<p>Polyamide (PA) membranes exhibit promising prospects for lithium extraction from spent lithium-ion batteries (LIBs) leachate but face challenges in breaking the trade-off between selectivity and permeability. Herein, we demonstrate that the nascent PA membrane post-grafted with triaminoguanidinium (TAG) monomers (PA-TAG membranes) can gain expanded ion passage channels (0.8–7.1 Å) with the enhanced positive charge, achieving high-performance lithium separation. The PA-TAG membrane exhibits pure water permeance (PWP) of 15.5 L m⁻² h⁻¹ bar⁻¹, superior divalent ions rejection (∼98%), and excellent separation factor (∼30), much higher than that of the pristine PA membrane. A simulated acid battery leachate was used to test the PA-TAG membrane, achieving a relative volumetric lithium recovery rate of 48.2%. After a two-stage nanofiltration process, the Li⁺/M²⁺ mass ratio of the 2nd permeate reaches 53.35, 445 times that of the feed solution (0.12), showing promising potential for extracting Li from the acidic battery leachate.</p>