Mechanism-based fatigue life prediction for polymer-based Cu current collectors in Li-ion batteries

The pursuit of ultra-high energy density and enhanced safety for next-generation Li-ion batteries has driven the shift from metallic to polymer-based current collectors, yet their ultrathin micron-scale metal layers are highly susceptible to fatigue-induced damage with no predictive design tools available. Here, fatigue cracking mechanism of polymer-based Cu current collectors fabricated via one-step and two-step routes is investigated using a real-time resistance method. Quantitative analysis of fatigue crack density reveals that slip-band-dominated cracking is gradually suppressed, with damage becoming localized at pre-existing defects when the strain amplitude exceeds the critical threshold of 0.5 %. Building on these mechanistic insights, a mechanism-based fatigue life model integrating the modified Tanaka-Mura model (for slip band-dominated crack initiation) and the Paris law (for defect-driven crack growth) is proposed to determine the upper and lower bounds of fatigue life. The model demonstrates good agreement with experimental data, partitioning fatigue behavior into two distinct regimes: a slip-band-dominated region, optimized by microstructure design, and a defect-dominated region, controlled by defect tolerance criteria to enhance fatigue resistance in current collectors. Additionally, the model quantifies the maximum tolerable defect sizes required to achieve theoretical slip-band-dominated fatigue limits. Together, these results establish a predictive framework and actionable design principles for engineering fatigue-resistant polymer-based current collectors in advanced battery systems.