High-purity　FePO4　with　small　particle　size　is　pivotal　for　improving　electrochemical　performance　of　LiFePO4-based　lithium-ion　batteries.　Here,　an　oscillating　feedback　microreactor　(OFM)　based　on　chaotic　advection　was　designed　to　prepare　high-quality　FePO4　by　coprecipitation.　Dye-tracer　experiments　and　CFD　simulations　were　adopted　to　investigate　fluid　mixing　mechanism　and　to　evaluate　mixing　efficiency　in　the　reactor.　The　results　indicated　that　a　＇flow-focused＇　OFM　(FOFM)　performed　better　than　a　＇Y-junction＇　OFM　in　mixing　performance,　owing　to　enhanced　premixing　in　focused　inlet　microchannel　of　the　former.　The　results　of　Villermaux-Dushman　experiments　illustrated　that　almost　complete　mixing　could　be　achieved　in　0.16　ms　in　FOFM.　Accordingly,　FePO4　nanoparticles　of　good　quality　was　synthesized　using　FOFM,　and　the　particle　size　can　be　easily　controlled　by　adjusting　flow　rate　or　reactant　concentration.　High-purity　(P/Fe　molar　ratio:　0.95–1.04)　and　ultra-small　FePO4　nanoparticles　(9　nm)　with　narrow　particle　size　distribution　were　generated　at　a　high　throughput　of　180　mL/min.　©　2023　Elsevier　Ltd