Gas-Phase and Condensed-Phase Synergy in a Nonflammable Electrolyte for Highly Stable Sodium-Ion Batteries

Mitigating electrolyte-induced thermal runaway to ensure safety while preserving superior electrochemical performance remains a critical challenge for secondary batteries, especially in electric vehicle applications. Herein, we design a synergistic electrolyte with trimethyl phosphate (TMP) and ethoxy-pentafluorocyclotriphosphazene (PFPN) as additives in a conventional commercial carbonate electrolyte, enabling dual gas-phase and condensed-phase flame retardancy via functional zone partitioning in the electrolyte solvation structure. This nonflammable electrolyte simultaneously enhances the battery safety and electrochemical properties. The Na_0.75Li_0.15Mg_0.05Ni_0.1Mn_0.7O₂||HC full cell delivers an improved specific capacity of 102.9 mAh g^-1 at 0.02 A g^-1 and a capacity retention of 75% after 1000 cycles at 0.1 A g^-1. The capacity retention rate of a single-stack pouch cell is 80% after 500 cycles, which also proves the applicability of the electrolyte. Specifically, TMP preferentially coordinates in the inner solvation sheath to suppress gas-phase flame propagation via radical scavenging, which is confirmed by differential electrochemical mass spectrometry (DEMS) through its effective suppression of H₂ release and interruption of the combustion chain reaction. In contrast, PFPN achieves condensed-phase flame retardancy by forming a dense protective carbon layer at high temperatures, with in-depth X-ray photoelectron spectroscopy (XPS) analysis revealing that P-N/P-F functional groups enable it to act as both a "fire barrier" and an "interfacial stabilizer", thereby preventing continuous decomposition of flammable components. This work provides novel insights for flame-retardant electrolyte design.