Fast Discharging Stabilizes Electrochemical Interfaces: Achieving Close-to-Unity Reversibility in "Dendrite-Forming"...

Developing resilient aqueous energy storage systems, such as Zn batteries, is essential for advancing energy sustainability. A key challenge in these systems is the dendritic growth of metals, which causes poor rechargeability in battery electrodes. Classical diffusion-limited dendritic growth is predicted to occur when the charging rate exceeds the limiting current density J_lim determined by electrolyte transport properties. Understanding the electrochemical behavior of dendrites is critical for designing fast-charging metal electrodes, where dendritic growth is not only likely but sometimes unavoidable due to uneven current distribution. Conventional wisdom suggests that high-aspect-ratio fractal dendrites are susceptible to bottom-initiated dissolution, leading to mechanical break-off from the current collector and the formation of "dead" metal. Surprisingly, our results show that near-unity charge-discharge reversibility can be achieved even with highly ramified classical diffusion-limited dendritic Zn metal structures. In particular, the reversibility improves with an increasing discharge rate, demonstrating a strong positive correlation. A two-orders-of-magnitude (∼200×) enhancement in cycle life is observed when Zn electrodes charged identically fast, but discharged at higher versus lower rates. Operando visualization reveals that dendrite fragmentation is significantly suppressed at higher discharge rates. Complementary post-mortem microstructural analysis shows that, consistent with predictions based on the Wagner number (Wa), high-rate discharges promote preferential tip-initiated stable retraction, whereas low-rate discharges induce "pitting" corrosion that mechanically weakens the dendrite backbone and promotes fragmentation. These findings challenge the prevailing assumption that dendritic growth necessarily limits the reversibility and offer new design principles for metal battery electrodes operating at unprecedented high rates approaching the diffusion limit.