Thermal Decomposition Pathways and Interfacial Reactivity in Potassium-Ion Batteries: Focus on the Electrolyte and Anode

Potassium (K)-ion batteries are an attractive alternative to lithium-ion batteries due to their resource abundance, graphite-anode compatibility, manufacturability, and reduced reliance on critical metals. However, their thermal safety remains poorly defined. Here, we investigate a widely accepted “safer” anode-electrolyte pair, a graphite anode with a low-flammable electrolyte, 2.5 M potassium bis(fluorosulfonyl)imide (KFSI) in triethyl phosphate (TEP), to clarify decomposition pathways and interfacial reactivity. This work shows that stand-alone TEP primarily volatilizes, whereas in the presence of KFSI it thermally decomposes via FSI-derived intermediates, producing exothermic reactions totaling ~264 J g-1 above 200 degrees C and generating organophosphate/fluorophosphate species (e.g., diethyl fluorophosphate) together with SO2, HNO3, and SOF2. This is roughly twice the heat released by a conventional LiPF6-based carbonate electrolyte, underscoring that low flammability does not equate to safety. With potassiated graphite (KC8), potassium leaching at ~63-80 degrees C triggers an early interfacial exotherm that builds an inorganic-rich secondary SEI and temporarily suppresses further anode attack up to ~200 degrees C. Beyond this temperature, electrolyte and anode-electrolyte reactions contribute a total of ~262 J g-1, which is lower than that of the Li-ion analogue (~431 J g-1) but occurs at an earlier onset (~65 vs. ~100 degrees C). Interfacial analysis shows that heating transforms the initially stratified SEI into a K-rich, chemically homogenized interphase. Our findings demonstrate that low flammability alone does not ensure thermal safety; rather, interfacial reactivity governs risk. Engineering the SEI composition, controlling salt-solvent coordination, and selecting suitable binders are essential for suppressing sub-100 degrees C reactivity while maintaining electrochemical performance.