A Novel Na ₁₅ Sn ₄ /NaI Biphasic Interface Layer: Synergistic Regulation of Sodium Deposition and Interface Stability for Superior Sodium Metal Anodes

Restricted by the fragile and unstable intrinsic interface layer, the sodium metal anode (SMA) struggles to withstand volume expansion, dendrites growth, and excessive electrolyte consumption during cycling, which severely hinders the practical development of sodium metal batteries. Herein, a novel biphasic interface layer consisting of Na 15 Sn 4 and NaI is in situ constructed on the SMA surface (Na 15 Sn 4 /NaI‐Na) via a simple solution treatment strategy. NaI provides efficient ion transport channels and effectively suppresses the electron tunneling, while the sodiophilic Na 15 Sn 4 promotes uniform Na nucleation and deposition. With their synergistic effect, the Na 15 Sn 4 /NaI biphasic interface layer significantly facilitates the Na + diffusion kinetics and inhibits the uncontrolled dendrites growth. Besides, the high Young's modulus (13.5 GPa) ensures the Na 15 Sn 4 /NaI‐Na anode possesses sufficient mechanical strength to avoid stress damage during volume fluctuations. Consequently, the Na 15 Sn 4 /NaI‐Na‐based symmetric cells operate stably for nearly 1000 h at 2.0 mA cm −2 and 2.0 mAh cm −2 with a low overpotential of 9.5 mV. Moreover, the full cells coupled with high‐mass‐loading Na 3 V 2 (PO 4 ) 3 cathode (15 mg cm −2 ) retain a high specific capacity of 106.6 mAh g −1 after 150 cycles at 2.0 C. This work provides a scalable approach for developing robust and dendrite‐free SMAs.