Unveiling Critical Insight into the Zn Metal Anode Cyclability in Mildly Acidic Aqueous Electrolytes: Implications for Aqueous Zinc Batteries.

The cost benefit and inherent safety conferred by the energy-dense metallic zinc anode and mildly acidic aqueous electrolytes are critical to aqueous zinc batteries' (AZBs) large-scale energy-storage ambition. Aggressive research efforts in the past five years have resulted in the discovery of several high-energy positive (cathode) host materials, but understanding of the Zn anode rechargeability and any influence of the electrolyte, which are critical for AZBs' practical development, remains limited. As we unravel here, under realistic test conditions, when parameters are set keeping practical applications in mind, Zn anode cycling appears vulnerable to dendritic failure in all common AZB electrolytes. While 3 M ZnSO4 delivers the best overall performance for the Zn anode cycling, viability of the oxidatively stable "water in salt" electrolyte appears gravely restricted. Defying the general understanding of metal electrodeposition, a high current density is found to dramatically prolong the Zn cycling lifetime, achieving >8000 cycles at 20 mA cm-2 for 1 mAh cm-2 capacity in 3 M ZnSO4. High current also allows prolonged cycling at capacities of 2 and 4 mAh cm-2. Such a striking improvement in lifetime on going from low to high currents is further confirmed through Zn|Zn0.25V2O5 and Zn|LiMn2O4 full-cell studies with practical electrode loading. Not surprisingly, all the parameters influence the cycled Zn morphology, which in turn dictates the propensity for short-circuit. These findings not only divulge previously unanticipated insight into the Zn anode cycling and electrolyte performance in AZBs but also offer a rational basis to gauge their practical development.