Traditional Electrochemical Zn2+ Intercalation/Extraction Mechanism Revisited: Unveiling Ion‐Exchange Mediated Irreversible Zn2+ Intercalation for the δ‐MnO2 Cathode in Aqueous Zn Ion Batteries

Abstract Rechargeable aqueous Zn/δ‐MnO2 batteries are extensively investigated owing to the low cost, safety and eco‐friendliness. However, the charge storage mechanism of δ‐MnO2 electrode is still in debate. In this paper, it is revealed that the Zn2+ intercalation in δ‐MnO2 electrode is an ion exchange process rather than the commonly‐conceived electrochemical process for the first time. Before the discharge/charge process, Zn2+ irreversibly intercalates into the structure of δ‐MnO2. The ion‐exchange mediated irreversible Zn2+ intercalation in δ‐MnO2 has no contribution to the capacity of δ‐MnO2 electrode during cycles. This study further reveals that the electrochemical H+ intercalation/extraction, the electrodissolution of δ‐MnO2 and the electrodissolution‐electrodeposition of vernadite dominate the charge storage process of δ‐MnO2 electrode. These findings shed new light on the fundamental understanding for the reaction mechanism of δ‐MnO2 electrode in aqueous batteries.