Silicon (Si) has attracted considerable interest as a negative electrode material for next-generation lithium (Li)–ion batteries because of its high capacity density. 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. 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. 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. 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 {111} planes. During the charge reaction, Li occupied the tetrahedral sites via intrusion along the 〈110〉 direction of Si, and amorphization proceeded as the Li concentration increased. Thus, the amorphous region grew preferentially in the 〈110〉 direction of Si.
