Molecular understanding of Li⁺ separation from divalent cations across GO-crown-ether functionalized membrane: A molecular dynamics study

A graphene oxide (GO) membrane functionalized with 15-crown-5 ether was investigated for its potential in separating Li⁺ from divalent cations (M²⁺) in the context of lithium-ion battery (LIB) recycling. This study employs molecular dynamics simulations to evaluate the efficacy of GO membranes functionalized with crown ether in Li⁺/M²⁺ selectivity. A GO membrane with 0.0133 wt% crown functional groups (GO-2-crown system) demonstrates exceptional Li ⁺ separation performance from M²⁺ cations in both binary and mixed solutions. In binary solutions, the membrane demonstrated 100 % rejection of Co²⁺ and Cu²⁺, with high rejection rates for Mn²⁺ (95 %) and Ni²⁺ (93 %). In a mixed solution, the membrane exhibits exceptional selectivity, allowing only Li ⁺ permeation while achieving complete rejection of all M²⁺ ions. Examining the separation of Li⁺ from divalent cations, we identified dual selectivity mechanisms operating at the molecular scale. Li⁺ demonstrates optimal residence times with both GO and crown oxygen sites in both Li–Co²⁺ and Li–Mn²⁺ systems, enabling binding without permanent trapping while maintaining superior water exchange dynamics compared to Cu²⁺ and Ni²⁺ cations in the Li–Cu²⁺ and Li–Ni²⁺ systems. The strategic incorporation of 15-crown-5 groups creates an environment that simultaneously enhances Li⁺ mobility while restricting competing cations through unfavorable crown interactions or restricted hydration shell dynamics.