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

Abstract

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 KPF 6 and KFSI) for electrolytes that do not contain FEC. Using a combination of 19 F 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 K 2 CO 3 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 KPF 6 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.

Tracking Passivation and Cation Flux at Incipient Solid‐Electrolyte Interphases on Multi‐Layer Graphene using High Resolution Scanning Electrochemical Microscopy

“…For the K + ‐SEI, the area under the F 1s peak decreased relative to the C 1s area, which indicated more extensive hydrocarbon/organic species content during build‐up of the SEI [51] . This agrees with previous reports suggesting differences in the degradation mechanism of the K + electrolytes compared with Li + and Na + [52] …”

“…[51] This agrees with previous reports suggesting differences in the degradation mechanism of the K + electrolytes compared with Li + and Na + . [52] Looking closer at the F 1s peaks, [53,54] in Figures 5b,c, and in the valence band spectra (Figure S20), the Li + -SEI was dominated by LiF at both formation potentials. For the K + -SEI, the KF peak remained overshadowed by PÀ F containing species (likely arising from KPF 6 crystals as shown in Figure S21 and S22), suggesting poorer incorporation of K + in the SEI structure (also evident from decreasing K 2p peak intensities relative to the graphite CÀ C peak, Figure S19c, d).…”

Vital roles of fluoroethylene carbonate in electrochemical energy storage devices: a review

“…ROCO 2 Na, ROSO 2 Na, and RSO 3 Na); this approach can reduce the irreversible decomposition of solvents. 148 However, FEC as the additive in the electrolyte solution of 0.8 M KPF 6 - or 0.8 M KFSI-PC/EC for a HC electrode resulted in a dramatic drop in capacity, whereas capacity remained high for those without FEC addition, 137 as shown in Fig. 6(d).…”

Less is More: Trace Amount of a Cyclic Sulfate Electrolyte Additive Enable Ultra-Stable Graphite Anode for High-Performance Potassium-Ion Batteries

“…In contrast, only a few studies demonstrated the improved SEI properties of K metal in PIBs by electrolyte additives. − Although the (de)­intercalation mechanisms of the graphite anode in LIBs and PIBs are similar, one cannot simply apply the knowledge of electrolyte additives obtained from LIBs to PIBs. For example, both potassium fluoride and potassium carbonate are recently demonstrated to be harmful to K + conduction in the SEI . Therefore, the frequently used LIB additives such as fluoroethylene carbonate) and vinylene carbonate) cannot be used in PIBs.…”

“…For example, both potassium fluoride and potassium carbonate are recently demonstrated to be harmful to K + conduction in the SEI. 21 Therefore, the frequently used LIB additives such as fluoroethylene carbonate) and vinylene carbonate) cannot be used in PIBs. To avoid the overproduction of KF in SEI, the PIB electrolyte additive must be turned into sulfur, nitrogen, and boron-containing compounds.…”