Thickness-Dependent Creep in Lithium Layers of All-Solid-State Batteries under Stack Pressures

Stack pressure is broadly explored in improving contact at the lithium metal-solid-state electrolyte interface of all-solid-state lithium-metal batteries (ASSLBs). The effectiveness of this procedure relies heavily on the time-dependent accommodation of lithium sheets under confined conditions. Herein, a continuum modeling framework coupling power-law creep and diffusion is developed to investigate the mechanical behavior of pressed lithium layers of different thickness. It is revealed that lateral shear stress arising from interfacial confinement retards plastic accommodation in lithium layers. This detrimental effect becomes increasingly significant as lithium layers' thickness H decreases or their diameter D to thickness H ratio (D/H) increases. For layers of higher D/H, the stack pressure to realize a constant strain rate is proportional to (D/H)^{(1 + m)/m}, where m is the power-law creep exponent. Diffusion is beneficial to lithium deformability through reducing interfacial shear stresses and boosting power-law creep at constant stack pressure. A critical thickness characterizing the dominance of diffusion over creep is theoretically determined and validated through modeling for a wide range of deformation rates. Collectively, these findings advance the fundamental understanding of confined lithium mechanics and provide quantitative guidelines for the structural design and pressure management of ASSLBs.