Fe³⁺ reduction during melt‐synthesis of LiFePO₄

Abstract

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.

Co-Sintering of Li1.3Al0.3Ti1.7(PO4)3 and LiFePO4 in Tape-Casted Composite Cathodes for Oxide Solid-State Batteries

“…The expansion might be correlated with the decomposition reactions of the carbon coating. Raising the temperature to T s > 940 • C results in the melting of LFP [51], which limits the temperature range for using this active material.…”

Melt Synthesis of Lithium Manganese Iron Phosphate: Part I. Composition, Physical Properties, Structural Analysis, and Charge/Discharge Cycling

“…Besides low reactant cost, low purity requirements and process simplicity, melt synthesis is advantageous because it is fast (several minutes at 1000 °C required to melt LFP), produces no solid or liquid waste (all precursors are completely converted in the liquid phase), and produced a highly dense product (a solid LFP ingot). [14][15][16][17] Challenges are the need for grinding of the as-obtained ingot to submicron powder and a separate carbon coating step. The extra mechanical processing is probably preferable to a complex chemical synthesis and the calcination during carbon coating is beneficial to the crystallinity as we will show in this study.…”

Melt Synthesis of Lithium Manganese Iron Phosphate: Part II. Particle Size, Electrochemical Performance, and Solid-State Lithium Diffusion

“…6 Melt synthesis is potentially superior to traditional methods, e.g., solid-state synthesis or hydrothermal synthesis, since it is fast (several minutes at 1000 °C required to melt LMFP), produces no solid or liquid waste (all precursors are completely converted in the liquid phase), and produces a highly dense product (solid LMFP ingots). [7][8][9][10] Challenges are the need for grinding of the as-obtained ingot to submicron powder and a separate carbon coating step. The extra mechanical processing is probably preferable to a complex chemical synthesis and the calcination during carbon coating is beneficial to the crystallinity, as we showed in the first part of this study.…”