Precisive lithium extraction based on synergistic effect of water-rich 3D network in hydrogel and covalent grafted crown ether

Abstract

Efficient lithium recovery from salt-lake brine continues to be a critical enabler for the global energy transition and sustainable low-carbon development. Although the current established crown ether molecules with comparable ring size present high selectivity for Li${}^{+}$, it still suffers from low recovery efficiency and partial dissolution of crown ether molecules. Herein, we develop a crown ether-based hydrogel, MDB14C4-PAA, interconnected through covalent bonds via an in-situ approach for efficient lithium separation from salt-lake brine with high Mg/Li mass ratio. Specially, the obtained hydrogel presents a prospective water-rich 3D network with hierarchical pores for water connection with aqueous environment and no obvious structural fracture after swelling in water with the optimal usage of cross-linker (0.1% in molar). Notably, this system exhibits excellent Li${}^{+}$ extraction performance with adsorption quantity and Li${}^{+}$/Mg${}^{2+}$ separation factor up to 5.5 mg/g and 5.23, respectively, under the optimal operation condition, and outstanding cycling stability. The nuclear magnetic titration experiments with DFT calculations and molecular dynamics simulations demonstrate the main active complex sites of oxygen atoms in the crown ether ring with 1:1 of complex ratio and 0.6 Å of complex distance. And the COO${}^{-}$ groups in PAA chains generated due to deprotonation of COOH can synergistically enhance the complex of Li${}^{+}$. Moreover, the high selectivity of MDB14C4-PAA for Li${}^{+}$ over Mg${}^{2+}$ is proposed to be ascribed to the lower complex energy barrier of Li${}^{+}$, 70.51 kcal/mol, than that of Mg${}^{2+}$, 116.19 kcal/mol. This work constitutes a substantial insight into selective interaction of Li${}^{+}$ with crown ether-based materials and develops a novel strategy based on super-hydrophilic 3D network structure for special ions transfer.