Thus, when observing and analyzing actual batteries, it is necessary to conduct ex situ experiments under strictly controlled environments that prevent the battery components from contacting the atmosphere during all processes, including battery disassembly, the preparation of specimens for electron microscopy, and specimen transport. Since the materials used in Li–ion batteries are generally extremely active when exposed to moisture and air, it is difficult to know how long the microstructure remains stable when a battery in a charged state is exposed to the atmosphere. Furthermore, it is necessary to enhance the use of electron microscopy in the analysis of the reactions that occur in Li–ion batteries using actual battery structures. 14, 15 ) It is important to clarify the causal relationship between the performance of functional materials and their microstructures using electron microscopy. The detection of light elements including Li has also become relatively easy because of improvements in the sensitivity of electron energy loss spectrometry (EELS) at the atomic level, and limited examples of in situ TEM observation have even been reported. In recent years, microscopic analyses using SEM, scanning transmission electron microscopy (STEM), and focused ion beam (FIB) have matured, and it has become possible to visualize and analyze the microstructures of composite materials from the micron level to the atomic level. 8 ) Although studies based on nuclear magnetic resonance 9, 10 ) and experiments on special Si structures 11 – 13 ) have also been reported, no atomic-level observations of Li intrusion and the resulting structural changes in Si negative electrodes within actual battery structures have been reported. This orientation-dependent anisotropy of the amorphous phase formed during the lithiation of Si crystals and its swelling rate were also clarified in an investigation of the charge reaction of single-crystal Si nanowires with ⟨100⟩, ⟨110⟩, ⟨111⟩, and ⟨112⟩ orientations with respect to the axial direction. 7 ) The authors found that the swelling rate of the amorphous Li–Si phase (a-Li–Si) differed based on the crystallographic orientation, with the ⟨110⟩ direction exhibiting the fastest swelling rate. In another study, single-crystal Si nanopillars with different crystal orientations were prepared, and their lithiation process was investigated with in situ scanning electron microscopy (SEM). The initial stage of amorphization was observed by high-angle annular dark field-scanning transmission electron microscopy, demonstrating that the Li atoms occupied the tetrahedral sites of Si crystals, and that the crystal structure was destroyed via the severing of Si–Si bonds between the planes. The orientation of the single-crystal Si powder in the charged state was observed by electron backscatter diffraction, indicating that lithiation occurred preferentially along the (110) plane of Si. All of the processes from disassembly of the charged battery and preparation of specimens for use in electron microscopy observation to specimen transport to the electron microscopes were performed under non-atmospheric exposure conditions. In this study, ex situ electron microscopy was applied to observe Si negative electrodes under different charge states within an actual battery structure to reveal the Li intrusion direction and the effects of Li concentration on the electrode structure. Silicon (Si) has attracted considerable interest as a negative electrode material for next-generation lithium (Li)–ion batteries because of its high capacity density.
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