Entrainer-Assisted Membrane Separation (EMS) Strategy for Circumventing Size-Azeotropes: Li/Na Separation

One of the most difficult challenges in valorizing spent batteries and seawater brine is the separation of Li from Na-rich environments due to their similar hydration radius and charge density. The Li/Na mixture, with a size difference of only 0.32 Å, could be considered as a size-azeotrope that must first be circumvented using an entrainer for effective separation. Analogous to the azeotropic distillation, we introduce an innovative Entrainer-assisted Membrane Separation (EMS) strategy that employs Li-selective ionophores as an entrainer to break the Li/Na size-azeotrope for effective membrane separation. The addition of a Li-selective entrainer improved the Li/Na yield selectivity (Y~s) from 1.2 to 14.2, allowing facile purification via membrane cascade diafiltration, resulting in 99% Li+ purity. The proposed EMS strategy presents a promising route for sustainable Li+ recovery from Na-rich environments, with further studies needed to improve the entrainer performance and stability in aqueous environments.