Overview of the Iron Oxide Red Method for High-Density LFP Preparation Process
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Process Route Overview
The Iron Oxide Red Method (hereinafter referred to as the "Iron Red Method"), also known as the carbothermal reduction iron red method, is a process route that uses ferric oxide (Fe₂O₃) as the iron source, composite salts such as lithium dihydrogen phosphate (LiH₂PO₄) as the lithium and phosphorus sources, and organic carbon sources like glucose as both the reducing agent and conductive coating agent. Through a high-temperature solid-state carbothermal reduction reaction, lithium iron phosphate (LiFePO₄/C) cathode material is synthesized in a single step. The Iron Red Method is one of the four mainstream solid-state synthesis routes for lithium iron phosphate, alongside the iron phosphate method (market share ~83%) and the ferrous oxalate method (market share ~7%). Representative companies adopting this method include GCL, Chongqing Terui, and Wanhua Chemical, which is currently advancing the industrialization of the Iron Red Method for high-density LFP preparation process.
Process Principles
The Iron Red Method employs ferric oxide (Fe₂O₃) as the iron source, lithium dihydrogen phosphate (LiH₂PO₄) as a dual source providing both lithium and phosphorus, and an organic carbon source (glucose/sucrose) serving as both the reducing agent and conductive coating agent. A single sintering step is carried out under an inert atmosphere, where carbothermal reduction converts Fe³⁺ to Fe²⁺. Simultaneously, LiH₂PO₄ decomposes to release Li⁺ and PO₄³⁻, which react with the reduced iron to form olivine-structured LiFePO₄, while pyrolytic carbon forms an in-situ coating on the particle surfaces, creating a conductive network.
Advantages:
Low raw material cost: Ferric oxide (Fe₂O₃) is widely available and inexpensive; the iron source is non-toxic, environmentally friendly, and highly safe.
Short process flow: The synthesis of lithium iron phosphate and carbon coating can be completed in a single solid-state reaction step, resulting in a short production cycle.
Good mixing uniformity: The morphology of iron oxide red can be controlled after wet grinding, leading to good uniformity and consistency in the final product.
Excellent product performance: The carbothermal reduction method improves the material‘s electrical conductivity, endowing the product with good low-temperature and rate performance.
Single-step sintering: The process is simple, with relatively low energy consumption and a high yield rate.
Disadvantages:
Iron oxide red often has a relatively high impurity content, requiring strict control over raw material purity.
The carbothermal reduction reaction from Fe³⁺ to Fe²⁺ requires precise control of reaction conditions (temperature, carbon content, atmosphere). The stoichiometric ratio of phosphate to lithium is difficult to control accurately.
High-temperature sintering must be conducted under an inert/reducing atmosphere, demanding high gas-tightness of the kiln.
Current market application is relatively limited, with insufficient accumulated industrialization experience.
Process Chemical Reaction Principles
The core chemical process of the Iron Red Method is the carbothermal reduction reaction. Taking lithium dihydrogen phosphate as the dual lithium and phosphorus source, the overall reaction is as follows:
This reaction involves two key processes:
Carbothermal reduction process: At high temperatures, carbon from the pyrolyzed carbon source or CO reduces Fe³⁺ to Fe²⁺.
Lithium iron phosphate synthesis process: The generated FeO undergoes a solid-state reaction with the lithium and phosphorus sources, forming olivine-structured lithium iron phosphate with the assistance of the carbon source, which simultaneously completes the carbon coating.
Key to the Iron Red Method process: The carbon source plays two roles simultaneously:(1) Reducing agent — reduces Fe³⁺ to Fe²⁺;(2) Carbon source — pyrolyzes to form a conductive carbon layer coating the surface of LFP particles. The pyrolysis of the carbon source also generates reducing CO gas, forming a "protective shield" that prevents the oxidation of Fe²⁺ to Fe³⁺, while also inhibiting excessive particle growth. This is more rational than the ferrous oxalate process.
Core Reaction Equations:
Overall Reaction:2LiH₂PO₄ + Fe₂O₃ + C → 2LiFePO₄ + 2H₂O↑ + CO↑
Stepwise Reaction Mechanism:
Stage 1 (Room Temperature ~ 200°C) — Low-Temperature Reduction:3Fe₂O₃ + C (or CO) → 2Fe₃O₄ + CO₂↑Fe₂O₃ is partially reduced by carbon to Fe₃O₄ (an intermediate product). Reduction is incomplete at this stage.
Stage 2 (200 ~ 600°C) — Medium-Temperature Deep Reduction:Fe₃O₄ + C (or CO) → 3FeO + CO₂↑Fe₃O₄ is further reduced to FeO, while LiH₂PO₄ begins to decompose.
Decomposition of LiH₂PO₄:LiH₂PO₄ → LiPO₃ + H₂O↑ (approx. 300~400°C)LiPO₃ (lithium metaphosphate) exists as a glassy molten phase, coating the surface of Fe₂O₃ particles and providing a diffusion interface for Li⁺ and PO₄³⁻.
Stage 3 (600 ~ 800°C) — High-Temperature Solid-State Reaction and Phase
Formation: FeO + LiPO₃ + C → LiFePO₄ + CO↑FeO reacts with LiPO₃ under carbothermal reduction conditions to form LiFePO₄, while the carbon source simultaneously cracks to form a coating layer.
Objective: To ensure that the quality of raw materials is acceptable and to accurately weigh each material according to the formula. In the Iron Red Method, the precise Li/Fe ratio and carbon source proportion are the most critical factors for ensuring product performance.
Operating Steps:
Upon arrival, battery-grade iron oxide red, lithium dihydrogen phosphate, and glucose are sampled and tested according to quality standards. Only materials passing inspection are warehoused.
Materials are fed from the feeding station into a pneumatic conveying system, which transports them to their respective metering bins.
The metering bins use a loss-in-weight metering method to accurately weigh each material according to the calculated ratio:
Iron oxide red: 535 ± 5 kg per metric ton of finished product
Lithium dihydrogen phosphate: 690 ± 5 kg per metric ton of finished product
Glucose: 100 ± 5 kg per metric ton of finished product
The weighed materials are fed via a screw feeder into a pre-mixing tank, where a calculated amount of purified water is added and pre-stirred to form a preliminary slurry.
Process 1: Raw Material Inspection and Weighing & Batching
Objective: To ensure that the raw materials meet quality requirements, and to accurately weigh each material according to the formulation. In the iron oxide red method, the precise Li/Fe ratio and the carbon source proportion are the most critical factors for guaranteeing product performance.
Operating Steps:
Upon arrival, battery-grade iron oxide red (Fe₂O₃), lithium dihydrogen phosphate (LiH₂PO₄), and glucose are sampled and tested in accordance with the quality standards. Only materials that pass the inspection are accepted into storage.
The materials are fed from a feeding station into a pneumatic conveying system, which transports them to their respective metering bins.
The metering bins utilize a loss-in-weight metering method to accurately weigh each material according to the calculated proportions:
Iron oxide red: 535 ± 5 kg per metric ton of finished product
Lithium dihydrogen phosphate: 690 ± 5 kg per metric ton of finished product
Glucose: 100 ± 5 kg per metric ton of finished product
The weighed materials are fed via a screw feeder into a pre-mixing tank, where a calculated amount of deionized water is added and pre-stirred to form a preliminary slurry.
Process 2: Please see part2 of this article