The performance of gas separation membranes is commonly limited by a trade-off relationship between gas selectivity and permeability. Here we report a design of alkali metal doped carbon nanotube membrane based on the carrier-facilitated transport mechanism. Using molecular simulations, we demonstrate the prominent separation capacity of a Lithium doped carbon nanotube membrane under flue gas conditions, far beyond the upper limit. Particularly, it shows an excellent CO2 selectivity, associated with an extraordinary permeability 2 orders of magnitude higher than most advanced membranes to date. Molecular snapshots reveal that the mobile dopants serve as atomic conveyors to selectively shuttle CO2 across channels of carbon nanotubes, allowing a dramatic gain of gas selectivity without a major loss of permeation. Such a separation scheme transcends the conventional trade-off correlation, and features general applicability to the development of various membrane materials for separating a wide range of gas mixtures. (C) 2015 Elsevier B.V. All rights reserved