Ph.D, Case Western Reserve University
Lithium-ion batteries (LIBs) are widely used in portable electronic devices. However, LIBs are reaching their fundamental limits and new battery chemistries become essential for wider applications. Lithium metal batteries offer an order of magnitude increase in energy density in theory, compared to LIBs. However, the high reactivity and the related safety hazard of the metallic lithium hinder its application in electrochemical energy storage devices. Ionic liquid (IL) electrolytes due to their unique properties including lack of flammability, negligible volatility, wide electrochemical window and high thermal stability are capable of resolving safety and stability issues in lithium metal batteries, thus enabling high-energy storage devices. The ionic conductivity of ILs, however, need to be improved to achieve practical charge/discharge rates in batteries. We investigate the non-idealities in physical and electrochemical properties of eutectic ILs and their lithium salt mixtures. Here, we will present the ionic interactions that can lead to increased conduction pathways for lithium ion in eutectic mixtures. ILs consist of ions and their interactions with small charged ions such as Li+ create heterogeneities in the solvation shell of these solutes. Eutectic IL mixtures including the [PYR13][TFSI] with high electrochemical stability, and imidazolium and sulfonium based ILs with low viscosities but lower stabilities were studied. Improved conductivities were obtained for the eutectic IL mixtures, compared to the parent ILs. When examined on a Walden plot, the eutectic IL electrolyte demonstrate moderate ionicity. Surface Enhanced Raman Spectroscopy (SERS) was used to investigate the Li+ interaction and coordination in the bulk and near electrode surface via confocal capability of SERS. Li+ coordination is observed to increase with increasing lithium salt concentration in the range of 0 – 1 M, as expected. Increasing the concentration further to 2 M, however, shows no further changes in coordination suggesting that there are free Li+. Our in-situ electrochemical SERS indicate that the coordination condition did not completely reverse to initial state after polarization. Measured electrochemical windows, thermal analysis results and conductivities will be presented for the eutectic systems studied. Further performance analysis of these electrolytes are being conducted for lithium metal batteries.
Abstract: We report an in situ measurement of the interaction of an imidazolium‐based room temperature ionic liquid with both pure silver and a graphene‐over‐silver electrode under an applied electrochemical potential. At a negative applied potential, overall signal intensity increased indicating enhanced ionic liquid concentration at both silver and graphene electrodes. Vibrational modes associated with the imidazolium ring exhibited greater intensity enhancements and larger peak shifts compared with the anion indicating that the cation adsorbs with the ring and alkyl chain parallel to the electrode surface for both silver and graphene. In contrast to the silver, the surface enhanced Raman spectra of the ionic liquid near graphene showed shifts in the cation peaks even at no applied potential because of the strong π–π interaction between the ionic liquid and the graphene. Furthermore, the intensity of the graphene peak decreased in the presence of ionic liquid possibly due to the interaction between the ionic liquid and graphene. These results illustrate the effectiveness of surface‐enhanced Raman spectroscopy to investigate electrolyte interactions with graphene at the liquid/electrode interface. Copyright © 2015 John Wiley & Sons, Ltd.
Pub.: 17 Dec '15, Pinned: 08 Jul '17
Abstract: Suppressing dendrite formation at lithium metal anodes during cycling is critical for the implementation of future lithium metal-based battery technology. Here we report that it can be achieved via the facile process of immersing the electrodes in ionic liquid electrolytes for a period of time before battery assembly. This creates a durable and lithium ion-permeable solid–electrolyte interphase that allows safe charge–discharge cycling of commercially applicable Li|electrolyte|LiFePO4 batteries for 1,000 cycles with Coulombic efficiencies >99.5%. The tailored solid–electrolyte interphase is prepared using a variety of electrolytes based on the N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide room temperature ionic liquid containing lithium salts. The formation is both time- and lithium salt-dependant, showing dynamic morphology changes, which when optimized prevent dendrite formation and consumption of electrolyte during cycling. This work illustrates that a simple, effective and industrially applicable lithium metal pretreatment process results in a commercially viable cycle life for a lithium metal battery.
Pub.: 13 Jun '16, Pinned: 08 Jul '17
Abstract: The dynamical and structural properties in two ionic liquid electrolytes (ILEs) based on 1-ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)-imide ([emim][TFSI]) and N-methyl-N-propylpyrrolidinium bis-(trifluoromethanesulfonyl)imide([pyr13][TFSI]) were compared as a function of lithium bis-(trifluoromethanesulfonyl)-imide (LiTFSI) salt concentrations using atomistic molecular dynamics (MD) simulations. The many-body polarizable APPLE&P force field has been utilized. The influence of anion polarization on the structure of the first coordination shell of Li+ was examined. In particular, the reduction of the oxygen of the TFSI anion (OTFSI) polarizability from 1.36 Å3 to 1.00 Å3 resulted in an increased fraction of the TFSI anion bidentate coordination to the Li+. While the overall dynamics in [pyr13][TFSI]-based ILEs was slower than in [emim][TFSI]-based ILEs, the exchange of TFSI anions in and out of the first coordination shell of Li+ was found to be faster in pyr13-based systems. The Li+ ion transference number is higher for these systems as well. These trends can be related to the difference in interaction of TFSI with the IL cation which is stronger for pyr13 than for emim.
Pub.: 16 Nov '15, Pinned: 08 Jul '17
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