Electrochemical Li⁺ Enrichment of Mixed-Cation Brines with Antiselective Polymer Membranes

Natural brines with dilute quantities of Li⁺ are attractive as feedstocks for battery electric vehicle supply chains because of their onshore abundance. Electrolytic membrane reactors employing Li⁺-antiselective membranes are ideal for continuous removal of Na⁺ and other metals, upgrading brine quality and value. Here, we show that the selectivity of microporous polymer membranes and the permeability of cation exchange membranes can be combined in sulfonated polymers of intrinsic microporosity to defy conventional selectivity-permeability trade-offs for mixed-cation separations. The concentration-normalized Na⁺/Li⁺ selectivity reaches 3.03 due to mixed-ion effects, enabling the removal of 29% of Na⁺ ions and 41% of K⁺ ions from a brine sample from Teel’s Marsh (NV, USA) in less than 1 h, while only extracting 9.5% of Li⁺, producing a Li-enriched effluent. We discuss these results in the context of electrochemical efficiency, offering design criteria for minimizing losses originating from competing transport processes.