Potassium Fluoride and Carbonate Lead to Cell Failure in Potassium-Ion Batteries

While Li-ion is the prevailing commercial battery chemistry, the development of batteries that use earth-abundant alkali metals (e.g., Na and K) alleviates reliance on Li with potentially cheaper technologies. Electrolyte engineering has been a major thrust of Li-ion battery (LIB) research, and it is unclear if the same electrolyte design principles apply to K-ion batteries (KIBs). Fluoroethylene carbonate (FEC) is a well-known additive used in Li-ion electrolytes because the products of its sacrificial decomposition aid in forming a stable solid electrolyte interphase (SEI) on the anode surface. Here, we show that FEC addition to KIBs containing hard carbon anodes results in a dramatic decrease in capacity and cell failure in only two cycles, whereas capacity retention remains high (> 90% over 100 cycles at C/10 for both KPF₆ and KFSI) for electrolytes that do not contain FEC. Using a combination of ¹⁹F solid-state nuclear magnetic resonance (SSNMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS), we show that FEC decomposes during galvanostatic cycling to form insoluble KF and K₂CO₃ on the anode surface, which correlates with increased interfacial resistance in the cell. Our results strongly suggest that KIB performance is sensitive to the accumulation of an inorganic SEI, likely due to poor K transport in these compounds. This mechanism of FEC decomposition was confirmed in two separate electrolyte formulations using KPF₆ or KFSI. Interestingly, the salt anions do not decompose themselves, unlike their Li analogues. Insight from these results indicates that electrolyte decomposition pathways and favorable SEI components are significantly different in KIBs and LIBs, suggesting that entirely new approaches to KIB electrolyte engineering are needed.