Understanding Separator Properties Governing Zincate Crossover in Rechargeable Alkaline Zn–MnO2 Batteries (Supporting Information)

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 cm2 s−1) and a moderate diffusion coefficient of hydroxide ions (DOH = 3.5 × 10−5 cm2 s−1), leading to the highest permselectivity (DOH/DZn = 1.7 × 103). 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.