Author ORCID Identifier

https://orcid.org/0000-0003-1243-9857

Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. Yang-Tse Cheng

Abstract

Lithium-ion batteries (LIBs) with high energy density and cycling stability play a critical role in developing electric vehicle (EV) and grid energy storage techniques. The electrochemical performance of LIBs can be improved by using high capacity positive (e.g., LiNi1/3Mn1/3Co1/3O2, i.e., NMC111) and negative (e.g., silicon) electrodes; both, however, experience severe electrochemical-mechanical degradation caused by the lithiation/delithiation induced volume changes. Understanding mechanical degradation mechanisms and their relationships with the capacity fading of electrodes is important for improving the cycling stability of electrodes as well as optimizing the design of electrodes with high capacity electrode materials.

As one of the current commercial positive electrodes, NMC degrades because of the structural disintegration of its secondary particles, which consists of submicron primary particles. The decohesion of primary particles leads to the loss of electronic conductivity and low utilization of NMC. Hence, the fracture behavior of NMC particles is crucial to optimizing the performance of NMC electrodes. Using flat punch indentation measurements, the intergranular dominating fracture behavior of single NMC secondary particles with different sizes and at various state-of-charge (SOC) was investigated. The critical load corresponding to the fracture of secondary particles increases with increasing particle size, while the fracture strength (St ) is statistically independent of the particle size. Electrochemical cycling has tremendous effects on the fracture behavior, as St decreases remarkably just after the first delithiation. In addition, St decreases during delithiation and increases during the subsequent lithiation process due to the SOC-dependent stress generated in secondary particles. Possible approaches to enhance the structural integrity of NMC secondary particles are proposed based on these findings.

Low-cost Si microparticles (SiMPs) are a promising high capacity negative electrode material for LIBs. The lithiation/delithiation-induced substantial volume change, inevitable fracture, and unstable solid electrolyte interphase (SEI) of SiMPs impede the applications of SiMP electrodes. Several recent studies have shown that using proper polymeric binders can mitigate the electrochemical-mechanical degradation of SiMP-based electrodes. Yet, a guidance for designing effective binders for SiMP electrodes is lacking. Herein, the effect of binders on the degradation behavior of SiMP electrodes and its correlation with binders’ properties were investigated. The comparison among three binders, i.e., polyvinylidene fluoride, Nafion, and sodium-alginate, shows that the strong adhesion between binders and Si is not the dominating parameter for the degradation of SiMP electrodes. Mechanical properties of binders are of critical importance. Furthermore, poly(acrylic acid) (PAA) and Li substituted PAA (PAA-xLi, x ≤ 1) were used as a model binder system to study the effects of binders on the electrochemical stability of SiMP electrodes. Due to the metal cation-induced electrostatic association of carboxyl groups, PAA-xLi binders exhibit different mechanical properties, adhesion with Si, and electrochemical stability. As a result, SiMP/PAA-xLi electrodes show different cycling stabilities and C-rate capabilities following the sequence of PAA-0.75Li > PAA-0.5Li > PAA-0.25Li > PAA-1Li > PAA-0Li. The correlation between the electrochemical performance of SiMP/PAA-xLi electrodes and the properties of PAA-xLi further suggests that the critical properties of binders for SiMP electrodes are robust mechanical properties, strong adhesion with Si, and electrochemical stability, all of which are more demanding than those of Si nanoparticle-based electrodes.

In full cell applications, SiMP electrodes, even made of the state-of-the-art binders, always suffer from the insufficient cycle life because the formation of SEI irreversibly consumes Li ions and electrolytes. To address this issue, a pre-cycling method was developed to stabilize SiMP electrodes before assembling full cells. During pre-cycling, SiMPs gradually pulverize into clusters consisting of nano-sized particles. SEI is generated in electrodes. The full cells made of the pre-cycled SiMP/PAA-0.75Li electrodes show much slower capacity fading than those made of fresh SiMP/PAA-0.75Li electrodes whereas pre-cycling has little improvement on the full cell performance of SiMP/PAA-0Li electrodes.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.342

Funding Information

National Science Foundation Award No. 1355438 (2016-2019)

and

the U.S. Department of Energy Battery Materials Research (BMR) Program under Contract Number DE-EE0007787 (2019-2020)

Available for download on Saturday, August 07, 2021

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