Study of the Expansion/Contraction Behavior of Si-Based Electrodes By Electrochemical Dilatometry
Authors: Tranchot Alix , Thivel Pierre-Xavier , Idrissi Hassane , Roué Lionel 
Published: February 2020

The replacement of graphite by silicon as the active material in negative electrodes of Li-ion batteries is very attractive since the specific capacity of Si is 10 times higher than that of graphite. However, Si suffers from huge volume variation (up to ~300% vs 10% for C) during its lithiation. This leads to the electrode/particle cracking which induces electrical disconnections in addition to cause an instability of the solid electrolyte interface (SEI), resulting in poor cycle life and low coulombic efficiency. A precise evaluation of the expansion/contraction behavior of the Si-based electrodes upon cycling is thus highly relevant to develop more efficient electrode formulations. For this purpose, a simple and efficient method consists of integrating a contact displacement transducer to the electrochemical cell, which permits a measurement of any thickness change of the working electrode upon its charge and discharge as shown hereafter.

In the present study, electrochemical dilatometry experiments are performed on silicon/carbon/carboxymethylcellulose (Si/C/CMC) composite electrodes. It is shown that the pH of the slurry (pH 7 vs. buffered pH 3) and the size of the Si particles (1-5 µm vs. 85 nm) have a major impact on their electrochemical performance (Fig. 1) and their expansion/contraction behavior (Fig. 2a-c). During the first discharge (lithiation), a maximum electrode thickness expansion of ~130% is observed for the pH3-micro Si electrode (Fig. 2b). compared to ~330% for the pH7-micro Si electrode. (Fig. 2a). A lower irreversible expansion is also observed at the end of the 1st cycle (~50% compared to ~180% for the pH7 electrode). It can be explained by the the formation of more cohesive (covalent) bonds between the Si particles and the CMC chains when the electrode is prepared with a slurry buffered at pH 3, increasing the mechanical strength of the composite electrode and its ability to reversibly sustain the volume variations of the silicon particles.
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However,when the 1-5µm Si is replaced by 85 nm Si, a very large and abrupt expansion/contraction of the electrode is observed (up to 500%, see Fig. 2c), resulting in a significant decrease of the electrode cycle life (Fig. 1). It may be related to the higher specific surface area and/or lower surface oxidation state of the nanosized Si powder, inducing a less efficient grafting of the CMC binder at the surface of the Si particles.
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Additionaldilatometric experiments with oxidized Si nanoparticles and with different CMC amounts will be presented to address this issue.

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Figure 1