The pulverization of silicon (Si) anode materials is recognized as a major cause of their poor cycling performance, yet a mechanistic understanding of this degradation from a full cell perspective remains elusive. Here, we identify an overlooked contributor to Si anode failure: mechanical shutdown of separators. Through mechano-structural characterization of Si full cells, combined with digital-twin simulation, we demonstrate that the volume expansion of Si exerts localized compressive stress on commercial polyethylene separators, leading to pore collapse. This structural disruption impairs ion transport across the separator, exacerbating redox nonuniformity and Si pulverization. Compression simulation reveals that a Young’s modulus greater than 1 GPa is required for separators to withstand the volume expansion of Si. To fulfill this requirement, we design a high modulus separator, enabling a high-areal-capacity pouch-type Si full cell to retain 88% capacity after 400 cycles at a fast charge rate of 4.5 mA cm−2.