Microstructure-Dependent Sodium Storage Mechanisms in Hard Carbon Anodes

Hard carbon (HC) is a leading anode material for sodium-ion batteries, but its complex microstructure complicates understanding of sodium storage mechanisms. Using X-ray total scattering and density functional theory calculations, this study clarifies how HC’s microstructural variations influence sodium storage across the slope (high potential) and plateau (low potential) regions of the potential-capacity curve. In the slope region, sodium initially adsorbs at high-binding energy defect sites and subsequently intercalates between graphene layers, adsorbing at low-binding energy defect sites, correlating with different slopes observed during initial sodiation. Initial irreversibility arises from sodium trapping at surface defects and solid electrolyte interface formation. In the plateau region, sodium simultaneously intercalates and fills pores, influenced by pore size, interlayer spacing, and defect concentration. HCs with larger pore sizes form larger sodium clusters. The proposed mechanism underscores the role of microstructure engineering in enhancing HC performance and advancing sodium-ion batteries for grid-scale energy storage.