Performance and failure mechanisms of alkaline zinc anodes with addition of calcium zincate (Ca[Zn(OH)3]2·2H2O) under industrially relevant conditions

Abstract

Additions of calcium zincate (CaZn₂(OH)₆·2H₂O, CaZn) to zinc (Zn) anodes in alkaline batteries have been investigated and were found to remarkably increase cycle life at high 50% Zn utilization of the anode's theoretical capacity, thereby significantly reducing anode costs. A spectrum of anode formulations with increasing CaZn (0%, 30%, 70%, 100%) in mixtures with metallic Zn is investigated along with minor additions of Bi₂O₃, acetylene carbon black, and CTAB. The total molar zinc content is normalized; thus, electrode capacity is kept comparable, resulting in electrodes relevant to real world use cases. We report details of the cell design, electrolyte composition, electrode design, and cycle testing procedure, all of which are kept close to industrially relevant values. A pure CaZn anode with acetylene carbon was shown to achieve 1062 cycles at 20% Zn utilization in ZnO saturated 20 wt% KOH whereas traditional Zn anodes only utilize 10% for similar cycle life. At high 50% Zn utilization, CaZn anodes achieved ∼280 cycles while Zn anodes achieved ∼50 cycles, resulting in a five-fold improvement in cycle life resulting in approximately ∼25% reduction in cost per cycle. Scanning electron microscopy analysis of cycled electrodes shows that adding CaZn reduces electrode failure by slowing down formation of a passivating zinc oxide layer at the surface of the electrode as well as decreases shape change. This appears to occur because zinc and calcium remain intimately mixed forming CaZn, which reduces dissolution and reprecipitation, but slowly will segregate as inactive materials are pushed to the surface where conductivity is lower.