Indium‐Mediated Glue‐Like Interlayer Enables Stable High‐Capacity Flexible Sodium Metal Batteries

Na metal is considered as a highly promising anode material for Na batteries, owing to its high theoretical capacity (1166 mAh g −1 ) and low redox potential (−2.71 V vs. SHE). However, its practical application is significantly hindered by interfacial instability and dendrite formation arising from uneven Na deposition, which severely compromise its cycling lifespan. Herein, we develop a robust three‐dimensional glue‐like interlayer via in situ electrochemical conversion of CuInS 2 on a flexible Cu current collector. This process yields a Na 2 S/Na 5 InS 4 /Cu composite interlayer of strong interphase connections, ensuring tight interfacial connections and exceptional mechanical stability throughout extended cycling, thereby effectively mitigating interfacial degradation. With this interlayer, the Na metal anode achieves a high Coulombic efficiency over 99.4%, outstanding symmetrical cell stability exceeding 2400 hours, and excellent high‐capacity full cell cycling (95.3% capacity retention after 1000 cycles). Flexible pouch cells retain 95% capacity over 200 cycles and endure more than 10,000 bending.