However, Gent was able to take only an initial stab at making the material before his doctoral clock ran out.
Cut to 2025: Three subsequent Stanford PhD students, Hari Ramachandran, Edward Mu, and Eder Lomeli, led an interdisciplinary team that built off of Gent's work to achieve a fundamental discovery that indeed created a higher potential energy state than what was previously thought possible for an iron-based material. The team comprises 23 scientists spanning three U.S. universities, four U.S. national laboratories, and universities in Japan and South Korea. Nature Materials published the results of this team's work earlier this month.
The immediate potential application of the team's findings is for lithium-ion batteries, but other possible uses include magnetism applications, like magnetic resonance imaging (MRI) machines in medicine and magnetic levitation technologies for high-speed trains. The findings might also aid the development of superconductors.
Iron routinely takes part in reactions where it releases and reabsorbs electrons, known as redox reactions. From transporting oxygen in your body to plants growing and bikes rusting, iron redox reactions are a huge part of our lives.
Iron atoms tend to limit their contributions to these reactions to two or three of iron's 26 electrons. Gent believed he could make it do more, pushing the material to repeatedly give up five electrons per iron atom and take five back during charging. The researchers think that keeping the iron atoms from getting next to each other in the crystal structure of the material is key. Otherwise, side reactions, like oxygen atoms bonding, prevent iron's higher oxidation state. If used in a lithium-ion battery cathode, this could enable the battery to store more energy and provide higher voltage.
When Ramachandran and Mu took up Gent's work in 2021, they initially couldn't keep the battery material's crystal structure from collapsing during charging. The two figured that making the particles for their materials extremely small could help.
That wasn't easy.
"Making the particles very small - just 300 to 400 nanometers, or billionths of a meter, in diameter, about 40 times smaller than before - turned out to be a challenge," said Ramachandran, PhD '25.
In 2022, he and Mu found a solution.
"Literally, a solution. We grew our crystals out of a carefully concocted liquid," Mu said. "In our electrochemical tests, the material seemed to get iron to reversibly give up and later take back five electrons while the crystal structure remained stable."
At least, that's what Ramachandran and Mu's spectral images suggested was increasing the material's energy potential. To confirm this, they teamed up with Lomeli in 2023. Lomeli's faculty advisor, Tom Devereaux, is a pioneer of modeling and interpreting X-ray spectra using numerical methods in condensed matter physics.
Based on his detailed modeling of the spectra, Lomeli was eventually able to show that the extra two electrons, it turns out, come not from the iron atoms but from the oxygen with help from the iron.
"It's too simple to say that iron is the hero or oxygen is the hero when it comes to contributing free electrons," said Lomeli. "The atoms in this very nicely arranged material behave like a single entity."
Over the past few years, iron has come to replace cobalt and nickel as the dominant metal in lithium-ion cathodes globally for both electric vehicles and stationary storage systems. Iron is much less expensive than cobalt and nickel. In addition, 70% of the global supply of cobalt comes from the Democratic Republic of the Congo, and China controls much of the Congo's output. These mines have been reported to employ children in working conditions that are hazardous for all miners, and the mining has contributed to deforestation and contaminated rivers and soil.
As a result of these issues, 40% of lithium-ion batteries manufactured today use cathodes made of lithium, iron, and phosphorus. This cathode "is rapidly growing into the most popular battery cathode chemistry for both electric vehicles and grid-scale stationary storage applications," said Ramachandran.
However, these cathodes are not high-voltage, and batteries made from them aren't either. Car makers and other manufacturers have worked around the low voltage to achieve commercial success without nickel and cobalt.
"A high-voltage, iron-based cathode could avoid the tradeoff between higher voltage and higher-cost metals that previously dominated cathode materials," Mu said. "The best of both worlds."
To get a high-voltage, iron-based, reversible, and stable cathode, Ramachandran and Mu carefully synthesized their material from - as Gent suggested - lithium, iron, antimony, and oxygen, or "LFSO." Initial tests in the SLAC-Stanford Battery Center, a joint effort of Stanford Doerr School of Sustainability's Precourt Institute for Energy and SLAC National Accelerator Laboratory, showed that the high-voltage cathodes were stable.
To investigate the structure and action of LFSO in more detail and compare it with earlier versions that didn't work, the team examined it with beams of X-rays and neutrons at Lawrence Berkeley, Oak Ridge, and Argonne national laboratories.
Even with that information, Ramachandran and Mu still couldn't pin down exactly what was going on. Their advisor, William Chueh, and Devereaux are faculty members in the departments of Materials Science and Engineering in the School of Engineering and of photon science at SLAC, but Chueh approaches his work more from a chemistry perspective, while Devereaux's research focuses on theoretical and computational materials science.
Forcing an iron-based cathode material to give up more electrons provided more useful energy but weakened the material, which collapsed when lithium flowed to the anode during charging (top); a new version of this cathode material (bottom) bends slightly to accommodate the retreating lithium and remains intact for its return.
In the end, the combined approach of experimental results and computational modeling agreed: Unlike in the previous material, which twisted and collapsed after lithium ions pulled out and headed for the anode during charging, the material made from nanoparticles bent a little to accommodate the vacated lithium spaces and remained intact during cycling.
"Scientists have rarely reported high-voltage iron-based materials," said Chueh. "Our detailed electronic structure exploration of this iron species provides conclusive evidence of oxidation beyond three electrons."
Now that they know how to push iron to a high oxidation state and keep it there, Chueh said, the remaining core team is working on solving practical engineering problems - tweaking the shapes of particles, the composition of the material, and the chemistry to find a combination that will work in a commercial application. High on the list is finding a replacement for the antimony in LFSO. Like cobalt, it's an expensive mineral with significant supply chain vulnerabilities, but the evolving research team has several substitute candidates in mind.
Research Report:A formal FeIII/V redox couple in an intercalation electrode
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