Demystifying the Influence of Precursor Structure on S‐Doped Hard Carbon Anode: Taking Glucose, Carbon Dots, and Carbon Fibers as Examples

Heteroatom doping is a promising strategy for adjusting the microstructure of hard carbon (HC) to promote its electrochemical sodium storage performance. However, clarifying the doping sites of heteroatoms and effectively regulating their doping levels remain serious challenges. Herein, this work reveals the impact of three distinct structural precursors on S‐doped hard carbon: namely glucose (small organic molecule), carbon dots (CDs, intermediate state between organic and inorganic), and graphitized carbon fibers (inorganic carbon materials). It is demonstrated that the S‐doped HC derived from CDs possesses a more significant number of C─S bonds within its carbon framework, which is attributed to the preferential bonding between sulfur and short polymeric chains abundant in unsaturated functional groups. And these chains cluster prominently on the surface of CDs, enhancing the affinity of sulfur. Furthermore, as a prominent feature of CDs, the extremely small size inherently distinguishes them from other precursors, enabling them to serve as fundamental units for constructing various carbon microstructures, such as three‐dimensional (3D) structure. In summary, this study explores the influence of different precursor structures on heteroatom doping, with CDs identified as the most useful precursor. The influence mechanism of various carbon precursors on the S‐doped carbon is revealed, and carbon dots are demonstrated to be the optimal precursor. It is found that the appropriate content of sulfur not only provides more sodium storage sites but also significantly enhances the solid electrolyte interface.