Thermal energy storage materials

A thermal energy storage material (TESM) system that reproducibly stores and recovers latent heat comprising:

• a. a container having a wall surface;
• and
• b. a TESM in at least partial contact with the wall surface, and including a eutectic composition of
• i) a lithium salt selected from the group consisting of lithium nitrate and lithium nitrite;
• ii) a sodium salt selected from the group consisting of sodium nitrate and sodium nitrite;
• wherein the TESM optionally including water, wherein the water concentration if any is present is less than about 2 wt. % based on the total weight of the TESM;
• wherein the TESM has a liquidus temperature, TL, from about 100° C. to about 250° C.;
• wherein the TESM exhibits a heat storage density from 300° C. to 80° C. of at least about 1 MJ/l;
• so that upon being used in a system that generates heat, at least a portion of the heat is captured and stored by the TESM and subsequently released for use, and wherein the absolute value of the change in mass of the wall surface in contact with the TESM is less than about 1 g per m2 of the wall surface in contact with the TESM after 45 days exposure to the TESM at 300° C. in an inert atmosphere.