Amide Additives Enhance the Understanding of Kinetic Reversibility in Zinc Anode Stability Using Ultramicroelectrodes

Abstract

Aqueous zinc metal batteries (AZMBs) offer safety and sustainability but face challenges from hydrogen evolution, corrosion, and dendrite formation in mildly acidic electrolytes. Electrolyte additives improve anode stability by modifying interfacial chemistry through surface adsorption or altering zinc ion solvation. However, the mechanisms by which trace amounts of additives, often less than one percent of total ions, yield large performance improvements remain unclear. This suggests highly specific interfacial effects that require deeper investigation of charge transfer kinetics and interfacial resistances. Using fast scan voltammetry on ultramicroelectrodes (UMEs), we show that additives affect both the exchange current density and kinetic reversibility, a parameter reflecting the steady-state regime at high scan rates. We propose kinetic reversibility as a complementary metric to evaluate anode stability. Three amide-based additives—hexamethylphosphoramide, trimethylphosphoramide, and phosphoramide differing only in methyl substitution on the amide nitrogen, serve as a model system to study how molecular structure influences solvation, adsorption, and plating behavior. Electroanalysis on UMEs, supported by density functional theory reveals the interplay of kinetics and interfacial chemistry. Galvanostatic cycling and morphological studies validate these findings. This work provides mechanistic insight and introduces kinetic reversibility as a valuable design criterion for stable zinc metal anodes.