High Modulus Na2SiO3‐Rich Solid Electrolyte Interphase Enable Long‐Cycle and Energy‐Dense Sodium Metal Battery

Abstract Sodium metal battery is considered as one of the most promising energy storage/conversion devices due to their high energy density, and abundant sodium reserves. However, its development is hampered by the limited metallic utilization and detrimental sodium dendrite growth ascribed to the unstable, and fragile solid electrolyte interphase (SEI). Here, the high stable and modulus SEI is constructed with a component of a thin and dense layer of high Na‐ion conductive Na 2 SiO 3 . It is in situ formed by the reaction between sodium anode and fish‐skin structure separator, which exhibits a 3D porous polyvinylidene fluoride‐based framework embedded with thin and robust vermiculite sheets. The robust interphase cannot only suppress sodium dendrite growth but also enable high energy density with small interphase‐consumed Na ions in anode‐less batteries. Accordingly, the as‐assembled NaNi 1/3 Fe 1/3 Mn 1/3 O 2 //Na cell shows 83% capacity retention after 5000 cycles at a rate of 5 °C. Moreover, the proof‐of‐concept pouch cells deliver an energy density of 205 Wh kg −1 (based on the total mass of the battery), and high safety (not catching fire after punctured) under anode‐less condition (with a low negative/positive capacity ratio of ≈2.1).