Reunderstanding aqueous Zn electrochemistry from interfacial specific adsorption of solvation structures

Abstract

Although sulfate- and sulfonate-based electrolytes have been widely used in the study of aqueous zinc-ion batteries (AZIBs), discrepancies in the faradaic reaction kinetics of cation interfacial chemistry including in Mn²⁺ redox reactions and Zn deposition are observed in these two systems, the mechanism of which is still unclear. Herein, through focusing on the electrolyte solvation structure, we constructed a specific adsorption model involving the coexistence of anions, cations, and water molecules at the electrode/electrolyte interface. Distinguished from the traditional investigation of isolated adsorbed particles, we demonstrate that the specific adsorption model enables a rational explanation of the reaction difference of cations with different solvation structures at the electrode/electrolyte interfaces (EEIs). Specifically, owing to the more intense adsorption of active solvated Mn²⁺ near the inner Helmholtz plane, the deposition reaction of Mn species is enhanced in a sulfate-based electrolyte, resulting in a stronger capacity increase and fluctuation in comparison with sulfonate electrolytes. Similarly, the more rapid Zn²⁺ deposition kinetics in the sulfate-based electrolyte can be attributed to its strong adsorption behavior at the EEI. Furthermore, as validation of the as-proposed model, a sulfate/sulfonate hybrid electrolyte system is proposed, in which the optimized adsorption behavior at the EEI gives it synergistic improvement both in terms of capacity and cycling stability for aqueous Zn–Mn cells. This work provides rationale for understanding the interfacial electrochemistry in various aqueous electrolyte systems from the view of the adsorption behavior of the solvation structure.