Iron Battery Breakthrough Promises Cheaper, More Sustainable Energy Storage

CORVALLIS, Oregon & STANFORD, California – Scientists are making significant strides in utilising iron, one of Earth's most abundant and cheapest metals, to create high-energy density battery cathodes, a development that could drastically reduce the cost and environmental impact of lithium-ion batteries. Recent breakthroughs from institutions like Oregon State University and Stanford University demonstrate that iron can be engineered to achieve unprecedented energy states, paving the way for a new generation of more affordable and sustainable energy storage solutions.

Understanding the Types of Iron Battery Technologies

To fully grasp the landscape of the modern iron battery, it is essential to distinguish between the three primary architectures currently under development. While some researchers focus on incorporating iron into the cathode structure of traditional lithium-ion systems to phase out cobalt and nickel, others are designing completely different systems—namely, iron-air and iron flow batteries—specifically optimised for long-duration grid storage.

The table below compares these three dominant iron battery technologies, highlighting their mechanisms, typical energy densities, and primary use cases:

Reimagining Battery Chemistry with Iron

Traditional lithium-ion batteries heavily rely on expensive and often unethically sourced metals like cobalt and nickel for their cathodes, which can account for up to 50 per cent of a battery cell's cost. The surging demand for these materials in electric vehicles (EVs) and grid-scale storage has raised concerns about supply shortages and environmental contamination. Iron, costing less than a dollar per kilogram, presents an attractive alternative due to its abundance and lower toxicity.

Oregon State's Reactivity Transformation

A collaboration co-led by Oregon State University chemistry researcher Xiulei "David" Ji has successfully transformed the reactivity of iron metal, enabling it to function effectively as a cathode material in lithium-ion batteries. Published in Science Advances, their findings detail a chemical environment built from a blend of fluorine and phosphate anions. This blend allows for the reversible conversion of a fine mixture of iron powder, lithium fluoride, and lithium phosphate into iron salts, creating an electrode that can offer higher energy density than current state-of-the-art cathode materials in electric vehicles. This innovation means that iron can be incorporated into battery cathodes without requiring significant changes to existing battery designs or production lines.

Stanford's Five-Electron Redox Breakthrough

Independently, researchers at Stanford University and their international collaborators have achieved another remarkable feat, demonstrating an iron-based cathode capable of undergoing redox transitions involving five electrons per iron atom. Historically, iron's redox chemistry in battery cathodes was limited to two or three electrons, constraining its energy storage capacity. This new discovery, detailed in Nature Materials, shatters that ceiling, promising to dramatically enhance the energy density and voltage of lithium-ion batteries. The breakthrough involved meticulously refining the synthesis and characterisation of a new lithium-iron-antimony-oxygen (LFSO) cathode material, where the spatial separation of iron atoms within the crystal structure prevented unwanted side reactions that previously limited higher oxidation states.

The Economic and Environmental Advantages of Iron-Based Batteries

The shift to iron-based cathodes carries substantial economic and environmental benefits:

Dramatically Reduced Costs

By replacing costly cobalt and nickel, iron-based cathodes can significantly lower the overall production cost of batteries. Since the cathode can represent up to half the cost of a lithium-ion battery cell, this change could make electric vehicles and grid-scale energy storage far more affordable. Some estimates suggest these innovations could reduce the cost of a finished battery by 50 to 60 per cent.

Enhanced Sustainability and Safety

Iron is an Earth-abundant element, ensuring a stable and secure supply chain, unlike the precarious supplies of cobalt (70 per cent of which comes from the Democratic Republic of Congo, often with ethical concerns). Furthermore, current high-energy lithium-ion batteries using nickel and cobalt are approaching their energy density limits; pushing them further risks oxygen release during charging, which can lead to fires. Cobalt is also toxic and can contaminate ecosystems if it leaches from landfills. Iron-based alternatives offer greater inherent safety and a reduced environmental footprint.

Beyond Lithium-Ion: The Promise of Iron-Air and Flow Batteries

While these advancements primarily focus on improving lithium-ion technology, iron is also a key component in other promising battery chemistries designed for large-scale and long-duration energy storage.

