A high-voltage cathode breakthrough for next-generation batteries

Engineers working in energy storage have long regarded iron as an attractive but fundamentally limited cathode material. Abundant, low-cost and geopolitically secure, iron has already displaced cobalt and nickel in many lithium-ion batteries through lithium iron phosphate (LFP) chemistries. Yet iron’s relatively low operating voltage has constrained energy density, forcing designers to trade performance against sustainability and cost

New research led by Stanford University and SLAC National Accelerator Laboratory now challenges that assumption, demonstrating an iron-based cathode material capable of reversibly exchanging five electrons per iron atom - far beyond the two or three electrons previously thought possible.

Published in Nature Materials, the work represents a significant advance in fundamental materials science with potentially wide-ranging engineering implications, from higher-energy lithium-ion batteries to future developments in magnetics and superconductivity.

BREAKING THE IRON CEILING

At the heart of the breakthrough is a long-standing question in redox chemistry: how many electrons can iron reliably donate and accept without destabilising its crystal structure? In most natural and engineered systems, iron cycles between oxidation states involving two or three electrons. In his 2018 doctoral thesis, Stanford alumnus William Gent proposed that, under the right structural conditions, iron might be pushed to a much higher oxidation state, enabling significantly greater energy storage.

Gent’s early work suggested a pathway but stopped short of a practical demonstration. That challenge was taken up several years later by Stanford PhD students Hari Ramachandran, Edward Mu and Eder Lomeli, who led a large interdisciplinary collaboration involving 23 researchers across universities and national laboratories in the US, Japan and South Korea.

Their target was a lithium-ion battery cathode material composed of lithium, iron, antimony and oxygen, referred to as LFSO. The goal was to stabilise iron in an unusually high-energy redox state without triggering the side reactions that typically limit performance, such as oxygen loss or irreversible structural collapse.

ENGINEERING STABILITY AT THE NANOSCALE

Early attempts to realise Gent’s concept ran into a familiar problem for battery engineers: structural degradation during cycling. When lithium ions were extracted during charging, the cathode lattice distorted and collapsed, rendering the material unusable.

The solution turned out to be one of scale. By reducing the cathode particles to the nanoscale – around 300 to 400 nanometres in diameter, roughly 40 times smaller than previous iterations – the researchers were able to fundamentally change how the material responded to lithium extraction.

“Making the particles that small was a significant challenge,” Ramachandran explains, but the team ultimately succeeded by growing the crystals from a carefully controlled liquid solution. Electrochemical testing suggested that the new material could indeed reversibly exchange five electrons per iron atom while maintaining structural integrity.

To confirm the underlying mechanism, the team combined advanced spectroscopy with computational modelling. Detailed X-ray and neutron studies carried out at Lawrence Berkeley, Oak Ridge and Argonne national laboratories revealed that the additional redox capacity was not attributable to iron alone. Instead, oxygen atoms within the crystal lattice also participated in the charge compensation process, enabled by the specific geometric arrangement of the material.

“The atoms in this very nicely arranged material behave like a single entity,” says Lomeli. This cooperative behaviour allows the system to reach a higher overall energy state than iron could achieve independently.

WHY VOLTAGE MATTERS

While LFP cathodes have become dominant – accounting for around 40% of lithium-ion batteries produced today – their lower voltage limits energy density at the pack level.

“A high-voltage, iron-based cathode could avoid the trade-off between higher voltage and higher-cost metals that previously dominated cathode materials,” Mu notes. In principle, this would enable higher energy density without reverting to cobalt- or nickel-rich chemistries that carry cost, ethical and supply-chain risks.

Around 70% of global cobalt supply originates from the Democratic Republic of the Congo, where mining has been linked to environmental damage and unsafe labour practices. Nickel supply chains also face volatility as demand rises from both battery and stainless steel markets. Iron, by contrast, is plentiful, inexpensive and globally distributed.

DISCOVERY TO DEPLOYMENT

Despite their promise, significant engineering work remains before LFSO-type materials could reach commercial use. While effective in stabilising the crystal structure, antimony is expensive and subject to its own supply constraints. The research team is now actively investigating alternative elements that could play a similar structural role without introducing new vulnerabilities.

Other practical considerations include particle morphology, manufacturability at scale, long-term cycling stability and compatibility with existing electrolyte systems. These are familiar hurdles for battery developers, but the underlying demonstration that iron can be pushed beyond a three-electron redox limit in a stable, reversible way represents a genuine step change.

“Scientists have rarely reported high-voltage iron-based materials,” says project lead William Chueh. “Our detailed electronic structure exploration provides conclusive evidence of oxidation beyond three electrons.”