Silicon nanowires (Si NWs) hold great promise as high-capacity anode materials for next-generation batteries. However, their application is severely hindered by anisotropic lithiation, which leads to structural failure and rapid capacity fading. Here, we introduce a novel in situ transmission electron microscopy (TEM) cross-sectional analysis technique that enables real-time visualization and quantitative analysis of the radial structural evolution of one-dimensional (1D) nanomaterials under external stimuli. Applying this method to Si NWs, we uncover a two-tiered mechanism for regulating anisotropic lithiation in Si NWs. First, selecting axial orientations with high in-plane crystallographic symmetry can effectively facilitate uniform lithium (Li) diffusion and suppress directional expansion. Second, rational cross-sectional design, such as faceted-engineered geometries, further suppresses anisotropy by constraining the effective interfacial area and diffusion path length in fast-lithiation directions. These findings provide new insights into the control of anisotropic lithiation and offer a geometry-guided strategy for enhancing the structural stability and performance of Si-based anodes. Moreover, the methodology and anisotropy regulation principles established here are broadly applicable to other 1D nanomaterials.
