Alkaline manganese dioxide (MnO₂)/Zinc (Zn) cells have been primarily used as AA batteries, where their complete capacity is discharged only once because the electrochemical irreversibility of its active materials. The energy density obtained from its single discharge is >400Wh/L because of the high capacity of MnO₂ (617mAh/g) and Zn (820mAh/g) based on two electron reactions.¹ This energy density is higher than other aqueous chemistries like lead acid, nickel-zinc, nickel-metal hydride and some varieties of lithium-ion as well, which could find its usage in important applications like grid-scale energy storage because of its other advantages like low cost, non-toxic material and nonflammable electrolyte; however, it has been relegated to the realm of primary applications like remote controls, etc. because of its electrochemical irreversibility. Recently, we discovered the complete reversibility of MnO₂ in alkaline electrolyte by using its layered polymorph called birnessite (δ-MnO₂) mixed with bismuth oxide and intercalating it with copper (Cu) ions [Cu-intercalated Bi-δ-MnO₂). This new material helped to achieve the theoretical two electron capacity of 617mAh/g for over 3000-6000 cycles.¹ When the Cu-intercalated Bi-δ-MnO₂ was tested against a Zn anode (15% utilization), energy densities of >160Wh/L were obtained; however, cycle life was limited because of Zn-ion (zincate) poisoning of the cathode to form ZnMn₂O₄.⁴ We implemented calcium hydroxide [Ca(OH)₂] interlayers in the cells which allowed to capture these zincate ions and prevent the poisoning of the cathode, which resulted in the highest cycle life of 900 yet achieved from this chemistry in literature. These results, however, were laboratory size cells with electrode sizes ranging from 1in² to 6in² and occasionally with 2 cathodes and 3 anode pairings. To understand the applicability of this chemistry to real world conditions, it is necessary to test them at much higher capacities (areal and volumetric), larger electrode sizes, in starved electrolyte conditions and much higher active mass loadings, where these batteries can be considered economical.

In this talk, we will present our experiences building and testing >70-100Ah large prismatic cells (18in²) of Cu-intercalated Bi-δ-MnO₂/Zn. The cells consisted of 10 cathodes and 11 anodes, where the cell capacity varied from >70-100Ah, the Zn utilization varied between 10-25% utilization and a number of different C-rates were tested. We found that the cycle life of the cells were usually affected by the high utilizations of the Zn anode, where causes of failure were due to shape change, drying of the electrode and passivation (probably Type 2-type ZnO formation). The drying of the electrode was a result of the expansion of the cathodes during cycling, which would result in limiting the electrolyte contact with the Zn anodes. Further drying of the anodes were also postulated to be due to pore plugging as a result of ZnO formation. The compactness and expansion of the cells could sometimes also result in minimizing the electrolyte contact or hydroxyl ion transport to the cathodes, which would result in affecting the cathode performance. When cellophane and Celgard 5550 were used as the only separators the cells occasionally experienced “soft” shorts, where Zn and Cu deposited on the separators. The changing pH of the electrolyte, the cross talk between dissolved Mn and Zn ions and minimized hydroxyl ion transport would often lead to formation of spinel-type structures, which would affect the potential curves of the cells. Cycling protocols were also found to be very important in improving the cycle life of the large prismatic cells, where inputting charge on constant voltage protocol was a driver in negatively affecting the discharge curves of the cell.

We developed a number of successful methods to improve the cycle life, where use of interlayers like Ca(OH)₂ and other types of layers prevented the “soft” short and minimized the effect of zincate ions on the cathode. New electrode designs were incorporated that improved electrolyte contact and maintained hydroxyl transport to the cathodes. Cycling protocols were designed to improve the plating efficiency of the Zn, the efficiency of the cathode formation and improve the cycle life of the cells. New novel membranes were designed that successfully blocked zincate ions from poisoning the cathode. We will present these results in detail and discuss our perspective and new directions for this chemistry.

Funding:

This work was supported by the New York State Research and Development Authority (NYSERDA) under Project Number 58068.

References:

1] Yadav, G. G.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Huang, J.; Wei, X.; Banerjee, S., Nat. Commun., 2017, 8, 14424