Elucidating Sodium Ion Storage Mechanisms in Hard Carbon Anodes at the Electronic Level

Abstract Sodium‐ion batteries (SIBs) are a promising technology for advanced energy storage systems. Hard carbon (HC) is a commonly used SIB anode material; however, the Na ion storage mechanism in HC remains poorly understood and highly debated. Here, the paramagnetic species in HC during Na ion storage are systematically studied to elucidate the underlying mechanism at an electronic level using high‐resolution electron paramagnetic resonance (EPR) spectroscopy, complemented by in situ Raman spectroscopy, in situ synchrotron X‐ray diffraction, and density functional theory calculations. This investigation identifies and characterizes the coexistence of two distinct intercalation processes in HC: Na ion intercalation and Na⁺‐solvent co‐intercalation, which are active across both the sloping and plateau voltage regions. Additionally, in the sloping region, Na ions are also stored at in‐plane Stone‐Wales defect sites, which transition into a quasi‐metallic state and subsequently to metallic Na as Na ion intercalation progresses. This transformation is driven by charge redistribution within the graphene layers. These insights establish a direct paramagnetic‐electronic structure‐electrochemical property relationship in HC, providing new insights into the Na ion storage mechanism. Furthermore, this study highlights the unique capability of EPR spectroscopy in elucidating the charge storage mechanism in electrode materials.