A poly(methacryloylamino propyl trimethylammonium chloride)/poly(dimethylacrylamide) (PMAPTAC/ PDMAAm) double-network (DN) ion-gel membrane containing triethyl(2-methoxymethyl)phosphonium indazole ([P-222(101)][Inda]) was fabricated via a casting method, displaying excellent mechanical strength and high CO2 separation performance. Changes in the cross-linker loading of the PMAPTAC network were found to significantly impact the CO2 separation performance of the ion-gel membrane. For instance, as the cross-linker loading was decreased from 4 to 0.5 mol.%, the CO2 permeability and CO2/N-2 selectivity increased from 2254 to 7569 Barrer and from 130 to 210, respectively. The network structure and mechanical properties of the fabricated ion gels was also strongly dependent on the cross-linker loading of the PMAPTAC network, with excellent strength obtained at decreased loadings. The CO2 transport properties of the DN ion-gel membranes were substantially improved by increasing the [P-222(101)][Inda] content, which was mainly attributed to two cooperative factors: (1) carrier content increase, resulting in a large driving force for the diffusion of the CO2-complex and (2) decrease of the polymer network content, resulting in lower diffusion resistance. The proportional enhancement of CO2 permeance with reducing ion-gel membrane thickness implied that diffusion is the rate-determining step of CO2 permeation.