Unveiling the structure, chemistry, and formation mechanism of an in-situ phosphazene flame retardant-derived interphase layer in LiFePO4 cathode

Abstract

The safety risks posed by the liquid electrolytes of lithium-ion batteries have drawn large-scale concern recently due to the increased reports of fires in electric vehicles and portable devices fuelled by these electrolytes. Efforts to address the fire safety of electrolytes have predominantly focused on the incorporation of flame retardant additives which generally lead to poor electrochemical properties when used in large quantities. Conversely, low amounts (∼5 vol%) of fluorinated phosphazene-based flame retardants have been reported to yield non-flammable electrolytes while improving the electrochemical properties. Herein, an advanced fire testing method, cone calorimetry, is employed to analyze a model electrolyte (ethoxy (pentafluoro) cyclotriphosphazene (EPCP) based electrolyte), which reveals that the flourinated phosphazene-based flame retardant electrolyte with up to 6 vol% only exhibits ignition delay rather than the widely acknowledged non-flammable behavior. Besides, for the first time, by employing a time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and transmission electron microscopy, we clarify the chemistry and structure of a flame retardant-derived cathod electrolyte interphase (CEI) layer formed on the LiFePO4 cathode surface. The CEI layer is characterized by a phosphorus and nitrogen (PN) rich layer, which inhibits the formation of a thick parasitic LiF layer, which improves the electrochemical integrity of cells.