteries with specialized functions can be fabricated by utilizing functional separators that consist of unique functional materials coated on the separator surface. However, currently commercialized separators are made of polyolefin fibers, which are hydrophobic in nature, making it difficult to utilize aqueous slurries for the coating process of functional materials due to dewetting of slurries on the separator surface. Herein, we present an intriguing fabrication method utilizing polyvinyl alcohol (PVA) binder for in-situ surface modification of hydrophobic separators, which results in favorable wetting behavior against all kinds of aqueous slurries. Dynamic contact angle measurement was employed to evaluate the wettability of the modified separator surface, and a near-zero degree receding contact angle was obtained for the PVA-treated separator. As a demonstration, aqueous slurries that could not be coated on hydrophobic separators by themselves were successfully coated uniformly using PVA as a surface modifier. In addition, using the proposed fabrication method, a ceramic-coated separator with enhanced thermal stability and adhesion strength was demonstrated with negligible deterioration of lithium-ion transport. This method, using cost-effective and eco-friendly aqueous binders, can successfully fabricate functional separators for various advanced batteries with superior features.