Design of a Hydrogel-Based Composite Evaporator for Effective Solar-Driven Lithium Extraction from Salt Lake Brine

Abstract Developing efficient lithium extraction technologies from salt lake brines is crucial to ensuring a stable lithium supply. Conventional lithium extraction from brines is energy-intensive and has high environmental costs. This study designed and fabricated a solar-driven membrane separation system for lithium extraction from brines, featuring a laminate structure. Poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels serve as photothermal top layers. Their strong water–polymer network interactions enable the hydrogels to generate negative pressures as high as 83.14 bar during rapid water evaporation, thereby driving transmembrane transport of water and ions. The bottom-layer ion-selective membrane efficiently separates Li+ from Mg2+ ions. The enriched Li+ ions are stored in the middle storage layer. Leveraging the synergistic effect of these layers, the hydrogel-based composite evaporator exhibits highly effective lithium extraction performance employing natural light as the sole energy source. When processing MgCl2–LiCl mixed salt solutions with concentrations of 50 g L–1 (MgCl2/LiCl mass ratio ∼20), the system achieved a LiCl flux of 5.17 g m–2 h–1 and a LiCl/MgCl2 separation factor as high as 16.02. Furthermore, a two-stage separation system increased the LiCl purity by 24.3 times, from 3.15% to 76.47%. This study presents a green, sustainable process for the efficient extraction of lithium from high-salinity brines.