Tailored metal oxide thin film on polyethylene separators for sodium-ion batteries

The development of polymer separators for Na⁺-ion batteries has not been of interest because conventional polyolefin (e.g., polyethylene or polypropylene) separators are not suitable for the solvation of Na⁺-ion-containing electrolytes. Here, we report a simple surface modification method based on chemical vapor deposition of SiO₂ applied to a polyethylene separator for Na⁺-ion batteries. A thin SiO₂ layer is coated uniformly onto a porous polymer separator with the negligibly increased total separator thickness. Improved wetting ability of the SiO₂-film-coated polyethylene separators with a polar electrolyte based on ethylene carbonate (EC) and dimethyl carbonate (DMC) solvents is demonstrated, with superior electrochemical performance characteristics, such as initial specific capacity, C-rate and cyclic stability. In addition, the thin SiO₂ coating film results in substantially suppressed thermal shrinkage, which may lead to improvements in the thermal and dimensional stability of Na⁺-ion batteries. Compared to a glass-fiber separator and the conventional PE separator, the metal-oxide-thin-film-coated polyethylene separator will accelerate the development of Na⁺-ion batteries for various electrochemical energy storage applications.