In Situ Constructed Co/NaCl Mixed Ion/Electron‐Conducting Interphase Enabling Dendrite‐Free Sodium Metal Anodes

Abstract The practical deployment of sodium metal anodes (SMAs) is hindered by dendrite growth, unstable solid‐electrolyte interphase, and severe volume fluctuations. Herein, an in situ constructed mixed ion/electron‐conducting (MIEC) artificial interphase comprising Co nanoparticles and NaCl nanocrystals is proposed to address these challenges. This layer forms spontaneously via a displacement reaction between CoCl 2 and Na, yielding a dual‐phase structure where ionic‐conducting NaCl domains are interspersed with electron‐conducting Co nanocrystals. The NaCl phase ensures ultralow Na⁺ diffusion barriers (0.058 eV), while the Co network enhances sodiophilicity (−5.98 eV for Na adsorption energy) and mechanical robustness (5.02 GPa for Young's modulus), enabling uniform Na deposition and dendrite suppression. Consequently, the Co/NaCl/Na||Cu half‐cell exhibits excellent cyclability, achieving an average Coulombic efficiency of 99.89% over 1000 cycles at 5 mA cm −2 and 5 mA h cm −2 . In symmetric cells, the Co/NaCl/Na electrode maintains stable operation for 1000 h at 4 mA cm −2 and 4 mA h cm −2 under a high depth of discharge of 75%. Full cell coupled with Na 3 V 2 (PO 4 ) 3 retains 82 mA h g −1 after 1000 cycles at 8 C. This work demonstrates a scalable strategy to stabilize SMAs through a robust MIEC interphase, paving the way for next‐generation sodium metal batteries.