Rechargeable alkaline Zn–MnO2 (RAM) batteries are a promising candidate for grid-scale energy storage owing to their high theoretical energy density rivaling lithium-ion systems (∼400 Wh/L), relatively safe aqueous electrolyte, established supply chain, and projected costs below $100/kWh at scale. In practice, however, many fundamental chemical and physical processes at both electrodes make it difficult to achieve commercially competitive energy density and cycle life. This review presents a detailed and timely analysis of the constituent materials, current commercial status, electrode processes, and performance-limiting factors of RAM batteries. We also examine recently reported strategies in RAM and related systems to address these issues through additives and modifications to the electrode materials and electrolyte, special ion-selective separators and/or coatings, and unconventional cycling protocols. We conclude with a critical summary of these developments and discussion of how future studies should be focused toward the goal of energy-dense, scalable, and cost-effective RAM systems.
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 (DZn (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 (DOH (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).
Rechargeable alkaline batteries containing zinc anodes suffer from redistribution of active material due to the high solubility of ZnO in the electrolyte, limiting achievable capacity and lifetime. Here, we investigate pre-saturating the KOH electrolyte with ZnO as a strategy to mitigate this issue, utilizing rechargeable Ni-Zn cells. In contrast to previous reports featuring this approach, we use more practical limited-electrolyte cells and systematically study ZnO saturation at different levels of zinc depth-of-discharge (DODZn), where the pre-dissolved ZnO is included in the total system capacity. Starting with 32 wt. % KOH, cells tested at 14%, 21%, and 35% DODZn with ZnO-saturated electrolyte exhibit 191%, 235%, and 110% longer cycle life respectively over identically tested cells with ZnO-free electrolyte, with similar energy efficiency and no voltage-related energy losses. Furthermore, anodes cycled in ZnO-saturated electrolyte develop more favorable compact zinc deposits with less overall mass loss. The effect of initial KOH concentration was also studied, with ZnO saturation enhancing cycle life for 32 wt % and 45 wt % KOH but not for 25 wt % KOH, likely due to cell failure by passivation rather than shorting. The simplicity of ZnO addition and its beneficial effect at high zinc utilization make it a promising means to make secondary alkaline zinc batteries more commercially viable.