Impact of Salt Chemistry and Interlayer Architecture on Solid Electrolyte Interphase Composition and Performance

Reversible lithium metal anodes require interphases that are chemically stable, mechanically robust, and compatible with scalable processing. Here, we use slot‐die‐coated, salt‐rich PEO interlayers to design SEI nanolayers on Cu current collectors and isolate how salt anion chemistry controls interphase composition and lithium plating/stripping behavior in architectures relevant to Li plating and anode‐free cells. Mesoscale modeling shows that dendrite initiation at SEI defects is governed by a tradeoff among interphase thickness, ionic conductivity, and stiffness through defect‐driven current focusing, motivating controlled formation from thin precursor layers. Using UHV‐transferred XPS and ToF‐SIMS depth profiling after controlled Li deposition, we find that fluorinated sulfonylimide salts (LiTFSI, LiFSI) form mixed inorganic/organic interphases containing LiF and PEO‐derived alkoxides, with LiFSI producing a more LiF‐dominant Li‐facing surface than LiTFSI under identical conditions. In contrast, LiNO 3 ‐containing interlayers yield a comparatively thick nitrate‐derived interphase that reduces upon Li contact to an N‐rich inorganic layer (Li–N/N 3− , LiN x O y ). In Cu||Li cells, these designed interlayers reduce interfacial resistance and improve Coulombic efficiency (CE) and critical current density (CCD) relative to uncoated controls, with LiFSI‐ and LiNO 3 ‐based interlayers providing the highest CE/CCD. Overall, the results demonstrate a manufacturing‐relevant approach to engineer SEI nanolayers by salt chemistry, providing pathways to improve reversible lithium metal anodes while reducing reliance on LiTFSI.