A review on advances in direct lithium extraction from continental brines: Ion-sieve adsorption and electrochemical methods for varied Mg/Li ratios

The unbalanced supply and demand of lithium (Li) has elevated the urge for its extraction owing to the accelerated surge of battery and electric vehicle (EV) industries to meet the carbon emission reduction target. As the cost of extracting Li from brine is typically 30–50% lower than conventional hard-rock sources, this work intends to critically analyze the evolution of direct lithium extraction (DLE) methods employed in Salt Lake brine with various magnesium/lithium (Mg/Li) mass ratios whereas the lithium brine concentration (LBC) methods seek to concentrate the Li brine and eliminate contaminants without isolating the Li from the brine. Solvent extraction, precipitation, adsorption, membrane technology, and electrochemical extraction are the developed methods for Li extraction from Salt Lake brine. This review focuses on the mechanism, workflow, and comparative analysis of different methods. Moreover, recent technological advancements to handle the high Mg/Li ratio, such as modification of adsorption using ion sieves, liquid-membrane electrodialysis, and efficient multicomponent doping electrode materials, have also been discussed in depth. Although it was previously believed that solvent extraction was only feasible for low Mg/Li ratio brines, it has recently been commercially applied for high Mg/Li ratio brines in China. Precipitation is more ecology-friendly and economically favorable because of its low cost. Li extraction from brines with high Mg/Li ratios also shows promising performance using aluminate (Al) precipitants and novel Mg precipitants. However, during Mg precipitation, there is a significant loss of Li. On the other hand, in the cost-effective adsorption method, aluminium salt adsorbents are industrially used, yet low adsorption capacities limit their application. Recently, ion-exchange methods have gained popularity, as 'Li sieves' exhibit remarkable selectivity and adsorption towards Li-ions and are effective at high Mg/Li ratios. Powdered ionic sieves have low fluidity and solution permeability despite their strong affinity and adsorption capacity. Membrane technology is promising because of the benefits of improved energy consumption, simple controls, high separation rates, and the continuity of the process, yet as an emerging technology, its commercial viability is not proven. Nevertheless, a coupled "adsorption-membrane" technique has been developed and used in China for Salt Lake brines with low Li grades. Furthermore, exceptional selectivity, low energy demand, and minimal impact on the environment of electrochemical methods make Li extraction from brine promising. Being a recent technology, there is ample scope for improving electrode materials and understanding the process mechanism and cell configuration. Lastly, perspectives on the future Li extraction from brines are conferred in this article. By combining the methods (i.e., adsorption and ion exchange, membrane technology, and electrochemical process), the growth potential exists for an efficient, cost-effective, green, and sustainable extraction technology for Li from Salt Lake brine with a high Mg/Li ratio.