Synergistic Interfacial Adsorption and Anion Enrichment for Low-Temperature Cycling of Sodium-Ion Batteries

Sodium-ion batteries (SIBs) offer a sustainable alternative for large-scale energy storage. However, their operation at low temperatures is severely limited by sluggish interfacial kinetics and unstable solid-electrolyte interphase (SEI). Here, we report an interfacial engineering strategy enabled by a trace amount (2 wt.%) of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) additive, which preferentially adsorbs on hard carbon (HC) anodes and strongly coordinates electrolyte anions. This dual effect induces anion enrichment at the interface and promotes the in situ formation of an inorganic-rich SEI with high Na⁺ conductivity and mechanical robustness, thereby accelerating interfacial charge transfer and suppressing dendritic growth. As a result, a 2 Ah HC||NaNi_1/3Fe_1/3Mn_1/3O₂ pouch cell retains 87.88% of its capacity after prolonged cycling at -20°C, compared with only 34.60% for the reference electrolyte. Even at 1 C and -20°C, the PFPN-modified cell maintains 45% of its initial capacity, far exceeding the 33% of the control. This work highlights a practical pathway to construct fast-ion-conducting and durable SEI via interfacial adsorption and anion regulation, advancing SIBs toward reliable operation in cold environments.