Ab-initio study of doped salt hydrates crystal stabilities for thermochemical heat storage

In order to move towards a sustainable energy economy, a reliable energy storage technology is necessary. Thermochemical heat storage based on reversible physico-chemical processes, like sorption of water in hygroscopic chloride-based salts, is a highly appealing approach. This concept allows one to store heat over long periods of time in relative compact systems. Promising materials for this process are MgCl2·nH2O and CaCl2·nH2O (n=0,1,2,4,6), which undergo chemical reactions between the different hydrates at the desired operating temperatures. However, challenges remain related to the kinetics and stability of these salt hydrates. Another challenge is the formation of HCl through hydrolysis of MgCl2·nH2O at high temperatures. Doped salts have the potential to overcome these challenges. Therefore, density functional theory (DFT) studies are performed on MgCl2·2H2O with varying contents of Ca, and on CaCl2·2H2O with varying contents of Mg. Advanced chemical bonding approaches (DDEC6 bond order and charges, Bader topological analysis) are used to describe the relevant chemical bonds within these doped structure. These tools allow to quantitatively and qualitatively describe the doped crystals, and provide insight in the strength and nature of its interatomic interactions. Important trends in structure stability of the doped salts are revealed. It is confirmed that doping MgCl2·nH2O increases its resistivity towards hydrolysis. However, too high amount of doping (above 39%), can result in metastable crystal structures.