Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells: A Postmortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES)

Durability and performance of Li-ion cells are impaired by undesirable side reactions, observed as capacity decreases and resistance increases during their usage. This degradation is caused by aging mechanisms on the material level including surface film formation, especially in the case of graphite-based anodes. The present study evaluates the applicability of glow discharge optical emission spectroscopy (GD-OES) as a powerful tool to study aging-induced film formation on graphite anodes of Li-ion cells, including deposition of metallic Li. The technique provides depth-resolved information on the elemental distribution in the samples from the anode surface to the current collector (through-plane resolution). Additionally, conducting GD-OES depth profiling at different positions of an aged graphite anode reveals differences in surface film growth across the anode plane (in-plane resolution). After verification of the GD-OES method by well-established analytical techniques, aged anodes from commercial state-of-the-art Li-ion cells are analyzed. The results show through-plane and in-plane inhomogeneity in surface film growth: local island-like Li deposition is revealed for 16Ah pouch cells cycled at 45 °C and high charging current density while a more homogeneous Li plating gradient is found for cycling 26650-type cells at −20 °C.