A thermochemical energy storage system and method of storing thermal energy are disclosed. The energy storing system described herein comprises a reactor comprising a CO2 sorbent comprising i) CaO and mayenite or ii) Li4SiO4, or a combination thereof, and b) a CO2 source, wherein the CO2 source is in fluid communication with the reactor to allow flow of CO2 between the CO2 source and the reactor. Further, methods are disclosed for storing thermal energy through a wide temperature range.