Engineering solid electrolyte interphases in anode-free sodium metal batteries

Anode-free sodium metal batteries (AFSMBs), which eliminate active anode materials and rely on in situ sodium plating on a bare current collector, offer a compelling route toward high-energy-density batteries. However, their practical deployment is fundamentally limited by interfacial instability. In AFSMBs, the solid electrolyte interphase (SEI) forms on a metal-free current collector during the initial plating process and continuously evolves upon repeated plating/stripping, rendering the interface highly susceptible to non-uniform deposition, dendrite formation, parasitic reactions and rapid sodium inventory loss. Therefore, rational SEI engineering is crucial for achieving reversible sodium nucleation/deposition and durable cycling. Here, we summarize the central challenges of SEI formation in AFSMBs, highlight recent advances in SEI engineering, and discuss the key design principles for stable interphases, with emphasis on fast Na + transport, mechanical robustness and low sodium consumption. We further outline future opportunities for advanced operando characterization and synergistic interfacial design to accelerate the development of high-performance AFSMBs.