Recently, Professor He Ping and Professor Zhou Haoshen of Nanjing University published a research paper titled "Lithium Metal Extraction from Seawater" online on July 27, 2018, in the academic journal "Joule" of Cell, a sub-journal in the field of energy. Combining the concept of hybrid electrolyte and the constant current electrolysis technology of ion-selective solid film, the metal lithium element was successfully extracted from seawater. The advent of this technology has opened up a new path for the development of marine lithium resources and the conversion and storage of solar energy into chemical energy.
Lithium is one of the most important mineral resources in modern society. It is widely used in ceramic chemical industry, medicine, nuclear industry and the well-known lithium battery industry. With the popularization of electric vehicles and portable electronic devices, the scale of the lithium battery market has grown significantly. It is estimated that in the next 30 years, it will consume 1/3 of the current global mineable lithium reserves (Figure 1A), which will lead to insufficient supply of lithium resources in the future. . At present, the mineable lithium reserves in the world all come from ore and brine, totaling about 14 million tons. Extracting lithium salts from ores and brines consumes a lot of energy and brings serious pollution problems. Compared with the limited lithium resources in ores and brines on land, there are 230 billion tons of lithium resources stored in seawater, which is 16,000 times the total amount of lithium resources that can be mined globally (Figure 1B). Therefore, if the simple, controllable and clean extraction of lithium from seawater is realized, human beings will obtain almost inexhaustible lithium resources.
The expected annual consumption and total consumption curve of lithium resources between 2015 and 2050; (B) the comparison map of lithium resource reserves in the ocean and land. The distribution of lithium resources on land is uneven, mainly in Chile, China, Argentina and Australia.
Although seawater contains extremely rich lithium resources, the lithium concentration in seawater is very low, only 0.1-0.2ppm, which makes it difficult to extract lithium from seawater. Researchers have proposed many solutions, including adsorption and electrodialysis. The adsorption method is to adsorb lithium elements from seawater through the exchange mechanism of hydrogen ions and lithium ions by some hydrogenated metal oxides. Electrodialysis is to promote the directional movement of positive and negative ions in seawater through an external electric field, and then achieve the enrichment of lithium ions by selectively permeating the membrane. The existing seawater lithium extraction technology has a slow extraction rate and is not easy to control. The obtained primary extract needs further processing to obtain metallic lithium or pure lithium compounds (such as Li2CO3). Therefore, the existing seawater lithium extraction technology may not be able to meet the large demand for lithium resources for new lithium battery technologies such as lithium-sulfur batteries and lithium-air batteries in the future.
Professor He Ping and Professor Zhou Haoshen from the School of Modern Engineering and Applied Science of Nanjing University proposed the concept of hybrid electrolyte (Hybridelectrolyte) as early as 2009. This concept combines the characteristics of organic and aqueous electrolytes and broadens the battery system compared with a single electrolyte. working voltage and application range. Based on the combined electrolyte, the team developed new high-capacity batteries such as aqueous lithium-air batteries, lithium-air fuel cells, lithium-copper batteries, and lithium flow batteries.
Recently, the research team applied the strategy of combining electrolytes to the technology of extracting metal lithium from seawater. The combined electrolyte designed by the team consists of a combination of positive and negative regions. The positive electrode area is a lithium-ion organic electrolyte protected by an argon atmosphere, and the copper foil immersed in the electrolyte is used as the positive electrode; the negative electrode area uses seawater as the working electrolyte, and the Ru@SuperP catalytic electrode is used as the negative electrode. The lithium ion solid electrolyte ceramic membrane is used as the lithium ion selective permeable membrane to separate the positive electrode area and the negative electrode area, and the ceramic membrane only allows lithium ions to pass through. A self-designed miniature tunable solar plate constant current power supply is used to apply a constant current between the positive electrode and the negative electrode, so that the lithium ions in the seawater in the negative electrode area continuously pass through the solid ceramic membrane, and are reduced on the surface of the positive electrode copper sheet to form lithium metal. , and successfully realized the extraction of lithium metal from seawater (Figure 2). The results were published online in the "FutureEnergy" column of "Joule".
Figure 2: (A) Schematic diagram of the principle of electrolytic seawater lithium extraction device driven by solar energy; (B) Schematic diagram of the single unit of the device, from top to bottom are solar plate, organic electrolyte positive electrode area, ceramic selective membrane, In the negative electrode area of seawater, the entire device can float on the sea surface with a rubber ring; (C) A hypothetical diagram of a large number of devices arranged on the sea.
During the electrolysis process, the reduction reaction of lithium ions occurs on the positive electrode:
On the negative electrode, the oxidation reaction of seawater:
Potential-time curves at current densities of 80, 160, 240 and 320 μA cm-2 (the inset is a photo of the electrode electrolyzed at a current density of 80 μA cm-2 for 1 h); (B) metal lithium on copper sheet per square centimeter Yield; (C) XPS characterization patterns of Li before and after argon ion etching of the deposition product; (D) XPS characterization patterns of Li and Na of the positive electrode deposition product before and after argon ion etching; (E) XRD characterization patterns of the deposition product (Al The peak comes from the sample stage of the atmosphere protection device)
During the process of extracting lithium from seawater, a silver-white substance is formed on the surface of the copper sheet. According to XPS and XRD analysis, the deposit on the surface of the copper sheet is metallic lithium. The electrolysis voltages were 4.52V, 4.75V, 4.88V and 5.28V at current densities of 80, 160, 240 and 320μA cm-2, respectively, and the yields of lithium metal were 1.9, 3.9, 5.7 and 1.2mg dm-2· h-1 (Figure 3). When the current density exceeds a certain threshold, such as 320 μA cm-2, serious side reactions (electrolyte decomposition) will occur at the positive electrode, resulting in a decrease in lithium production. It can be seen that the technical advantage of extracting lithium from seawater is that it can directly obtain elemental lithium metal, which already contains chemical energy converted from solar energy, which can be released by new battery systems such as lithium-sulfur batteries or lithium-air batteries. In addition, the constant current electrolysis method is fast and tunable, and is suitable for large-scale production. The invention of this technology has opened up a new way for the development of marine lithium resources and the conversion and storage of solar energy into chemical energy.