Congener-welded crystalline carbon nitride membrane for robust and highly selective Li/Mg separation

Extracting lithium from salt-lake brines critically relies on the separation of Li⁺ and Mg²⁺, which could combat the lithium shortage. However, designing robust sieving membrane with high Li⁺/Mg²⁺ selectivity in the long-time operation has remained highly challenging. Here, we demonstrate a bioinspired congener-welded crystalline carbon nitride membrane that can accomplish efficient and stable monovalent ion sieving over divalent Mg ion. The crystalline carbon nitrides have uniform and narrow pore size to reject the large hydrated Mg²⁺ and rich ligating sites to facilitate an almost barrierless Li⁺ transport as suggested by ab initio simulations. These crystals were then welded by vapor-deposited congeners, i.e., amorphous polymer carbon nitride, which have similar composition and chemistry with the crystals, forming intimate and compatible crystal/polymer interface. As a result, our membrane can sieve out highly dilute Li⁺ (0.002 M) from concentrated Mg²⁺ (1.0 M) with a high selectivity of 1708, and can be continuously operated for 10 days.