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.