Zn-anode interphase regulation by electrolyte additives in aqueous two-electron Zn–I2 batteries

Abstract

Aqueous zinc-iodine (Zn-I2) batteries are attractive for grid-scale energy storage owing to their intrinsic safety and low cost; however, their practical implementation is hindered by instability of the Zn metal anode in chloride (Cl -)-containing electrolytes needed to access high-energy iodine redox chemistry. Although ZnCl2 electrolytes stabilize iodine species via interhalogen complexation (e.g., iodine monochloride (ICl)) and enable the two-electron I -/I 0 /I + regime (422 mA h g -1 theoretical capacity), they simultaneously accelerate Zn dendrite growth and corrosion. Here, we elucidate how additivedriven interphase chemistry on Zn governs cell performance in the two-electron I -/I 0 /I + regime by systematically comparing tetramethylurea (TMU), dimethyl sulfoxide (DMSO), and N,N-dimethylacetamide (DMA) as electrolyte additives in a ZnCl2based electrolyte. To decouple interphase effects from Zn 2+ solvation, we select additives with comparable Gutmann donor numbers (DN), yielding similar solvation structures across the three electrolytes. Electrochemical and interfacial analyses reveal that DMSO and DMA are electrochemically stable within the operating voltage window, leading to negligible solidelectrolyte interphase (SEI) formation. In contrast, TMU undergoes preferential reductive decomposition on Zn surface, forming a robust organic-rich SEI that suppresses dendrite formation and mitigates corrosion in Cl --rich media. Consequently, aqueous Zn-I2 cells with TMU sustain the high-energy I -/I 0 /I + chemistry, achieving stable capacity retention over 2000 cycles. These findings establish controlled reductive additive chemistry as an effective strategy to tailor a protective Zn/electrolyte interphase while preserving ICl-mediated high-energy redox, providing practical design principles for durable aqueous Zn-I2 batteries.