Engineering Biomimetic Sub-Nanostructured Ion-Selective Nanofiltration Membrane for Excellent Separation of Li+/Co2+

Nanofiltration (NF) membranes with exceptional ion selectivity and permeability are needed for the recovery of lithium from waste lithium-ion batteries. Herein, inspired by the homogeneous microchannels in the skeletal structure of glass sponges, an innovative biomimetic sponge-like sub-nanostructured NF membrane was designed using an alkali-induced MXene (AMXene)-ethyl formate (EF)-induced bulk/interfacial diffusion decoupling strategy to simultaneously improve Li⁺/Co²⁺ selectivity and membrane permeability. The Li⁺/Co²⁺ separation factor (S_Li,Co = 24) of the engineered membrane was improved by an order of magnitude compared to that of an NF270 membrane (S_Li,Co = 2). The selectivity of Mg²⁺/Na⁺ ($B_{\text{NaCl}}/B_{{\text{MgCl}}_2}$ = 286) and ${\mathrm{SO}}_4^{2-}$/Cl⁻ ($B_{\text{NaCl}}/B_{{\text{NaSO}}_4}$ = 941) increased by 3 ~ 12 times, and the permeability (25.8 L m⁻² h⁻¹ bar⁻¹) remained at a desirable level, beyond the current upper bound of the other cutting-edge membranes. The superior performance was attributed to the limited release of amine in bulk phase and the boosted interfacial diffusion by reducing interfacial energy barrier during the interfacial polymerization reaction, which were realized via the synergetic effects of AMXene and EF. This approach yielded a biomimetic sponge-like sub-nanostructured NF membrane with controlled homogeneous pore radii (0.202 nm) and a thickness as small as 16.08 nm, which led to high ion selectivity and permeability. The engineered membrane was capable of efficient separation and recovery of Li⁺/metal ions.