Decoupling Ion Selectivity and Transport via Dual-Scale Interlayer Nanochannels in Reduced Graphene Oxide Membranes for Lithium Separation

Selective separation of Li+ from coexisting Ni2+, Co2+, and Mn2+ is an important step in lithium recovery from spent lithium-ion batteries. Reduced graphene oxide (rGO) membranes offer a promising platform for this separation through their ångström-scale interlayer nanochannels. However, the same narrow nanochannels hinder ion diffusion, inducing a severe permeability–selectivity trade-off. To decouple ion selectivity from transport, we report a nanoconfined sol–gel strategy for constructing a dual-scale interlayer architecture in rGO membranes. Intercalated between graphene oxide layers, aluminosilicate (AS) sols serve as cross-linkers and in situ vapor sources. During thermal aging, sol–gel condensation generates vapor that expands un-cross-linked galleries into transport highways, while cross-linked domains retain ion-selective confinement. The architecture is tunable across ion-selective interlayer spacing (dS, 7.1–8.3 Å) and expanded interlayer spacing (dE, 34–94 Å). Compared with conventional rGO membranes, the optimized dual-scale spacing membrane increases Li+ permeability 5.4-fold in mixed-ion solution while achieving high divalent-ion selectivity (Li+/Ni2+ = 246, Li+/Co2+ = 391, Li+/Mn2+ = 448). Molecular dynamics simulations attribute this selectivity to asymmetric energy barriers imposed on Li+ and divalent cations within the cross-linked spacing, while the expanded spacing facilitates low-resistance transport. This work shows how nanoconfined sol–gel chemistry can open a design space for fabricating high-performance ion recovery membranes.