Bromoaniline‐Derived Modulator Enabling Robust Interphase Construction for All‐Solid‐State Sodium Metal Batteries

ABSTRACT Poly(ethylene oxide) (PEO)‐based all‐solid‐state sodium metal batteries (ASSMBs) hold significant promise for safe, large‐scale energy storage, yet suffer from fragile electrode–electrolyte interfaces. Here, we address the interfacial instability through a position‐specific substituent engineering strategy applied to bromoaniline derivatives. By tuning the electronic interplay between para‐fluorine and ortho‐methyl groups, an optimal modifier, 2‐bromo‐4‐fluoro‐6‐methylaniline (BrFMA), is identified from ten candidates. The amino group of BrFMA forms NH···O hydrogen bonds with ether oxygen in PEO and sulfonyl oxygen in TFSI − , disrupting the Na + solvation shell and lowering desolvation energy. At the anode, BrFMA anchors at defect protrusion sites, suppressing dendrite growth and directing Na + deposition along the (110) orientation. Concurrently, its accessible LUMO and high nucleophilic Fukui index drive reductive C‐Br cleavage to generate NaBr, which catalyzes TFSI − decomposition into NaF, assembling a robust NaBr‐NaF bilayer interphase with rapid Na + conduction. Symmetric cells sustain stable plating/stripping beyond 6000 h at 0.1 mA cm −2 . Paired with Na 3 V 2 (PO 4 ) 3 cathodes, full cells achieve 90.39% capacity retention over 1550 cycles at 0.5C with 99.64% average Coulombic efficiency. Flexible pouch cells further demonstrate reliable operation under mechanical abuse scenarios. This work establishes precise molecular control for the interfacial design of sodium metal batteries.