Inhomogeneous Degradation of Li-Ion Cells: A Post Mortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES)
Authors: Ghanbari Niloofar , Waldmann Thomas , Kasper Michael , Axmann Peter , Wohlfahrt-Mehrens Margret 
Published: February 2020

Commercialization of many Li-ion cell chemistries is hindered by undesirable processes occurring during usage of Li-ion batteries, limiting their lifetime. One such process is the development of temperature gradients, especially in large-format cells, leading to uneven utilization of electrode material.1 Such inhomogeneity could locally accelerate or change the aging phenomena.2 Correlation of the electrochemical data with the post mortem analysis of cell components enables root cause analysis of aging mechanisms under the given conditions.3The present study looks into the applicability of GD-OES as a powerful method in post mortem analysis of Li-ion cell electrodes.
GD-OES provides direct depth-resolved elemental analysis of the complete electrode coating (~100µm) within hours.4Sensitivity to light elements (e.g. Li and H), fast analysis, and quantification of the recorded elemental intensity, are among the features of GD-OES that compare with similar analytical methods (SIMS, XPS, and AES). A method is developed using reactive sputtering for investigating aged graphite electrodes. Calibration is performed using homemade reference coatings with systematic variation of Li-containing salt in graphite slurries.
GD-OES depth profiling is employed to study SEI formation during the initial charge of a graphite/Li(NixCoyMnz)O2 cell. It is discovered that SEI formation is not limited to the reduction potential of electrolyte components and additives, but continues during Li intercalation at lower cell potentials.5,6 Furthermore, the method is used for quantitative detection of Li plating (Figure 1).7Applying the method to different planar positions of aged graphite electrodes harvested from large-format commercial cells, the impact of temperature gradients on Li distribution across the electrode is studied.

Acknowledgement

This research has been performed within the MAT4BAT project (http://mat4bat.eu/) and received funding from the European Community's Seventh Framework Program (FP7/2007-2013) under grant agreement n°608931.

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Figure 1