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Etude par émission acoustique et dilatométrie d'électrodes à base de silicium pour batteries Li-ion

Abstract : To increase the energy density of Li-ion batteries, especially for the electric vehicle market, the development of new electrode materials is required. Silicon is a particularly interesting material, thanks to its high specific capacity (3579mAh/g, ten times higher than the capacity of graphite). Nevertheless, upon lithiation, silicon undergoes an important expansion (300% vs 10% for graphite). This leads to the cracking of the Si particles and fracturing of the electrode film. These induces electrical disconnections upon cycling, resulting in a poor cycle life. To improve the cyclability of the Si based electrodes, it is important to better understand/quantify their mechanical degradation. Conventional post mortem analyses are insufficient for that purpose. The objective of this work is to develop and use in operando analyses techniques. Therefore, we established protocols to characterize composite electrodes by electrochemical measurements coupled with either acoustic emission (AE) or dilatometry measurements. The evolution of the acoustic activity upon cycling showed that the cracking of the micrometric Si particles and of the composite film mainly occurs during the first cycle and is initiated in the early stage of the lithiation. Very few AE signals are detected in the following cycles. The signal analysis leads to the identification of three types of signals depending to their peak frequency. High frequency signals were associated with surface micro-cracking of the Si particles at the beginning of lithiation. Medium and low frequency signals were respectively attributed to the fracturing of the electrode film and bulk macro-cracking of the Si particles at the end of lithiation. An electrode thickness expansion of 170% was measured by electrochemical dilatometry for our electrodes prepared at pH3 versus 300% for electrodes prepared at pH7. The different mechanical behavior is explained by the formation of covalent bonds between the CMC binder and Si particles at pH3, which increases the mechanical stability of electrodes. This was confirmed by the measurement of their hardness and Young’s modulus. Therefore, pH3 electrodes display a higher capacity retention. It was also demonstrated that a decrease of the Si particle size does not necessarily lead to an improvement of the electrode cycle life. Indeed, we observed a significant decrease of the electrode cycle life when the Si particle size is decreased from 230 to 85 nm. This can be explained by a lack of CMC binder in relation with the higher surface area of the smaller Si particles, leading to a lower mechanical resistance of the electrode film. Within the first cycles, Si 85 nm based electrodes suffer from important cracking and exfoliation. This was confirmed by in operando dilatometry and acoustic measurements, and post mortem SEM observations.
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Alix Tranchot. Etude par émission acoustique et dilatométrie d'électrodes à base de silicium pour batteries Li-ion. Matériaux. Université de Lyon; Institut national de la recherche scientifique (Québec, province), 2016. Français. ⟨NNT : 2016LYSEI101⟩. ⟨tel-01755208⟩

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