Compositions suitable for reversibly storing heat in thermal energy systems (TES) include a salt hydrate represented by the formula: MX.sub.q.Math.nH.sub.2O. M is a cation selected from Groups 1 to 14 of the IUPAC Periodic Table, X is a halide of Group 17, q ranges from 1 to 4, and n ranges from 1 to 12. The cation (M) may have an electronegativity of about 1.8 and a molar mass about 28 g/mol. The anion (X) may have an electronegativity of about 2.9 to about 3.2. A distance between a cation (M) and coordinating water molecules (H.sub.2O) is about 2.1 . Thermal energy systems (TES) incorporating such compositions are also provided that are configured to reversibly store heat in the thermal energy system (TES) via an endothermic dehydration reaction and to release heat in in the thermal energy system (TES) via an exothermic hydration reaction.

Compositions suitable for reversibly storing heat in thermal energy systems (TES) include a salt hydrate represented by the formula: MX.sub.q.Math.nH.sub.2O. M is a cation selected from Groups 1 to 14 of the IUPAC Periodic Table, X is a halide of Group 17, q ranges from 1 to 4, and n ranges from 1 to 12. The cation (M) may have an electronegativity of about 1.8 and a molar mass about 28 g/mol. The anion (X) may have an electronegativity of about 2.9 to about 3.2. A distance between a cation (M) and coordinating water molecules (H.sub.2O) is about 2.1 . Thermal energy systems (TES) incorporating such compositions are also provided that are configured to reversibly store heat in the thermal energy system (TES) via an endothermic dehydration reaction and to release heat in in the thermal energy system (TES) via an exothermic hydration reaction.