From Blast Furnaces to Battery Packs: A Metallurgist’s Introduction to Battery Materials — Part 2

Metallurgy Process (Photo by Nicholas Dariel)

Olivine Structure: The Crystal Heart of LFP

LFP is often described as having an “olivine” structure. This term comes from mineralogy. Natural olivine minerals, such as (Mg,Fe)₂SiO₄, also have this type of crystal framework. In the case of LFP, the structure can be represented as LiFePO₄, where lithium ions occupy specific sites in a stable three-dimensional framework composed of FeO₆ octahedra and PO₄ tetrahedra.

Structure of LiFePO₄ (Source: Crystallography365, image generated by VESTA)

The key features of the olivine structure are:

• Strong Fe–O and P–O bonds: The phosphate groups provide excellent structural stability and help prevent oxygen release at high temperature, which improves safety compared to layered oxides such as LCO.
• One-dimensional lithium diffusion channels: Lithium ions move mainly along certain crystallographic directions within the olivine framework. This makes diffusion pathways more anisotropic, so particle size, orientation, and defects play an important role in rate capability.
• Fe²⁺/Fe³⁺ redox couple: During charge, LiFePO₄ is partially converted to FePO₄, and iron is oxidized from Fe²⁺ to Fe³⁺. During discharge, the reverse happens and lithium reinserts, restoring LiFePO₄. This two-phase reaction between LiFePO₄ and FePO₄ was clearly described in early work by Padhi, et. al. [1], who first proposed phospho-olivines as positive-electrode materials for rechargeable lithium batteries.

Because the structure remains robust during cycling, LFP cells can reach thousands of cycles with limited capacity fade when properly designed. However, the olivine structure also has relatively low electronic conductivity, so many studies focus on improving conductivity by carbon coating, particle size control, and doping. [2] [3]

Processing LFP Cathodes: A Metallurgical View

LFP Processing (Source: WinAck Battery Technology)

From a processing standpoint, making high-performance LFP cathode powder is not very different from preparing advanced ceramic or oxide materials. In general, advanced ceramics are produced by carefully selecting high-purity raw material powders, mixing them in precise proportions, shaping the mixture, and then sintering it at high temperatures in a controlled atmosphere to achieve the desired crystal structure and microstructure. As for LFP, typical steps include:

1. Selection of precursors: Iron sources such as FeSO₄ or FeC₂O₄, lithium salts such as Li₂CO₃ or LiOH, and phosphate sources such as H₃PO₄ or NH₄H₂PO₄ are used.
2. Mixing and synthesis: Solid-state reaction, sol–gel routes, coprecipitation, or mechanical alloying are used to synthesize precursor powders where the elements are homogeneously mixed before high-temperature treatment.
3. Calcination and sintering: The mixture is heated in a controlled atmosphere at temperatures around 600–800 °C to form the final LiFePO₄ olivine phase. Careful control of oxygen partial pressure is needed to keep iron in the correct oxidation state and to avoid impurity phases. This is similar in spirit to controlling atmosphere in a DRI shaft furnace, although the temperature and phases are different.
4. Carbon coating and particle engineering: To overcome the low electronic conductivity of LFP, carbon is deposited on particle surfaces, often by decomposing organic precursors or gases during heat treatment. This creates a conductive network that improves rate capability and cycle life.

These processing steps strongly influence particle size distribution, porosity, phase purity, and conductivity, which in turn control the electrochemical performance. From a metallurgical point of view, this is similar to how sintering conditions, cooling rate, and alloy additions influence microstructure and properties in steels.

Iron-based and Iron–Air Batteries: Iron as an Energy Carrier

Interest in using iron as the main element in the batteries and not just a supporting ingredient is growing fast. It is mainly because iron is incredibly cheap and found almost everywhere on Earth compared to cobalt and nickel which are rare, expensive, and often mined under harmful conditions. Some new battery designs literally just “rust” and “un-rust” iron using water and air, which sounds simple but can actually store energy for days at a time. On top of that, iron-based batteries don’t catch fire the way some other batteries can, making them far safer to use at large scale.

Iron–air batteries are one example. In these systems, iron metal is oxidized to iron oxides during discharge while oxygen is supplied from air at the cathode. During charge, electrical energy is used to reduce the iron oxides back to metallic iron. This concept has been studied for many decades and is now being reconsidered for long-duration grid storage because iron is abundant, cheap, and non-toxic.

