Analysis of surface morphological changes induced by the electrochemical reduction of ZnO and Zn(OH)42− during the recharge process of Zn anodes

Alkaline Zn rechargeable batteries are attractive for large-scale energy storage. However, their application is limited by the performance degradation of Zn anodes owing to morphological changes during charge–discharge cycles. This study investigates the morphological change mechanism, focusing on the electrochemical reactions involving the ZnO passivation layer during the recharge process (i.e., charge after discharge). The initial stage is dominated by ZnO reduction, during which Zn 2+ is reduced to Zn 0 within the lattice, inducing structural reorganization into a Zn-rich defective ZnO phase. Owing to the low electrical conductivity of ZnO, the current distribution concentrates at the Zn substrate/ZnO layer interface. By tuning the layer resistivity with a Li + additive, we experimentally demonstrate that the conductivity of ZnO governs the current distribution and thereby dendrite growth. As charging progresses, ZnO particles become covered by the Zn-rich defective ZnO or metallic Zn, and Zn(OH) 4 2− reduction dominates at only 36–54% of the charge required for complete ZnO reduction, at both −200 and − 5 mA cm −2 . The accompanying Zn deposition causes irreversible densification of the passivation layer, which is not recovered by the subsequent discharge and degrades the rechargeability in the following charge. These findings offer rational guidance for mitigating Zn anode degradation.