Iron-Air Batteries

Iron-air batteries utilise iron as the anode and atmospheric oxygen as the cathode. These systems promise significantly higher energy densities than present-day lithium-ion batteries (theoretical energy densities of over 1,200 Wh/kg compared to lithium-ion's 600 Wh/kg) and leverage incredibly abundant, inexpensive materials.

The chemical reactions within an iron-air system mimic the natural rusting and unrusting processes. During discharge, the iron anode oxidises, reacting with oxygen to form iron hydroxide and releasing electrons. During charging, an electrical current reverses this reaction, converting the rust back into metallic iron and releasing pure oxygen.

The fundamental chemistry during discharge can be expressed through the following equations. At the iron anode, oxidation occurs:

$$\text{Fe}+2{\text{OH}}^{-}\to {\text{Fe(OH)}}_2+2e^{-}$$

At the air-breathing cathode, oxygen reduction occurs:

$${\text{O}}_2+2{\text{H}}_2\text{O}+4e^{-}\to 4{\text{OH}}^{-}$$

This results in the following reversible overall cell reaction:

$$2\text{Fe}+{\text{O}}_2+2{\text{H}}_2\text{O}\rightleftharpoons 2{\text{Fe(OH)}}_2$$

Where $\text{Fe}$ represents metallic iron, ${\text{O}}_2$ represents atmospheric oxygen, and ${\text{Fe(OH)}}_2$ represents the discharged iron(II) hydroxide (rust) state.

The simplified loop of this process is visualised in the flowchart below:

Companies like Form Energy are developing iron-air batteries that are reportedly ten times cheaper than lithium batteries, can last longer, and are safer, with commercial production having commenced at their facility in 2024. These batteries are particularly well-suited for grid stabilisation and storing renewable energy for extended periods of up to 100 hours. Form Energy's commercial-scale manufacturing facility in Weirton, West Virginia (Form Factory 1), is designed to produce multi-megawatt systems to support utilities transitioning away from fossil fuels.

Iron Flow Batteries

Iron-based flow batteries, designed for grid-scale energy storage, have also seen recent advancements. Rather than storing energy in solid electrodes, flow batteries store chemical energy in liquid electrolytes contained in external tanks, which are pumped through an electrochemical cell stack.

In an all-iron flow battery, the electrochemical reactions rely on the redox couple of iron in different oxidation states. At the negative electrode (anode side), iron ions plate onto the substrate or dissolve back into the solution during cycling:

$${\text{Fe}}^{2+}+2e^{-}\rightleftharpoons {\text{Fe}}^0$$

At the positive electrode (cathode side), iron oxidises or reduces between its divalent and trivalent ionic states:

$$2{\text{Fe}}^{2+}\rightleftharpoons 2{\text{Fe}}^{3+}+2e^{-}$$

Where ${\text{Fe}}^{2+}$ and ${\text{Fe}}^{3+}$ represent the soluble iron ions in the liquid electrolyte, and ${\text{Fe}}^0$ represents plated iron metal.

Researchers at Pacific Northwest National Laboratory (PNNL) have developed a new all-liquid iron flow battery that combines charged iron with a neutral-pH phosphate-based liquid electrolyte. This design exhibited remarkable cycling stability, retaining 98.7 per cent of its capacity over a thousand charging cycles, and utilises a commercially available chemical typically used in water treatment facilities.

In the commercial sector, Oregon-based ESS Inc. has successfully deployed commercial iron flow batteries (such as the Energy Warehouse and Energy Center). These systems avoid the toxic, corrosive acids common in traditional vanadium flow systems, utilising an environmentally benign iron-chloride electrolyte. These batteries offer a safe, economical, and water-based solution for integrating intermittent renewable energy sources into the electric grid without toxic run-off or thermal runaway risks.

The Future of Energy Storage

These breakthroughs in iron-based battery technology represent a pivotal step towards a more sustainable and affordable energy future. By overcoming previous limitations in iron's electrochemical performance, scientists are unlocking the potential for batteries that are not only cheaper and safer but also rely on materials that are readily available globally. The ongoing research and commercialisation efforts in both enhanced lithium-ion, iron-air, and iron flow batteries underscore a global movement towards weaning the world off expensive, scarce, and often environmentally damaging battery components. As these innovations move from the lab to industrial scale, they could accelerate the adoption of electric vehicles, bolster grid stability with renewable energy, and foster a new era of sustainable energy technologies.