Impact of electrolyte decomposition products on the electrochemical performance of 4 V class K-ion batteries

Abstract

In the pursuit of long-life K-ion batteries (KIBs), half-cell measurements using highly reactive K metal counter electrodes are a standard practice. However, there is increasing evidence of electrolyte decomposition by K metal impacting electrode performance. Herein, we systematically explored the K metal-treated electrolytes KPF₆, KN(SO₂F)₂ (KFSA), and their combination in ethylene carbonate/diethyl carbonate (EC/DEC), referred to as K-KPF₆, K-KFSA, and K-KPF₆:KFSA, respectively, after storage in contact with K metal. Through mass spectrometry analysis, we identified significant formation of carbonate ester-derived decomposition products such as oligocarbonates for K-KPF₆, while K-KFSA predominantly generates anions combining FSA⁻ with the solvent structures. Using three-electrode cells, we delineated the positive effects of the K-KFSA and K-KPF₆:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF₆ on K₂Mn[Fe(CN)₆] positive electrodes. The interactions between the decomposition products and the electrodes were further evaluated using density functional theory calculations. Full cell measurements using K-KPF₆:KFSA showed an improved energy density and capacity retention of 78% after 500 cycles compared with an untreated electrolyte (72%). Hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which was not observed in K metal-free cells. Overall, this work indicates further complexities to consider in KIB measurements and suggests the potential application of decomposition products as electrolyte additives.