Evaluation of the Impact of Conductive Additives on the EPR Spectra of Hard Carbon Anodes

Hard carbon (HC), a prime anode candidate for sodium-ion batteries, exhibits unresolved charge-state-microstructure debates reveal critical sodium storage mechanism gaps. This study employs electron paramagnetic resonance (EPR) spectroscopy as a principal characterization tool, capitalizing on its unique capability to probe electronic configurations and detect subtle structural transformations in carbon matrices. Through systematic EPR investigations of sodium storage dynamics in varied carbon architectures, the quasi-metallic state of sodium ions stored in closed pores exhibits distinct signal characteristics due to size effect-induced structural confinement, compared to surface storage mechanisms. Furthermore, the underappreciated influence of conductive carbon additives, a ubiquitous component in electrode formulations is specifically addressed, on spectroscopic interpretations. This findings reveal that sodium's distinctive storage states (ionic vs quasi-metallic) and their spatial distribution within carbon matrices induce quantifiable modifications in EPR spectral parameters, including characteristic linewidth broadening and lineshape evolution. The comparative analysis demonstrates that trace amounts (≤10 wt.%) of conductive additives can substantially distort ex situ EPR measurements, with interference patterns exhibiting strong material-dependent behavior. Therefore, the application of conductive additives demands rigorous consideration in EPR-based investigations of energy-storing carbon materials, given their methodological implications.