Dehydration Pathway of CoF₂·4H₂O Revisited by Integrated ex Situ and in Situ Calorimetric and Structural Studies

Cobalt(II) fluoride (α-CoF₂) has potential for application as a high-performance electrode material in lithium-ion batteries. α-CoF₂ is synthesized by the thermal heat treatment of CoF₂·4H₂O, commonly synthesized in an aqueous environment. There exists disagreement in the literature upon the mechanism, intermediate hydration states, and temperatures of the reaction. Here, we resolve this discontinuity by using integrated structural, thermogravimetric, and calorimetric analyses to elucidate the dehydration pathway of CoF₂·4H₂O in both ex situ and in situ experimental conditions. Specifically, the decomposition of CoF₂·4H₂O to α-CoF₂ has been investigated using isothermal thermogravimetry (ex situ TG), thermogravimetry (TG)-differential scanning calorimetry (DSC), kinetic analysis, and ex situ and in situ X-ray diffraction (XRD). We deduce that in two irreversible steps CoF₂·4H₂O completely decomposes into α-CoF₂, with an amorphous intermediary phase of CoF₂·0.5H₂O. Under DSC conditions with a heating rate of 10 °C/min, CoF₂·4H₂O dehydrates to CoF₂·0.5H₂O from 80 to 175 °C, and further dehydration between 175 and 300 °C leads to α-CoF₂. The α-CoF₂ phase remains stable up to the highest temperature recorded, 400 °C.