Microfibrillated cellulose (MFC)-SiO 2 nanopapers were prepared using a rapid spray deposition technique. Large area (*310 cm 2 ) composite nanopapers with thickness and SiO 2 content varying from 16 to 92 lm 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, SiO 2 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 %, SiO 2 clusters induced a net increase in air permeability and ionic conductivity up to 12 nm 2 and 1.5 mS cm -1 for a SiO 2 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-SiO 2 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.