Coupling Tof-SIMS and XPS for a Better Understanding of Lithiation Mechanism of Silicon Electrodes for Li-Ion Batteries

Li-ion batteries technology is an attractive energy storage system for many specific applications like portable devices. To follow the growing energy demand of new devices, their energy density needs to be improved. Silicon-based anode is a serious option since it offers a specific capacity almost ten times higher than carbonaceous materials. However, cycling performances of Si electrodes, particularly in term of coulombic efficiency, remain so far inadequate for a use in practical LIBs, due to huge volume changes upon alloying and de-alloying with lithium. A fine understanding of the lithiation mechanism of silicon electrodes will help to design more robust architectures. Indeed, the degradation mechanisms upon electrochemical cycling are still under debate.

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 Li_xSi alloy indicates a higher lithium amount compared to literature[1]. 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.

1.            Radvanyi, E., De Vito, E. and al., Study of lithiation mechanisms in silicon electrodes by Auger Electron Spectroscopy. Journal of Materials Chemistry A, 2013. 1(16): p. 4956-4965.