Intensified ion-to-channel interactions within pyridinic covalent organic framework membranes towards exclusive lithium-ion sieving†

Membrane separation technologies demonstrate outstanding potential for achieving efficient lithium-ion (Li⁺) extraction from the battery leachate in a high-value and eco-friendly way, but, up to date, the rare focuses on developing one kind of specific Li⁺-filter applicable in this context. Herein, we prepared a pyridinic two-dimensional covalent organic framework (2D COF-Py) membrane featuring non-angstrom-sized 1D channels for exclusive Li⁺ sieving. The 2D COF-Py membrane enabled an excellent Li⁺ permeance (∼30 mmol m⁻² h⁻¹) with impressive Li⁺/M²⁺ selectivity of over 47 under any mixed salt conditions (e.g. LiCl-CoCl₂, LiCl-NiCl₂, and LiCl-MnCl₂). The experimental measurements and theoretical calculations revealed the dual roles of pyridine groups in dominating the ion transport behavior across the COF membrane. One role was to ensure fast Li⁺ transmembrane activity via electrostatic attraction and the other was to suppress M²⁺ free diffusion by forming strong coordination interactions. When stimulated battery leachate (a quaternary cation solution) was used as the feed solution, the COF-Py membrane not only sustained a striking separation performance under a long-term operation test without losing any trade-off but also maintained structural stability under high-salinity conditions.