Cyclophosphazene-ring-opening polymerization enables long-life, wide-temperature and safe sodium-ion batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs); yet the simultaneous achievement of long cycle life, a wide operating temperature range, and high safety remains a critical challenge. Herein, we exploit the unique ring-opening polymerization behavior of ethoxy (pentafluoro) cyclotriphosphazene (PFPN) in a methyl acetate (MA) based electrolyte to construct an inorganic-polymeric homogeneous mixed interface. Furthermore, the flame-retardant PFPN preferentially interacts with MA molecules rather than Na + , facilitating desolvation and endowing the electrolyte with excellent low-temperature performance and fast-charging capability. The designed electrolyte enables hard carbon (HC) anodes and Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) cathodes to exhibit stable cycling over 1,000 cycles. Notably, the NFPP cathode delivers a high specific capacity of 100.4mAh g −1 at an ultrahigh rate of 20C. Moreover, the commercial NFPP||HC pouch cell retains 87.4 % of its initial capacity after 1,200 cycles, and even 80 % at − 20 °C. The commercial NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM)||HC high-voltage pouch cell achieves a capacity retention of 91.9 % after 400 cycles. This study clarifies the unique interfacial chemistry of cyclophosphazene-based additives in ester-based electrolytes and provides a practical strategy for designing high-performance electrolytes for SIBs.