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.