Prospects on Mixed Tutton Salt (K0.86Na0.14)2Ni(SO4)2(H2O)6 as a Thermochemical Heat Storage Material

In this paper, a novel mixed Tutton salt (K0.86Na0.14)2Ni(SO4)2(H2O)6 was successfully synthesized as a single crystal and evaluated as a thermochemical heat storage material. Its thermal and thermochemical properties were correlated with the structure, which was determined by powder X-ray diffraction using the Le Bail and Rietveld methods. The elemental ratio between the K+ and Na+ monovalent cations was established by energy-dispersive X-ray spectroscopy. Similar compounds such as Na2Ni(SO4)2(H2O)4 and K2Ni(SO4)2(H2O)6 were also synthesized and used for structural comparisons. The (K0.86Na0.14)2Ni(SO4)2(H2O)6 salt crystallizes in monoclinic symmetry with the P21/c-space group, typical of hexahydrate crystals from the Tutton salt family. The lattice parameters closely resemble those of K2Ni(SO4)2(H2O)6. A comprehensive analysis of the intermolecular contacts, based on Hirshfeld surfaces and 2D fingerprint mappings, revealed that the primary interactions are hydrogen bonds (H···O/O···H) and ion-dipole interactions (K/Na···O/O···Na/K). The unit cell exhibits minimal void space, accounting for only 0.2%, indicative of strong atomic packing. The intermolecular molecular and atomic packing are important factors influencing crystal lattice stabilization and thermal energy supplied to release crystallographic H2O. The thermal stability of mixed Tutton salt ranges from 300 K to 365 K. Under the dehydration of its six H2O molecules, the dehydration reaction enthalpy reaches 349.8 kJ/mol, yielding a thermochemical energy storage density of 1.79 GJ/m3. With an H2O desorption temperature ≤393 K and a high energy storage density ≥1.3 GJ/m3 (criteria established for applications at the domestic level), the (K0.86Na0.14)2Ni(SO4)2(H2O)6 shows potential as a thermochemical material for small-sized heat batteries.