In Situ Focused Ion Beam-Scanning Electron Microscope Study of Crack and Nanopore Formation in Germanium Particle during (De)Lithiation
Authors: Zhou Xinwei , Li Tianyi , Cui Yi , Liu Yuzi , Mullins Charles Buddie , Meyerson Melissa , Zhu Likun 
Published: February 2020

Germanium (Ge) has been considered a promising anode active material, due to its high capacity, low voltage, fast lithium ion diffusion, and high electrical conductivity. However, Ge based anode materials suffer from pulverization due to a large volume change, up to 300%. Although pulverization issues can be alleviated by nano-structuring the Ge-based anode materials, recent studies have shown that nanometer-sized pores are still formed in the nano-structured electrode due to vacancy-mediated diffusion, which could lead to pulverization during repeated cycling.

In recent years, the reaction mechanism of the Ge anode has been studied by using different in situ and operando characterization methods such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). For instance, Lim et al. proposed the phase transformations of Ge that occur during lithiation and delithiation.1 They also studied the effect of cycling rate on Ge phase transformation during the charge/discharge process and proposed a mechanism to explain the C‐rate dependence phase transformation phenomenon. However, it is still necessary to correlate the microstructural evolution of the Ge electrode and its electrochemical performance. To this end, some advanced imaging technologies have been used to monitor the microstructural evolution of the Ge electrode during cycling such as transmission electron microscopy (TEM)2 and transmission X-ray microscopy (TXM).3
In situ TEM studies have shown the volume expansion during lithiation and formation of nanopores during delithiation in Ge nanowires in real time. But due to the small size of the sample used in the TEM, it is difficult to control the current rate. In situ TXM can obtain two dimensional (2D) or three dimensional (3D) microstructures of Ge particles during cycling with a large field of view. However, the resolution of TXM is not sufficient for monitoring nanometer-sized structures, such as nanopores.

In this study, we developed an approach to build a single particle battery in the chamber of focused-ion beam-scanning electron microscope (FIB-SEM) to monitor the microstructural evolution of a single Ge micro-particle during cycling. The experiment was performed within a ZEISS Nvision FIB-SEM at the Center for Nanoscale Materials, Argonne National Laboratory. A Ge particle was attached to the tungsten probe by carbon coating using FIB deposition as positive electrode. The lithium metal was placed on the top of SEM stage as negative electrode. One drop of ionic liquid electrolyte (ILE) was placed on the top of lithium metal. The ILE was made by dissolving the lithium salt, lithium bis (trifluoromethylsulfonyl) imide (LiTFSI), in a solvent of 1-butyl-1-methylpyrrolidinium bis (trifluoromethyl -sulfonyl) imide (P14TFSI). The tungsten probe and the SEM stage were connected to a Keithley 6430 sub-femtoamp remote sourcemeter. Galvanostatic mode was used in all electrochemical cycling. The particle was immersed in the ILE drop during cycling and lifted out for imaging at different states of charge. The particle was polished by FIB before imaging to remove ILE on the surface.

This study reveals that germanium electrodes with low and high cycling rates have better microstructure integrity, which leads to better cycling performance. The nanopores tend to aggregate into large porous structures during cycling which leads to particle pulverization and capacity fading of the electrode.

1. Lim, L. Y.; Fan, S.; Hng, H. H.; Toney, M. F., Storage Capacity and Cycling Stability in Ge Anodes: Relationship of Anode Structure and Cycling Rate. Advanced Energy Materials
2015,
5 (15), 1500599.

2. Liu, X. H.; Huang, S.; Picraux, S. T.; Li, J.; Zhu, T.; Huang, J. Y., Reversible Nanopore Formation in Ge Nanowires during Lithiation-Delithiation Cycling: An In Situ Transmission Electron Microscopy Study. Nano Letters
2011,
11 (9), 3991-3997.

3.Weker, J. N.; Liu, N.; Misra, S.; Andrews, J. C.; Cui, Y.; Toney, M. F., In situ nanotomography and operando transmission X-ray microscopy of micron-sized Ge particles. Energy & Environmental Science
2014,
7 (8), 2771-2777.