Microfibrillated cellulose-SiO₂ composite nanopapers produced by spray deposition

Microfibrillated cellulose (MFC)-SiO₂ nanopapers were prepared using a rapid spray deposition technique. Large area (~310 cm²) composite nanopapers with thickness and SiO₂ content varying from 16 to 92 µm and 0 to 33 %, respectively, were prepared in less than 30 min with nearly complete nanoparticle retention in the cellulose mat. In the presence of an excess of MFC, SiO₂ nanoparticles formed large clusters embedded in a dense and continuous cellulose matrix which conferred to the composite an extremely low permeability to air, i.e., below 2 nm². For silica mass fraction above 20 %, SiO₂ clusters induced a net increase in air permeability and ionic conductivity up to 12 nm² and 1.5 mS cm⁻¹ for a SiO₂ content of 33 %. Despite the addition of an inert phase, composite nanopapers displayed mechanical properties, viz. Young’s modulus and internal cohesion higher than 2.2 GPa and 913 J m⁻², outperforming those of most conventional papers. This study demonstrates that MFC-SiO₂ nanopapers fabricated by spray deposition can be an alternative to PE/PP membranes as separators in Li-ion batteries and, in general, that spray deposition is a promising method for the rapid fabrication of large area composite nanopapers.