Piperidinium-incorporated fabrication of high-performance polyamide nanofiltration membrane with high free volume for magnesium/lithium separation

Efficient magnesium-lithium separation is key to extracting lithium resources from salt lake brines. However, efficient magnesium-lithium separation is constrained by the high magnesium-to-lithium ratios on the nanofiltration separation performance due to weakened Donnan effect and inherent permeability-selectivity trade-off behaviour. To address this challenge, a dual methylpiperidinium ion liquid was designed in this work and incorporated into polyamide networks to manipulate the structural properties of polyamide networks membrane for improved magnesium/lithium separation. We demonstrate that the piperidinium modification of polyamide networks not only modulated and enhanced the morphology, hydrophilicity, free volume and electropositivity but also synergized the steric hinderance differentiation inside the membrane nanochannels. These structural advantages enabled the modified membrane to achieve high-separation performance with enhanced water permeance of 37.3 L m−2 h−1 bar−1 and Mg2+/Li+ selectivity of 30.6 (for Mg/Li mass ratio of 31.2). Molecular dynamics simulations further confirmed that the fast water transport and the difference in the ion separation behaviour is strongly correlated to the enhanced structural properties of membranes. We expect this work to provide insightful guidance for engineering high-performance membranes and make contributions in the application of nanofiltration in lithium mining from high Mg/Li ratio brines.