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Solid–gas thermochemical sorption heat storage is an efficient heat storage technology. Due to its high theoretical heat storage density, magnesium sulfate (MgSO4) is considered as one of the most potential sorption heat storage materials. Using zeolite 13X as the matrix, zeolite 13X/MgSO4 composite adsorbents were prepared, and the sorption–desorption performance was studied to reveal the potential heat storage capacity of the composite adsorbents. The addition of zeolite 13X aimed to enhance the hydration reaction rate by increasing the reaction surface area. The heat storage capacity of zeolite 13X/MgSO4 composite adsorbents with the salt mass fraction increasing from 5% to 20% was experimentally studied. Results showed that the composite sorbent with 20% MgSO4 offers the best heat storage performance. Under conditions of 25 °C and 60% RH, the mass sorption capacity of the composite sorbent containing 20% MgSO4 was 0.21 g g−1, and a heat storage density of 438.4 kJ kg−1 was achieved. With the mass fraction of MgSO4 being reduced from 20% to 5%, the sorption capacity and heat storage density of the composite decreased to 0.187 g g−1 and 378.8 kJ kg−1, respectively. To further enhance mass transfer, an appropriate amount of LiCl was added into the composite sorbent with 20% MgSO4, verifying that LiCl can not only improve the adsorption rate, but also increase the sorption capacity by 8.6%. [ABSTRACT FROM AUTHOR]