Prominently improved CO2/N2 separation efficiency by ultrathin-ionic-liquid-covered MXene membrane

•The ultrathin-IL-covered MXene membrane was proposed.•The [BMIM][TF2N]/MXene membrane has a CO2 permeance of up to 104 GPU, and no N2 passed.•The permselectivity of the composite membrane is dominated by the diffusion selectivity.•The excellent CO2/N2 separation performance is ascribed to the synergistic effect of IL on the surface of MXene and anions in the nanoslit.