Publication Date

2023

Document Type

Dissertation/Thesis

First Advisor

Li, Tao

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Chemistry and Biochemistry

Abstract

The fundamental understanding of the liquid electrolytes (LEs) solvation structure and electrode-electrolyte interface behavior is important to the entire electrochemical energy storage field. Understanding the solvation structures from a few angstroms to hundreds of nanometers will undoubtedly lay a good foundation for studying the macroscopic transport properties, such as viscosity and ionic conductivity, of the LEs."Water-in-salt" electrolytes that contain 21 m lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) with excellent electrochemical and physical properties have been extensively investigated. However, the structural understanding of the LiTFSI in water is still lacking. Here, we perform synchrotron X-ray scattering to systemically study the structural variation of TFSI anions in an aqueous solution under a variety of concentrations and temperatures. There are two different solvation structures in the solution: TFSI- solvated structure and TFSI- network. As the concentration increases, the TFSI- solvated structure gradually disappears while TFSI- network gradually forms. Even at relatively low concentrations, TFSI- network can be observed. Our experimental results show that these two structures can co-exist at a particular concentration, and temperature changes will lead to one structure's formation or disappearance. Also, the TFSI- network is the key to obtain a stable electrochemical window under relatively high temperatures. Moreover, we also used small-angle X-ray scattering (SAXS) and molecular dynamics (MD) simulation to investigate the solvation structure of a series of lithium salts with four different symmetric anions: (bis(fluoro sulfonyl)imide (FSI), bis(trifluoromethane sulfonyl)imide (TFSI), bis(pentafluoroethane sulfonyl) imide (BETI) and bis(nonafluorobutane sulfonyl)imide (BNTI)), which have similar parent structures but different lengths of the fluorocarbon chains. Two competing structures were found in the four lithium salts aqueous solutions: anion solvated structure and anion network. The anion network plays a crucial role in obtaining a stable and enlarged electrochemical window. The d spacing of the anion solvated structure follows a linear correlation with the number of carbons in the fluorocarbon chains and an exponential correlation with the concentrations. We also observed that the transition from one structure to another is predominantly controlled by the salt volume fraction for all imide-based aqueous solutions. This work provides a systematic study of the atomistic scale structures of a series of imide-based lithium salt aqueous solutions that could extend the knowledge of WIS electrolytes. However, the correlations between the solvation structures and transport properties of those materials remain uncertain. Herein, we measured SAXS, small-angle neutron scattering (SANS) and pair distribution function (PDF) of LiTFSI aqueous solutions at a wide range of concentrations. Combining with molecular dynamics simulations, the detailed solvation structures from long to short length scale were resolved. We found that the TFSI- solvation structures consist of TFSI- solvated structures and TFSI- networks, the former corresponds to solvent separated ion pairs while the latter corresponds to contact ion pairs and cation-anion aggregates. In addition, we found that the relaxation time in the q range associated with the anion network structure exhibits the same concentration dependence as the viscosity. By combining the results from the experiments and simulations, this study revealed a correlation between the solvation structures of LiTFSI and the transport properties of the solutions, which could help understand the relation between the transport properties and the dynamics of the ions for imide-based lithium-ion salt aqueous electrolytes. Even though “Water-in-salt” (WIS) electrolytes exhibit excellent safety and electrochemical performance, they possess high concentrations, relatively low diffusion coefficient, and high viscosity. “Acetonitrile/water in salt” (AWIS) electrolyte can overcome the disadvantages of WIS electrolytes. Under relatively low concentrations, AWIS electrolytes show good electrochemical performance comparable to the WIS electrolytes and low conductivity. Herein, we investigate the relationship between the solvation structures and the transport properties using SAXS and MD simulation. We observed two solvation behaviors of AWIS: anions dissolved in acetonitrile forming small acetonitrile/anion clusters, and additional water further dissolving the small acetonitrile/anion clusters. The introduction of acetonitrile weakens the water-solute interaction and enhanced the cation-anion interaction, which results in an enhanced dynamical slowdown as the concentration increases. This work provides molecular-level understanding of the connection between two-stage solvation structures and transport properties for imide-based lithium salt solutions. Finally, Combining ultra-small-angle X-ray scattering (USAXS), SAXS, liquid cell transmission electron microscopy (TEM) and flow cell SAXS technique, this work provides the evidence that the nanoheterogeneity of high concentration LiTFSI aqueous electrolytes is generated from beam damage and electron damage. We should be very careful when we try to use X-ray or electrons to study the LiTFSI aqueous solutions, under some circumstances the flow cell should be considered to use.

Extent

141 pages

Language

en

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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