Insights into the pre-oxidation process of phenolic resin-based hard carbon for sodium storage

Phenolic resin as a typical hard carbon precursor has attracted much attention towards sodium-ion batteries (SIBs) due to its high carbon yield and large reversible capacity. However, phenolic resin-based hard carbon commonly suffers from limited cycling performance and low initial Coulombic efficiency. Herein, we synthesized a hard carbon material from phenolic resin and manipulated its microstructure through tuning the pyrolysis temperature and pre-oxidation procedure. In particular, hard carbon materials with pre-oxidation deliver much improved capacity retention in contrast with those without pre-oxidation. Further analysis shows that with the pre-oxidation procedure, the enlarged d002 spacing, the much disordered microstructure, and the increased content of oxygen-containing functional groups are responsible for the performance enhancement of hard carbon. This study provides an in-depth understanding of the microstructure feature of phenolic resin-based hard carbon and opens up a feasible avenue for the design of high-performance carbon anode materials for their application in SIBs.