Counter-ion controlled ionic sieving in graphene oxide/polyelectrolyte membrane for Li⁺/Mg²⁺ separation from low-quality brines

Precise control of the interlayer spacing of graphene oxide (GO) membranes at the sub-nanometer scale offers size-exclusion-based separation of Li⁺/Mg²⁺, a critical step towards efficient lithium resource management and energy sustainability. While previous GO-based membranes have shown high Mg²⁺ rejections, their performance was largely limited to static diffusion settings or diluted solutions where Donnan exclusion dominates. Here, we report a GO membrane intercalated with a uniform polyelectrolyte layer. The membrane shows substantially enhanced MgCl₂ rejection over a wide concentration range, while allowing LiCl permeation under pressure-driven filtration. This enables direct lithium recovery from low-quality brines with high MgCl₂ concentrations and high Mg²⁺/Li⁺ ratios. Mechanistic study reveals that the adsorption of halide counter-ions onto the charged GO channel walls narrowed the interlayer spacing and enabled size-exclusion-based ionic separation. At higher ionic concentrations, a partial exchange of these adsorbed counter-ions with those in the second hydration shell of Mg²⁺ allowed moderate Mg²⁺ permeation. We further demonstrate a streamlined GO-based membrane filtration process, which rejects 99.53% Mg²⁺ from a low-quality brine, effectively upgrading it towards a high-quality lithium source, and highlighting the potential of this approach for sustainable lithium production.