Cellulose, due to its abundance and propensity to form spherical structures, serves as an exceptional precursor for the synthesis of high-performance hard carbon materials used in sodium-ion batteries. This study explores the interplay between different cellulose types, their structural evolution during hydrothermal spherization, and the subsequent impact on the microcrystalline characteristics and electrochemical performance of the resulting hard carbon for sodium storage. The results observed that the crystallinity of various cellulose precursors-namely natural cellulose, α-cellulose, and microcrystalline cellulose-and their corresponding hard carbons are positively correlated. Among these, hard carbon derived from α-cellulose demonstrated superior attributes, including the highest closed pore volume and a balanced defect density, which contribute to its enhanced sodium storage capacity. Furthermore, the hydrothermal treatment of α-cellulose at 220 °C was optimized to achieve a spherical morphology, which significantly benefited both the capacity and rate performance of the hard carbon. The reversible capacities at current densities of 20 and 2 000 mA/g are 329.4 and 53.9 mA·h/g, respectively.