Electron Paramagnetic Resonance Spectroscopy in Carbon Materials for Energy Storage: A Review

Given that carbon-based materials serve as the crucial electrode materials in electrochemical energy storage devices, it is of significance to comprehensively understand their energy storage mechanisms and optimize the performance of electrodes. In recent years, a diverse array of characterization techniques, including synchrotron radiation, have been employed to elucidate the complex structure-property relationships in these carbon electrodes. Among these techniques, electron paramagnetic resonance (EPR) spectroscopy stands out due to its high sensitivity to unpaired electrons, making it a powerful tool for characterizing the electronic structures of complex carbons and tracking electron transfer characteristics at the carbon electrode/electrolyte interface during electrochemical processes. In this review, the spectral differences resulting from molecular structural variations in carbon materials used for energy storage are systematically explored and the storage mechanisms based on ex situ analyses are interpreted. The significant advancements in in situ electrochemical characterization using EPR technology, providing new insights into the behavior of carbon electrodes are highlighted. Additionally, the current challenges facing EPR spectroscopy in the context of carbon-based energy storage are discussed and potential solutions are proposed. This review aims to serve as a valuable resource for understanding the complex structures and energy storage mechanisms of carbon materials through EPR spectroscopy.