Today, all commercial lithium is sourced from ores or brines on land, where the total lithium reserves amount to 14 million tons. In contrast, the oceans contain 230 billion tons of lithium, although the lithium concentration in seawater is very low — 0.1 to 0.2 parts per million (ppm). Several methods have been proposed for extracting lithium compounds from seawater, including adsorption- and dialysis-based methods, but the lithium extraction rates by the current techniques are relatively low. Now, researchers from Nanjing University (Nanjing, China; www.nju.edu.cn) have developed a solar-powered electrolysis process to extract lithium from seawater that is said to be faster and more controllable than the current methods, and that it could overcome the limit of the concentration difference present in the dialysis method.
The Nanjing team used a lithium superionic conductor Nasicon-type solid-state electrolyte as the Li+-selective membrane, with an aprotic electrolyte instead of an aqueous solution in the anode side of the electrolysis cell to create a proton-free compartment. The prototype device can be powered by a solar panel.
The electrolyte is divided into two parts by the solid-state electrolyte. A lithium-perchlorate (LiClO4)/propylene-carbonate solution is on the cathode side and seawater is on the anode side. Cations in seawater move from the anode toward the Li+-selective membrane, but only lithium ions are transported to the cathode. The other cations are blocked and remain in the anode compartment.
On the cathode side, lithium ions are reduced to metallic lithium on a copper foil. Chloride (Cl–) or hydroxide (OH–) ions are oxidized into chlorine or oxygen gas on the anode side. Some of the Cl2 gas may further react with water to form hypochlorite, according to the researchers.
