Coupling Tof-SIMS and XPS for a Better Understanding of Lithiation Mechanism of Silicon Electrodes for Li-Ion Batteries
In this work, an amorphous silicon thin film has been used as a model for a better understanding of lithiation mechanism appearing in more complex systems such as Si composites electrodes. Lithium distribution in the Si layer has been thoroughly investigated by coupling powerful characterization tools: X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectroscopy (ToF-SIMS). In particular, cross-analyses of different lithiation states have been carried out by using an airtight transfer vessel between a glove box and XPS and ToF-SIMS spectrometers.
Results reveal a lithiation front moving forward over the state of charge. The quantification of the LixSi alloy indicates a higher lithium amount compared to literature. This anomaly leads to a description of the lithiation mechanism based on the presence of fast diffusion paths for Li throughout the Si layer reaching the Cu collector. These paths would be a second driving force for silicon alloying. To corroborate this mechanism, complementary analyses were performed: SEM observation of a FIB cut, TEM involving cryo-ultramicrotomy under protected atmosphere and Auger spectrometry.