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Hard carbon is a non-graphitizable carbon material built from randomly oriented graphene-like nanodomains, curved or fullerene-like fragments, and internal closed pores, and it cannot be converted into graphite even after treatment above about 2500 °C ^[1]. It is obtained mainly by pyrolysis of biomass, sugars, resins, and polymers at 1000–1600 °C, and its structure is usually described by the interlayer spacing (0.36–0.40 nm, against 0.335 nm for graphite) together with the volume fraction of closed micropores ^[2]. For sodium-ion batteries the material serves as the negative electrode, where reversible capacities of roughly 250–350 mAh/g are commonly reported ^[3]. Two storage processes are distinguished in the galvanostatic curve: adsorption of sodium at defect sites and pore surfaces, which gives the sloping region, and insertion or pore filling, which is associated with the low-potential plateau near 0.1 V ^[4]. Because plateau capacity depends on closed-pore volume, the balance between the sloping and plateau regions is governed by precursor selection, pyrolysis temperature, and heteroatom doping rather than by any single synthesis parameter ^[5]. Initial Coulombic efficiency, typically 70–90%, and the contribution of sodium metal plating to the measured capacity remain debated ^[3][4].