Producing nuclear fusion fuel is banned in the US for being too toxic, but these researchers found an alternative

"This is a step towards addressing a major roadblock to nuclear energy," says chemist and senior author Sarbajit Banerjee of ETH Zürich and Texas A&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."

The conventional method used to isolate lithium-6, called the COLEX process, involves liquid mercury and has been banned in the United States since 1963 due to pollution concerns. Since then, almost all lithium-6 used in US research has relied on a diminishing stockpile maintained at Oak Ridge National Laboratory in Tennessee. Having a safe method of isolating lithium-6 will be key to unlocking nuclear fusion as a sustainable energy source.

The researchers stumbled upon their method of isolating lithium-6 isolation while developing membranes for cleaning "produced water" -- groundwater that is brought to the surface during oil and gas drilling and that must be cleaned before it can be pumped back underground. They noticed that their cleaning membrane captured disproportionate quantities of lithium in the water.

"We saw that we could extract lithium quite selectively given that there was a lot more salt than lithium present in the water," says Banerjee. "That led us to wonder whether this material might also have some selectivity for the 6-lithium isotope."

The membrane's lithium-binding properties are due to a material called zeta-vanadium oxide (ζ-V₂O₅), a lab-synthesized inorganic compound that contains a framework of tunnels running in a single dimension.

"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," says Banerjee.

To test whether the material could separate lithium-6 from lithium-7, the team set up an electrochemical cell with a zeta-V₂O₅ cathode. When they pumped an aqueous solution containing lithium ions through the cell while applying a voltage, the positively charged lithium ions were drawn towards the negatively charged zeta-V₂O₅ matrix and into its tunnels. Because lithium-6 and lithium-7 ions move differently, the zeta-V₂O₅ tunnels preferentially captured lithium-6 ions while the more mobile lithium-7 ions escaped capture.

"Lithium-6 ions stick a lot stronger to the tunnels, which is the mechanism of selectivity," says 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."

As lithium ions are integrated into the zeta-V₂O₅, the compound gradually changes color from bright yellow to dark olive green, which enables the degree of lithium isolation to be easily monitored.

The team shows that a single electrochemical cycle enriched lithium-6 by 5.7%. To obtain fusion-grade lithium, which requires a minimum of 30% lithium-6, the process needs to be repeated 25 times, and 90% lithium-6 can be obtained in about 45 sequential cycles.

"This level of enrichment is very competitive with the COLEX process, without the mercury," says Ezazi.

"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," says Banerjee.

The researchers say that their results suggest that materials like zeta-V₂O₅ could be used to isolate other substances, for example, to separate radioactive from non-radioactive isotopes.

Now, the team is taking steps to scale their method up to an industrial level.

"I think there's a lot of interest in nuclear fusion as the ultimate solution for clean energy," says Banerjee. "We're hoping to get some support to build this into a practicable solution."