A New Method to Study Depth Profiles in Silicon/Carbon Composite Electrodes

Abstract

Si as active material has become a prominent candidate as anode material for high-energy Li-ion cells. Unfortunately, Si in Si/C anodes is hard to quantify by established methods, e.g. with ICP-OES no depth profile can be obtained and HF is needed for digestion. Here, we present a semi-quantitative depth profiling method based on glow discharge optical emission spectroscopy (GD-OES). The GD-OES calibration covers Si contents in the range of 0-100 wt.-% in the anode. The results are validated with lab and pilot line coated anodes with known Si contents. The quantified depth profiles with different pristine anodes show a homogeneous distribution of Si and therefore the presented GD-OES method is capable of assessing the quality of composite active materials, from graphite dominant up to pure Si1. The GD-OES method is further applied to anodes obtained from Post-Mortem analysis of commercially available 18650 and 21700 cells. The measurements reveal that the anodes of state-of-the-art Li-ion cells contain up to 5% wt.-% Si. GD-OES depth profiles of anodes after formation (anodes coated in our labs as well as anodes from commercial cells) show a Si peak near the anode surface (near the anode | separator interface, see Figure 1a. This peak changes with aging and is decreased when the anodes are washed with DMC (see Figure 1b). The presence of this peak is further investigated by EDX of ion milling cross-sections. ICP-OES of the DMC washing solution indicates soluble Si species. This aging mechanism is further investigated by Raman spectroscopy. Acknowledgement: The research leading to these results has been performed within the project LIB.DE and received funding from the federal ministry for economics and energy Germany (BMWi) under contract n° 03ET6081A. Literature: K. Richter, T. Waldmann, M. Memm, M. Kasper, and M. Wohlfahrt-Mehrens, J. Electrochem. Soc., 165, A3602–A3604 (2018). Figure 1