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

Abstract

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.