A solid electrolyte interphase (SEI) makes a critical impact on the cyclic property of anodes for both lithium- and sodium-ion batteries (LIBs/SIBs), which is more significant for high-capacity anodes. Unfortunately, it is arduous to control the components of the SEI and its arrangement due to the unstable interface resulting from the decomposition, rupture, and regeneration of the SEI and the complex chemical environment. Here, we report a synergistic in situ and ex situ strategy to tailor the interface of an active material, which enables an SEI with superb mechanical behavior to form in the Helmholtz region around the interface. The amorphous TiO₂ as a protection layer shows strong interaction with the reduction product (i.e., NaF) of fluoroethylene carbonate additive, which enhances the adhesion of the SEI to the electrode material. The shedding of the SEI during the huge volume expansion of the active material is substantially suppressed. Furthermore, the toughness of the SEI is increased by the binding of NaF with the organic components. When incorporating this synergistic strategy into a red phosphorus/carbon nanotube composite, the dual layers containing mostly inorganics in the interior and organics in the exterior constitute a thin, uniform, and robust SEI film with high ionic conductivity, which leads to ultrastable sodium storage with increased initial Coulombic efficiency. In addition, this research indicates that increasing the amount of some crucial SEI components (such as NaF) is not necessarily better; rather, the judicious configuration of such SEI components looks more important for battery performance. This universal approach for engineering the SEI component and structure suggests new insights into SEI chemistry and can be extended to unraveling most of the interface or SEI issues for other rechargeable batteries.