Mechanism on exothermic heat of FeF ₃ cathode in Li-ion batteries

To identify the mechanism of exothermic reactions of cycled FeF ₃ electrodes in an electrolyte with temperature ramp-up, the structure and thermal behavior of cycled FeF ₃ electrodes were investigated using a combination of XRD and DSC. The combination of XRD and DSC revealed that FeF ₃ can be restored and the electrochemical was reversible even after the conversion reaction. In the discharged electrodes, a structural transformation to LiFeF ₃ was observed after 1Li insertion, and two compounds (LiF and Fe metal) were formed during the conversion reaction. Accordingly, the differences of thermal behavior in the electrolyte were presented in the DSC curves. It was clear that no reaction occurred between LiF and the electrolyte at elevated temperatures. In contrast, Fe powder was found to react with the electrolyte at around 300 °C, indicating that Fe metal generated in the electrode had a direct effect on the thermal behavior of the discharged FeF ₃ electrode. Moreover, Fe powder was found to react with PF ₅ produced by LiPF ₆ decomposition. The mixture of discharged FeF ₃ electrode and electrolyte showed less heat generation than the electrolyte alone because some PF ₅ was consumed by the generated Fe metal in the electrode.