Recent work [4] shows that modern iron–air systems can store energy for many hours to several days at a cost per kWh that is significantly lower than conventional lithium-ion for long-duration applications. From a metallurgical perspective, these batteries are interesting because they involve reversible oxidation and reduction of iron, phase transformations between different iron oxides, and the design of electrodes that can survive repeated rusting and de-rusting cycles=

Battery Recycling: Pyrometallurgy and Hydrometallurgy for LFP and Other Chemistries

Just like steelmaking, battery technology must also confront sustainability challenges. End-of-life batteries contain valuable and sometimes hazardous elements. For nickel and cobalt rich chemistries, recycling focuses on recovering Co, Ni, and Li. For LFP, the focus is more on lithium and phosphate recovery, because the iron value is relatively low, but the volume is very high.

Generally, there are two main metallurgical approaches used for recycling:

1. Pyrometallurgical Routes Pyrometallurgical recycling uses high-temperature treatment, often in furnaces similar in concept to non-ferrous smelters. Spent batteries are fed into a furnace, where metals from different cell components form an alloy or separate phases that can later be refined. This approach is robust, but for LFP it may not always be economically optimal because the cathode contains less high-value metals such as cobalt.

2. Hydrometallurgical and “Closed-loop” Routes Hydrometallurgical routes treat crushed or preprocessed battery materials in aqueous solutions. For LFP, many studies focus on:

• Acid leaching of delithiated LiFePO₄ to dissolve lithium and sometimes iron and phosphorus.
• Precipitation or crystallization of battery-grade FePO₄·2H₂O and lithium carbonate, which can be reused as cathode precursors.
• Process water and reagent reuse to reduce waste and cost.

For example, a recent study demonstrated an economical hydrometallurgical process that converts delithiated LFP cathode scrap into high-purity FePO₄·2H₂O with Fe and P leaching efficiencies near 99 percent, followed by crystallization and reuse of the mother liquor as leachant in a closed-loop scheme. A separate critical review in Green Chemistry analyzed various lixiviants and showed that reusing process water from the second cycle onward can significantly reduce chemical consumption and improve profitability for LFP recycling plants.[5] [6]

Closing Thought

The journey from blast furnace to battery pack is shorter than it first appears. The same basic understanding with iron and steelmaking is completely accessible, which includes controlling redox reactions, engineering crystal structures, managing phase transformations, and designing sustainable process cycles, turns out to be exactly what the battery industry needs to solve its biggest challenges. Whether it is understanding the olivine framework that gives LFP its remarkable stability, designing hydrometallurgical closed-loop processes to recover FePO₄ and lithium from spent cells, or engineering iron-air electrodes that can survive thousands of rusting and de-rusting cycles, metallurgical thinking is at the core of it all.

[1] Padhi, A. K., Nanjundaswamy, K. S., Goodenough, J. B. “Phospho-olivines as positive-electrode materials for rechargeable lithium batteries.” Journal of the Electrochemical Society 144, 1188–1194 (1997).

[2] Belharouak, I., Yan, Q., Sun, Y.-K., & Amine, K. (2010). Phospho-olivine as advanced cathode material for lithium batteries. Eurasian ChemTech Journal, 12, 201–205.

[3] Kim, C. W., Lee, M. H., Jeong, W. T., & Lee, K. S. (2005). Synthesis of olivine LiFePO₄ cathode materials by mechanical alloying using iron(III) raw material. Journal of Power Sources, 146(1–2), 534–538. https://doi.org/10.1016/j.jpowsour.2005.03.058

[4] Li, Z., Yu, J., & Liu, X. (2025). Materials and devices for iron batteries: Recent progress and perspectives. ACS Applied Energy Materials, 8(14), 9965–9982. https://doi.org/10.1021/acsaem.5c00946

[5] Choi, S., & Shim, H. (2025). Toward closed-loop hydrometallurgy: A critical review of wastewater reuse strategies for end-of-life LiFePO₄ battery recycling. Green Chemistry. https://doi.org/10.1039/d5gc02987b

[6] Yang, L., Wang, D., Zhang, J., Chen, Y., & Wang, C. (2024). An economical and closed-loop hydrometallurgical method to prepare battery-grade iron phosphate from delithiated LiFePO₄ cathode scrap. Journal of Cleaner Production, 141194. https://doi.org/10.1016/j.jclepro.2024.141194