Development of a reactive force field for [formula omitted]n[formula omitted], and the application to thermochemical energy storage

Abstract

Calcium chloride salt hydrates (CaCl2·nH2O) have a high potential to be used as thermochemical storage material (TCM). However, specific material properties – e.g., slow diffusion, low thermal conductivity, melting temperature, and crystal stability – inhibits further implementation as robust TCM. Inherent bulk crystal defects like cracks, pores, and grain boundaries promoted during the (de) hydration cycle of the TCM, affect these material properties. Reactive force field molecular dynamics (ReaxFF-MD) is used to investigate CaCl2·nH2O (physical and chemical) properties, as well as the effect of crystal defects on them. In this sense, a new ReaxFF force field is developed, which can describe stable CaCl2·nH2O structures, accurate descriptions of crystal surface energies, and multiple material indicators like charges, reaction enthalpies, and radial distribution functions. The new force field is further used to investigate the thermal conductivity, dehydration echanisms/kinetics, and crack formation upon heating of the crystal. The thermal conductivity is found to be 1.1 and 0.5 W/mK for respectively CaCl2 and CaCl2·2H2O, which is in good agreement with experimental results. Additionally, we investigated the influence of grain boundaries and the salts’ anisotropic crystal morphology and found that both grain boundaries and the typical layered structure in z-direction lower the thermal conductivity. By investigating dehydration mechanisms, it is shown that initial dehydration is 1.9–2.5 times lower in the z-direction, also due to the typical layered morphology. For all directions, superficial dehydrated CaCl2 layers impede dehydration of core layers, but cracks and pores significantly promote it. These molecular-scale findings reveal nanoscale opportunities that could benefit the TCM.