Thermal energy storage takes a pivotal role in the renewable energy application and waste heat recovery through adjusting the instability and discrepancy between energy supply and demand. For the purpose of the long-duration storage application based on thermochemical sorption, SrCl₂ composite materials were prepared and a demonstrative prototype has been constructed. The long-duration storage capacity of thermochemical sorption heat storage was investigated with SrCl₂/NH₃ as the working pair. The experimental results show that the maximum heat storage density of SrCl₂/NH₃ thermochemical sorption heat storage system is about 1630.8 kJ kg⁻¹ SrCl₂ (or 1386.2 kJ kg⁻¹ composite sorbent) under the operation condition of the charging temperature of 92 °C, the condensation/evaporation temperature of −5 °C and the discharging temperature of 40 °C. The heat storage efficiency of thermochemical sorption system decreases when the discharging temperature increases, whilst the coefficient of performance is raised with the increasing discharging temperature. Under the experimental conditions, the highest values of sorption heat storage efficiency and coefficient of performance are 0.687 and 0.405, respectively. Experimental results reveal that the thermochemical sorption heat storage is an effective method for the long-duration heat storage applications. It can facilitate the large-scale solar thermal energy harvesting and the industrial waste heat recovery.