Phosphate-based lithium materials, such as lithium iron phosphate (known as lithium ferrophosphate, LiFePO 4 , LFP), are among the safest materials for large-scale lithium-ion batteries due to the stability of the phosphate-bound oxygen at elevated temperatures. LFP can be meltsynthesized where the kinetics is faster, allowing for coarser and lower cost reactants. The most common lithium-and phosphate-bearing reactants can react violently upon heat-up and release a large volume of gaseous by-product. Lithium metaphosphate (LiPO 3 , LPO) can improve the processability and safety of the operation. In this work, we investigate the thermal decomposition of lithium dihydrogenphosphate (LiH 2 PO 4 , LHP) to LPO up to 400°C. The decomposition was analyzed by isothermal and constant rate differential thermogravimetric (DTG) experiments. Activation energy profiles were estimated by an isoconversional model-free approach and kinetic model fitting. Li 5 H 4 P 5 O 17 (L2.5) was determined to be the most stable reaction intermediate and can be isolated at temperatures between 200 and 240°C. The resulting reaction is comprised of 6 reactions, where the LHP is progressively polymerized by condensation reactions leading successively to L2.5, Li 3 H 2 P 3 O 10 (L3), Li 4 H 2 P 4 O 13 (L4), and LPO. The first reaction step (LHP → L2.5) was fitted with 3 reactions series/parallel describing the solid surface reaction, the viscous/liquid surface reaction, and the bulk reaction. Limiting the reaction temperature to 400°C results in a solid product that can be advantageous if LPO is to be prepared in advance and dosed for LFP synthesis.
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.
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