Sodium-ion batteries (NIBs) are an attractive alternative to lithium-ion batteries (LIBs) due to their lower environmental impact, greater safety, and competitive energy density. A key factor in improving these batteries is the choice of negative electrode material, which significantly affects performance, stability, and the formation of the solid electrolyte interphase (SEI). Hard or non-graphitized carbons have become the main candidates for negative electrode materials for NIBs. In recent research, the process of Na$^+$ ion insertion into/from non-graphitized carbons has attracted a lot of attention. As this is a relatively new technology compared to LIBs, the exact mechanism of action and degradation pathways of NIBs remain unclear. In my doctoral thesis, I am investigating the mechanisms of sodium storage in non-graphitized carbon negative electrodes using advanced characterization techniques. In addition to operando, in-situ, and ex-situ nuclear magnetic resonance (NMR) spectroscopy in the solid-state, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements were also used. These methods provided insights into the sodium structure, dynamics, and composition of SEI. Key findings include the effect of carbonization temperature on sodium storage. At higher temperatures, larger pores facilitate the formation of quasi-metallic sodium clusters, which improves sodium storage capacity. In addition, it was found that the SEI consists of sodium fluoride (NaF) formed on the surface of the non-graphitized carbon, while sodium carbonate (Na$_2$CO$_3$) forms within the layers. A comprehensive mechanism for the behaviour of Na$^+$ ions in non-graphitized carbon was proposed: during initial sodiation, Na$^+$ ions are adsorbed on the surface of non-graphitized carbon and inserted between graphene layers, then fill the pore walls, and finally, if the pores are large enough, quasi-metallic Na clusters are formed. During desodiation, this process is reversed. In the case of underpotential sodiation, metal Na nucleates on the surface of the negative electrode, causing metal Na plating. The chemical environments of the plated Na and Na dendrites differ due to their different structural and chemical properties and can be distinguished by solid-state NMR. I tested the negative electrode made of non-graphitized carbon together with the positive electrode made of Na$_3$V$_2$(PO$_4$)$_2$F$_3$ using operando NMR in a full cell, thus demonstrating the battery's performance under real conditions. Using ex-situ NMR, I further investigated the effect of FEC addition in the electrolyte on SEI formation and stabilization. Finally, using ex-situ and operando NMR, I investigated the effect of heteroatom decoration of non-graphitized carbon with nanoparticles and the impact of the use of different precursors on the performance and degradation mechanisms of the battery.
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