Ultrathin Lithium Foil Prelithiated Silicon Carbon Anode with the Formation of Li₂₂Si₅ Alloy Phase Enables High‐Energy‐Density Pouch Cells Lifespan over 900 Cycles

Silicon‐carbon anodes with high capacity show great promise for advancing lithium‐ion battery technology, yet they face significant challenges. Their practical application is hindered by poor initial Coulombic efficiency and swift capacity loss, primarily due to erratic phase changes and unstable solid electrolyte interphases (SEIs). While prelithiation techniques have been investigated to mitigate these problems, the fundamental principles behind their effectiveness remain unclear. Herein, the contact‐prelithiation mechanism in Si‐based anodes is investigated using spectroscopy techniques, multiscale imaging, and first‐principles calculations. The results demonstrate that solid‐state lithiation at the Li/Si interface forms a stable Li 22 Si 5 phase that buffers volumetric strain, weakens the interaction with Li 2 CO 3 , and promotes the formation of a LiF‐rich inner SEI. This self‐evolving bilayer SEI enhances cycling stability, maintains fast Li + transport, and preserves electrical connectivity, leading to high energy density and superior cycling performance in pouch cells. Specifically, the prelithiated‐Si/C (pre‐Si/C) anodes exhibit 74.00% capacity retention over 910 cycles in industrial 7.2 Ah pouch cells, highlighting the practical potential of this approach. The insights provided here not only elucidate the key role of interface evolution in silicon anodes but also offer a rational phase–interphase co‐engineering strategy for developing durable, high‐performance silicon‐based anodes for next‐generation lithium‐ion batteries.