Elucidation of the Sodium-Storage Mechanism in Hard Carbons

Hard carbons (HCs) are the most promising candidate anode materials for emerging Na-ion batteries (NIBs). HCs are composed of misaligned graphene sheets with plentiful nanopores and defects, imparting a complex correlation between its structure and sodium-storage behavior. The currently debated mechanism of Na+-ion insertion in HCs hinders the development of high-performance NIBs. In this article, ingenious and reliable strategies are used to elaborate the correlation between the structure and electrochemical performance and further illuminate the sodium-storage mechanism in HCs. First, filling sulfur into the micropores of HCs can remove the low-voltage plateau, providing solid evidence for its association with the pore-filling mechanism. Along with the decreased concentration of defects/heteroatoms at higher treatment temperature, the reduced sloping capacity confirms the adsorption mechanism in the sloping region. Finally, the similar sodium-insertion behaviors of HCs with ether-based and ester-based electrolytes indicate that no Na+ ions intercalate between the graphene layers. The determined adsorption-pore-filling mechanism encourages the design of more efficient HC anode materials with high capacity for high-energy NIBs.