High capacity sodium (Na) metal anodes open up new opportunities for developing next-generation rechargeable batteries with both high power and high energy densities. However, many challenges still plagued their practical application, including low plating/stripping Coulombic efficiency (CE) and dendrite growth after repeated cycle inducing safety issue. Especially, the sodium metal is less stable in organic (i.e. carbonate-based) electrolytes than lithium metal, due to the more unstable organic solid–electrolyte interface (SEI). Herein, we report a facile technology to stabilize sodium metal anode and inhibit the growth of sodium dendrites. The in-situ ultrathin NaI SEI layer successfully endows best-performance Na/I2 metal batteries (>2200 cycles) with high capacity (210 mA h g−1 at 0.5 C) based on the conversion reaction chemistry with higher discharge voltage plateau (> 2.7 V) and lower overpotential (134 mV) due to the fast charge transfer dynamics and interfacial stability compared with pristine Na anode. The detailed theoretical calculations and experimental results elucidate that NaI layer has a much lower diffusion barrier compared to that of NaF (NaF as one the most commonly found inorganic components in Na-based SEI layer), and actually facilitates more uniform sodium deposition. This work provides a new avenue for designing low-cost, high-performance and high-safety sodium metal-iodine batteries and other metal-iodine batteries.