Influence of Nanostructuring on the Dehydration Mechanism of NiF₂·4H₂O: Structural, Thermal, and Kinetic Studies

Self-assembled architectures of NiF2 nanoparticles, which were obtained from the thermal decomposition of NiF2·4H2O, have performed better as conversion-type cathode materials for lithium-ion batteries. However, due to the lack of systematic insight into the microstructural and morphological evolution of NiF2·4H2O, inconsistencies have always existed in its decomposition temperature and pathway. Furthermore, nanoscale NiF2·4H2O has never been considered in such studies, let alone its self-assembled architectures and their comparison with bulk counterparts. Additionally, as an interesting thermochemical material, NiF2·4H2O lacks experimental evaluation. Here, we present these aspects of self-assembled NiF2·4H2O architectures and evaluate the nanostructuring effect on their dehydration mechanism and thermal energy storage capacity. Marked differences were found between the decomposition temperature and mechanism of nano- and bulk NiF2·4H2O. Both samples decomposed in two irreversible steps, which involved mostly the loss of water, giving the end product t-NiF2–x(OH)x. However, the two samples followed different decomposition pathways. Nano NiF2·4H2O converted directly to t-NiF2–x(OH)x, while the diaspore form of o-NiF2–x(OH)x was found as an intermediate phase in the case of the bulk sample. Furthermore, the thermal energy storage capacity of nanoscale NiF2·4H2O was not only larger than that of the bulk but was also comparable to that of other top-performing thermochemical materials.