Decomposition of Binary Mixtures of DMC/EC, EMC/EC, and DEC/EC on Potassium Surfaces; GC, XPS, and Calculation.

Potassium-ion batteries (KIBs) have emerged as promising candidates for low-cost, high-energy storage systems, driven by their fast ionic conductivity and high operating voltage. To develop advanced KIBs, the performance is usually evaluated in half-cell tests using highly reactive potassium metal, which often leads to misinterpretation of the results due to degradation processes between metal anode and electrolyte components. Here, we systematically investigated the surface reactivity of potassium metal, which is in contact with commonly used solvent combinations, namely, mixtures of ethylene carbonate and linear bis(alkyl)carbonates. Mass spectrometry analysis identified the main electrolyte degradation species, namely, di- and trifunctionalized carbonates, ether-bridged carbonates, and ether-like compounds. Possible reaction pathways for the formation of these products were evaluated by using density functional theory calculations (DFT). X-ray photoelectron spectroscopy showed that potassium metal favors the formation of electrode degradation species, leading to an inorganic rich solid electrolyte interphase composed of K₂CO₃, KOH, and R-OK species. Additionally, we were able to show how the potassium metal itself forms an initial surface layer containing KOH and K₂CO₃. This study highlights the complexity of KIB measurements and clearly reveals the challenges of interpreting half-cell tests.