Dynamic Mechanism of Short Peptide Additive Regulating Solvation Microenvironment of Zinc Ions

The optimization electrolyte strategy through molecular additives to improve the stability of aqueous zinc-ion batteries (AZIBs), which changes the solvation structure of hydrated zinc ions (Zn²⁺), generally relies on experimental trial and error, because the precise mechanism by which these additives alter the coordination environment of Zn²⁺ remains elusive. Here, we select the oligopeptide of mono-, di-, tri-, and tetra-glycine, as electrolyte additives to optimize the Zn²⁺ solvation microenvironment in AZIBs. Contrary to traditional views, we find that these additives modify the solvated structure of the Zn²⁺ by substituting sulfate ion (SO₄²⁻) in the preexistence of Zn²⁺-SO₄²⁻ ion pair, rather than water molecules in the first solvation shell, due to a high energy barrier to replace one of the coordinated water molecules of Zn²⁺. This observation is consistent with recent experimental result of the attenuating influence of glycine on the interaction between Zn²⁺ and SO₄²⁻ confirmed by Fourier-transform infrared spectroscopy. For the multifunctional triglycine, its favorable conformation is disrupted to accommodate the direct coordination of oxygen atoms with Zn²⁺, and Zn²⁺ is observed to migrate between distinct sites along the triglycine backbone. This work provides theoretical principles to rationally design advanced electrolytes for solvation modulation with high performance AZIBs.