Ex-situ Fourier Transform Infrared Spectroscopy Method for Quantitative Measurement of Salt and Solvent Distributions in Lithium-Ion Batteries

Automotive lithium-ion battery performance can be degraded by the formation of non-uniform gradients in the electrolyte salt and solvent concentrations. It is important from a performance and life-cycle standpoint that the formation and characteristic time scales of these gradients be understood as a function of electrode design, cycling history, and environmental conditions. In this work, a Fourier Transform Infrared (FTIR) spectrometer is used in open air to produce a rapid and easy to implement method for the quantitative analysis of salt and solvent gradients frozen in the battery separator post cell teardown. The method ignores electrolyte gradients between positive and negative electrodes and instead measures longer-scale gradients that exist within the separator of a given electrode layer. The separator is extracted from the cell and a 1-D transmission mode scan of the separator is conducted with a FTIR. In the first step of the method, cells are torn down inside an argon-filled glovebox, and the separator pieces of interest are allowed to dry for 15 minutes. This amount of time is sufficient for volatile components of the solvent (e.g., dimethyl carbonate, ethyl methyl carbonate, etc.) to fully evaporate. The presented method is not applicable to such highly volatile constituents as they rapidly evaporate and are therefore difficult to “freeze” in place for the time scale necessary to conduct a complete measurement. Concentrations of less volatile constituents of the solvent, such as ethylene carbonate (EC), were shown to be relatively unchanged over a one-hour period, which is sufficient for the FTIR measurement. However, sublimation of ethylene carbonate is appreciable over longer time scales (> hours), which was confirmed in the FTIR and through TGA experiments. Lithium hexafluorophosphate (LiPF 6 ) salt concentration was shown to be stable over short time scales (on the order of an hour), although 20-30% reductions in absorbance were observed over one to two days of air exposure. Water affinity for LiPF6 is immediately present in the FTIR spectrum and must be discriminated against the P-F vibration and F-P-F bending modes of LiPF 6 during calibration. Spectral bands are identified for calibration of LiPF 6 and EC, and it is shown that the LiPF 6 absorbance is independent of the local concentration of EC. However, the opposite is true of EC, where its absorbance is a strong function of the local LiPF 6 concentration. As such, a quantitative calibration is developed where the LiPF 6 concentration is determined first and is then used to determine the local EC concentration. The calibration table is populated by soaking pieces of new separator material in calibration electrolytes of 30-60% EC and 1-2M LiPF 6 to correlate the measured absorbance with the salt or solvent concentration. In addition to specifying the analysis method, this work will characterize electrolyte gradients formed during long term consecutive cycling of lithium-ion pouch cells, and the results will be compared to existing methods such as x-ray fluorescence. Figure 1