Alkaline Potassium Polyacrylate Electrolytes for Rechargeable Zinc-Manganese Dioxide Batteries

Author

Jungsang Cho, CUNY City CollegeFollow

Date of Award

2024

Document Type

Dissertation

Department

Chemical Engineering

First Advisor

Sanjoy Banerjee

Second Advisor

Robert Messinger

Keywords

Alkaline Potassium Polyacrylate, Zinc, Manganese

Abstract

Zinc (Zn)–manganese dioxide (MnO₂) rechargeable batteries have high specific theoretical capacity as well as being environmentally friendly, intrinsically safe and low-cost. Liquid electrolytes, such as potassium hydroxide (KOH), are historically used in these batteries; however, many failure mechanisms of the Zn–MnO₂ battery chemistry result from the use of liquid electrolytes, including the formation of electrochemically inactive compounds and shape change of Zn electrodes during charge and discharge cycling. This led to incorporation of an alkaline polymer electrolyte as an alternative to the liquid KOH electrolyte in Zn-MnO₂ rechargeable batteries, with the aim of making them non-spillable and enhancing their transportability and maintainability.

The alkaline polymer electrolyte was made ‘in situ’ (i.e., within the battery) by filling with a mixture of acrylic acid and KOH bearing electrolyte together with a polymerization initiator and in some cases, a crosslinker. Two types of experiments were conducted with the alkaline potassium polyacrylate electrolytes (PPAE) that were formed by this process. The first focused on the 1-electron reaction region for the MnO₂ cathodes. The fundamental and commercial aspects of use in rechargeable Zn–MnO₂ batteries as an alternative to liquid electrolytes were investigated using PPAE that was not cross-linked to form a gel but formed a highly viscous liquid. PPAE of this type was found to limit overdischarge of the Zn anodes and provide good performance under solar microgrid testing protocols. The development of the in situ polymerization process for the PPAE required control (and optimization) of the rate of reaction to provide adequate soaking time of porous Zn and MnO₂ electrodes to ensure contact between the active material and the electrolyte for the electrochemical reactions. Potentiostatic and galvanostatic tests with the optimized PPAE showed higher capacity retention compared to the tests with the liquid electrolyte, suggesting that PPAE helps reduce the rate of Mn³⁺ dissolution of MnO₂ cathodes and zincate ion migration from the Zn anode, improving reversibility. Cycling tests for commercially sized prismatic cells showed PPAE had exceptional cycle life, showing 100% capacity retention for >700 cycles at 10% utilization of the MnO₂ (9.5 Ah) and for >300 cycles at 20% utilization (19 Ah), while the 19 Ah prismatic cell with a liquid electrolyte showed discharge capacity degradation at 100th cycle. Over discharge protection tests were performed with a commercialized prismatic cell, which was discharged to 0 V and continued to achieve stable discharge capacities, while the liquid electrolyte cell showed discharge capacity fade in the first few cycles. Finally, PPAE-containing batteries were tested under the IEC solar microgrid protocol. It was noted that the PPAE-containing Zn–MnO₂ batteries outperformed the Pb–acid batteries. Additionally, a system nameplated at 2 kWh with a 12 V system with 72 prismatic cells was successfully tested with the same protocol. This suggests that Zn–MnO₂ rechargeable batteries with PPAE will be a good candidate for solar microgrid systems and grid storage.

To achieve a high degree of commercial acceptability, Zn-MnO₂ rechargeable batteries should access the second electron reaction of MnO₂ while maintaining reversibility of the active materials and avoiding side reactions. In a second set of tests, PPAE was tested for MnO₂ electrodes undergoing two-electron region cycling to investigate performance. These cathodes contained bismuth and copper to enhance cyclability in the two-electron discharge range. In these tests cross-linker was added to form a hydrogel rather than a very viscous liquid as in the first series of tests. Improved safety was achieved because the cells were made non-spillable by these means, according to standards from the US Department of Transportation. The cycling of MnO₂ cathodes vs NiOOH counter electrodes with a Hg-HgO reference electrode was tested with PPAE as well as a liquid KOH electrolyte. These half cells with PPAE achieved ≥700 cycles with 99% coulombic efficiency and 63% energy efficiency at C/3 based on the 2-electron capacity of MnO₂. Other cycling tests with PPAE for Zn anodes vs MnO₂ cathodes achieved ~300 cycles before reaching 50% capacity fade. When using 25 wt.% liquid KOH electrolyte, experiments with the Zn anodes in particular achieved a longer cycle life (~500 cycles for Zn vs MnO₂) compared to when PPAE was used. This could perhaps be due to PPAE cross-linking somewhat adversely affecting passivation of the Zn anodes. However, the emphasis lies here in PPAE’s advantage in ensuring transportability and in mitigating Cu migration, thereby maintaining the reversibility of MnO₂ and resulting in a non-spillable battery.

As mentioned earlier, the cathodes consisted of MnO₂ with copper and bismuth additives, which are necessary to allow stable cycling. Electrodes dissected after cycling showed that the liquid electrolyte allowed Cu ions to migrate more than PPAE. Migration of these ions is a complex phenomenon involving convection, diffusion and electrical effects. Even though Cu migration was markedly reduced in PPAE, measurements of the Cu diffusion coefficient (in the absence of electrical and convection effects) showed no difference between liquid KOH electrolyte and PPAE. This was perhaps due to the high water volume fraction in PPAE. Significant electrode volume changes occur during cycling in the two-electron region and this tends to pump the electrolyte in and out of the space between the electrodes. Perhaps the convective effects associated with such phenomena are mitigated in the hydrogel electrolyte. In any case, analysis of the separators following cycling indicates much reduced Cu ion migration and much lower Cu deposition on the separator when using PPAE. Besides ensuring cell non-spillability during transportation, this supports MnO₂ reversibility under two-electron cycling conditions. Work is continuing to improve zinc anode performance under high utilization conditions using PPAE by optimizing both anode and electrolyte compositions.

Recommended Citation

Cho, Jungsang, "Alkaline Potassium Polyacrylate Electrolytes for Rechargeable Zinc-Manganese Dioxide Batteries" (2024). CUNY Academic Works.
https://academicworks.cuny.edu/cc_etds_theses/1303