Redox-Programmed Bipolar Membranes from Electronically Wired Ion-Insertion Hosts for Selective Ion Transport

Abstract Selective ion separation is critical for resource recovery, water purification, and electrochemical energy technologies, yet it remains difficult to combine high selectivity, high flux, and simultaneous transmembrane uptake and release in a single-membrane architecture. Conventional polymer membranes are scalable but often lack molecular specificity. Redox-active ion-insertion materials provide lattice- and redox-defined ion selectivity, but their transport requires electronic charge compensation, limiting their use as single-stage transport membranes. Here, we establish electronically wired ion insertion hosts as active electrochemical membranes for redox-programmed ion separation. Li+-insertion particles are connected into a composite electronic–ionic conductor (CEIC), transforming a battery-type host into a membrane that stores, relays, and releases Li+ through spatially coupled redox reactions. Li+ transport is dominated by Li1–xMn2O4-mediated insertion/deinsertion, with a minor contribution from direct ionic transport through the porous membrane. Under an applied driving potential, bipolar electrochemistry couples Li+ insertion at the brine side interface with Li+ release at the recovery side interface, producing directional transmembrane transport. A LiMn2O4-based CEIC selectively transports Li+ from Na+-rich brine; integration with a Li+-selective polymer layer further suppresses nonselective ion transport and increases the Li+/Na+ separation factor to ∼2000. Spatially resolved electron energy loss spectroscopy reveals a lithiation gradient across the membrane, providing direct structural evidence of redox-coupled transmembrane transport. Replacement of LiMn2O4 with LiFePO4 further shows that modularity extends to the insertion host itself such that membrane selectivity can be programmed by the choice of insertion chemistry. This work shows that inorganic insertion chemistry can program membrane selectivity beyond the polymer design.