Microstructure Dependent Sodium Storage Mechanisms in Hard Carbon Anodes

Sodium storage mechanisms within hard carbon (HC) anodes for sodium-ion batteries are strongly dependent on the HC microstructure. The capacity curve of HC is composed of a high voltage slope and a low voltage plateau region.The HC microstructure ultimately determines the total capacity and the ratio between the capacities of the slope and plateau regions. It has been established that sodium can be stored via three processes: adsorption, intercalation, and pore filling with the consequent sodium cluster formation in the pores. However, the actual sequence and details of the sodium storage mechanisms still a subject of debate. Using X-ray pair distribution function analysis, this study clarifies how microstructural variations in HC influence sodium storage across both the slope and plateau regions of the capacity curve. During the slope region, sodium ions initially adsorb at high-energy defect sites and subsequently intercalate between graphene layers to adsorb in defect sites, which correlates with distinct electrochemical gradients observed during initial sodiation. In the plateau region, our findings reveal simultaneous intercalation and pore filling, dictated by the microstructure's characteristics such as pore size distribution, interlayer spacing, and defect concentration. This is especially notable in HC synthesized at higher pyrolysis temperatures, where larger sodium clusters form, indicating a preference for filling larger pores. The proposed 'surface adsorption – defect-assisted intercalation – intercalation/pore filling' mechanism highlights the critical role of microstructure engineering in optimizing HC performance. These insights are crucial for advancing HC anode design in sodium-ion batteries, particularly for large-scale energy storage applications, making a significant stride toward sustainable energy solutions.