Interlayer‐Confined LiNO3 Reservoir Enables the Formation of a Li‐Containing Hybrid SEI for Robust Sodium Metal Anodes

Sodium metal batteries hold great promise for energy storage applications, but are constrained by limited charge transfer kinetics and unstable solid electrolyte interphases (SEI). Here, we construct a montmorillonite‐based sustained‐release separator by confining within interlayer galleries and integrating it onto a glass‐fiber separator. This interlayer‐confined reservoir provides a sustained local nitrate supply during cycling, continuously regulates the solvation environment, and maintains anion‐participated interfacial reactions. The sustained‐release process induces the formation of a dense Li/Na hybrid SEI enriched with LiF, , and together with Na‐based inorganic species. DFT calculations indicate that representative Li‐containing phases exhibit lower diffusion barriers than their Na‐based counterparts, facilitating interfacial ion transport and homogenizing Na deposition. As a result, the modified separator enables highly reversible cycling for over 4000 h at 0.25 mA /0.25 mAh . In full cells, retains 83.7% capacity after 2300 cycles at 5 C, while high‐loading cells maintain 91.5% capacity retention. This work highlights interlayer‐confined sustained release as an effective separator‐engineering strategy for programming solvation chemistry and constructing Li/Na hybrid SEIs in sodium metal batteries.