Nanoscale Chemical Evolution of Silicon Negative Electrodes Characterized By Low-Loss STEM-Eels
Authors: Boniface Maxime , Quazuguel Lucille , Danet Julien , Guyomard Dominique , Moreau Philippe , Bayle-Guillemaud Pascale 
Published: February 2020

Silicon represents one of the most promising anode materials for next generation lithium-ion batteries. However its colossal volume expansion (up to 300%) upon electrochemical reaction with lithium repeatedly exposes fresh surfaces to electrolyte solvent oxidation. This leads to very high irreversible capacities1. Deeper insight into these degradation phenomena is critical to engineer adequate electrodes and/or electrolytes.
Little is known about the SEI’s morphology at the particle scale. Attempts to characterize this system through transmission electron microscopy (TEM) have been severely limited by the radiolysis and sputtering damage respectively undergone by the SEI and lithium-silicon alloys (LixSi).
In this work we demonstrate the possibility to map major SEI phases2 as well as quantifying LixSi compositions3 and Si crystallinity from a single dataset by combining scanning transmission electron microscopy and low-loss electron energy loss spectroscopy2 (STEM-EELS)4. The protocol we developed allows for large spectrum image acquisitions within short timeframes (~10 ms/voxel), making this method a robust and practical diagnostics tool for battery electrodes and other beam-sensitive nanostructured systems.
Results on silicon nanoparticle (SiNP)-based electrodes shed light on the SEI’s deposition mechanism and morphological as well as chemical evolution along cycling (figure 1). Revealing the morphology of lithium fluoride (LiF) - large chunk-like deposits, lithium carbonates (such as Li2CO3) - thin conformal layers and carbon black allows us to get unprecedented insight into the SEI's formation mechanism at the particle scale, while mapping lithium content in Li-Si alloys shed light on the lithiation mechanisms at the particle and aggregate scale. Strong correlations between the SEI's local chemistry and our nanoparticles cycling performances were observed. Alloy formation appeared to make particles fuse together into what became a-Si/c-Si composite networks after delithiation (figure 2d), as revealed through careful examination of the EELS low-loss data. Furthermore, lithiation is shown to proceed in a very heterogeneous manner on the aggregate and particle scale, with particles at the aggregate/porosity boundary exhibiting higher lithium content (figure 2e), pristine and lithiation particles side to side (figure 2a), and composition gradients across single shells. Lithiation was also observed to proceed preferentially along grain boundaries (figure 2c), resulting in different behaviours between mono- and polycrystalline silicon powders, with polycristalline silicon showing higher lithium contents on average in charged electrodes.
These tools were used to study the influence of the electrolyte solvent on the SEI's morphology and chemistry along cycling. Observations from electrodes cycled in pure carbonate electrolyte (1M LiPF6 in EC:DMC 1:1 + 10%FEC) at the 1st, 10th and 100th cycle were obtained and show clear trends in SEI accumulation and chemical evolution. These insights into both the SEI and the behavior of SiNPs are an exceptional asset in understanding the rapid capacity decay of silicon-based electrode.

Reference
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[1] Delpuech, N.; Dupre, N.; Moreau, P.; Bridel, J.-S.; Lestriez, B.; Guyomard, D. ChemSusChem 2016.
[2] Yakovlev, S.; Libera, M. Micron 2008, 39 (6), 734–740.
[3] Danet, J.; Brousse, T.; Rasim, K.; Guyomard, D.; Moreau, P. Phys. Chem. 2010, 12 (1), 220–226.
[4] Boniface, M., Quazuguel, Guyomard, D., Moreau, P., & Bayle-Guillemaud, P. (2016). Nano Letters.

Figure 1