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The essential transformations in the wake of the climate crisis and the associated shift away from fossil fuels are leading to completely changed demands for resources that were previously perceived as rather exotic. Lithium, for example, has long been one of the most sought-after raw materials, and demand for this white gold will continue to rise exponentially. The conversions in the mobility sector alone will make lithium a scarce resource. In principle, there is enough lithium on earth for the time being; for example, there are still considerable reserves in Chilean salt deserts or in Australian mines. But in the long term, there will be no getting around the need to develop new sources in order to meet the growing demand. For literally obvious reasons, regional lithium sources would be ideal, the exploitation of which would also have a positive effect on the CO2 footprint. Corresponding geological investigations are therefore taking place worldwide. Recycling lithium that has already been used also promises to relieve procurement pressure, but the cycle is still too much in its infancy for this to yield any significant return. Research is therefore also being devoted to new approaches, such as the extraction of the alkali metal from aqueous solutions. There are, for example, approaches to extract lithium from thermal or mine water. Finally, the world's oceans are an almost inexhaustible reservoir of lithium. Although the concentration of lithium in seawater is extremely low, there is a total of around 230 billion metric tons of this coveted raw material.

Potential recognized...

This is a potential that cannot simply be ignored. Researchers at the INM - Leibniz Institute for New Materials in Saarbrücken, Germany, have therefore developed a new electrochemical process for extracting lithium ions from seawater in collaboration with the Chinese Academy of Sciences in Shanghai. In ACS Energy Letters, the German-Chinese team led by Prof. Volker Presser has now presented the process, which, on the one hand, requires little energy input and, on the other, should ensure continuous separation of lithium.

...now it has to be lifted

The basis of the process for lithium extraction from aqueous solutions presented in ACS Energy Letters is a combination of a redox flow battery, a polymer membrane for the exchange of anions and two lithium-selective ceramic membranes (LISICON). Unlike the electrochemical reaction in solid electrodes of conventional batteries, redox flow batteries store energy by oxidation and reduction of a liquid electrolyte. The liquid medium has the advantage that the redox electrolyte can be pumped, ensuring continuous operation. Depending on the desired size of the system, the volume of the electrolyte tanks can also be easily adjusted.

Make four out of two

Basically, such an electrochemical cell consists of two chambers: one for electrochemical oxidation and a second for reduction - separated by an ion exchange membrane. In the INM system now presented, there are two additional channels between the two chambers for the redox electrolyte for the inflow of water containing lithium and for the enrichment of lithium ions, so that the overall system now has a total of four chambers. Meanwhile, the aforementioned LISICON ceramic membranes ensure that other cations, such as sodium or potassium ions, are effectively blocked.

Consistent lithium harvest

"You can think of our process as a bus driving in circles. Lithium ions, for example from seawater, are taken up by the reduction of a red potash solution in one chamber and released again during oxidation in another chamber," explains Prof. Presser, adding, "This 'on-and-off' has many advantages: First, we can run the system continuously, much like any other redox flow battery. This is very important for a consistent lithium harvest. And for another, it allows us to use different sources of lithium ions." Stefanie Arnold, a doctoral student in INM's Energy Materials Group, adds, "The process is suitable for natural water, for example from the oceans or from hydrothermal sources. But we can also use it for mine water or for the extraction of lithium ions in the hydrometallurgical recycling of used batteries."

Good prospects

The next step is to further improve the electrochemical system. "Currently, the ceramic LISICON membrane is the focus of our optimization strategy. Thinner lithium-ion membranes based on other materials will make the process much faster and result in lower costs while improving mechanical stability," Presser said. According to the scientists, such a technology will be able to make an important contribution to the lithium circular economy in the future.