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/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.