Mini-Review on Advanced Characterization Techniques for Insights into the Sodium Storage Mechanism of Hard Carbon Anodes: Recent Advances and Future Perspectives

Non-graphitizable hard carbon (HC) possesses numerous surface imperfections, functional groups, and randomly arranged graphene sheets (turbostratic), generating micro/meso/macropores. The sodium-ion storage mechanism in HC anodes remains unclear as it heavily relies on the diverse structures resulting from different precursor materials and heat treatment temperatures used for HCs. Various models have been anticipated to comprehend the analysis of structural and sodium storage mechanisms in HCs. Amidst conflicting reports, the prevailing storage model suggests an “adsorption–intercalation–filling” mechanism. It is widely accepted that lithium insertion into the graphene interlayer forms “graphitic intercalation compounds” (GICs), which involve an intercalation mechanism. Conversely, the formation of sodium-based GICs is thermodynamically unstable, and most sodium storage is attributed to the pore-filling mechanism, resulting in the formation of pseudo-metallic clusters at a lower potential, near 0 V. The primary topic of debate regarding the storage mechanism revolves around whether the low-potential plateau capacity arises from intercalation, pore filling, or metallic deposition. In this review, we mainly highlight and discuss the advanced characterization techniques, including operando techniques, such as Raman spectroscopy, X-ray diffraction, electron paramagnetic resonance, nuclear magnetic resonance, and small-angle X-ray scattering, to monitor the detailed mechanism. Therefore, we hope that this review can assist readers in understanding the charge storage relation in detail to enhance the HC performance to advance its practical application for sodium-ion batteries.