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

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 (similar to 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.

Automated summary

The safety risks associated with the liquid electrolytes of lithium-ion batteries have become a major concern due to an increase in fires related to these electrolytes in electric vehicles and portable devices. Efforts to address the fire safety of electrolytes have primarily focused on incorporating flame retardant additives, which often result in poor electrochemical properties. However, studies have shown that the addition of low amounts (around 5 vol%) of fluorinated phosphazene-based flame retardants can yield non-flammable electrolytes while also improving electrochemical properties. This research utilizes advanced testing methods to analyze a model electrolyte, revealing that a fluorinated phosphazene-based flame retardant electrolyte with up to 6 vol% only delays ignition, contrary to widely believed non-flammable behavior. Additionally, for the first time, the chemistry and structure of a flame retardant-derived cathode electrolyte interphase (CEI) layer formed on the LiFePO4 cathode surface are clarified, demonstrating that the CEI layer, characterized by a phosphorus and nitrogen-rich composition, inhibits the formation of a thick parasitic LiF layer and improves the electrochemical integrity of cells.