A gelation-phase separation strategy to create membranes with highly selective angstrom-scale transport pathways

Ion-selective membranes are among the most vital components in separation processes and electrochemical devices, such as seawater desalination and flow batteries. To overcome the permeability and selectivity trade-off of membranes, the construction of angstrom-scale transport pathways within ultrashort structures is essential. Herein, we report a novel gelation-phase separation strategy to fabricate ultrathin composite membranes with sub-nanometer pathways. The electrostatic interactions between anions of additives and dipoles of polymers drive the gelation of polymer solutions. Then, the interaction between the gel network and nonsolvent molecules induces unconventionally uniform phase separation at the molecular level, forming tunable angstrom-scale pathways within an ultrashort structure. Apart from high strength, the angstrom-scale pathways enable the membrane to achieve precise sieving of ions with slight size differences, while the ultrashort transport pathway minimizes transport resistance. The as-prepared robust free-standing 1.4 µm-thick composite membrane is demonstrated in a vanadium flow battery. It delivers a high energy efficiency exceeding 80% at a high current density of 240 mA cm^-2. This work introduces a novel gelation-phase separation strategy for molecular-level precise regulation of membrane microstructures, enabling high-performance membrane design.