Unlike lithium-ion batteries (LIBs), where crystalline graphite is commonly used as the negative electrode material, disordered carbons are regarded as more promising for sodium-ion batteries (SIBs). However, further advances towards better reversibility and higher specific capacity are still needed to match or even exceed the properties of graphite in LIBs. The main challenge is the complex and unpredictable Na-storage mechanism in disordered carbons. [1]
Recently, Matsukawa et al. have reported that the reversibility of (de)sodiation processes of ultra microporous carbons is higher compared to carbon materials with larger pore sizes.[2] Ultra micropores are accessible only to Na+-ions and not to solvent molecules. Therefore, ultra micropores can be used for Na-storage, however, do not significantly contribute to side reactions caused by solid-electrolyte interphase (SEI) formation, which reduces the related irreversible capacity loss.
Building on this concept, the origins of specific capacity and irreversible losses were further explored in the current work by modifying the chemical composition of zeolitic imidazolate framework (ZIF-8) derived carbons, while maintaining comparable porosity. This involved adjusting the nitrogen content as well as the types of nitrogen sites through temperature variation. Subsequently, these ZIF-8 derived carbons were modified with a protective ion-sieving carbon shell formed by chemical vapor deposition (CVD). This synthesis methodology enables effective distinction between reversible Na-storage and irreversible processes, e.g., SEI-formation. The former and later are mainly dictated by the properties of the ZIF-derived carbon core and the CVD based shell, respectively.
The structure and morphology of the materials were characterized by electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. In addition, different gas sorption techniques were carried out to analyze the porosity, pore sizes and specific surface areas. The electrochemical performance of the prepared materials was evaluated in coin cells vs. Na-metal.
The results of the tailor-made core-shell carbons reveal that a higher nitrogen content leads to a greater capacity in the sloping region, but to a lower capacity in the plateau region of the voltage profile. In addition, the overall capacity is higher for materials with lower nitrogen content. Lastly, it is important to highlight that the incorporation of ion-sieving carbon shell results in a significant overall increase in capacity compared to materials without a carbon shell. The highest reversible capacities (i.e., 381 +/- 4 mAh g–1) were obtained for the core-shell carbon comprising a ZIF-derived core pyrolyzed at 1000°C.
Apparently, the N-sites in the as-synthesized materials facilitate the adsorption of Na+ ions, indicating that Na+ ions preferentially adhere to these active sites during the sodiation process. It is worth noting that these results were observed while minimizing side reactions, making them more reliable and providing conclusive evidence for the long-debated Na-storage mechanism. Moreover, the lower capacity observed in materials with higher nitrogen content may be attributed to a lower electrical conductivity.
References:
[1] Nuria Tapia-Ruiz et al., J. Phys. Energy. 2021, 3, 031503.
[2] Y. Matsukawa, Beilstein J. Nanotechnol. 2020, 11, 1217–1229.