Mechanistic Insights into Degradation of Lead Dioxide Electrode in Soluble Lead Battery

Recent research has extended the cycle life of the soluble lead redox flow battery to several hundred cycles at the laboratory scale. These improvements have largely resulted from empirical experimentation rather than a mechanistic understanding of the degradation processes. In this study, we investigate the structural evolution of the positive electrode during cycling and identify key phenomena contributing to performance loss using cyclic voltammetry, impedance spectroscopy, and SEM imaging. Specifically, we demonstrate that the lead dioxide deposit formed during charging undergoes electrodissolution during discharge, accompanied by distinct morphological transformation. The discharged electrode exhibits a heterogeneous surface, characterized by bare regions and porous PbO2 residues---highly conductive but largely overlaid by an impervious, poorly conducting lead oxide PbO passivation layer, formed by the merging of growing two-dimensional islands. Upon recharging, all regions show formation of needle-shaped PbO2 crystals and nucleation-induced voltage peaks. Repeated cycling with recursive nucleation and growth leads to the accumulation of a fragile residue. Furthermore, we show that PbO, previously assumed to oxidize independently to PbO2, requires the presence of Pb2+ ions for oxidation. Ongoing work focuses on suppression of nucleation and understanding the role of ion depletion in deep deposit regions on flaking and electrode degradation.