Unveiling the Crucial Role of Dissolved Fe²⁺ on the Solid Electrolyte Interphase in Long‐life LiFePO₄/Graphite Batteries

Abstract The dissolution of iron from the cathode significantly contributes to the accelerated degradation of LiFePO 4 /Graphite batteries, particularly at elevated temperatures. However, a systematic understanding of the spatial distribution and impact of Fe ions on the dynamic solid electrolyte interphase (SEI) layer is lacking. In this study, a comprehensive and quantitative investigation is conducted into the effects of transition metals (TM) and thoroughly examined the interaction between dissolved Fe 2+ and SEI in long‐life LiFePO 4 /Graphite pouch cells. The dissolved Fe in the electrolyte is more prone to deposition at the negative electrode at elevated temperatures, leading to an accelerated loss of active lithium. Additionally, Fe deposition on the SEI catalyzes the decomposition of EC and contributes to an increase in organic components, particularly lithium alkyl carbonates within the SEI, as evidenced by mass spectrometry titration (MST) analysis. Neutron imaging (NI) provides more insights into the impacts of dissolved Fe 2+ on active lithium loss, SEI components, and electrolyte decomposition, resulting in greater macroscopic heterogeneity in the electrode regions of the cells. This research sheds light on the mechanisms underlying the degradation of LiFePO 4 /Graphite batteries and provides valuable insights for the development of strategies to mitigate capacity fade and enhance battery performance and longevity.