Investigating interfacial structures of highly concentrated electrolytes

Hoane, Alexis Grace

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https://hdl.handle.net/2142/129262 Copy

Description

Title

Investigating interfacial structures of highly concentrated electrolytes

Author(s)

Hoane, Alexis Grace

Issue Date

2025-05-02

Director of Research (if dissertation) or Advisor (if thesis)

Gewirth, Andrew A

Doctoral Committee Chair(s)

Gewirth, Andrew A

Committee Member(s)

• Murphy, Catherine J
• Rodriguez-Lopez, Joaquin
• Espinosa-Marzal, Rosa M

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Electrolyte
• battery
• multivalent
• SEIRAS

Abstract

As global demand for both electric vehicles and electric power generated from renewable sources continues to grow, advanced energy storage solutions are increasingly necessary. While state-of-the-art Li-ion batteries feature high energy density, improved Li-ion battery and beyond Li-ion battery technology is necessary to accommodate the global demand for energy storage. This dissertation features work relevant to the development of high-performance Li-ion and alternative battery systems via the investigation of highly concentrated electrolyte interfaces. Highly concentrated electrolytes (HCEs) exhibit special properties due to their bulk solvation and interfacial chemistry. However, the composition and potential dependence of highly concentrated electrolytes near the interface is not well understood, particularly for beyond Li-ion battery system. This research investigates the impact on interfacial structures in HCEs by the modulation of electrolyte ionic and solvent association properties. Impact of Multivalent Cations on Interfacial Layering in Water-in-Salt Electrolytes. Water-in-salt electrolytes (WiSEs) are of interest for use as aqueous multivalent electrolytes due to their potential to address reversibility and passivation concerns common in multivalent batteries. The impact of the addition of multivalent cation salts, including Zn(TFSI)2, Mg(TFSI)2, Ca(TFSI)2, and Al(TFSI)3 on the double layer behavior in LiTFSI WiSE was investigated. Surface-enhanced infrared absorption spectroscopy (SEIRAS) was utilized to observe the potential-dependent double-layer composition. TFSI– is enriched at relatively positive potentials for LiTFSI WiSE, and water is enriched at negative potentials for mixed electrolytes containing Mg2+ and Ca2+, but this shift does not hold for mixed electrolytes containing Zn2+ or Al3+. Ultramicroelectrode (UME) voltammetry shows confinement of a probe molecule Fe(CN)64– at the interphase in the presence of Mg2+ and Ca2+, an effect that is eliminated by the addition of 1.75 and 1.25 mM of Zn2+ or Al3+, respectively, to LiTFSI WiSE. Atomic force microscope (AFM) measurements showed the presence of smaller interlayer distances at positive potentials relative to those seen without the presence of Zn2+. These effects are correlated to cation pKa, highlighting the importance of the water structure at the interphase of WiSE for multivalent electrolytes. Effect of Salt on Interfacial Structures of Highly Concentrated Acetonitrile Electrolytes. HCEs which contain acetonitrile (AN) are increasingly investigated for Li-ion and high voltage Li-metal anode batteries due to the ability of HCEs to increase reductive stability of the AN solvent. However, chemical speciation at the interface, and the impact of salt choice on speciation of these electrolytes is largely unknown. The impact of anion on potential-dependent interfacial speciation within the electric double layer (EDL) of AN HCEs was first investigated through comparison of AN-LiTFSI and AN-LiFSI HCEs. Then, the impact of decreasing ionic association by substituting the LiFSI salt with NaFSI and KFSI was investigated. SEIRAS was utilized to probe the potential dependence of surface concentration, ionic association and conformation of the anions and AN molecule. FSI- in the AN-LiFSI HCE exhibited a high degree of ionic association at the interface, which was not observed in the AN-LiTFSI, AN-NaFSI, or AN-KFSI HCEs, and was related to a change in anion conformation at the interface. Electrochemical impedance spectroscopy was used to calculate the differential capacitance in the electrolytes, which was higher and more potential dependent for the AN-LiFSI HCE than other HCEs investigated, attributed to increased ion pairing and reduced conformational shifts of the anion. This work shows that in these AN HCE systems, both anion and cation can have a large impact on the interrelated factors of interfacial ionic association and anion conformation.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129262

Copyright and License Information

Copyright 2025 Alexis Hoane

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