Angstrom-sized, lithiophilic and positively charged covalent organic framework membrane for high-efficiency Li+/Mg2+ separation

Covalent organic frameworks (COFs) are promising candidates for fabricating ion-selective membranes towards Li + extraction from brines. However, most COF membranes feature nanoscale pores, limiting their capability to sieve angstrom-sized ions. Herein, a DG-TBB membrane with angstrom-sized, lithiophilic and positive channels was fabricated via the condensation of a small, positively charged amine (1,3-diaminoguanidine monohydrochloride, DG) with an aldehyde containing lithiophilic groups (2,4,6-trimethoxy-1,3,5-benzenetricarbaldehyde, TBB). For comparison, the DG-TFB membrane based on 1,3,5-triformylbenzene (TFB) without lithiophilic groups was adopted to clarify the role of Li + -affinitive sites, while the ODH-TBB membrane containing neutral oxalyl dihydrazide (ODH) was employed to evaluate charge effect. Benefiting from the synergistic effects of Li + affinity and Donnan exclusion, DG-TBB membrane delivered the highest Li + permeation rate of 73.6 mmol m −2  h −1 and Li + /Mg 2+ selectivity of 1200, which were superior to DG-TFB (4.3 mmol m −2  h −1 , selectivity of 238) and ODH-TBB (64.9 mmol m −2  h −1 , selectivity of 39). Moreover, DG-TBB exhibited robust separation performance in diverse practical scenarios and for various mono-/divalent cations. This study demonstrates that the rational integration of angstrom-sized pores, lithiophilicity and positive charge into COF channels enables highly efficient ion separation.