Surface Overpotential as a Key Metric for the Discharge–Charge Reversibility of Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are receiving increasing attention for power-grid energy storage systems. Nevertheless, warranting long-term reversible operation is not trivial owing to uncontrolled interfacial phenomena related to zinc dendritic growth and parasitic reactions. Herein, the addition of hexamethylphosphoramide (HMPA) to the electrolyte revealed the surface overpotential (|η_s|) to be a key metric of the reversibility. HMPA adsorbs onto active sites on the zinc metal surface, raising the surface overpotential toward lowering the nucleation energy barrier and decreasing the critical size (r_crit) of nuclei. We also correlated the observed interface-to-bulk properties by the Wagner (Wa) dimensionless number. The controlled interface enables a Zn|V₆O₁₃ full cell to retain 75.97% capacity for 2000 cycles, with a capacity loss of only 1.5% after 72 h resting. Our study not only delivers AZIBs with unparalleled cycling and storage performance but also proposes surface overpotential as a key descriptor regarding the sustainability of AZIB cycling and storage.