Unraveling the effect of carbon morphology evolution in hard carbons on sodium storage performance

Sodium storage by hard carbons in the low potential region is closely correlated with their internal structure. Reasonable microstructural composition and carbon layer spacing are key to effectively boosting sodium storage capacity. Herein, chitosan-derived hard carbons (CHCs) were prepared via tuning the carbonization temperatures. With the increase of carbonization temperature, the internal structure evolved to be more ordered and the carbon atoms rearranged to form a more pseudo-graphitic structure. The pseudo-graphitic domains with suitable carbon layer spacing play a crucial role in sodium storage. Carbon layers can significantly accelerate Na-ion insertion/extraction, thus leading to an increased plateau capacity, while the defects and functional groups are conducive to ion transfer, resulting in a good rate. As a result, the optimized material delivers a high specific capacity of up to 317.4 mA h g-1 at 0.5 A g-1 and excellent cycling stability by retaining 238.9 mA h g-1 at 5 A g-1 after 1000 cycles. This study reasonably reveals the relationship between the microstructural evolution of hard carbons and their sodium storage performance, which may provide guidance for practical application. Na ions can be effectively inserted into carbon layers with suitable spacing and pseudo-graphitic domains play a crucial role in sodium storage. The plateau capacity is positively correlated with the proportion of pseudo-graphitic domains.