Author

Ehsan Faegh

Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

William E. Mustain

Abstract

Today Lithium-ion (Li-ion) batteries are the most-emphasized battery technology among the many different battery systems in the market. However, due to their high cost (especially for electric vehicle applications), flammability and toxicity, the development of inexpensive and safer alternative battery chemistries has been the focus of a significant amount of recent research. Among various battery chemistries, Aluminum-based (Al-based) and Zinc-based (Zn-based) batteries have been touted as promising options to compete with Li-ion batteries. However, the practical realization of Al battery chemistries has been difficult over a long period of time (170 years) due to a number of fundamental and intrinsic limitations of Al anodes. On the other hand, Zn anodes have been a reliable electrode material in primary alkaline batteries since the 1960s due to their intrinsic safety, low cost of Zn and use of aqueous, non-flammable electrolytes.

However, as elaborated on in Chapter 1, Zn based anodes are not perfect and there are modern issues that need to be addressed. First, Zn corrosion during discharge leads to the rapid release of H2 gas, which can cause cells to rupture and leak. Second, due to an inability to control the in-cell morphology during recharging (Zn plating), Zn-MnO2 alkaline batteries have shown limited rechargeability. Third, aqueous Zn batteries are prone to passivation, which can limit their achievable depth of discharge. In recent years, the most common research to address these issues have been: i) manipulating the interaction of Zn with the electrolyte by introducing electrolyte additives; ii) modifying the Zn bulk and surface compositions; and iii) designing exotic electrode architectures to improve mass transport. Although some approaches have shown modest success, many of them have failed to truly identify and address the root cause for corrosion and alkaline cell leakage. Additionally, many of the solutions that have been posed have not been economically feasible, either because of the material chemistry or the fabrication method.

Therefore, my dissertation has focused on: i) identifying the fundamental cause for rapid Zn corrosion and H2 release (gassing); ii) using that understanding to pose engineering solutions to this problem; and iii) using a new understanding about the role of Zn structure to improve rechargeability. First, multiple operando cells were designed and built to probe and visualize the reaction dynamics of Zn-MnO2 alkaline cells. It was discovered that a previously un-reported redox electrolyte mechanism is what enables the current to be carried through the anode column to the current collector – despite the fact that the anode lacks true electronic percolation. This leads to a dynamic stripping and deposition of Zn in the direction of current flow, which results in the formation of high surface area Zn – driving corrosion and H2 evolution. The development of the operando cells and redox mechanism are detailed in Chapter 2.

Next, a method to control the crystallographic orientation of Zn was developed, which is discussed extensively in Chapter 3. The resulting electrolytic Zn (e-Zn) allowed for the study of which of the Zn facets were preferentially responsible for gassing (basal plane) and Zn architectures were then made that possessed lower gassing and corrosion rate compared to commercial Zn powder. Control over the orientation also enabled record cycling performance – where secondary cells were operated for more than 2000 h in symmetric cells and 400 h in full cells. In Chapter 4, the discharge capacity of Zn was enhanced by controlled partial inclusion of Al atoms in the e-Zn (e-Zn/Al). Al has been a commonly discussed replacement for Zn in alkaline batteries due to its higher theoretical gravimetric capacity (2980 mAh g-1) than Zn (820 mAh g-1). However, such replacement has not been practically achievable due to even more severe corrosion and passivation. Here, partial inclusion of Al coupled with electrolyte engineering, e.g. adding ZnO as an electrolyte additive, reduced the Al corrosion rate by 2 orders of magnitude. The resulting e Zn/Al anodes significantly increased the discharge capacity and energy density by 53% (581 mA g-1anode) and 56% (~784 Wh kg-1anode), accordingly, compared to pure Zn. This was also accompanied by superior reversibility – up to 800 h – in full cells. Finally, the major findings accomplished by this study are summarized in Chapter 5 and some recommendations for future work are provided in Chapter 6.

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