Spin-Resolved Insights into Fast Al-Ion Intercalation in Graphite via <i>Operando</i> Electron Paramagnetic Resonance Approach in Al Batteries

Graphite is widely used as an anode material in lithium-ion batteries, yet its fast-charging capability is constrained by sluggish Li+ intercalation kinetics. In contrast, graphite can store AlCl4– anions very fast when being considered as electrode for aluminum-ion batteries. However, the mechanism underlying this behavior remains unclear due to the lack of suitable Operando probes. Here, we combine Operando electron paramagnetic resonance (EPR) and Raman spectroscopy to investigate the intercalation of AlCl4– in graphite. Operando Raman confirms the reversible insertion of AlCl4–, establishing the structural basis for comparison with Li+ systems. Operando EPR reveals distinct electronic signatures: unlike the monotonic increase in signal intensity observed in Li+ intercalation, AlCl4– intercalation produces a nonmonotonic evolution. EPR line-shape analysis shows consistently larger A/B ratios in the aluminum-ion system, indicating higher metallicity, enhanced conductivity, and faster electron exchange. These spin-level fingerprints demonstrate that electronic delocalization and spin–orbit interactions contribute to the intrinsic ultrafast kinetics of AlCl4– anion intercalation. Operando EPR thus provides an important electronic probe to track the charge transportation in graphite during ion intercalation and deintercalation and offers new guidance for the design of high-rate aluminum-ion batteries.