Paper EN+NS-ThM11
The Contribution of Auger Electron Spectroscopy to a Better Understanding of the Lithiation Process Occurring in Si-based Anodes Designed for Li-ion Batteries

Thursday, November 1, 2012, 11:20 am, Room 15

With a specific capacity of almost 3580 mAh.g^-1 at room temperature (corresponding to the Li₁₅Si₄ alloy), silicon is a promising element for designing new efficient anodes in Li-ion battery technology. However, because of huge material volumic expansion (around 300%) during the lithiation process¹, leading to a quick pulverization of the electrode², silicon has been used only as an additive to graphite in commercial cells so far³. In order to improve Si-based electrodes cyclability, a better understanding of the lithium insertion mechanisms is among the key issues⁴. The study of the lithiation process in silicon particles is particularly challenging. Indeed, a complete amorphization of the material during cycling⁴ makes difficult the use of surface characterization techniques based on beam diffraction (e.g. X-rays or electrons). In this study, we have used Auger Electron Spectroscopy (AES) to study these mechanisms. Poor attention has been focused on AES in the battery research field so far^5,6. In this work we emphasize its interest, notably for the study of the lithiation mechanisms in silicon particles. The first part of this work is dedicated to the study of Li-Si alloys by using AES. Several crystalline Li_xSi alloys (Li₇Si₃, Li₁₃Si₄ and Li₂₂Si₅) have been synthesized and characterized by X-Ray diffraction and AES. It appears that the atomic relative concentrations obtained by AES for the elements Li and Si are accurate with a tolerance of approximately 10%. The effect of Ar⁺ sputtering has also been investigated. After this initial work, six electrodes based on silicon particles have been analyzed by using AES for different "State Of Charge" (SOC) within the first electrochemical cycle.

For each SOC, several silicon particles have been investigated individually. Li and Si depth concentration profiles have been achieved by following Li (KLL) and Si (LVV) Auger transitions. Thanks to the good spatial resolution (17 nm at 10kV/1nA), it is possible to detect inhomogeneities of Li concentration at the particle surface. Based on the results obtained on different silicon particles at several SOC, a lithiation model is proposed and discussed.

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(2) Kasavajjula, U.; Wang, C.; Appleby, A. J. J. Power Sources 2007, 163 (2), 1003-1039

(3) www.hitachi.com

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(5) Morigaki, K. I.; Ohta, A. J. Power Sources 1998, 76 (2), 159-166

(6) Kim, Y. J., Lee, H.; Sohn, H. J. Electrochem. Comm. 2009, 11 (11), 2125-2128