A Novel Na₁₅Sn₄/NaI Biphasic Interface Layer: Synergistic Regulation of Sodium Deposition and Interface Stability for...

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 Na₁₅Sn₄ and NaI is in situ constructed on the SMA surface (Na₁₅Sn₄/NaI-Na) via a simple solution treatment strategy. NaI provides efficient ion transport channels and effectively suppresses the electron tunneling, while the sodiophilic Na₁₅Sn₄ promotes uniform Na nucleation and deposition. With their synergistic effect, the Na₁₅Sn₄/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 Na₁₅Sn₄/NaI-Na anode possesses sufficient mechanical strength to avoid stress damage during volume fluctuations. Consequently, the Na₁₅Sn₄/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 Na₃V₂(PO₄)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.