Microstructure, Pore Accessibility, and Sodium Storage in Hard–Soft Carbon Composites: Implications of Hard-Carbon Precursor Morphology

Hard carbon (HC) is a leading anode material for sodium-ion batteries (SIBs), and combining HC with soft carbon (SC) has been widely explored to tune electrochemical performance. However, it remains unclear how the precursor morphology of HC influences the microstructure ordering and pore accessibility of HC-SC composites and, consequently, their sodium storage behaviour. In this study, HC derived from microcrystalline cellulose (MCC) was co-carbonised with pitch-derived SC to produce composites with varying HC/SC ratios, which were evaluated under identical electrochemical conditions. Structural characterisation shows that SC addition modifies the degree of structural ordering in the carbon matrix, while Raman analysis indicates that the overall defect population remains largely unchanged across the series. Gas adsorption and NMR wetting experiments further reveal that SC addition restricts access to confined pore domains within the HC framework, limiting sodium storage in the low-voltage plateau region. Electrochemically, the sloping capacity and diffusion coefficients remain broadly similar across the composites, whereas the low-voltage plateau capacity and initial Coulombic efficiency (ICE) progressively decrease with increasing SC content. Furthermore, rate capability improves with moderate SC addition, with the MCC HC-SC composite containing 15 wt% SC delivering the highest rate performance (117 mAh g -1 at 500 mA g -1 ). These results show that the electrochemical response of HC–SC composites is not universal but is controlled by the morphology-dependent accessibility and the utilisation of low-voltage sodium storage domains.