Ostwald‐Ripening Induced Interfacial Protection Layer Boosts 1,000,000‐Cycled Hydronium‐Ion Battery

Collapsing and degradation of active materials caused by the electrode/electrolyte interface instability in aqueous batteries are one of the main obstacles that mitigate the capacity. Herein by reversing the notorious side reactions include the loss and dissolution of electrode materials: as we applied Ostwald ripening (OR) in the electrochemical cycling of a copper hexacyanoferrate electrode in a hydronium‐ion batteries, the dissolved Cu and Fe ions undergo a crystallization process that creates a stable interface layer of cross‐linked cubes on the electrode surface. The layer exposed the low‐index crystal planes (100) and (110) through OR‐induced electrode particle growth, supplemented by vacancy‐ordered (100) superlattices that facilitated ion migration. Our design stabilized the electrode–electrolyte interface considerably, achieving a cycle life of one million cycles with capacity retention of 91.6%, and a capacity retention of 91.7% after 3000 cycles for a full battery.