Microfibrillated cellulose (MFC)-SiO2 nanopapers were prepared using a rapid spray deposition technique. Large area (similar to 310 cm(2)) composite nanopapers with thickness and SiO2 content varying from 6 to 92 A mu m 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 nm(2). For silica mass fraction above 20 %, SiO2 clusters induced a net increase in air permeability and ionic conductivity up to 12 nm(2) 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.