SAVANTS START EFFICIENT MINING OF LITHIUM FROM SEAWATER
Booming electric vehicle sales have spurred a growing demand for lithium, a light material essential for making power-packed rechargeable batteries that is not abundant in nature. Now, researchers report a major step toward tapping a virtually limitless lithium supply: pulling it straight out of seawater.
“This represents substantial progress” for the field, says Jang Wook Choi, a chemical engineer at Seoul National University who was not involved with the work. He adds that the approach might also prove useful for reclaiming lithium from used batteries, wrote Robert F. Service for Science on July 13, 2020 (July 14, 2020) in Manila.) Lithium is prized for rechargeables because it stores more energy by weight than other battery materials. Manufacturers use more than 160,000 tons of the material every year, a number expected to grow nearly 10-fold over the next decade.
S eawater could come to the rescue. The world’s oceans contain an estimated 180 billion tons of lithium. But it’s diluted, present at roughly 0.2 parts per million. Choi and other researchers have also tried to use lithium-ion battery electrodes to pull lithium directly from seawater and brines without the need for first evaporating the water. Those electrodes consist of sandwich-like layered materials designed to trap and hold lithium ions as a battery charges. In seawater, a negative electrical voltage applied to a lithium-grabbing electrode pulls lithium ions into the electrode. But it also pulls in sodium, a chemically similar element that is about 100,000 times more abundant in seawater than lithium. If the two elements push their way into the electrode at the same rate, sodium almost completely crowds out the lithium.
To get around this problem, researchers led by Yi Cui, a materials scientist at Stanford University, looked for ways to make electrode materials more selective. First, they coated an electrode with a thin layer of titanium dioxide as a barrier. Because lithium ions are smaller than sodium, it is easier for them to wriggle through and into the electrode sandwich. The researchers also changed the way they controlled the electric voltage. Instead of applying a constant negative voltage to the electrode, as others had done, they cycled it.
First, they applied a negative voltage, and then they briefly turned it off. Next, they applied a positive voltage, turned it off again, and repeated the cycle. The change in voltage, Cui explains, causes lithium and sodium ions to move into the electrode, stop, and then start to move back out when the current reverses.
However, because the electrode material has a slightly higher affinity for lithium than sodium, lithium ions are the first to move into the electrodes and the last to leave. So, repeating this cycle concentrates lithium in the electrode. After 10 such cycles, taking just minutes, Cui and colleagues ended up with a one-to-one ratio of lithium to sodium, they report this month in Joule.