Operando Decoding of Stress Relaxation Behavior in LiFePO ₄ /Graphite Pouch Cells during Resting States

Lithium-ion batteries develop substantial mechanical stress under fast charging, which constrains cycle life and operational safety. Although stress evolution during electrochemical cycling has been widely investigated, how mechanically stored stress relaxes during rest remains poorly understood, particularly in large-format commercial cells. Here, fiber Bragg grating sensors are integrated into LiFePO4/graphite pouch cells to resolve stress relaxation during post-cycling rest, revealing a pronounced size dependence in nonequilibrium mechanical behavior. The post-cycling rest period is shown to be mechanically active rather than inert. Stress relaxation during rest is governed by coupled lithium-ion redistribution and viscoelastic binder creep, driving the system from a nonequilibrium state toward a quasi-equilibrium configuration. Large-format (10 Ah) cells exhibit apparent mechanical stability under fast charging, characterized by relatively low peak stress during current application. However, immediately after current interruption at 1.5 C, these cells undergo pronounced transient stress relaxation exceeding 6 MPa, indicating the presence of a hidden nonequilibrium internal stress field arising from steep lithium concentration gradients. Incorporating intermittent rest into pulse fast-charging protocols reduces cumulative mechanical stress by approximately 10%, thereby extending the mechanical lifetime of large-format cells. This finding establishes a practical framework for stress-aware fast-charging strategies in high-capacity lithium-ion batteries.