Étude physico-chimique du silicium amorphe méthylé pour l'électrode négative de batteries Li-ion

Abstract : Previous studies showed that the incorporation of methyl groups in silicon thin films obtained by Plasma Enhanced Chemical Vapor Deposition (PECVD) improves their stability as negative electrodes in Li-ion batteries. In this thesis, several approaches have been used to understand phenomena occurring in the so-called "methylated amorphous silicon" during realistic cycling condition.We have shown that infrared spectroscopy operando in attenuated total reflection geometry (ATR-FTIR) provides quantitative information on the phenomena involved during the lithiation and the delithiation of the active material. The gradual increase of the absorbance during the first lithiation makes it possible to follow the formation of a lithium-rich phase LizSi which gradually invades the material. We have shown that the concentration z of lithium in this phase depends on the content of methyl groups. This behavior has been explained by two distinct effects: (1) the weakening of the material due to the methyl-induced lowering of its reticulation degree and cohesion. (2) the increase of the material porosity at high enough methyl content. Transmission Electron Microscopy (TEM) measurements revealed that a porosity is present at the nanometer scale in the active material and increases with methyl content. Time-Of-Flight Secondary Ion Mass Spectroscopy measurements performed at Chimie ParisTech have shown that the lithium invasion of the pristine active material from the lithium-rich phase is an utterly slow phenomenon (corresponding to a diffusion coefficient D ~10-19 cm2).We observed Solid Electrolyte Interphase (SEI) evolution during first cycle depends on cycling rate and methyl content, using operando ATR-FTIR spectroscopy and electrochemical methods.The evolution of the material after interruption of the electrochemical lithiation at several states of charge has been monitored using operando ATR-FTIR spetroscopy and Electrochemical Impedance Spectroscopy (EIS). Those measurements show that part of the lithium initially inserted in the layer returns to the electrolyte during relaxation and contributes to the growth of the SEI.
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Bon Min Koo. Étude physico-chimique du silicium amorphe méthylé pour l'électrode négative de batteries Li-ion. Matériaux. Université Paris-Saclay, 2017. Français. ⟨NNT : 2017SACLX092⟩. ⟨tel-01712032⟩

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