A fundamental understanding of structure evolution in the synthesis of hard carbon from coal tar pitch for high-performance sodium storage

Abstract

Coal tar pitches (CTPs) as byproducts of the coal chemical industry can be used to fabricate low-cost hard carbon anodes in sodium-ion batteries (SIBs) via pre-oxidation methods; however, an in-depth analysis of their synthesis processes is still scarce in literature. In this study, three typical isotropic CTPs (denoted as P1, P2, and P3) with different physicochemical properties (glass transition temperature, softening point, aromaticity, and others) were employed to fabricate amorphous carbons and a systematic investigation of the structural evolution in pre-oxidation and carbonization processes and the relationship between the microstructures of the CTP-derived carbons and their sodium storage behaviors was carried out for the first time. The results demonstrated that the air-blown pre-oxidation was diffusion dominated rather than reaction controlled, and the high glass transition temperature range of P3 provided convenient oxygen diffusion channels for sufficient oxidative stabilization. The significantly increased C–O/CO groups in pre-oxidized P3 (POP3) lead to coking and carbonization behaviors that were distinctly different from those of the other CTPs. The abundant oxygen cross-linking structures in POP3 prevented the condensation of aromatic hydrocarbons (below 500 °C) and the occurrence of a distinct liquid-phase transition, which resulted in a solid-state carbonization process and the formation of a more amorphous texture of the resulting carbon product. The P3-derived hard carbon manifested the best sodium-storage performance, especially the highest reversible capacity (276.8 mA h g⁻¹ at 0.1C), and a long cycling life of up to 500 cycles, while the products from P1 and P2 presented the typical electrochemical behaviors of soft carbons.