Anisotropic lithiation onset in silicon nanoparticle anode revealed by in situ graphene liquid cell electron microscopy.

Recent real-time analyses have provided invaluable information on the volume expansion of silicon (Si) nanomaterials during their electrochemical reactions with lithium ions and have thus served as useful bases for robust design of high capacity Si anodes in lithium ion batteries (LIBs). In an effort to deepen the understanding on the critical first lithiation of Si, especially in realistic liquid environments, herein, we have engaged in situ graphene liquid cell transmission electron microscopy (GLC-TEM). In this technique, chemical lithiation is stimulated by electron-beam irradiation, while the lithiation process is being monitored by TEM in real time. The real-time analyses informing of the changes in the dimensions and diffraction intensity indicate that the very first lithiation of Si nanoparticle shows anisotropic volume expansion favoring the ⟨110⟩ directions due to the smaller Li diffusion energy barrier at the Si-electrolyte interface along such directions. Once passing this initial volume expansion stage, however, Li diffusion rate becomes isotropic in the inner region of the Si nanoparticle. The current study suggests that the in situ GLC-TEM technique can be a useful tool in understanding battery reactions of various active materials, particularly those whose initial lithiation plays a pivotal role in overall electrochemical performance and structural stability of the active materials.