A bidirectional interfacial engineering strategy for highly stable sodium metal batteries

Sodium (Na) metal batteries (SMBs) are regarded as some of the most promising next-generation energy storage systems due to their high energy density. However, their practical application is severely hindered by interfacial instabilities at both the anode and cathode, which result in rapid capacity degradation during cycling. Here, we proposed a bidirectional interfacial regulation strategy that simultaneously stabilizes both electrode interfaces. We found that the additive sulfolane features highly polar sulfone groups, effectively tailors the Na⁺ solvation structure and mitigates excessive anion decomposition under high-voltage conditions at the cathode. Concurrently, another additive fluoroethylene carbonate preferentially decomposes at the Na metal anode to form a dense, NaF-rich inorganic layer, which suppresses dendrite growth and inhibits parasitic side reactions. As a result, Na‖Na symmetric cells with this mixed electrolyte exhibit an ultra-long cycling lifespan of 1400 h at 0.5 mA cm^-2/0.5 mAh cm^-2, and Na‖Cu cells deliver stable cycling over 500 cycles. Furthermore, the Na‖Na₃V₂(PO₄)₃ full cell also achieves over 88% capacity retention after 1100 cycles at 80 mA g^-1. We believe that our work offers a viable pathway for designing high-stability Na metal anodes through synergistic interfacial engineering.