Mechanical Relaxation Dynamics of Confined Lithium Metal in Solid-State Batteries

Abstract The rate-dependent mechanics of lithium metal under confinement critically influences interfacial contact evolution and stress buildup at the lithium metal−solid-state electrolyte interface. Here, we investigate the relaxation dynamics of confined lithium using dynamic mechanical analysis within a continuum framework coupling power-law creep and diffusion. Two distinct stress relaxation modes with separated timescales are identified. Under fully constrained conditions, only deviatoric stress relaxes, producing a single loss peak Pd in the mechanical relaxation spectrum. Under interfacial confinement, an additional low-frequency peak Ph emerges from hydrostatic stress relaxation. The deviatoric stress relaxation time τd is controlled by power-law creep kinetics. The hydrostatic stress relaxation time τh depends on length scale L, aspect ratio L/H, and lithium mobility MLi, following τh∝L2MLi(LH)1m in the intermediate diffusion-assisted creep regime, where m is the power-law creep exponent. These findings establish a mechanistic framework for understanding confined lithium relaxation relevant to interfacial stability in solid-state batteries.