Understanding Separator Properties Governing Zincate Crossover in Rechargeable Alkaline Zn–MnO<sub>2</sub> Batteries

Suppressing zincate crossover is necessary for rechargeable alkaline Zn-MnO2 rechargeable batteries using electrolytic manganese dioxide (EMD).We benchmarked six commercial separators for their ability to block zincate ions while maintaining the transport of hydroxide ions.Among the separators, FAAM-75-PK (FP75), a commercial anion-exchange membrane, exhibited moderate conductivity (5.8 mS cm -1 ) with the lowest diffusion coefficient of zincate ions (DZn = 2.0 10 -8 cm 2 s -1 ) and a moderate diffusion coefficient of hydroxide ions (DOH = 3.5 10 -5 cm 2 s -1 ), leading to the highest permselectivity (DOH/DZn = 1.7 10).In the galvanostatic discharge-charge tests, FP75 exhibited the best capacity retention of 68% at the 7th cycle.Characterization of the discharged EMD electrodes revealed that FP75 effectively suppressed ZnMn2O4 formation at the cathode, favoring Mn3O4 formation instead.The low Zn concentration of 6 ppm in the catholyte is consistent with the favored Mn3O4 formation.Beaker-cell tests identify a system-dependent zinc concentration threshold.When the Zn/Mn molar ratio exceeds ~0.05-0.15,product formation shifts toward ZnMn2O4 from Mn3O4, and thus, the capacity decay accelerates.