Self-assembled nano-silica-embedded polyethylene separator with outstanding physicochemical and thermal properties for advanced sodium ion batteries

Sodium ion batteries (NIBs) provide significant advantages for large-scale energy conversion and storage devices. NIBs are extensively investigated because of the global abundance of Na resources. The Na ion is similar to the Li ion; however, the conventional poly(ethylene) (PE) separator used in Li ion batteries is unsuitable for NIBs because of its extremely poor wettability by the Na+ electrolyte and its high thermal shrinkage. These properties can cause severe safety issues at elevated temperatures, as well as poor ionic conductivities. Therefore, the PE separator needs modification for use in high-performance NIBs. Here, we propose a unique nano-SiO2 impregnation method that preserves the thickness of the separator to ensure a high volumetric energy density. Silica matrices are incorporated into the PE separator through a simple chemical modification that includes embedding the hydroxyl group by benzoyl peroxide treatment, followed by Si-O condensation to introduce 3-(trimethyloxysilyl)propyl methacrylate auxiliaries. These steps allow the introduction of multiple Si-O functional groups onto the PE backbone through chemical reactions with tetraorthosilicate. The proposed separator exhibits high wettability and ionic conductivity while maintaining the initial separator thickness. The nano-SiO2 -impregnated PE has significantly improved thermal stability and high dimensional stability even at an elevated temperature of 140 degrees C. Na full-cell systems, consisting of Na0.9Li0.05Ni0.25Fe0.25Mn0.5O2 and hard carbon electrodes, exhibit considerably increased cycle lives and rate capabilities when fabricated with this nano-SiO2-impregnated PE separator.

Automated summary

A unique nano-SiO2 impregnation method for polyethylene separators was proposed to enhance the performance and safety of high-performance sodium ion batteries.