We report an in situ analysis of nano-micro sodium deposition in an ester-based electrolyte using atomic force microscopy. It is found that sodium dendrites are effectively suppressed by adding fluoroethylene carbonate (FEC) as an electrolyte additive. The decomposition of FEC provides a NaF-containing solid electrolyte interphase with homogenous morphology and high modulus, leading to stable sodium deposition and high Coulombic efficiency (88% over 50 cycles) of the sodium metal anode.