Inducing Hard Carbon-like Sodium Storage Behavior in Graphitizable Carbon via Post-Synthetic Introduction of Closed Porosity

Abstract

Hard carbon is commonly used as negative electrode in sodium-ion-batteries (SIBs). Another type of disordered carbon, soft carbon (SC, also known as graphitizable carbon), is considered as unsuitable for SIB applications, due to sodium storage at higher potentials and with lower capacities. However, SCs exhibit structural flexibility, enabling graphene rearrangements at higher temperatures. This property was utilized in the current study to introduce closed porosity in carbon black (CB) and to alter the nanostructure to achieve a beneficial sodium storage mechanism for higher energy densities. For that, CB is CO₂ activated at 900 °C and different holding times to generate different porosities. High-temperature treatment (HTT) at 1500 °C induces the graphitization process and closure of pore entrances. N₂ and CO₂ physisorption confirm the pore generation after activation and reduced porosity after HTT. X-ray diffraction, Raman spectroscopy, and transmission electron microscopy show no other structural alteration compared to nontreated CB. Galvanostatic charge-discharge experiments reveal an extension of the low-voltage plateau, showing a “hard carbon like” storage in correlation with the micropore volume introduced. These findings add to the knowledge of the sodium storage mechanism and showcase the possible need for revising the common carbon classification in the context of SIB research.