Effect of Local Atomic Structure on Sodium Ion Storage in Hard Amorphous Carbon

The fundamental understanding of sodium storage mechanisms in amorphous carbon is essential to develop high-performance anode materials for sodium-ion batteries. However, the intrinsic relation between the structure of amorphous carbon and Na⁺ storage remains to be debated due to the difficulty in controlling and characterizing the local atomic configurations of amorphous carbon. Here we report quantitative measurements of Na⁺ storage in a low-temperature dealloyed hard carbon with a tunable local structure from completely disordered micropores to gradually increased graphitic order domains. The structure-capacity-potential correlation not only verifies the disputing "adsorption-intercalation" mechanisms, i.e., Na⁺ intercalation into local graphitic domains for the low-voltage plateaus and adsorption in fully disordered carbon for the sloping voltage profiles, but also unveils a new mechanism of Na⁺ adsorption on defective sites of graphitic carbon in the medium-potential sloping region. The quantitative investigations provide essential insights into the reaction mechanisms of Na⁺ with amorphous carbon for designing advanced sodium-ion battery anodes.