Thermal decomposition pathways and interfacial reactivity in potassium-ion batteries: focus on the electrolyte and anode

Abstract

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 °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 °C triggers an early interfacial exotherm that builds an inorganic-rich secondary SEI and temporarily suppresses further anode attack up to ∼200 °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 °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 °C reactivity while maintaining electrochemical performance.