Dual‐Phase Interpenetrating Reinforcement Enables an Ultrathin Sodium Metal Anode for High‐Energy‐Density Batteries

ABSTRACT Ultrathin sodium (Na) metal anodes (< 40 μm) with superior electrochemical performance are essential in developing high‐energy‐density sodium metal batteries (SMBs). Nonetheless, Metallic Na's poor processability hinders its practical application. Hence, a dual‐phase interpenetrating reinforcement strategy is proposed to fabricate sodium/sodium phosphide–carbon nanotube (Na/Na 3 P5‐CNT10, NNP5C10) composite foils with controllable thickness (40−15 μm) by mechanical kneading operation. Sodium phosphide (Na 3 P) is in situ formed through a pressure‐induced reaction between red phosphorus (RP) and metallic Na. The resulting Na 3 P and carbon nanotube (CNT) are uniformly embedded within the Na matrix to form an interpenetrating Na 3 P‐CNT framework. Na 3 P impedes dislocation motion, whereas CNT provides interfacial lubrication and crack‐bridging, enabling smooth separation and reliable thinning of the composite foil. This mixed‐conductive framework accelerates ionic/electronic transport, lowers the Na nucleation barrier, and homogenizes Na deposition. As a result, a symmetric cell employing 30 μm‐thick NNP5C10 composite foils achieves a long‐term cycling stability of 2500 h at 0.5 mA cm −2 and 0.5 mAh cm −2 . When paired with a Na 3 V 2 (PO 4 ) 3 (NVP) cathode, the full cell with a 20 μm‐thick NNP5C10 anode achieves a high energy density of ∼276 Wh kg −1 at 0.2C with a low negative‐to‐positive (N/P) capacity ratio of ∼1.9. This study demonstrates a viable pathway in fabricating ultrathin Na metal anodes with high capacity, thereby promoting the development of high‐energy‐density SMBs.