Author ORCID Identifier

https://orcid.org/0000-0002-1156-0324

Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. Yang-Tse Cheng

Abstract

Rechargeable lithium ion and lithium (Li) metal batteries with high energy density and stability are in high demand for the development of electric vehicles and smart grids. Intensive efforts have been devoted to developing high capacity battery electrodes. However, the known high capacity electrode materials experience fast capacity fading and have limited cycle life due to electromechanical degradations, such as fracture of Si-based electrodes and dendrite growth in Li metal electrodes. A fundamental understanding of electromechanical degradation mechanisms of high capacity electrodes will provide insights into strategies for improving their electrochemical performance. Thus, this dissertation focuses on mechanical properties, microstructure changes, and degradation mechanisms of Si composite electrodes and Li metal electrodes. Based on these findings, possible coping strategies are proposed to improve the cycling stability of both electrodes.

The poor cycling life of Si-based electrodes is caused by the repeated lithiation/delithiation-induced huge volumetric change in Si particles, which leads to the fracture of particles, excessive formation of solid electrolyte interphase on the newly exposed surface, as well as the loss of electronic conductivity between Si particles and the conductive matrix. The expansion/contraction of Si particles during cycling also causes the changes in the mechanical properties, microstructure, and porosity of Si composite electrodes. Understanding the relationship between mechanical property evolution, microstructure degradation, and capacity fading is essential for the design of Si composite electrodes. Using an environmental nanoindentation system, in situ microscope cell, and electrochemical impedance spectroscopy, I investigated the mechanical properties, cracking behavior, and lithiation/delithiation kinetics of Si composite electrodes made with different polymeric binders, including polyvinylidene fluoride, Nafion, sodium-carboxymethyl cellulose, and sodium-alginate, in their realistic working environment. The mechanical property evolution is determined by the state-of-charge, porosity, irreversible volume change, and mechanical behavior of binders. Periodical crack opening and closing happens in Si composite electrodes prepared with binders that have strong adhesion with Si. Mechanical degradations, e.g., irreversible volume change, cracking, and debonding between binders and Si particles, are correlated with the evolution of lithiation/delithiation kinetics and the capacity fading of Si composite electrodes. Based on these findings, a partial charging approach is proposed and confirmed experimentally to improve the cycling stability of Si composite electrodes.

Li metal electrodes suffer from the low Coulombic efficiency, high electrochemical reactivity with the electrolytes, and the safety hazards caused by the uncontrollable dendrite growth during cycling. Mechanical suppression by using solid electrolytes and artificial SEI is a promising strategy to inhibit the formation of Li dendrites. Mechanical properties of bulk and mossy Li are required for designing mechanical inhibitors and improving the stability of the Li | inhibitor interface. Using an environmental nanoindentation system, I studied the mechanical behavior, especially the time-dependent behavior, of bulk Li and porous mossy Li at ambient temperature. By combining finite element (FE) modeling with experiments, a constitutive law was determined for the viscoplastic deformation of Li metal. FE modeling also demonstrates that the elasticity has a negligible influence on the indentation deformation of bulk Li. Flat punch indentation measurements showed that mossy Li has significantly higher deformation and creep resistance than bulk Li despite of its porous microstructure. The mechanical parameters of bulk and mossy Li may be helpful to develop of dendrite-free Li metal electrodes.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.315

Funding Information

This work is supported by the National Science Foundation Award No. 1355438 and Vehicle Technologies Office of the U.S. Department of Energy Battery Materials Research (BMR) Program under Contract Number DE-EE0007787.

Available for download on Sunday, January 26, 2020

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