In Situ Construction of NaF‐rich Solid Electrolyte Interphase with Metallic Ce Sites for Stable Anode‐Free Sodium Metal Batteries

Abstract Developing reversible Na plating/stripping under high depth of discharge (DOD) is critical for stable anode‐free sodium metal batteries (AFSMBs). Achieving high reversibility relies on a robust inorganic‐rich solid electrolyte interphase (SEI), yet current strategies depend on high‐concentration salts or fluorinated solvents, limiting their practicality. Herein, we demonstrate an in situ NaF‐rich SEI with metallic Ce sites that bypasses electrolyte decomposition. Operando synchrotron X‐ray absorption spectroscopy and TOF‐SIMS revealed a conversion reaction between Na + and CeF 3 @NC, forming NaF and metallic Ce sites during cycling. This substrate‐driven well‐defined NaF‐rich SEI ensures interfacial stability even under high DOD. Moreover, metallic Ce sites exhibit strong PF 6 − affinity, facilitating Na + desolvation by lowering the energy barrier for step‐by‐step anion extraction. Consequently, the anode achieved ultra‐long stability of 5800 h (20% DOD) and 1600 h (90% DOD). The full cells delivered the impressive cycling stability for 500 cycles with 93.3% capacity retention, while a pouch cell retained 90.98% capacity after 300 cycles. Notably, the AFSMBs with high cathode loading (>9 mg cm −2 ) exhibited excellent stability, retaining 83.6% capacity over 100 cycles. This work provides new insights into engineering robust in situ SEI, paving the way for accelerating practical application of AFSMBs.