Weakening Chaotropic Effect of Li ⁺ in Two-Dimensional Confined Channels via Coulomb Interactions for Efficient Li ⁺ /Mg ²⁺ Separation

The poorly solvated nature of Li⁺ induces a strong interfacial affinity at the walls of the two-dimensional (2D) confined channels, a phenomenon referred to as the "chaotropic effect". This phenomenon severely hinders the transport of Li⁺ by creating substantial interfacial barriers, thereby compromising the Li⁺/Mg²⁺ separation efficiency. To tackle this challenge, we developed a strategy of grafting charged groups, such as sulfonates, onto the walls of graphene oxide (GO) channels. Theoretical simulations demonstrate that the Coulomb attraction between the negatively charged sulfonates and Li⁺ effectively repositions Li⁺ away from the channel walls toward the central region. This strategic redistribution of Li⁺ reduces the unfavorable Li⁺-wall interaction energy from -31.18 kJ/mol to -5.26 kJ/mol and suppresses the Li⁺'s hydration shell reconfiguration by approximately 49%. We experimentally engineered a sulfonated GO membrane that yields an almost 2-order-of-magnitude enhancement in Li⁺/Mg²⁺ selectivity and concurrently boosts Li⁺ flux by a factor of 5 compared with the pristine GO membrane, further firmly validating the feasibility of our strategy. This work establishes a conceptual framework for realizing highly efficient ion separation through 2D membranes.