Recently, Professors He Ping and Zhou Haoshen of Nanjing University published a research paper entitled "Lithium Metal Extraction from Seawater" online in *Joule*, a Cell sub-journal of leading energy journals, on July 27, 2018. The paper proposes a galvanic negative energy-driven, constant-current electrolysis technique based on a hybrid electrolyte and ion-selective solid-state thin films, successfully extracting elemental lithium from seawater. This technology opens up new avenues for the development of marine lithium resources and the conversion and storage of galvanic negative energy into chemical energy.
Lithium is one of the most important mineral resources in modern society, widely used in ceramics, chemicals, pharmaceuticals, nuclear industry, and the well-known lithium battery industry. With the popularization of electric vehicles and portable electronic devices, the lithium battery market has grown significantly and is expected to consume one-third of the world's current exploitable lithium reserves in the next 30 years, leading to a future shortage of lithium resources.
Currently, the world's exploitable lithium reserves all come from ores and brines, totaling approximately 14 million tons. Extracting lithium salts from ores and brines consumes vast amounts of energy and causes serious pollution problems. Compared to the limited lithium resources in terrestrial ores and brines, seawater contains 230 billion tons of lithium resources, 16,000 times the total current global exploitable lithium resources (Figure 1B). Therefore, if a simple, controllable, and clean extraction of lithium from seawater could be achieved, humanity would have access to a virtually inexhaustible lithium resource.
Although seawater contains extremely abundant lithium resources, the lithium concentration is very low, only 0.1~0.2 ppm, which makes it difficult to extract lithium from seawater. Researchers have proposed many solutions, including adsorption and electrodialysis methods.
Adsorption methods utilize hydrides of metal oxides to adsorb lithium from seawater through the exchange of hydrogen and lithium ions. Electrodialysis, on the other hand, uses an external electric field to induce the directional movement of positive and negative ions in seawater, followed by the enrichment of lithium ions through a selectively permeable membrane.
Existing seawater lithium extraction technologies suffer from slow extraction rates that are difficult to control. The initial extracts require further processing to obtain metallic lithium or pure lithium compounds (such as Li₂CO₃). Therefore, current seawater lithium extraction technologies may not be able to meet the large demand for lithium resources from future new lithium battery technologies such as lithium-sulfur batteries and lithium-air batteries.
Professors He Ping and Zhou Haoshen of the School of Modern Engineering and Applied Sciences at Nanjing University proposed the concept of hybrid electrolyte as early as 2009. This concept combines the characteristics of organic and aqueous electrolyte systems, broadening the operating voltage and application range of battery systems compared to single electrolytes. Based on hybrid electrolytes, the team has developed novel high-capacity batteries such as aqueous lithium-air batteries, lithium-air fuel cell batteries, lithium-copper batteries, and lithium redox flow batteries.
Recently, this research team applied a combined electrolyte strategy to the extraction of metallic lithium from seawater. The combined electrolyte designed by the team consists of a positive electrode region and a negative electrode region. The positive electrode region uses an argon-protected lithium-ion organic electrolyte, with a copper foil immersed in the electrolyte serving as the positive electrode; the negative electrode region uses seawater as the working electrolyte, with a Ru@SuperP catalytic electrode as the negative electrode. A lithium-ion solid electrolyte ceramic membrane is used as a lithium-ion selectively permeable membrane to separate the positive and negative electrode regions, allowing only lithium ions to pass through. A self-designed micro-tunable negative plate constant current power supply is used to apply a constant current between the positive and negative electrodes, allowing lithium ions from the seawater in the negative electrode region to continuously pass through the solid ceramic membrane and be reduced to metallic lithium on the surface of the copper foil at the positive electrode, successfully achieving the extraction of metallic lithium from seawater.
During electrolysis, a reduction reaction of lithium ions occurs at the positive electrode:
Li++e-→Li
Meanwhile, at the negative electrode, seawater undergoes an oxidation reaction:
2Cl-→Cl2+2e-
2OH-→H2O+0.5O2+2e-
Cl₂ + H₂O → HClO + H⁺ + Cl⁻
During the lithium extraction process from seawater, a silvery-white substance was generated on the surface of the copper sheet. XPS and XRD analyses confirmed that the deposit on the copper sheet surface was metallic lithium. The electrolysis voltages at current densities of 80, 160, 240, and 320 μA·cm⁻² were 4.52 V, 4.75 V, 4.88 V, and 5.28 V, respectively, and the metallic lithium yields were 1.9, 3.9, 5.7, and 1.2 mg·dm⁻²·h⁻¹, respectively (Figure 3).
When the current density exceeds a certain threshold, such as 320 μA·cm⁻², severe side reactions (electrolyte decomposition) occur at the positive electrode, leading to a decrease in lithium production. Therefore, the technological advantage of this seawater lithium extraction method lies in its ability to directly obtain elemental lithium metal. Elemental lithium metal already contains chemical energy converted from negative energy, which can be released through novel battery systems such as lithium-sulfur batteries or lithium-air batteries.
Furthermore, the constant current electrolysis method offers rapid and tunable preparation, making it suitable for large-scale production. This invention opens up entirely new avenues for the development of marine lithium resources and the conversion and storage of negative energy into chemical energy.
