Unlocking the local structure of hard carbon to grasp sodium-ion diffusion behavior for advanced sodium-ion batteries

Clarifying the microstructure of hard carbon is essential to reveal its sodium storage mechanism and to develop hard carbon negative electrodes for high-performance sodium ion batteries. Currently, although various sodium storage mechanisms for hard carbon models are proposed, they are still controversial. Besides, the puzzling and abnormal variation of a Na⁺ diffusion coefficient during the discharge process cannot be well explained. Inspired by amorphous alloys, we propose and confirm the dispersion region at the junction between amorphous structures and graphite microcrystals, which is closely related to the structure of graphite microcrystals. The special dispersion region plays a buffer role in the sodium ion diffusion process and provides satisfactory storage capacity. Therefore, the effect of synthesis conditions on the local structure in the dispersion region should be considered when designing hard carbon. In this work, a specific graphite microcrystalline structure of hard carbon is precisely synthesized by screening organic molecules, and the constraint relationship between the parameters of the graphite microcrystalline structure is revealed. Importantly, this work is of great significance for resolving the current controversy about the sodium storage mechanism and making clear the anomalies of sodium ion diffusion in the low-voltage interval (<0.1 V) in hard carbon.