Fe 3+ reduction during melt‐synthesis of LiFePO 4

LiFePO 4 (LFP) is a safe and low cost cathode material for Li‐ion batteries. Its solid‐state synthesis requires micron‐sized reactants yielding high production costs. Here, we melt‐synthesized up to 5 kg batches of LFP from low‐cost coarse Fe 2 O 3 (509 µm) in an induction furnace. Graphite from the crucible was an effective reducing agent. Adding metallic Fe or CO increased the Fe 2+ content and reaction kinetics. Metallic Fe improves the lifetime of the graphite crucible but requires a premixing step for it to be effective, otherwise the Fe powder agglomerates due to the presence of a eutectic in the LiPO 3 ‐Fe‐Fe 2 O 3 system. In a pushout furnace configuration, for an hour‐long holding period, injecting CO into the melt increased the Fe 2+ content from 0.301 to 0.315 g/g, which we attributed to melt protection. Likewise, graphite powder floating on top of the melt further improved the Fe 2+ content to 0.331 g/g. The Fe 2+ content reached 0.325 g/g when using fine Fe 3+ (142 µm) and CO as reducing agent at half the holding period at 1150 °C. We attribute the higher reaction rate to the improved contact between the suspended Fe 3+ and the CO reducing gas. When the graphite crucible is the unique reducing agent, the reaction rate was proportional to the crucible base surface area. A zero‐order kinetic model characterized the solids disappearance with time. A thermal model developed to compare lab‐scale data against small pilot‐scale demonstrated that the charge lagged the furnace temperature by as much as 22 min at 1000 °C.