Abstract text (incl. figure legends and references)

The improvement of modern Li-ion batteries (LIBs) is an important step in the development of energy storage systems with regard to the growing market of electric vehicles and the realization of the European goal for climate neutrality by 2050 [1].

Silicon (Si) is one of the most promising anodes for next-generation LIBs, with its bestselling point being the theoretical capacity of 3579 mAh/g which is approximately ten-fold than that of the commonly used graphite-based materials used nowadays [2-4]. However, the ~300% volume change during (de)lithiation, which also results in crystalline-amorphous transition, limits the commercialization of Si-based anodes, particularly the Si microparticles. This transition takes place during the first lithiation cycle of the crystalline Si, which is amorphized and stays amorphous after delithiation, causing degradation. One approach to reducing the degradation is to operate the Si anode under its capacity limit, e.g., by using only ~30 % of the capacity the volume expansion is reduced to only one-third of the maximal expansion and leaving part of the crystalline Si phase unchanged during lithiation. This allows the Si anode to cycle over 200 times [5].

In order to further understand the lithiation process and the resulting capacity fading due to degradation within the Si microparticles, one needs to have an in-depth insight into the structural arrangements within the Si microparticles. Transmission electron microscopy (TEM) is the method of choice to study morphology and chemical composition of the crystalline and amorphous phases within partially lithiated polycrystalline Si microparticles. For the TEM investigation, FIB lamellas are prepared from the 5-10 µm pristine and cycled (lithiated/delithiated) particles. Bright-field (BF) imaging, selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX) revealed the presence of the crystalline phase and the complex vein-like amorphous Si phase within the cycled microparticles, the latter not being present in the pristine Si microparticles (Figure 1). This shows the complex expansion of the amorphous phase not only present at the Si microparticle shell but also along stacking faults and grain boundaries.

Figure 1: Bright field (BF) TEM image and corresponding diffraction images of Si particles. The images in the upper row a) correspond to the pristine Si anode and in the lower row b) the cycled Si anode is visible.

[1] European Commission Communication (COM(2019) 640) "The European Green Deal".

[2] U. Kasavajjula, C. Wang, and A. J. Appleby, J. Power Sources, 163, 1003 (2007).

[3]. M. N. Obrovac and V. L. Chevrier, Chem. Rev., 114, 11444 (2014).

[4] J.R. Dahn, T. Zheng, Y. Liu, J.S. Xue, Science 270 (1995) 590e593.

[5] Maximilian Graf et al 2022 J. Electrochem. Soc. 169 020536.