Worse Interference of Fe³⁺ than Fe²⁺ on Degrading the Interphase and Performance of LiFePO₄||Graphite Battery

The detrimental effects of Fe-ion crosstalk on LiFePO₄||Graphite battery performance, coupled with limited mechanistic insights into solid electrolyte interphase (SEI) evolution under such interference, warrant systematic investigation. Herein, advanced characterization techniques-including X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and cryogenic transmission electron microscopy-are employed to reveal SEI evolution on graphite anodes under Fe²⁺/Fe³⁺ influence. Results demonstrate that Fe³⁺ exerts more severe adverse effects than Fe²⁺. Specifically, Fe²⁺ primarily promotes conventional electrolyte reduction reactions, increasing H₂, CH₄, and CO₂ generation. In contrast, Fe³⁺ facilitates radical combinations and catalyzes two-electron reductions, triggering side reactions releasing CO, C₂H₄, and C₂H₆. The SEI thickness under Fe-ion interference (Fe²⁺: 7.20-13.76 nm, Fe³⁺: 9.12-17.55 nm) significantly exceeds that of the base electrolyte (3.15-7.64 nm), with Fe deposits accumulating in organometallic forms. Critically, the presence of Fe³⁺ and its adverse impacts on battery safety and cycling stability are validated in practical pouch cells. This study uncovers the chemical nature of the dissolved Fe ion and its deposits, provides mechanistic insights into its interference with SEI properties and Li-ion battery performances, which in turn helps to mitigate such detrimental effects.