Multiscale investigation of the rate-dependent degradation mechanisms in LiFePO₄/graphite lithium-ion batteries

Understanding rate-induced degradation mechanisms is essential for ensuring the reliable operation of lithium-ion batteries under fast-discharging conditions. This study systematically examines LiFePO₄/graphite cells cycled across a broad spectrum of discharge rates (1C–10C) to elucidate rate-dependent aging behaviors and failure mechanisms. A multiscale characterization framework—spanning from the electrode to the nanoscale and integrating electrochemical, thermal, morphological, and interfacial analyses—was developed to uncover the progressive degradation processes. Electrochemical analyses indicate that capacity fade is governed primarily by lithium inventory loss (LLI) rather than active material degradation (LAM), with lithium plating occurring above 3C and dendritic growth dominating at 5C. Morphological observations demonstrate that while the cathode structure remains largely intact, high-rate cycling induces particle cracking, interfacial instability, and CEI thickening. In contrast, the anode exhibited pronounced surface degradation, SEI growth, and increased severity of lithium plating, as confirmed by surface-sensitive chemical analyses and fluorescence imaging. Finite element simulations further revealed increasing spatial inhomogeneity in the potential distribution and electrode utilization with increasing discharge rates, aligning with the observed electrochemical and structural gradients. The study provides a quantitative and mechanistic understanding of how high-rate operation accelerates anode failure and interfacial degradation, offering critical insights for lifetime modeling and the optimization of fast-discharging protocols in commercial lithium-ion batteries.