Organic Redox Flow Device for Sustainable Lithium Extraction from Seawater Using a Flexible LLZO/PVDF-HFP Composite Membrane

The global transition to green energy has precipitated a surge in the demand for lithium resources. However, terrestrial lithium reserves are limited and unevenly distributed. While the ocean harbors abundant lithium, its ultralow concentration and the interference from high concentrations of background ions pose significant challenges for efficient extraction. This study introduces a strategy for lithium extraction from seawater, leveraging the synergistic effect of a flexible LLZO/PVDF-HFP composite membrane and a TEMPO redox couple. A cubic phase garnet-type Li_6.5Ga_0.25La_2.75Ba_0.25Zr₂O₁₂ (LLZO) solid electrolyte, prepared via a sol-gel method, was compounded with PVDF-HFP to create a flexible membrane with high lithium-ion selectivity. With integration into a redox flow device, the TEMPO redox couple drives the transmembrane migration of lithium ions. The effects of the applied voltage, TEMPO concentration, and operating time on the lithium extraction performance were systematically investigated. At an optimal constant voltage of 1.5 V, the lithium concentration in the enriched solution reached 20.6 times that of the initial seawater. The system demonstrated excellent selectivity with Li/Na, Li/Ca, and Li/Mg selectivity factors of 2309.3, 11164.2, and 3808.9, respectively. After 72 h of continuous lithium extraction, the Li⁺ concentration in the recovered solution reached 130.6 times that of the feed, with an average energy consumption as low as 34.1 Wh/g_(Li). This research demonstrates a promising path for the development of low-cost, highly selective seawater lithium extraction technologies.