Research on the synthesis of lithium iron phosphate using vivianite prepared from municipal sludge

Sewage treatment facilities are widespread in cities throughout China, and municipal sludge, a byproduct of wastewater treatment, often leads to resource waste and secondary pollution. Concurrently, the global community faces a critical shortage of phosphorus resources. In this study, we propose an innovative resource recycling strategy to address the challenges presented by global resource scarcity and municipal sludge disposal. Specifically, we used a self-designed dual-chamber electrolytic cell to synthesise vivianite (Fe₃(PO₄)₂·8 H₂O), followed by high-temperature solid-phase synthesis of lithium iron phosphate (LiFePO₄) in an atmosphere tube furnace. Using this system, we systematically investigated the effects of different molar ratios of lithium-iron-phosphorus (1:1:1, 2:1:1, 3:1:1, 1:1:2, and 1:1:3), different types of reducing agents (glucose and ascorbic acid), a range of sintering temperatures (500, 550, 600, 650, and 700 °C), different amounts of reducing agent (10%, 15%, 20%, and 25%), and positional differences within the tube furnace on the efficiency of LiFePO₄ synthesis. The experimental results revealed that optimal synthesis was achieved under the following conditions: a lithium-iron-phosphorus molar ratio of 1:1:1, with ascorbic acid as the reducing agent, added at 20% of the actual mass of LiFePO₄, and a 10-h high-temperature solid-phase synthesis at 700 °C in the central region of the tube furnace.