Dehydration Pathway of CoF 2 ·4H 2 O Revisited by Integrated ex Situ and in Situ Calorimetric and Structural Studies Cody B. Cockreham, Xianghui Zhang, Vitaliy G. Goncharov, Xiaofeng Guo, Hongwu Xu, and Di Wu* Cite This: J. Phys. Chem. C 2020, 124, 3551-3556 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: Cobalt(II) fluoride (α-CoF 2 ) has potential for application as a high-performance electrode material in lithium-ion batteries. α-CoF 2 is synthesized by the thermal heat treatment of CoF 2 ·4H 2 O, 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 CoF 2 ·4H 2 O in both ex situ and in situ experimental conditions. Specifically, the decomposition of CoF 2 ·4H 2 O to α-CoF 2 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 CoF 2 ·4H 2 O completely decomposes into α-CoF 2 , with an amorphous intermediary phase of CoF 2 ·0.5H 2 O. Under DSC conditions with a heating rate of 10 °C/min, CoF 2 ·4H 2 O dehydrates to CoF 2 ·0.5H 2 O from 80 to 175 °C, and further dehydration between 175 and 300 °C leads to α- CoF 2 . The α-CoF 2 phase remains stable up to the highest temperature recorded, 400 °C. ■ INTRODUCTION In the face of climate change, electric vehicles offer a significant reduction in carbon emissions over traditional transportation. 1 To enable commercially feasible and competitive electric vehicles, electrode materials with higher energy density are needed. 2 Transition metal fluorides have potential as lithium- ion battery cathode materials due to their high specific capacities. 2,3 Transition metal fluorides commonly exhibit multiple hydration states which can strongly affect their electrochemical performance. 4 As research continues into transition metal fluorides as electrode materials, it is important to examine not only the electrochemical properties of each of their hydration states but also the phase evolution and stability for better design and processing. CoF 2 (α-CoF 2 ) has recently received interest for application as an electrode material for energy storage 5-12 due to its high theoretical capacity (553 mA·h/g). 7 For optimal Li-ion insertion and greater performance, CoF 2 is employed as nanoparticles 5,6,8,12 or nanostructured hybrids 13 to increase its reactive surface area. One common strategy to create these nanostructures is by synthesis of CoF 2 ·4H 2 O in an aqueous environment followed by heat treatment to remove the waters of crystallization to form α-CoF 2 (see Figure 1). 5,13 To best enable the controllable synthesis of α-CoF 2 materials from CoF 2 ·4H 2 O, it is important to understand the mechanisms of crystalline water loss. Previous studies of the thermal decomposition of CoF 2 · 4H 2 O to the anhydrous α-CoF 2 have produced a considerable disagreement in the mechanism and temperature ranges of decomposition. Moreover, ex situ and in situ analyses usually lead to different conclusions. In 2008, Berdonosov et al., using thermogravimetric analysis (TGA), reported a three-step mechanism in which no water loss was seen below 120 °C, followed by the loss of about three water molecules to CoF 2 · 0.9H 2 O up to 310 °C and the loss of about one-half a water molecule to CoF 2 ·0.4H 2 O up to 440 °C, and finally, complete loss of crystalline water at 500 °C. 14 In 2016, Nasriddinov et al., using nonequilibrium and equilibrium tensiometry with a membrane null manometer, reported a two-step decomposi- tion mechanism including dehydration to the intermediate CoF 2 ·H 2 O at 72 °C and complete dehydration at 107 °C. 15 Most recently, in 2018, using thermogravimetry (TG) coupled with differential scanning calorimetry (DSC), Khan et al. Received: August 27, 2019 Revised: December 26, 2019 Published: December 31, 2019 Figure 1. Crystal structures of CoF 2 ·4H 2 O and α-CoF 2 , cobalt in blue, fluoride in green, oxygen in white, and hydrogen in red. Article pubs.acs.org/JPCC © 2019 American Chemical Society 3551 https://dx.doi.org/10.1021/acs.jpcc.9b08175 J. Phys. Chem. C 2020, 124, 3551-3556 Downloaded via WASHINGTON STATE UNIV on February 13, 2020 at 17:04:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.