Solid polymer electrolytes based on plastic crystals are promising for solid-state sodium metal (Na⁰) batteries, yet their practicality has been hindered by the notorious Na⁰-electrolyte interface instability issue, the underlying cause of which remains poorly understood. Here, by leveraging a model plasticized polymer electrolyte based on conventional succinonitrile plastic crystals, we uncover its failure origin in Na⁰ batteries is associated with the formation of a thick and non-uniform solid electrolyte interphase (SEI) and whiskery Na⁰ nucleation/growth. Furthermore, we design a new additive-embedded plasticized polymer electrolyte to manipulate the Na⁰ deposition and SEI formulation. For the first time, we demonstrate that introducing fluoroethylene carbonate (FEC) additive into the succinonitrile-plasticized polymer electrolyte can effectively protect Na⁰ against interfacial corrosion by facilitating the growth of dome-like Na⁰ with thin, amorphous, and fluorine-rich SEIs, thus enabling significantly improved performances of Na//Na symmetric cells (1,800 h at 0.5 mA cm⁻²) and Na//Na₃V₂(PO₄)₃ full cells (93.0 % capacity retention after 1,200 cycles at 1 C rate in coin cells and 93.1 % capacity retention after 250 cycles at C/3 in pouch cells at room temperature). Our work provides valuable insights into the interfacial failure of plasticized polymer electrolytes and offers a promising solution to resolving the interfacial instability issue.