Electrochemical cell extracts lithium from sea water
Lithium is a vital element in the batteries that power electric vehicles, but soaring lithium demand is expected to exhaust land-based reserves by 2080. Researchers at King Abdullah University of Science and Technology (KAUST) have now developed an economically viable system that can extract high-purity lithium from sea water, described in the journal Energy & Environmental Science.
The oceans contain about 5000 times more lithium than the land, but at extremely low concentrations of about 0.2 ppm. Larger ions, including sodium, magnesium and potassium, are all present in sea water at much higher concentrations; however, previous research efforts to tease lithium from this mixture have yielded little.
The KAUST team solved this problem with an electrochemical cell containing a ceramic membrane made from lithium lanthanum titanium oxide (LLTO). Its crystal structure contains holes just wide enough to let lithium ions pass through while blocking larger metal ions. As explained by KAUST postdoc Zhen Li, who developed the cell, “LLTO membranes have never been used to extract and concentrate lithium ions before.”
The cell contains three compartments. Sea water flows into a central feed chamber, where positive lithium ions pass through the LLTO membrane into a side compartment that contains a buffer solution and a copper cathode coated with platinum and ruthenium. Meanwhile, negative ions exit the feed chamber through a standard anion exchange membrane, passing into a third compartment containing a sodium chloride solution and a platinum-ruthenium anode.
The researchers tested the system using sea water from the Red Sea. At a voltage of 3.25 V, the cell generates hydrogen gas at the cathode and chlorine gas at the anode. This drives the transport of lithium through the LLTO membrane, where it accumulates in the side chamber. This lithium-enriched water then becomes the feedstock for four more cycles of processing, eventually reaching a concentration of more than 9000 ppm. Adjusting the pH of this solution delivers solid lithium phosphate that contains mere traces of other metal ions — pure enough to meet battery manufacturers’ requirements.
The researchers estimate that the cell would need only US$5 of electricity to extract 1 kg of lithium from sea water. The value of hydrogen and chlorine produced by the cell would more than offset this cost, and residual sea water could also be used in desalination plants to provide fresh water.
“We will continue optimising the membrane structure and cell design to improve the process efficiency,” said group leader Zhiping Lai. His team also hopes to collaborate with the glass industry to produce the LLTO membrane at large scale and affordable cost.
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