"This is a step towards addressing a major roadblock to nuclear energy," explained chemist and senior author Sarbajit Banerjee of ETH Zurich and Texas A and M University. "Lithium-6 is a critical material for the renaissance of nuclear energy, and this method could represent a viable approach to isotope separation."
Lithium-6 is vital for nuclear fusion but is difficult to extract from its more abundant sibling, lithium-7. The traditional separation method, known as the COLEX process, relies on liquid mercury, a substance banned in the U.S. since 1963 due to its environmental toxicity. Since then, American researchers have depended on a shrinking stockpile of lithium-6 stored at Oak Ridge National Laboratory in Tennessee.
The newly developed method emerged from an unrelated project aimed at purifying wastewater generated during oil and gas extraction. The team observed that their filtration membrane captured a significant amount of lithium from the saline water, prompting them to investigate further.
"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 6-lithium isotope."
The key component is zeta-vanadium oxide (zeta-V2O5), an inorganic compound characterized by its one-dimensional tunnel-like structure. These tunnels allow it to bind lithium ions with high precision.
"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 added.
In tests, the team used an electrochemical cell outfitted with a zeta-V2O5 cathode. When they flowed a lithium-ion solution through the system under an electric current, lithium ions migrated toward the cathode. Due to differences in mobility, lithium-6 ions were preferentially absorbed into the zeta-V2O5 tunnels, while lithium-7 ions mostly passed through.
"Lithium-6 ions stick a lot stronger to the tunnels, which is the mechanism of selectivity," said co-first author Andrew Ezazi of Texas A and 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."
The process visibly alters the material's color from yellow to olive green as lithium accumulates, allowing researchers to gauge progress at a glance. Initial results showed a single cycle enriched lithium-6 by 5.7%. Achieving 30% lithium-6 concentration-suitable for fusion applications-requires about 25 cycles, while 90% purity could be reached after 45 cycles.
"This level of enrichment is very competitive with the COLEX process, without the mercury," Ezazi noted.
Banerjee emphasized that while the method is not yet scaled for industrial use, it has significant 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, but within a bunch of flow cycles, you can get fusion-grade lithium for quite cheap," he said.
Beyond lithium, the team believes zeta-V2O5 and similar materials could be harnessed for other isotope separations, including applications involving radioactive substances. They are now exploring ways to industrialize the process.
"I think there's a lot of interest in nuclear fusion as the ultimate solution for clean energy," Banerjee concluded. "We're hoping to get some support to build this into a practicable solution."
Research Report:Electrochemical 6-Lithium Isotope Enrichment Based on Selective Insertion in 1D Tunnel-Structured V2O5
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ETH Zurich
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