Structural changes during pyrolysis of petroleum pitch: in situ environmental TEM studies

The understanding of structural evolution of carbonaceous materials during pyrolysis is crucial for developing high performance hard carbon anode materials for sodium-ion batteries. Our study presents a detailed investigation of hard carbons synthesized from pre-oxidized petroleum pitch, and especially highlights the powerful role of in situ environmental TEM (ETEM) in elucidating structural evolution during pyrolysis. In situ ETEM experiments conducted under vacuum and nitrogen flow reveal distinct structural evolution pathways: vacuum condition promotes the formation of open graphene edges and aligned turbostratic domains, whereas nitrogen flow preserves a more disordered structure with abundant closed micropores. Electrochemical characterization demonstrates that these textural differences critically govern sodium storage behaviours: carbon prepared under vacuum exhibits lower plateau contribution on the galvanostatic profile and lower initial coulombic efficiency (ICE), while carbon prepared under nitrogen flow shows a longer plateau and improved ICE. By directly linking synthesis atmosphere, local microstructure, and electrochemical performance, this work establishes in situ ETEM as a powerful tool for guiding the rational design of hard carbon anodes for Na-ion batteries.