Zincate-Blocking-Functionalized Polysulfone Separators for Secondary Zn-MnO₂ Batteries

Alkaline zinc-manganese dioxide (Zn-MnO₂) batteries are well suited for grid storage applications because of their inherently safe, aqueous electrolyte and established materials supply chain, resulting in low production costs. With recent advances in the development of Cu/Bi-stabilized birnessite cathodes capable of the full 2-electron capacity equivalent of MnO₂ (617 mA h/g), there is a need for selective separators that prevent zincate (Zn(OH)₄)^2- transport from the anode to the cathode during cycling, as this electrode system fails in the presence of dissolved zinc. Herein, we present the synthesis of N-butylimidazolium-functionalized polysulfone (NBI-PSU)-based separators and evaluate their ability to selectively transport hydroxide over zincate. We then examine their impact on the cycling of high depth of discharge Zn/(Cu/Bi-MnO₂) batteries when inserted in between the cathode and anode. Initially, we establish our membranes' selectivity by performing zincate and hydroxide diffusion tests, showing a marked improvement in zincate-blocking (D_Zn (cm²/min): 0.17 ± 0.04 × 10^-6 for 50-PSU, our most selective separator vs 2.0 ± 0.8 × 10^-6 for Cellophane 350P00 and 5.7 ± 0.8 × 10^-6 for Celgard 3501), while maintaining similar crossover rates for hydroxide (D_OH (cm²/min): 9.4 ± 0.1 × 10^-6 for 50-PSU vs 17 ± 0.5 × 10^-6 for Cellophane 350P00 and 6.7 ± 0.6 × 10^-6 for Celgard 3501). We then implement our membranes into cells and observe an improvement in cycle life over control cells containing only the commercial separators (cell lifetime extended from 21 to 79 cycles).