This study develops a facile and generic way to fast and effectively produce and separate nanoparticles of water-insoluble inorganics from an aqueous medium, aiming to fulfill massive industrial demands for inorganic nanoparticles. As a model study, herein, PbSO4 nanosuspension with an average particle diameter of similar to 50 nm is produced via the in-situ reactive flash nanoprecipitation (RFNP) with chitosan as a pH-sensitive surface stabilizer, whose hydrophobicity can be tuned by varying its pH. By increasing pH of the suspension, the produced chitosan/PbSO4 nanoparticles could rapidly aggregate and settle down. After filtration and dry, the particles are able to be fast and effectively separated from water so as the size enlargement due to Ostwald ripening and recrystallization is significantly inhibited. The dried nanoparticles as an excellent active material are then used to fabricate an negative electrode, with which the assembled lead acid battery shows similar to 150% of a regular discharge capacity (similar to 100 mA h g(-1) at 120 mA g(-1) of current density in this study), a doubled cycle number of charge-discharge (> 700 cycles), and thus a much longer durability than a commercial battery does. This success in the battery application well demonstrates feasibility and bright prospects of the in-situ RFNP technique in large-scaled synthesis of nanoparticles.