Xinyi Zhao

Date of Award

Summer 2019

Document Type

Open Access Thesis

Department

Chemical Engineering

First Advisor

William E. Mustain

Abstract

Zn-MnO2 primary alkaline batteries stood out immediately among all batteries after it was introduced in the late 1980s and has attracted world-wide attention. These batteries have dominated the portable power market for decades as a highly effective energy storage system with high energy density, long life and low self-discharge, dependable safety and other superiorities.

In order to achieve higher cell-level energy density, Aluminum has long been considered a very attractive anode material for alkaline batteries because of its high theoretical capacity (2980 mAh/g; 2.60 Ah/cm3) and low price. However, some early efforts to utilize aluminum anodes did not succeed in any commercial battery products. The primary hurdle in the adoption of aluminum anodes in alkaline batteries has been the extremely high Al corrosion rates in concentrated KOH electrolytes. At the same time, zinc is the most common anode for primary alkaline batteries, because it is relatively inert in highly concentrated KOH and is one of the most electropositive metals in the electrochemical series.

Therefore, the underlying goal of this thesis is to investigate various methods and environments to deposit Zn layers on top of Al and to evaluate their efficacy in reducing the corrosion rate. Also, if a thin layer of Zn were achieved, the resulting Zn-coated Al materials would maintain the high energy density offered by Al. Different approaches – including electrodeposition and electroless deposition – have been employed to deposit Zn onto Al substrates (including wires and foils).

Zn was first deposited onto Al wires in a 3-electrode cell. Electrolytic bath additives, including chloride ions (Cl-, from HCl) and polyethylene glycol (PEG), were introduced into the 1M ZnSO4 solution during electrodeposition. The structure and electrochemical behavior of the resulting Zn-coated Al were characterized. The adhesion and crystallinity of the deposits improved with adding Cl- ions and PEG, and a lower porosity deposit was achieved. All of the films prepared from a 1%PEG + 1M ZnSO4 + (≤ 5 ppm) HCl deposition electrolyte showed a 99% reduction in H2 gassing compared to bare Al wires, and more than twice the capacity of Zn was achieved – though 2.5 ppm Cl- did show the highest capacity. It was even shown that this deposition process was repeatable with partial discharges.

After successful demonstration of Zn onto Al wires, the project then focused on moving away from wires and examining the creation of Zn-coated Al particles for use in realistic battery systems. 1M ZnSO4 + 1%PEG + 2.5PPM HCl+ 0.3M Sodium hypophosphite bath was explored for electroless deposition to form Zn-coated Al. The microparticles that were grown were too large to form a thin and efficient layer and due to their wide inter-particle spacing; hence, the sample had a relatively high H2 gassing rate. It was concluded that electrodeposition was a far more effective method to produce Zn particles than electroless deposition. However, no approaches resulted in Zn-coated Al active materials with acceptably low degradation rates to be seriously considered for commercial applications. Therefore, an alternative approach to utilizing a portion of the Al capacity while maintaining a low-corrosion material was conceived – to create Zn-rich Al-Zn alloys. With higher Al content in the alloy, it was confirmed that a higher capacity could be achieved. This result clearly showed that the Al incorporated into the Zn particles could be electrochemically utilized during discharge.

Rights

© 2019, Xinyi Zhao

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