Here we report on the significant improvements made in the energy efficiency and cycle life of full-cell soluble lead flow batteries (SLFBs). We describe energy efficiency loss mechanisms, particularly in context to the deposition of PbO₂ at the positive electrode. The morphology and crystal structure of deposits formed at the positive electrode, under galvanostatic and potentiostatic conditions, were characterized using both powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Rietveld refinements were performed to quantitatively determine the phase fraction of α- and β-PbO₂ formed. In addition, electrochemical impedance spectroscopy (EIS) was used to describe the charge-transfer reaction occurring at the positive electrode during conditions that promote the formation of various PbO₂ morphologies. These features were used to evaluate and predict the long-term cycling stability of SLFBs as well as to diagnose potential problems arising during battery operation. We demonstrate that conditions optimized to preferentially deposit nanoscale PbO₂ leads to long battery lifetimes, exceeding 2000 cycles at 79% energetic efficiency.