Percolating Anode Microstructures Underpin the Choice of Electrolyte Composition for a Stable Alkaline Zn Battery

Rechargeable alkaline Zn batteries are promising alternatives to Li-ion batteries, but the cycle lives remain short at a moderate depth of discharge. The choice of electrolyte has a strong impact, whose exact mechanism has yet to be deciphered. Here, we understand the electrolyte effect from the perspective of passivation, the formation of a ZnO layer in the anode during battery discharging. We reveal that the porosity of the layer determines the failure mechanism. Too low porosity blocks ion transport in 4 M KOH, whereas too high porosity renders a large volume change in 6 M KOH and disrupts electron transport, both of which conform to continuum percolation theory. A Ca(OH)2-containing electrolyte results in a medium porosity and enables a stable NiOOH/Zn battery. The work provides not only a quantitative approach to understanding a Zn anode through its microstructure but also a guide to selecting electrolytes for stable Zn batteries.