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 CeF3@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 PF6 - 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.