Quantifying Electrokinetics of NaCa0.6V6O16·3H2O Cathode in Aqueous Zinc-Ion Batteries with ZnSO4 Electrolyte

Abstract

Aqueous zinc-ion batteries (AZIBs) have been actively studied in recent years as a promising solution for next-generation stationary energy storage due to their inherent safety, low cost, and high energy density. However, their practical deployment remains hindered by the limited cycling stability of cathode materials. Overcoming this challenge requires a detailed understanding of cathodic electrokinetics and degradation mechanisms. In this study, we investigate the electrokinetic behavior of a NaCa0.6V6O16·3H2O (NaCaVO) cathode in ZnSO4 electrolyte through a combined application of the galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). For the first time, we quantify the exchange current density (i0) and interfacial charge-transfer resistance (RCT) of NaCaVO as a function of states of charge (SOCs). The results reveal that the V4+ ⇌ V3+ redox reaction exhibits significantly slower kinetics than the V5+ ⇌ V4+ counterpart. Further GITT-EIS studies using D2O-ZnSO4 electrolyte, complemented by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), indicate that the sluggish V4+ ⇌ V3+ process is predominantly associated with proton insertion. Distribution of Relaxation Time (DRT) analysis correlates the increased interfacial resistance with the intermediate phase formation induced by this proton insertion. The electrokinetic insights obtained in this work fill critical knowledge gaps in AZIB research and provide a foundation for designing more durable and efficient cathode materials in the future.