Highlights: •A free IP method featuring easy operation is proposed based on a newly-designed plate-and-frame assembly.•Ultrathin charged polyamide nanofilms are prepared to reduce the distance for monovalent ion permeation.•Different effect of membrane thickness reduction on transports of ions with different valence leads to enhanced ion selectivity.•A rigidly-contorted second monomer is introduced to enhance mechanical strength, microporosity, and electronegativity, simultaneously.