Dissertations, Theses, and Capstone Projects

Date of Degree

2-2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Chemistry

Advisor

Elizabeth J. Biddinger

Advisor

Robert J. Messinger

Committee Members

Sharon Lall-Ramnarine

Mark Kobrak

George John

John-Paul Jones

Subject Categories

Analytical Chemistry | Other Chemistry | Physical Chemistry

Keywords

Ionic liquids, Battery, Lithium, Electrolyte

Abstract

Lithium-ion battery utilization is widespread due to relatively high capacity and long cycle life. Advanced technologies have specific demands of their energy storage devices that traditional Li-ion batteries are unable to meet. Lithium-ion batteries utilize a graphite anode which has a limited theoretical capacity. Traditional lithium-ion batteries also utilize flammable solvents in their electrolyte mixtures. Due to the flammable electrolyte, lithium-ion batteries are not considered practical for large scale applications. Additionally, the lithium-ion battery has a narrow optimal operating temperature window. The temperature limitation of lithium-ion batteries leads to issues for many applications such as operating in extreme temperature environments, such as space exploration. These limitations have led researchers to explore other battery chemistries such as lithium metal batteries. Improving and adapting Li-ion batteries for a wider range of applications has also attracted attention.

This dissertation investigates alternative electrolytes for lithium-ion batteries as well as studying electrolytes for lithium metal batteries. To address the issue of flammability, this work iii focuses on ionic liquids as an alternative electrolyte component. Ionic liquids are molten salts at and below 100 °C, are composed completely of ions and have negligible flammability. The functionality of ionic liquids can be modified due to a vast number of cation and anion combinations, but the high viscosity and poor transport properties of ionic liquids limit their applicability as a battery electrolyte. This thesis investigates several ionic liquid-based electrolytes. The transport parameters of the ionic liquid 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([Pyr14][TFSI]) was addressed through the addition of an organic co-solvent methyl propionate. The addition of methyl propionate improved the conductivity and maintained a sufficiently large electrochemical window for battery operation. Methyl propionate also facilitated an undesirable side reaction with lithium metal and the addition of a SEI forming cyclic carbonate co-solvent was used to suppress the side reaction and enable efficient lithium plating and striping behavior.

The functionality of ionic liquids was investigated in mixtures of functionalized pyrrolidinium ionic liquids and solvate ionic liquids. Solvate ionic liquids are a subclass of ionic liquids composed of lithium salts and short polyethers called glyme. By mixing LiTFSI and tetraethylene glycol dimethyl ether (tetraglyme, G4) in equimolar amounts the solvate ionic liquid LiG4TFSI was made. Solvate ionic liquids require multidentate solvation of the glyme to Li+ to form a cation complex and achieve ionic liquid like properties with high lithium salt concentrations. Due to this requirement the functionalization of glyme and the anions are more limited than traditional ionic liquids. In this thesis, mixtures of LiG4TFSI and pyrrolidinium-based bis(trifluoromethylsulfonyl)imide ionic liquids with polyether side chains containing either one ether (EO1) functional group or three ether (EO3) functional groups were studied. We sought to understand the effect of the ionic liquid functionality on the solvate ionic liquid cation complex. To further investigate the effect of the functionality of the ionic liquids the TFSI anion was replaced. The effect of (perfluoroalkylsulfonyl)imide with varying fluorination and symmetry was investigated in LiG4[X] and EO1:Li[X]:G4 1:1:0.8 mixtures. The anions studied were bis(fluorosulfonyl)imide (FSI), bis(pentafluoroethylsulfonyl)imide (BETI), (fluorosulfonyl)(trifluoromethylsulfonyl)imide (IM01), and (trifluoromethylsulfonyl)(nonafluorobutylsulfonyl)imide (IM14). The effect of the anions on the physiochemical and electrochemical properties was investigated and analyzed. How the various combinations of cations and anions affected the [LiG4]+ cation complex was probed using spectroscopic techniques such as NMR and Raman spectroscopy.

Though lithium metal batteries and ionic liquids have many intriguing and desirable benefits, significant hurdles exist before their practical implementation for many applications is realized. Traditional carbonate-based Li-ion electrolytes remain the most reliable, high-capacity rechargeable battery chemistry for many modern uses, despite the previously mentioned concerns. This work also highlights adapting the Li-ion battery electrolyte for 100 °C operation for harsh environments found in space. The high temperature performance of the Li-ion was improved using salt additives and a highly fluorinated co-solvent, 1,1,2,2-tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether (TTE). The formulation studied in this thesis was identified by team member Jonah Wang through careful screening of different electrolyte formulations. Through variable temperature cycling paired with electrochemical impedance spectroscopy, it was determined that the additives and co-solvent helped form a stable interphase at the cathode which facilitated improvements in 100 °C cycling capacity retention.

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