• • The functionally gradient ductile SEI enables Li|PALA|Li cells to cycle for over 4500 h at an ultrahigh current density of 15 mA cm−2 and areal capacity of 15 mA h cm−2, achieving a cumulative capacity of 33750 mA h cm−2—an order of magnitude higher than prior SLMBs (e.g., 11500 mA h cm−2 for Li|porous garnet|Li).
• • At −30 °C, the cells stably operate for over 7000 h under practical conditions (5 mA cm−2, 5 mA h cm−2), demonstrating exceptional low-temperature performance critical for cold-climate applications.
• • The ductile SEI exhibits a high Li-ion diffusion coefficient of 3.8 × 10−8 cm2 s−1 and low activation energy, facilitating rapid ion transport even at high current densities.
• • The SEI's ductility is confirmed by a low generalized stacking fault energy (E_gsf) for AgF/Ag2S interfaces, and it maintains structural integrity at a bending angle of 150°, whereas brittle SEIs fracture—highlighting the importance of mechanical robustness in preventing dendrite growth and contact loss.