Artificial Solid Electrolyte Interphases in Sodium Metal Batteries: Classification, Properties, and Challenges

Sodium metal anodes (SMAs) are promising candidates for high-energy and cost-effective batteries, yet their practical application is hindered by the instability of naturally formed solid electrolyte interphase (SEI). The SEI is often structurally heterogeneous, mechanically fragile, and partially soluble, leading to uneven ${\mathrm{Na}}^{+}$ flux, dendrite growth, dead sodium, and increased interfacial resistance, which together compromise safety and cycling stability. Artificial SEIs have emerged as an effective strategy to overcome these challenges by stabilizing the interface and regulating Na deposition. In this review, we systematically classify artificial SEI materials based on their functional characteristics and provide detailed discussions of how each category contributes to interface stabilization, dendrite suppression, and electrochemical performance enhancement. Finally, remaining challenges and future perspectives in rational design and practical implementation are highlighted. This work provides a comprehensive overview of artificial SEI mechanisms and offers guidance for the development of safe, durable, and high-performance sodium metal batteries (SMBs).