Understanding the Dynamics of Zinc Deposition/Dissolution Processes in Aqueous Zinc-Ion Batteries by In Operando Electrochemical Quartz Crystal Microbalance with Dissipation

Interfacial reactions at the electrolyte/Zn anode critically influence the reversibility and cycling stability of aqueous Zn-ion batteries (AZIBs). Here, we employ an in operando electrochemical quartz crystal microbalance with dissipation monitoring (E-QCMD) to investigate the dynamics of Zn deposition/dissolution in three representative ZnSO4, Zn(OTf)2, and Zn(ClO4)2 electrolytes at concentrations of 0.5, 1.0, and 2.0 M. Using a three-electrode configuration under a constant overpotential of ± 100 mV, current density, mass change, and dissipation were simultaneously monitored in real time. A comparison between the measured mass change (ΔMf) and the theoretical Faradaic mass (ΔMq) reveals that both electrolyte anion identity and concentration strongly affect Zn redox behavior. Zn(ClO4)2 promotes the formation of rigid, highly reversible Zn deposits, although accompanied by hydrogen evolution, whereas concentrated Zn(OTf)2 forms viscoelastic interfacial layers, consistent with the accumulation of alkaline byproducts and the solid electrolyte interphase (SEI). ZnSO4 exhibits intermediate behavior with improved reversibility at higher concentrations. Complementary analyses of Coulombic efficiency and mass-per-electron (MPE) further show that concentrated Zn(ClO4)2 and ZnSO4 electrolytes provide more reversible Zn deposition/dissolution than Zn(OTf)2. Ex situ X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations corroborate this picture by resolving the anion-dependent interphase chemistry and the underlying solvation and hydrogen-bond structures, respectively. These findings provide mechanistic insights into electrolyte-dependent Zn interfacial chemistry and demonstrate the capability of in operando E-QCMD for elucidating Zn redox processes in AZIBs.