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

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