Researchers at Stanford University and the SLAC National Accelerator Laboratory have developed a high-voltage, iron-based cathode material capable of achieving a reversible iron redox transition beyond the conventional ferric ion, also known as iron (III) (Fe³⁺). It represents a step toward high-voltage, cobalt-free, and nickel-free lithium-ion batteries.

The new lithium–iron–antimony oxide compound, identified as Li₄FeSbO₆ (LFSO), exhibits a formal Fe³⁺ to Iron pentacarbonyl (Fe⁵⁺) transition and demonstrates exceptional structural stability and electrochemical performance.

Iron’s Redox Limits

Iron is the most abundant transition metal in the Earth’s crust and a well-established redox element in battery cathodes, such as in LiFePO₄. However, until now, iron-based cathodes have been limited to relatively low-voltage performance due to the Iron (II) (Fe²⁺)/Fe³⁺ redox couple.

Attempts to push iron to higher oxidation states, such as Fe⁴⁺, have faced challenges including structural collapse, oxygen instability, and irreversible side reactions. The Stanford-SLAC team achieved a stable Fe³⁺/Fe⁵⁺ redox process in a lithium-ion intercalation host by precisely controlling the cation ordering of Fe and Sb within the LFSO lattice.

This control enables a two-phase crystallographic transition during lithium extraction and reinsertion, thereby stabilizing high-valent Fe species without triggering oxygen dimerization or other degradation pathways.

Synthesizing the Nanoscale

The researchers synthesized nanoscale LFSO crystals composed of lithium, iron, antimony, and oxygen. The material was produced through a liquid-phase synthesis route designed to yield particles measuring only 300 to 400 nanometers in diameter—approximately forty times smaller than earlier versions.

The reduction in particle size proved critical in maintaining structural integrity during lithium extraction and insertion cycles. Larger crystals tested in prior studies collapsed during charging, as lithium ions departed the lattice, leading to capacity loss and irreversible structural damage.

The nanoscale LFSO, however, exhibited the ability to flex slightly, “bend” rather than break, accommodating the vacancies created by lithium removal and thus preserving the framework for continued operation.

The high-voltage redox couple operates at approximately 4.5 volts versus Li⁺/Li, which is significantly higher than that of conventional iron phosphate cathodes, and delivers a capacity of around 230 mAh per gram. The cell displayed minimal voltage hysteresis, less than 0.1 volts, and excellent resistance to calendar ageing, indicating strong reversibility and long-term stability.

Atomic-Scale Mechanism

To understand the atomic-scale mechanisms behind this performance, the team combined advanced X-ray and neutron diffraction studies with theoretical modeling. Valence-sensitive X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) revealed a direct Fe³⁺ to Fe⁵⁺ transition described as a 3d⁵ to 3d⁵L² configuration, where “L” denotes ligand holes associated with oxygen atoms.

This indicates that the oxidation process involves metal–ligand charge transfer rather than purely metal-centered oxidation. The negative charge-transfer character of the system suppresses oxygen dimer formation, a common cause of voltage hysteresis and degradation in other lithium-rich oxides.

The team’s computational models confirmed that electron redistribution occurs cooperatively between Fe and O atoms, forming a delocalized redox network across the lattice.

This cooperative charge-compensation mechanism allows Fe ions to reach a formal Fe⁵⁺ state without destabilizing the structure. When cation ordering is disrupted, either by defects or compositional disorder, the Fe³⁺/Fe⁵⁺ redox couple collapses, and unwanted oxygen redox processes become dominant.

These findings establish cation ordering as a fundamental design principle for achieving stable high-valent redox behavior in transition-metal oxides.

According to the study’s co-lead authors, Ramachandran and Mu, the success of this approach depended on precise control over particle size, lattice structure, and synthesis conditions. “Making the particles very small, just 300 to 400 nanometers, was a significant challenge,” said Ramachandran. “We had to grow our crystals out of a carefully formulated liquid to maintain order and stability.”

Industrial Implications

From an industrial perspective, the implications of this discovery are significant. Iron-based lithium-ion batteries currently dominate the market for electric vehicles (EVs) and stationary grid storage, primarily in the form of lithium-iron-phosphate (LFP) cathodes.

These batteries are inexpensive and safe, but their limited operating voltage restricts their energy density. By contrast, high-voltage LFSO could bridge the performance gap between LFP and nickel–cobalt-based cathodes while retaining the cost and supply-chain advantages of iron chemistry.

Global production of LFP batteries has surged amid concerns about the supply of cobalt and nickel. Approximately 70% of the world’s cobalt originates from the Democratic Republic of the Congo, and much of its output is controlled by China.

Cobalt mining is associated with environmental degradation, deforestation, and child labor, raising significant ethical and sustainability concerns. As a result, more than 40% of lithium-ion batteries manufactured today use lithium–iron–phosphate cathodes. However, these offer relatively low voltage and energy density, forcing manufacturers to design larger or heavier battery packs to meet performance targets.

By pushing the iron oxidation limit further, LFSO could enable lighter, more energy-dense batteries suitable for electric vehicles and large-scale grid applications without relying on cobalt or nickel.

The researchers note that the material’s electronic structure may be relevant to other technologies such as magnetic resonance imaging, magnetic levitation systems, and possibly superconductors.

The high oxidation state of Fe achieved in LFSO could expand the range of energy levels accessible for magnetic and quantum materials. Antimony, a key component of LFSO, remains an expensive and supply-constrained element.

In 2023, Researchers from Stanford University and SLAC National Accelerator Laboratory said they had solved the mystery behind the persistent problem of short circuits and failures in new lithium-metal batteries.