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-NiF_2-x(OH)_x. However, the two samples followed different decomposition pathways. Nano NiF₂·4H₂O converted directly to t-NiF_2-x(OH)_x, while the diaspore form of o-NiF_2-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.