Microstructure of Silicon Anodes in Solid‐State Batteries ‐ From Crystalline to Amorphous

Abstract Silicon offers great promise as a potential anode active material and the optimum alternative to lithium metal in all‐solid‐state lithium‐ion batteries. However, its practical application is limited by severe volume expansion (≈300%) during lithiation, leading to cracking upon delithiation. In this study, the microstructural evolution of microcrystalline silicon electrodes in a solid‐electrolyte‐free environment is investigated using cryogenic scanning transmission electron microscopy (STEM) after electrochemical cycling. A controlled workflow prevents ambient exposure, and cryo‐TEM ensures structural integrity. After the first lithiation, the electrode shows a heterogeneous mix of crystalline Li 15 Si 4 , various amorphous Li x Si phases, and residual crystalline silicon. After the first delithiation, the silicon becomes largely amorphous, showing a heterogeneous texture with pronounced thread‐like features and only traces of crystallinity. By the tenth delithiation, the bulk microstructure is far more uniform, with thread‐like features largely eliminated and persisting only in small regions near grain boundaries. These results indicate that, although silicon begins in a crystalline state, a more homogeneous bulk silicon amorphous microstructure develops only after several cycles. These findings highlight that stabilizing the microstructure and minimizing cracking during cycling requires not only optimization of electrode architecture but also careful selection of the silicon phase.