An Oligomeric Additive Bridges Inner and Outer Helmholtz Planes to Enable Reversible Zn Anodes via Spatial and Functional Decoupling

ABSTRACT Precise molecular engineering of the electrode/electrolyte interface remains a key bottleneck for aqueous Zn‐ion batteries (ZIBs). Here, we report an oxidative polymerization strategy to synthesize oligomeric chromotropic acid disodium salt (OCAD) as a multifunctional additive. The oligomeric architecture bridges the inner and outer Helmholtz planes (IHP and OHP), shifting regulation from IHP‐localized adsorption to full IHP/OHP coverage. The expanded dimeric framework (1.51 vs. 0.65 nm) displaces water molecules and promotes Zn 2+ desolvation at the OHP via steric hindrance and hydrophobic effects, while sulfonic and phenolic hydroxyl groups repel SO 4 2− and buffer pH at the IHP through electrostatic repulsion and proton donation. This functional decoupling overcomes the consumption‐driven failure of conventional small‐molecule additives, suppressing side reactions and guiding uniform Zn deposition. Consequently, symmetric cells with OCAD‐modified Zn anodes achieve exceptional cycling stability exceeding 4300 h at 1 mA cm −2 /1 mAh cm −2 and operate at an 85.6% depth of discharge. A full battery paired with a Na 2 V 6 O 16 cathode retains 81.6% capacity after 4700 cycles at 20 A g −1 . The performance translates effectively to high‑mass‑loading cathodes and pouch cells, underscoring practical viability. This work establishes molecular oligomerization as a versatile paradigm for long‑range interface regulation in durable aqueous ZIBs.