A major roadblock to nuclear fusion energy may soon be overcome, thanks to a newly developed method for isolating lithium-6, a key ingredient in fusion fuel. Researchers have discovered a mercury-free process that is as effective as the traditional method but without the environmental risks. The findings, published in the journal Chem, could pave the way for a more sustainable future for nuclear energy.
“This is a step towards addressing a major roadblock to nuclear energy,” senior author and chemist Sarbajit Banerjee of ETH Zürich and Texas A&M University said in a recent statement. “Lithium-6 is a critical material for the renaissance of nuclear energy, and this method could represent a viable approach to isotope separation.”
The Challenge of Lithium-6 Separation
Lithium-6 is an essential component in producing tritium, a key fuel for nuclear fusion. However, isolating lithium-6 from its more abundant counterpart, lithium-7, has long been a challenge. Since 1963, the United States has banned the conventional lithium-separation method, known as the COLEX process, because it relies on toxic liquid mercury. As a result, U.S. researchers have been forced to rely on a dwindling stockpile of lithium-6 maintained at Oak Ridge National Laboratory in Tennessee.
A Serendipitous Discovery
The researchers stumbled upon their lithium-6 isolation method while developing membranes to clean “produced water”—groundwater brought to the surface during oil and gas drilling. While testing their membranes, they noticed that the material was particularly effective at capturing lithium.
“We saw that we could extract lithium quite selectively given that there was a lot more salt than lithium present in the water,” said Banerjee. “That led us to wonder whether this material might also have some selectivity for the lithium-6 isotope.”
The team traced the lithium-binding properties to a material called zeta-vanadium oxide (ζ-V₂O₅), an inorganic compound with a unique tunnel-like structure.
“Zeta-V₂O₅ has some pretty incredible properties—it’s an amazing battery material, and now we’re finding that it can trap lithium very selectively, even with isotopic selectivity,” Banerjee explained.
Using Electrochemistry to Isolate Lithium-6
To test the material’s ability to separate lithium isotopes, the team built an electrochemical cell with a ζ-V₂O₅ cathode. When they pumped a lithium-containing solution through the cell while applying a voltage, positively charged lithium ions were drawn into the zeta-vanadium oxide tunnels.
Because lithium-6 is lighter than lithium-7, it interacts differently with the material. The researchers found that lithium-6 ions formed a stronger bond with the ζ-V₂O₅ tunnels, while the heavier lithium-7 ions moved more freely and were less likely to be captured.
“Lithium-6 ions stick a lot stronger to the tunnels, which is the mechanism of selectivity,” explained co-first author Andrew Ezazi of Texas A&M. “If you think of the bonds between V₂O₅ and lithium as a spring, you can imagine that lithium-7 is heavier and more likely to break that bond, whereas lithium-6, because it’s lighter, reverberates less and makes a tighter bond.”
An added advantage of this process is that it provides a visual indicator of lithium capture. As lithium ions are absorbed, the ζ-V₂O₅ material changes color from bright yellow to dark olive green, allowing researchers to monitor the degree of lithium isolation in real time.
Competitive with Traditional Methods—Without Mercury
In a single cycle, the process enriched lithium-6 by 5.7%. To achieve fusion-grade lithium—where at least 30% of the lithium is lithium-6—the process must be repeated about 25 times. After 45 cycles, the researchers demonstrated they could obtain lithium-6 concentrations of 90%, making the method comparable to the mercury-based COLEX process.
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“This level of enrichment is very competitive with the COLEX process, without the mercury,” noted Ezazi.
Though the method has yet to be implemented at an industrial scale, the researchers are optimistic about its potential.
“Of course, we’re not doing industrial production yet, and there are some engineering problems to overcome in terms of how to design the flow loop,” Banerjee said. “But within a bunch of flow cycles, you can get fusion-grade lithium for quite cheap.”
Lithium-6 Provides a Step Toward the Future of Fusion Energy
Beyond lithium-6 separation, the researchers believe their method could be applied to other isotope separations, including those used in medical and industrial applications.
“I think there’s a lot of interest in nuclear fusion as the ultimate solution for clean energy,” said Banerjee. “We’re hoping to get some support to build this into a practicable solution.”
Kenna Hughes-Castleberry is the Science Communicator at JILA (a world-leading physics research institute) and a science writer at The Debrief. Follow and connect with her on BlueSky or contact her via email at kenna@thedebrief.org