Applied Single Particle Measurement Technique for One Si Particle

Abstract

Si is very attractive material for next-generation negative electrode of Li batteries, because of very large capacity. However, Si has very large volume change during the lithiation and delithiation. Such large volume change induces very rapid electrode degradation. There are many researches for the restriction and accommodation of the volume change of Si based materials [1]. Also, in-situ TEM observation technique has been utilized to understand the lithiation and delithiation mechanism of nano-sized Si particles [2]. In-situ TEM technique is very powerful technique, however it needs high vacuum condition or no vapor pressure liquid like ionic liquids. It is also very important to find the electrochemical characteristics in actual electrolytes system. Single particle measurement technique is very useful to reveal the intrinsic electrochemical characteristics of a micro-scale particle in commercial Li-ion battery electrolytes. In this study, the single particle measurement technique is used to understand the volume expansion mechanism of a micro-size Si particle in the first lithiation reaction.The detailed experimental conditions for the single particle measurement were described elsewhere [3]. After the electrochemical characterization, the lithiated Si particle was picked-up by micro-tweezers in order to transfer the particle to other analytical equipment, for example, SEM, FIB-SEM, TEM and micro-Raman. In this study, the relationship between the volume expansion behavior of a micro-sized Si particle and the morphological and structural variations is focused. Figure 1 shows the drastic volume expansion behavior of a Si particle during the first lithiation and the charging curve. The measured apparent volume expansion ratio is much larger than the theoretical values [3, 4]. The Si powders used in this study is composite of crystalline and amorphous Si, and micro-sized secondary particles are composed of several hundred nm primary particles. Therefore, the Si particle used in this study may have the space inside the secondary particle. Furthermore, Cui et al. have reported the crystalline Si shows the anisotropy volume expansion during the lithiation. Such anisotropy volume expansion may induce the extraordinary apparent volume expansion. In order to reveal the volume expansion of a Si particle, ex-situ SEM, TEM observation was conducted. Fig.2 shows TEM images of cross section of the lithiated Si particle. The images show the lithiated Si is mainly amorphous phase, and only a very small parts remain crystalline Si phase. It is caused by the local electrical disconnection because of very large volume expansion. The TEM observation also shows surface SEI layer composition. The detailed results will be shown in the presentation.[1] M. Wu, et. al., J. Am. Chem. Soc., 135 (2013) 12048[2] M. T. McDowell, et al., Nano Lett., 13 (2013) 758-764[3] K. Nishikawa, et. al., J. Power Sources, 243 (2013) 630[4] K. Nishikawa, et al., J. Power Sources 302 (2016) 46 Figure 1