To ensure safe and long-term operation of Li-ion batteries (LIBs), one important key strategy is to understand the underlying aging mechanisms. In order to fulfill the demand of increased energy density, the introduction of high capacity active materi-als like Silicon (Si) is attractive, but this approach is accompanied by severe Si relat-ed aging mechanisms, which have to be investigated in further detail. This work presents the development and implementation of the glow discharge optical

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... sion spectroscopy (GD-OES) depth profiling to analyze aging phenomena in Si/C composites used for Li-ion battery negative electrodes. The GD-OES method revealed a new Si related film formation with parasitic Lithium ion (Li+) consumption. Moreover, GD-OES depth profiling meets high standards with regard to measurement time, reproducibility and depth resolution. These high stand-ards make the GD-OES method highly suitable for Post-Mortem analysis of interfac-es and interphases of battery electrodes. Detailed analysis of the atomic Si emission lines demonstrated that Hydrogen (H/H2) influences the specific emission intensity and the Si depth profiles, which needs to be considered within the correct analysis routine. In contact with ambient molecular oxygen, Si metal forms an oxide layer resulting in the formation of Si-O bonds on the Si particle surface. GD-OES depth profiling and Raman spectroscopy showed the important participation of these surface Si-O bonds to the electrochemical film formation during the initial cycle. The periodic vol-ume change of Si during cell operation leads to further electrolyte decomposition, supplying oxygen (O) for further Si corrosion. GD-OES is capable of tracking the progress of film formation, since the amount and the depth evolution of the film de-pend on external physical parameters, e.g. temperature and state-of-health. Moreo-ver, the elemental ratios of Li, Si and O, introduced by the formed Lithium silicates (LixSiyOz), can be analyzed depth resolved with GD-OES. Similar to the film f [...]