Low-quality brines containing dilute lithium collectively represent an enormous untapped resource for this critical element. Effective extraction technologies must process large brine volumes with high selectivity while minimizing the need for downstream purification. Targeting these requirements, we developed a scalable stirred tank electrochemical reactor for direct electrosynthesis of lithium iron phosphate from low-grade brine. The reactor employs a slurry of iron phosphate and carbon black particles suspended in brine within a cathode tank. Active stirring drives particle collisions with a solid cathode electrode, promoting Li intercalation into iron phosphate with a Li/Na selectivity of 280 and a maximum current density of 1 mA/cm2. Anodic current is supplied by oxidation of ferrocyanide redox molecules in brine solution circulated through a Nafion membrane tube. This stirred tank architecture using slurry-phase intercalation hosts offers a potentially scalable pathway for lithium recovery from low-quality brines, with the produced lithium iron phosphate serving as a potential feedstock for battery manufacturing.