Impact of the Salt Anion on K Metal Reactivity in EC/DEC Studied Using GC and XPS Analysis

To develop K-ion batteries, the potassium metal reactivity in a half-cells must be understood. Here, it is shown first that the K metal leads to the migration of the electrode degradation species to the working electrode surface so that half-cells' solid electrolyte interphase (SEI) studies cannot be trusted. Then, the K metal reactivity was studied by combining gas chromatography (GC)-mass spectrometry, GC/Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis after storage in ethylene carbonate/diethylene carbonate (EC/DEC) wo/w 0.8 M KPF₆ or KFSI. A comparison with Li stored in EC/DEC wo/w 0.8 M LiPF₆ was also performed. Overall, full electrolyte degradation pathways were obtained. The results showed a similar alkali reactivity when stored in EC/DEC with the formation of a CH₃CH₂OCO₂M-rich SEI. For a MPF₆-based electrolyte, the reactivity was driven by the PF₆^- anion (i) forming mostly LiF (Li metal) or (ii) catalyzing the solvent degradation into (CH₂CH₂OCOOK)₂ and CH₃CH₂OCOOK as main SEI products with additional C₂H₆ release (K metal). This highlights the higher reactivity of the K system. With KFSI, the reactivity was driven by the FSI^- anion degradation, leading to an inorganic-rich SEI. These results thus explain the better electrochemical performance often reported in half-cells with KFSI compared to that with KPF₆. Finally, the understanding of these chemically driven electrolyte degradation mechanisms should help researchers to design robust carbonate-based electrolyte formulations for KIBs.