Recently, Professor He Ping and Professor Zhou Haoshen of Nanjing University published a research paper entitled 'Lithium Metal Extraction from Seawater' on July 27, 2018 in the Cell publication "Joule", the top academic journal of the energy field, and proposed an etheric energy. Driven energy, based on the idea of composite electrolyte (hybridelectorlyte) and constant-current electrolysis of ion-selective solid film, successfully extracts metallic lithium from seawater. The technology is developed for the development of marine lithium resources and chemical energy. Transforming storage opens up a whole new path.
Lithium is one of the most important mineral resources in modern society. It is widely used in ceramics, medicine, nuclear industry and the well-known lithium battery industry. With the popularity of electric vehicles and portable electronic devices, the scale of the lithium battery market has increased significantly. It is estimated that in the next 30 years, it will consume one-third of the current global recoverable lithium reserves (Figure 1A), which will lead to insufficient supply of lithium resources in the future.
At present, the world's recoverable lithium reserves are from ore and brine, totaling about 14 million tons. Extracting lithium salt from ore and brine will consume a lot of energy and cause serious pollution problems. Compared with onshore ore and brine With limited lithium resources, 230 billion tons of lithium resources are stored in seawater, which is 16,000 times of the total available lithium resources in the world (Fig. 1B). Therefore, if simple, controlled and clean extraction of lithium from seawater is achieved, humans Will get almost inexhaustible lithium resources.
Figure 1: (A) Expected annual consumption and total consumption curve of lithium resources between 2015 and 2050; (B) Comparison of lithium resource reserves in ocean and land, uneven distribution of lithium resources on land, mainly distributed in Chile, China, Argentina and Australia.
Although seawater contains extremely abundant lithium resources, the concentration of lithium in seawater is very low, only 0.1 to 0.2 ppm, which makes it difficult to extract lithium from seawater. The researchers have proposed many solutions, including adsorption methods. And electrodialysis.
The adsorption method is to realize the adsorption of lithium from seawater through the exchange of hydrogen ion and lithium ion by some hydrogenated metal oxides. The electrodialysis method is to promote the directional movement of positive and negative ions in seawater by an applied electric field, and then realize the selective permeation through the membrane. Enrichment of lithium ions.
The existing extraction technology of seawater lithium is slow and difficult to control, and the obtained primary extract needs further treatment to obtain metallic lithium or pure lithium compound (such as Li). 2CO 3Therefore, the existing seawater lithium extraction technology may not be able to meet the large demand for lithium resources in the future of new lithium battery technologies such as lithium-sulfur batteries and lithium-air batteries.
Professor He Ping and Professor Zhou Haoshen from the School of Modern Engineering and Applied Science of Nanjing University proposed the concept of combined electrolyte (Hybridelectrolyte) in 2009. This concept combines the characteristics of organic and water system electrolytes to broaden the battery system compared with single electrolyte. Working voltage and application range. Based on the combined electrolyte, the team developed new high-capacity batteries such as water-based lithium-air batteries, lithium-air fuel cells, lithium-copper batteries, and lithium liquid batteries.
Recently, the research team applied the strategy of combining electrolytes to the technology of seawater extraction metal lithium. The combined electrolyte designed by the team is composed of a positive electrode region and a negative electrode region. The positive electrode region is a lithium ion organic electrolyte protected by an argon atmosphere. The copper foil immersed in the electrolyte is used as the positive electrode; the seawater is used as the working electrolyte in the negative electrode region, 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, and the positive electrode region is separated. And the negative electrode region, the ceramic film only allows lithium ions to pass through. A self-designed micro-tunable too negative energy plate constant current power supply applies a constant current between the positive electrode and the negative electrode, so that the lithium ions in the seawater in the negative electrode region continuously pass through the solid Ceramic membrane, on the surface of the positive copper sheet to reduce the formation of metallic lithium, successfully achieved the extraction of metallic lithium from seawater (Figure 2).
Figure 2: (A) Schematic diagram of the electrolysis seawater extraction device driven by too negative energy; (B) Schematic diagram of the monomer of the device, from the top to the bottom, the negative electrolyte plate, the organic electrolyte positive electrode region, the ceramic selective film, In the seawater negative zone, the entire device can be floated on the surface with a rubber band; (C) An imaginary map of a large number of devices at sea.
During the electrolysis process, the reduction of lithium ions occurs on the positive electrode:
Li ++e-→Li
On the negative electrode, the oxidation of seawater:
2Cl -→Cl2+2e-
2OH -→H 2O+0.5O2+2e-
Cl 2+H2O→HClO+H ++Cl-
Figure 3: (A) at 80, 160, 240 and 320 μA·cm -2Potential-time curve at current density (illustrated at 80μA·cm -2Photograph of the electrode electrolyzed at current density for 1 h); (B) Metallic lithium production per square centimeter of copper; (C) XPS characterization of the deposited product before and after argon ion etching; (D) Positive electrode deposition product in argon ion engraving XPS characterization of Li and Na before and after etch; (E) XRD characterization of deposited products (Al peak from the sample stage of the atmosphere protection device)
In the process of extracting lithium from seawater, silvery white matter 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. At 80, 160, 240 and 320 μA·cm -2The electrolysis voltage at current density is 4.52V, 4.75V, 4.88V and 5.28V, respectively. The output of lithium metal is 1.9, 3.9, 5.7 and 1.2mg·dm respectively. -2·h -1(image 3).
When the current density exceeds a certain threshold, for example 320μA·cm -2The positive electrode will have a serious side reaction (decomposition of the electrolyte), which leads to a decrease in lithium production. It can be seen that the technical advantage of lithium extraction from the seawater is that the metal lithium can be directly obtained, and the metal lithium has a chemical that has been converted from too negative energy. Yes, can be released through 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 and preparation. The invention of this technology opens up a new path for the development of marine lithium resources and the conversion of too negative energy to chemical energy.
