Mechanism on exothermic heat of FeF3 cathode in Li-ion batteries

To identify the mechanism of exothermic reactions of cycled FeF3 electrodes in an electrolyte with temperature ramp-up, the structure and thermal behavior of cycled FeF3 electrodes were investigated using a combination of XRD and DSC. The combination of XRD and DSC revealed that FeF3 can be restored and the electrochemical was reversible even after the conversion reaction. In the discharged electrodes, a structural transformation to LiFeF3 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 FeF3 electrode. Moreover, Fe powder was found to react with PF5 produced by LiPF6 decomposition. The mixture of discharged FeF3 electrode and electrolyte showed less heat generation than the electrolyte alone because some PF5 was consumed by the generated Fe metal in the electrode.

► The electrochemical reaction of FeF3 electrode was reversible after cycling.
► The structural transformation was observed during the discharge process.
► No reaction occurred between LiF and the electrolyte at elevated temperatures.
► The Fe metal in the FeF3 electrode can consume some of PF5 in the electrolyte.