Date of Graduation

12-2019

Document Type

Thesis

Degree Name

Master of Science in Chemistry (MS)

Degree Level

Graduate

Department

Chemistry & Biochemistry

Advisor/Mentor

Jie Xiao

Committee Member

Ryan Tian

Second Committee Member

Robert Coridan

Third Committee Member

Jingyi Chen

Keywords

Diffusion, Stoke-Einstein equation, lithium metal batteries

Abstract

Lithium-ion batteries are reaching the specific theoretical capacity limit, while lithium metal batteries are regarded as the ideal energy storage system for the next generation “beyond lithium-ion” battery systems. The lithium metal anode is considered as the “Holy grail” of anodes due to its relatively low electrochemical potential (-3.04 V vs SHE) and high theoretical capacity (3860 mAh g-1). However, the application of lithium metal anodes is hindered because of significant reaction between metallic lithium and electrolytes, as well as uneven electro-plating, which leads to dendrite formation, causing safety problems.

The kinetic parameters of Li ions such as diffusion coefficient are strongly related to dendrite growth. In this thesis, micro-electrodes are employed to determine the diffusion coefficients in various electrolytes. Compared with SEM images of lithium metal microstructures obtained by electro-deposition, the lower diffusion coefficient can promote lithium dendrite growth at a similar exchange current density, which is indicated by Sand’s equation. The low electro-chemical potential of lithium metal leads to reactions between the metallic lithium and the conventional electrolytes. However, the benzene possesses a lower electro-chemical potential (-3.42 V vs SHE). Therefore, benzene is applied as a co-solvent to adjust ionic solvation structures of the electrolytes for lithium anode protection.

In chapter 1, the background of lithium metal anodes is introduced. In chapter 2, the micro-electrodes are employed to determine the kinetic parameters of the electrolytes. Then the relations among these parameters and electro-plating morphologies are explored. In chapter 3, benzene is applied as a co-solvent in the electrolyte. The improved performance of the lithium metal anode protection is shown in this chapter. Chapter 4 further investigates the benzene-based electrolytes as applied in lithium metal full-cell batteries, such as lithium-sulfur batteries and NCM-lithium metal batteries, and the performance is noted.

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