Operando and In Situ Study on the Lithium Redistribution within Silicon-Graphite Composite Electrodes in Li-Ion Full Cells

Blended electrodes are known for buffering effects due to the different kinetic and thermodynamic properties of the combined active materials, causing an inhomogeneous distribution of Li between the different active materials during operation.[1] In this study, different operando, in situ, and ex situ methods, as well as 3D microstructure-resolved simulations were used to investigate the charge redistribution during relaxation periods in silicon-graphite (SiG) composite electrodes at different state-of-charges (25%, 50%, 75%). Therefore, two different cell chemistries, graphite/NMC622 and SiG (~21 wt.-% Si)/NMC622, at two different C-rates (0.1C and 0.5C) were analyzed. Operando X-ray diffraction (XRD) in bilayer pouch cells and cross-sectional in situ optical microscopy (CS-IOM) were used to investigate the differences in the lithiation behavior of the two cell chemistries by tracking the graphite component during operation. Relaxed cells at 0%, 25%, 50%, 75%, and 100% SOC were analyzed with in situ XRD in bilayer pouch cells. By comparing operando and in situ XRD, significant differences in the lithiation state of graphite in the SiG cells were found. The relaxation process at 75% SOC after a 0.1C charge was directly recorded in CS-IOM measurements and the observed lithiation states were confirmed by ex situ optical microscopy. All these measurements indicate a redistribution of charge from the graphite component into silicon at 0.1C. For validation, simulations using digital twins of the two cell chemistries was performed and allowed analyzing the Li concentration in both the graphite and Si components of the blended electrode. This enabled validation of the indirect assumptions on the Li concentration changes in Si from XRD and microscopy as well as linking the results of the surface sensitive microscopy measurements and averaged bulk values from XRD. The gained insights not only highlight what is accomplishable by applying several different experimental and simulative methods but also can support a better understanding of aging of blended SiG anodes during operation.
[1] C. Heubner, T. Liebmann, O. Lohrberg, S. Cangaz, S. Maletti and A. Michaelis, Batteries & Supercaps, 5(1) (2022).