Designing Realistic Operando Raman Battery Experiments: Examining Measurement Validity and Spatial Inhomogeneities

Operando Raman spectroscopy measurements are becoming increasingly common in battery materials research. Yet, from a measurement science perspective, there is little consistency in the literature in terms of experimental design, and methodological validity is often not thoroughly considered. In this paper, we experimentally critique the most commonly used operando Raman approach to examining commercial battery electrodes─the “through-hole” method, in which optical access to the electrode under investigation is gained via a hole in the opposing electrode and separator. We benchmark the electrochemical performance of the operando cell against a conventional 2032 coin cell, identifying critical factors that can negatively impact electrochemical performance. Using the well-established lithiation of graphite as a test system, we demonstrate systematic spatial variations in the localized operando Raman response upon moving from the center to the edge of the optical access hole. These variations highlight the importance of sampling location and statistical analysis in connecting local spectral metrics with the global electrochemical characteristics of the cell. Secondary ion mass spectrometry imaging also indicates spatial variations in surface chemistry resulting from the reduced stack pressure at the position of the optical window, which is an important consideration when interpreting the observed operando Raman response.