The soluble lead flow battery (SLFB) is a promising energy storage system. In comparison to conventional flow batteries, the membrane-less and single-flow design of SLFBs is potentially much more economical to scale up for utility-scale applications. However, SLFB lifespan reported so far is less than 200 cycles under normal flow conditions. This study reports a method for significantly extending the cycle life and expanding capacity of SLFBs. By adding an adequate amount of sodium ethanoate to the electrolyte, lead dioxide (PbO2) deposition stability is materially improved and shed PbO2 particles are substantially reduced. Lifespan of ethanoate-added SLFBs is shown to extend by over 50%, and under optimal condition exceeds 500 cycles at over 65% energy efficiency. This improvement in SLFB performance is primarily attributed to the stabilization of both the electroplated PbO2 layers and proton activity at the electrodeposit-electrolyte interphase. We demonstrate a novel and economical approach for advancing performance of membrane-less flow batteries that involve redox reactions associated with acidity variation and operate through electrodeposition.