Date of Award

Fall 2024

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Jamil Khan

Abstract

Rechargeable aqueous zinc-ion batteries (ZIBs) have garnered significant attention in recent years as a promising candidate for stationary large-scale energy storage due to their distinct safety features and cost-effectiveness compared to conventional lithium-ion batteries. Despite their great potential, ZIBs are currently facing critical challenges for commercialization, including poor cycle stability at low discharge rates, lower energy density (Wh/kg) and higher self-discharge rate. These issues must be addressed for them to become a viable energy storage solution. The above challenges are fundamentally rooted in the bulk properties of electrolytes and interactions with electrodes, such as; 1) formation of insulating layered double hydroxide (LDH)-based solid electrolyte interphases (SEIs) at the Zn/electrolyte interface due to Zn-H2O reactions, accompanied by the localized hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR); 2) dissolution of cathode materials, such as MnO2 and V2O5, into the aqueous electrolyte, resulting in the loss of active materials. The LDH-based SEIs are also formed at the oxide cathode/electrolyte interface due to the co-intercalation of protons into layered cathodes.

My PhD study is aimed at understanding the degradation mechanisms and develop viable solutions to mitigate the degradation. First, I systematically measured the bulk properties, including conductivity, pH, water activity and signatures of vibrational spectroscopy, of various aqueous Zn-ion electrolytes as a function of concentration. Then, I focused on investigation of the interfacial reaction mechanisms between Zn-anode and electrolytes. Based on the initial results and understanding, I decided to develop gel polymer electrolytes (GPE) to address the degradation problems caused by the uncontrollable electrode and electrolyte interactions and formation of excessive LDH. Three distinct GPEs were studied: carboxymethyl chitosan (CMCS) crosslinked with polyacrylamide (PAM), donated as CSAM, polybenzimidazole, denoted as p-PBI, and another new form of PAM having higher acrylamide content and zinc salt inclusion. I started the work by first synthesizing CSAM and PAM membranes and characterized their chemical, mechanical and electrochemical properties. I also prepared Zn-ion functionalized commercial PBI membrane and performed similar properties characterization for comparison. The Zn-salts that were used to functionalize GPEs are: Zn(ClO4)2 and Zn(OTf)2. Among these polymer electrolytes, CSAM exhibited the highest ionic conductivity, reaching 26 mS/cm with Zn(ClO4)2 and 13 mS/cm with Zn(OTf)2. Both CSAM and PAM demonstrated anti-dendrite resistance, with stability lasting up to 4000 hours and high Zn-ion transference numbers of 0.65 and 0.52, respectively, along with lower activation energy compared to PBI membrane. Due to reduced water activity, PAM and CSAM widened the operating voltage windows (2.7 V and 2.66 V, respectively) compared to PBI and glass fiber, particularly when paired with Zn(OTf)2.

Throughout this PhD research, I also had an opportunity to develop a novel hydrogel cathode using polyacrylamide (PAM) hydrogel as an ion-conducting binder. With this new binder, the cathode becomes a mixed ion and electron conductor, significantly increasing the number of reactive sites (or triple phase boundaries). I used ammonium vanadate as the active Zn-ion storing material, carbon black as the electronic conductor, and PAM embedded with Zn(ClO4)2 as the ion conductor. This innovative hydrogel cathode has led to both high stability and excellent capacity of ZIBs. More importantly, it can also be applied to other aqueous batteries for performance and stability enhancement.

To comprehend my fundamental understanding of the newly emerged ZIB chemistry, I also performed a mathematical modeling of ZIB performance using COMSOL Multiphysics. The study includes building a physical model, establishing governing equations and boundary conditions, and validation with experimental data. With a validated model, I further performed sensitivity analysis to search for the most impactful parameters on performance. I found that ionic conductivity, electrode thickness, and exchange current density of the electrodes are the most influential parameters on ZIB performance. The sensitivity analysis provides guidance for designing high-performance ZIBs. It was found that increasing the ionic conductivity beyond 13 mS/cm does not significantly improve battery performance, particularly at lower current densities. Additionally, the analysis highlights the importance of high exchange current densities to maintain good capacity at higher current densities.

Rights

© 2025, Roya Rajabi

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