The CO2 capture properties of M2CO3 (M = Na, K)-promoted and CaCO3-promoted MgO sorbents are investigated by first-principles density functional theory complemented with lattice phonon calculations. The calculated thermodynamic properties indicate that by forming double salts (M2Mg(CO3)2 and CaMg(CO3)2), compared to pure MgO, the maximum allowable CO2 capture temperatures of the M2CO3- and CaCO3- modified MgO sorbents are shifted to higher temperature ranges. Under pre-combustion conditions with PCO2 = 10 bar, the Na2CO3-promoted and CaCO3-promoted MgO sorbents can capture CO2 at temperatures as high as 915 K and 740 K respectively. While under post-combustion conditions with PCO2 = 0.1 bar, their maximum allowable CO2 capture temperatures are 710 K and 600 K respectively. However, when adding K2CO3 into MgO, under both pre- and post-combustion conditions, its maximum CO2 capture temperatures only increased about 10 K relative to pure MgO. These results indicate that by mixing another solid into MgO, it is possible to shift its CO2 capture temperature to fit practical industrial needs.