Chelation-Buffer Co-Additives Enable Compact Zn Deposition and High-Rate Cycling in Zn-Br₂ Hybrid Flow Batteries

Zinc-bromine (Zn-Br₂) hybrid flow batteries offer nonflammable aqueous electrolytes with earth-abundant reactants but remain constrained by nonuniform Zn deposition, parasitic interfacial reactions, and efficiency losses at practical rates. Herein, we address these limitations by introducing a dual-additive anolyte that couples a chelating ligand (EDTA^2-) with a pH-buffering anion (acetate from NH₄OAc) to comodulate Zn²⁺ solvation and interfacial acidity. Results demonstrate that this simple, scalable strategy using chelation-buffer coadditives can stabilize Zn electrodeposition, enabling durable, high-rate operation in hybrid flow batteries. Zn K-edge X-ray absorption spectroscopy (XAS) and X-ray tomographic microscopy (XTM) reveal that Zn plating modifies the primary Zn-O shell (coordination number and bond distance), coinciding with the emergence of compact, laterally continuous Zn networks within three-dimensional carbon felt (CF). These structural evolutions correlate with reduced iR-corrected overpotentials and high Coulombic efficiency (CE ≥ 95%) during rate tests up to 200 mA cm^-2. At a practical areal capacity of 35 mAh cm^-2, the dual-additive electrolyte sustains superior capacity retention over ≥300 cycles compared with single-additive and additive-free controls. By quantitatively linking coordination restructuring to 3D deposit connectivity and cell-level metrics (CE, VE, EE), this work establishes a mechanistic basis for electrolyte design in mildly acidic Zn-Br₂ systems.