Revisiting Interfacial Enhancement Effects of Fluorinated Cyclophosphazene in Practical Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have garnered significant attention due to their cost-effectiveness and excellent low-temperature performance. However, practical applications of SIBs face critical challenges, such as sluggish kinetics and even sodium plating, particularly when they are paired with carbon-based anodes. In this study, we introduce ethoxy (pentafluoro) cyclotriphosphazene (PFPN), a flame-retardant additive, into the electrolyte and demonstrate its application in a Na_0.98Ni_0.5Fe_0.2Mn_0.3O₂-hard carbon (NFM-HC) practical pouch cell. During cycling, PFPN decomposes on the electrode surface, forming a thin and uniform solid electrolyte interphase (SEI) enriched with phosphorus and nitrogen elements. This optimized SEI structure not only enhances interfacial kinetics but also effectively suppresses sodium plating on the hard carbon electrode, significantly improving the cell's performance. The pouch cell exhibits remarkable cycling stability, with a capacity retention of 85% after 450 cycles at 0.2 C and 0 °C and 84% after 600 cycles at 0.5 C and 30 °C. This work highlights the potential of PFPN as a high-performance electrolyte additive for long-life and low-temperature SIBs, offering new insights into electrolyte design for advanced energy storage systems.