Advanced Single Particle Measurement Technique for Fundamental Study of First Charging Mechanism of One Silicon Particle
Authors: Nishikawa Kei , Li Chunyan , Kanamura Kiyoshi 
Published: February 2020

Silicon is very attractive material for the negative electrode of next-generation Li batteries, because of its very large capacity (3580 mAh g-1). However, very large volume change during the lithiation and delithiation causes very rapid electrode degradation. There are many researches for the restriction and accommodation of the volume change of Si based materials by using nano-sized Silicon and functional binders [1]. Also, in-situ TEM observation technique has been utilized to understand the lithiation and delithiation mechanism of nano-sized Si particles [2]. The in-situ TEM technique is excellent technique, however it needs high vacuum condition or no vapor pressure liquid like ionic liquids. It is also indispensable to find the electrochemical characteristics in actual organic 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. Furthermore, the effect of binders and conductive agents which are used in standard composite electrode can be removed in this technique. In this study, the single particle measurement technique is used to understand the volume expansion mechanism of a micro-size silicon 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 silicon particle was picked-up by micro-tweezers in order to transfer the particle to other analytical equipment, for example, SEM, FIB-SEM, TEM, micro-Raman spectroscopy, and Auger electron spectroscopy. In previous study, the volume expansion behavior of a micro-sized silicon particle was focused. Figure 1 shows the drastic volume expansion behavior of a silicon particle during the first lithiation and the charging curve. The measured apparent volume expansion ratio is much larger than the theoretical values [3, 4]. In this study, the further analysis about lithiated silicon particle was conducted. In order to reveal the volume expansion mechanism of a silicon particle, ex-situ SEM, TEM observation was conducted. For TEM observation, the lithiated silicon particle was tailored to be thin slice by FIB treatment. TEM observation revealed that the lithiated silicon particle is mainly amorphous Si-Li phase and only a small parts remain crystalline silicon phase. It is caused by the local electrical disconnection because of very large volume expansion. EDS analysis with SEM and TEM shows surface SEI layer composition, and TEM-EELS analysis reveals chemical species in the SEI layer. The detailed results and the discussion for SEI formation during the initial charging of a silicon particle 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.
[3] K. Nishikawa, et. al., J. Power Sources, 243 (2013) 630.
[4] K. Nishikawa, et al., J. Power Sources 302 (2016) 46.

Figure 1