  Attaining high reversibility of electrodes and electrolyte is essential for
the longevity of secondary batteries. Rechargeable zinc-air batteries (RZABs),
however, encounter drastic irreversible changes in the zinc anodes and air
cathodes during cycling. To uncover the mechanisms of reversibility loss in
RZABs, we investigate the evolution of zinc anode, alkaline electrolyte, and
air electrode through experiments and first-principles calculations. Morphology
diagrams of zinc anodes under versatile operating conditions reveal that the
nano-sized mossy zinc dominates the later cycling stage. Such anodic change is
induced by the increased zincate concentration due to hydrogen evolution, which
is catalyzed by the mossy structure and results in oxide passivation on
electrodes, and eventually leads to low true Coulombic efficiencies and short
lifespans of batteries. Inspired by these findings, we finally present a novel
overcharge-cycling protocol to compensate the Coulombic efficiency loss caused
by hydrogen evolution and significantly extend the battery life.
