Author ORCID Identifier

https://orcid.org/0000-0002-7330-3091

Date Available

10-20-2024

Year of Publication

2023

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Chemical and Materials Engineering

First Advisor

Dr. Yang-Tse Cheng

Abstract

Rechargeable batteries have become a staple of modern society, driving the development and expansion of portable electronics and electric vehicles. The long lifetime and high energy density provided by batteries have made them excellent candidates for use in the emerging field of long duration, grid-scale energy storage. However, traditional battery chemistries, such as Li-ion, are not an ideal solution for grid-scale storage, due to high-cost raw materials that are often sourced from volatile markets. Sodium-based batteries are a promising alternative for long duration storage, owing to the domestic abundance of raw materials and energy densities that nearly rival that of Li-ion cells. One of the most significant obstacles to widespread adoption of sodium batteries is premature failure of the solid-state electrolyte, which significantly reduces their lifetime. Therefore, this dissertation primarily focused on understanding failure modes in solid electrolytes for sodium batteries and how these modes related to quantifiable electrochemical and mechanical properties.

First, the primary modes of failure in NaSICON solid electrolytes were probed using molten sodium symmetric cells. The dominant mode of electrolyte failure in molten sodium cells was traditionally considered to be pressure-driven cracking (Mode I) at the interface where sodium metal is plated. Surface flaws and imperfect interfacial contact were viewed as the culprit for this failure mode. However, we identified that recombination of ions and electrons in the electrolyte interior (Mode II) was an equal contributor to failure in NaSICON, particularly at high (dis)charge capacities. These results demonstrated that electronic conductivity and anticipated (dis)charge capacity of the battery should be acknowledged as key parameters in the design of future electrolytes.

An interfacial tin coating on NaSICON was explored to alleviate the effects of both Mode I and Mode II failure. The coating essentially eliminated any interfacial resistance from the electrolyte and effectively suppressed Mode I (pressure-driven) failure at current densities up to 500 mA cm-2. However, Mode II (ion-electron recombination) failure was still prevalent at these current densities, again highlighting the importance of electronic conductivity in NaSICON electrolytes. Eventually, at current densities close to 1000 mA cm-2, interfacial defects succumb to Mode I failure, demonstrating the multiple, current-dependent regimes of failure in solid electrolytes.

These findings are then explored in the context of long duration energy storage by examining the performance of symmetric cells tested at different (dis)charge capacities. It was observed that symmetric cells subjected to high-capacity half-cycles failed much earlier than those at low-capacity half-cycles. These findings stress the importance of developing new test methods to accurately predict performance in this new class of long duration batteries.

Throughout this work, relevant mechanical properties of NaSICON solid electrolytes were explored. It was found that mechanics at multiple length scales (and particularly large length scales) were necessary to accurately assess material performance and failure. The mechanical properties of a new, low-cost electrolyte, montmorillonite, were also evaluated. These findings, coupled with an understanding of electrochemical failure, provide a path for better design and characterization of solid electrolytes.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2023.420

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Available for download on Sunday, October 20, 2024

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