Flexible lithium selective composite membrane for direct lithium extraction from high Na/Li ratio brine

Monovalent selective ion-exchange membranes have been widely used for lithium extraction from brine with a high Mg/Li ratio. However, extracting Li + directly from raw brine with high Na/Li ratio remains a significant challenge. This study introduces a flexible lithium selective composite membrane by incorporating lithium-ion conductor with a quaternized polymer. The lithium-ion conductor provides pathways for lithium transmission, while the quaternized polymer effectively hinders the leakage of impurity cations. As a result, with the applied voltage of 2.0 V, the membrane achieved a high Li + /Na + separation coefficient of 35.48 with a Li + flux of 8.33 × 10 −10  mol cm −2  s −1 in the simulated Yiliping brine, which are 56.3 and 2.8 times higher than that of commercial monovalent selective cation exchange membrane (CIMS). We proposed a lithium transport mechanism involving an adsorption-dehydration-diffusion process through the membrane. Theoretical calculations indicate that the combined effects of significant adsorption energy (−0.47 eV), low dehydration energy (515 kJ mol −1 ), and low diffusion barrier (0.37 eV) leads to the high selectivity for lithium. These findings provide promising prospects for the large-scale preparation of lithium ion-selective membranes, enabling the direct lithium extraction from raw brine.