Impact of Using Flame-Retardant Electrolyte Additives in Li-ion Batteries: A Comprehensive Evaluation of PFPN

Li-ion batteries (LIBs) are seeing increasingly widespread adoption across consumer electronics, electric vehicles, and grid-scale energy storage systems, yet their susceptibility to thermal runaway remains a concern. This study evaluates ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as an electrolyte additive to improve electrolyte flammability and thermal stability without compromising electrochemical performance. Electrolyte flammability was quantified using Self-Extinguishing Time (SET) measurements, which revealed that PFPN significantly suppresses combustion. At 4 wt% PFPN, 67% of electrolyte samples failed to ignite despite extended ignition exposure, and the average SET decreased by 43% (from 51 s g⁻¹ to 29 s g⁻¹). Differential Scanning Calorimetry (DSC) further demonstrated improved thermal stability, with the onset of solvent decomposition delayed by ~30 °C at 4 wt% PFPN. Ionic conductivity modestly decreases (11%, from 10.26 to 9.12 mS cm⁻¹ at 4 wt% PFPN). Electrochemical testing showed negligible impact on battery performance. Graphite||Li and NMC811||Li half-cells containing PFPN exhibited comparable capacity retention to baseline cells. NMC811||Graphite pouch cells were used to further evaluate extended cycling and rate capability, PFPN containing cells demonstrated similar capacities even after prolonged cycling and high-rate operation. Overall, PFPN provides effective flame retardance at concentrations as low as 4 wt% while maintaining electrochemical compatibility, making it a promising additive for enhancing thermal stability of LIB electrolytes.