Calcium Hydroxide Membrane As a Separator to Immobilize Zincate Ions in Secondary Alkaline Batteries
Authors: Huang Jinchao , Yadav Gautam Ganapati , Gallaway Joshua W , Nyce MIchael , Banerjee Sanjoy 
Published: February 2020

The rechargeable alkaline zinc/manganese dioxide battery is an attractive candidate for large-scale energy storage, as it is inexpensive, safe, and able to provide high energy density. Recent commercialization success with the Zn/MnO2 rechargeable batteries has been possible by limiting the depth of discharge (DOD)1. The reasons for the low DOD are tied to the inherent material properties of MnO2 and Zn, as well as the poisoning of the MnO2 cathodes by zincate ions. With the formation of an electrochemically inactive material hetaerolite (ZnMn2O4), zincate poisoning has become a crucial factor that limits the rechargeability of the battery2.
In order to immobilize the zincate ions, a new inorganic separator has been invented. The inorganic material used is calcium hydroxide, which has been reported to be an effective additive in Zn electrodes to mitigate the shape change problem3. Ca(OH)2 is able to localize the zincate ions by forming an insoluble complex calcium zincate (CaZn2(OH)6•2H2O). The formation and decomposition kinetics have been well studied4,5. However, the addition of Ca(OH)2 sacrifices the electrode’s conductivity, and its low density adds to the thickness of the electrode. Therefore, in this work a separator sheet was fabricated out of Ca(OH)2, instead of adding it directly to the electrode. By doing this, its negative effect on the electrode could be avoided, while its function as a “zincate reservoir” was kept.
The lab-fabricated Ca(OH)2 sheet has been characterized in electrolytes of different KOH concentrations. Its properties have been compared with those of widely applied commercial separators, including Celgard 5550 (Celgard, LLC, USA), Freudenberg FSWR104 (Freuden-berg Non-wovens LP), and Cellophane 350PØØ (Innovia Films Company). Results are shown in Table 1.
The permeabilities of zincate through different separators were tested in prismatic cells during battery cycling. We found that at the end of the first discharge, the Ca(OH)2 membrane, compared with other tested membranes, was able to reduce the amount of zincate ions in the cathode side by around 50%. After running for 20 cycles at full one-electron DOD of MnO2, the MnO2 electrodes were characterized by XRD and EDS. The XRD patterns (Fig.1) clearly showed that when normal commercial separators were used, the reflections corresponding to the MnO2 phase vanished and new reflections belonged to hausmannite (Mn3O4) or hetaerolite (ZnMn2O4) occurred after 20 cycles. However, in the cell with Ca(OH)2 membrane, no noticeable change could be found, indicating little material phase change. The EDS elemental analysis results on the MnO2 electrodes’ surfaces and cross-sectional areas also supported this conclusion, as the atomic ratio of Zn to Mn was only 0.05 when Ca(OH)2 membrane was applied, while for those without Ca(OH)2 membranes, a value close to 0.5 was found.
The Ca(OH)2 membrane has also been applied in a battery with lab-modified MnO2 electrodes. The battery was able to achieve more than 800 cycles at 80% of the 2-electron capacity, where the problem of zincate contamination is more severe. The curves of specific discharge capacity change are plotted in Fig.2. We can see that performance of the cell with Ca(OH)2 membranes is much better compared with the other two cells. With such a high retention of the second electron capacity being accessible, the Zn/MnO2 battery has achieved a major breakthrough.
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[5] Y. M. Wang, J. Electrochem. Soc., 137, 9, 2800 (1990)

Figure 1