Mn²⁺ Ions Confined by Electrode Microskin for Aqueous Battery beyond Intercalation Capacity

Abstract In aqueous rechargeable zinc–manganese dioxide batteries (ZMBs), some irreversible side reactions, such as Mn²⁺ dissolution, often lead to capacity fading over cycling. These side reactions play a crucial role in the capacity and cycle performance of the battery. The implementation of a bionic electrode microskin (EMS) composed of collagen hydrolysate to convert the irreversible side reactions into reversible reactions is reported. The proposed EMS effectively adsorbs and confines the Mn²⁺ ions around the cathode through van der Waals forces, hydrogen bonds, and/or ionic interactions, which makes the MnO₂/Mn²⁺ reactions reversible during the charge/discharge process. Such Mn²⁺ dissolution reactions, with an ultrahigh theoretical capacity (617 mAh g^‐1), contribute a large amount of capacity, ≈44% of the total specific capacity at a low scan rate. Based on these fundamental findings, the assembled ZMBs with an EMS display an unprecedented discharge capacity of 415 mAh g^‐1 at 20 mA g^‐1, which overcomes the theoretical capacity (308 mAh g^‐1) limitation of the Zn²⁺ intercalation mechanism. More significantly, the EMS on all α‐, β‐, and γ‐MnO₂ cathodes exhibits similar high capacity beyond the theoretical capacity of Zn intercalation and capacity retention enhancement after 3000 cycles.