In the context of increasing energy density of lithium-ion batteries, silicon is of high interest with his high theoretical gravimetric capacity, ten times higher than the commonly used carbon. However the use of silicon is faced to huge hurdles such as poor life time of those electrodes and not sufficient sustainability versus high current density (commonly used in EVs and HEVs).

The poor cycle life of Si-based electrodes is mainly due to their large volume variation upon cycling, inducing electrical disconnections and instability of the solid electrolyte interface (SEI). The study of the morphological variation of Si-based electrodes upon cycling is thus highly relevant to evaluate their degradation and to optimize their formulation and architecture. However, this is challenging considering their complex three-dimensional structure and their major evolution with cycling. Furthermore, samples are fragile and reactive and therefore difficult to prepare for bulk observations. In this context, X-ray tomography appears as an effective non-destructive and 3D observation tool.

In this communication, in-situ synchrotron  X-ray tomography analyses are performed on Si-based electrodes prepared from a pH3 buffered slurry of ball-milled Si powder + carbon black + carboxymethylcellulose (CMC) (80/12/8) loaded into a carbon paper (AvCarb EP40) by impregnation, in order to get a clear view of the 3D architecture of the electrode with cycling. From the initial state, represented in Fig. 1, to fully lithiated and then fully delithiated state, X-ray scans were performed each thirty minutes to continuously follow the morphological evolution of the electrode structure.

After an appropriate image reconstruction and segmentation procedure, phase identification has been achieved. Moreover the separation of a void porosity and an electrolyte phase was possible and the quantifying of the pore size distribution evolution with cycling, as shown in Fig. 2.

Also key morphological parameters of these Si-based electrodes and their evolution with cycling are determined, such as the electrode thickness and volume fraction of the pores as shown in Fig. 3. Those results may greatly enlighten the understanding of degradation phenomena in the Si-based electrodes and help develop new composite electrodes formulation for sustainable applications.

Figures:

Figure 1: X-ray attenuation imaging of the Si-based anode composite electrode (diameter = 3mm ; 1,4 mgSi/cm²) at the initial state (before lithiation) cut along thickness, recorded at the TOMCAT beamline, SLS facility (PSI)

Figure 2: 3D pore size visualization and evolution with cycling for the electrolyte phase and for the void phase (volume: 293x293x137 µm3, pixel size: 0,326µm)

Figure 3: Porosity, void and electrolyte, and volume evolution of the material phase with cycling (1,4mgSi/cm²) at i = 400 mA/gSi in LiPF6 1M EC : DMC (1 : 1)

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EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-study-of-the-degradation-phenomena-induced-by-lithiationdelithiation-cycle-of-a-composite-si-based-anode-by-the-mean-of-x-ray-tomography/