Researchers from Nanjing University have developed a new method for the extraction of lithium metal from seawater using solar-powered electrolysis with a NASICON solid-state electrolyte as the selective membrane.

In a paper in the journal Joule, they report that their electrolysis method made the lithium extraction process faster and more controllable than in adsorption- and dialysis-based methods and that it could overcome the limit of the concentration difference that is present in the dialysis method.

Additionally, with an aprotic electrolyte in the cathode side, they were able directly to obtain metallic lithium during the lithium extraction process.

The team also said that their method is extensible by using different types of ion-selective membranes or anodes to recover other elements in seawater.

Shortage of future lithium supply and the uneven distribution of current lithium resources. Yang et al.

Currently all commercial lithium is sourced from ores and brines on land, which contains a total lithium reserve of 14 million tons according to the latest survey conducted this year. … the geographic distribution of land-based lithium resources is uneven, with more than 98% of the total reserves concentrated in Chile, Argentina, China, and Australia. In addition, lithium extraction from ores and brines has a significant environmental impact, including water pollution and depletion, soil damage, and air contamination.

… In contrast, the ocean contains 230 billion tons of lithium, an amount four orders of magnitude larger than the lithium reserves on land. Since the amount of lithium in this massive reserve is far higher than the amount consumed annually by human activity, the impact of lithium extraction from seawater on the lithium concentration in the ocean would be negligible. In other words, the omnipresent seawater can act as a nearly infinite global lithium resource, making it a promising source for the future lithium supply.

Even though the lithium reserves in the ocean are immense, its concentration in seawater is very low (0.1–0.2 ppm). Researchers have proposed several strategies for extracting lithium compounds from seawater, such as adsorption- and dialysis-based methods. … the lithium extraction rates of the current techniques are relatively slow.

—Yang et al.

In their study, the Nanjing researchers used a lithium superionic conductor (NASICON)-type solid-state electrolyte as the lithium-ion selective membrane, with an aprotic electrolyte instead of an aqueous solution being employed in the anode side of the electrolysis cell to create a proton-free compartment.

The prototype device can be powered by a solar panel and metallic lithium can be directly generated during the lithium extraction process.

Schematic Diagram of the Solar-Powered Lithium Extraction Device. (A) Mechanism of the lithium extraction device; (B) Single unit; (C) Scale-up device array on the sea. Yang et al.

The electrolyte of the electrolysis cell is divided into two parts by the solid-state electrolyte. A LiClO₄-propylene carbonate solution is on the cathode side and seawater is on the anode side. During electrolysis in the study, the cell was charged under constant current by a solar panel. Driven by the electric field, cations in seawater (Na⁺, Li⁺ …) move from the anode toward the lithium-ion-selective membrane. Only lithium ions transported to the cathode side; the other cations were blocked and remained in the anode compartment.

On the cathode side, lithium ions are reduced to metallic lithium on a copper foil. Cl^- or OH^- is oxidized to Cl₂ or O₂ on the anode side, and part of the Cl₂ may further react with water to form hypochlorite.

Further study is required to clarify the impact of the current density on the lithium production rate (or energy efficiency) of the lithium extraction device.

—Yang et al.

Resources

• Yang et al. (2018) “Lithium Metal Extraction from Seawater,” Joule doi: 10.1016/j.joule.2018.07.006