Investigation of an Ionic Liquid As a High-Temperature Electrolyte for Silicon-Lithium Systems

Electrolytes for lithium ion batteries which work over a wide range of temperatures are of interest in both research and applications. Unfortunately, most traditional electrolytes are unstable at high temperatures. As an alternative, solid state electrolytes are sometimes used. These are inherently safer because they have no flammable vapors, and solid state electrolytes can operate at high temperatures, but they typically suffer from very low conductivity at room temperatures. Therefore, they have had limited use. Another option which has been previously explored is the use of ionic liquids. Ionic liquids are liquid salts, with nominally zero vapor pressure. Many are liquid over the temperature of interest (20–200°C). And, there is a tremendous range of available chemistries that can be incorporated into ionic liquids. So, ionic liquids with chemistries that are compatible with lithium ion systems have been developed and demonstrated experimentally at room temperature. In this study, we examined a silicon-lithium battery cycling at room temperature and over 150°C. Using half-cell vial and split-cell structures, we examined a standard electrolyte (LiPF6) at room temperature, and an ionic liquid electrolyte (1-ethyl-3-methylimidazolium bis(trifluorosulfonyl)imide) at room temperature and up to ∼150°C. The ionic liquid used was a nominally high purity product purchased from Sigma Aldrich. It was selected based on results reported in the open literature. The anode used was a wafer of silicon, and the cathode used was an alumina-coated lithium chip. The cells were cycled either 1 or 5 times (charge/discharge) in an argon environment at constant current of 50 μA between 1.5 and 0.05 volts. The results for the study showed that at room temperature, we could successfully cycle with both the standard electrolyte and the lithium ion electrolyte. As expected, there was large-scale fracture of the silicon wafer with the extent of cracking having some correlation with first cycle time. We were unable to identify any electrolyte-specific change in the electrochemical behavior between the standard electrolyte and the ionic liquid at room temperature. Although the ionic liquid was successfully used at room temperature, when the temperature was increased, it behaved very differently and no cells were able to successfully cycle. Video observations during cycling (∼1 day) showed that flocs or debris were forming in the ionic liquid and collecting on the electrode surface. The ionic liquid also discolored during the test. Various mechanisms were considered for this behavior, and preliminary tests will be presented. All materials were stable at room temperature, and the degradation appeared to be linked to the electrochemical process. As a conclusion, our working hypothesis is that, particularly at elevated temperatures, ionic liquid cleanliness and purity can be far more important than at room temperature, and small impurities can cause significant hurdles. This creates an important barrier to research efforts, because the “same” ionic liquids could cause failure in one situation and not in another due to impurities.