Na-Ion Storage and Diffusion Behavior in Coal-Based Hard Carbon on the View of Molecular Structure

Sodium storage mechanisms and microstructures play a key role in improving the sodium storage capacity of hard carbon (HC) anodes; however, the storage mechanisms of sodium ions in coal-carbon-derived HC and the effective regulation of microstructures at the molecular level are still scarce. In this work, it is proposed for the first time that the coaling effect affects the microstructure and the Na+ diffusion coefficient in coal-derived HCs during their discharge by grafting aryl rings and oxygen-containing functional groups within and between the main chains of the precursors. We propose and confirm two Na+ storage mechanisms that are closely related to the coalisation effect. Aromatic rings and oxygen-containing functional groups induce Na+ aggregation during Na+ diffusion, leading to the formation of metal clusters in low-voltage regions. Therefore, the effects of aromatic rings and oxygen-containing functional groups on the local microstructure of HCs should be considered when designing HCs. In this work, HCs with specific graphite microcrystalline structures were prepared by screening coal precursors, and constraints between graphite microcrystalline parameters and precursors were revealed. This work provides theoretical guidance to study the storage mechanism of Na+ through the coalisation effect and offers new ideas for the development of high-performance coal-derived anodes for sodium-ion batteries.