Na-ion batteries (NIBs) have recently attracted extensive attention from the scientific community, as a cost effective and environmentally friendly alternative to Li-ion batteries (LIBs). Meanwhile, they also share the same working principle with LIBs, which makes the manufacturing of NIBs easy to scale-up to large volumes using the well-established LIBs production knowledge. However, the remaining challenges for NIBs commercialization are the lack of suitable materials for the negative electrode and a realistic focus on exploring electrolytes that bring these NIBs to market. Unfortunately, graphite, which is the most commonly used anode material in LIBs, shows poor electrochemical performance in NIBs. Hard carbon (non-graphizatible carbon), due to its relatively low cost and good electrochemical performance, is the most promising anode material for NIBs. Due to the structural complexity of hard carbons, understanding the correlation between carbon structure and the Na-ion storage mechanism is still a challenge to be overcome for commercialization of hard carbons as anodes in NIBs. There have been reports regarding the Na-ion storage mechanism in hard carbon anodes in NIBs, however discrepancies still exist.
Herein, this thesis addresses two main topics. The first focuses on understanding the structure-performance correlations of Na-ion in hard carbons and improving the performance of hard carbon anodes. For this purpose, a series of hard carbons with tuned structure was synthesized by changing the carbonization temperature to vary systematically the porosity, number of defects, and graphitic structure. The microstructure of hard carbon and sodium storage behavior was demonstrated with X-ray and neutron scattering, Raman spectroscopy, transmission electron microscopy (TEM) and corroborated with density functional theory (DFT) calculations. Additionally, in-situ electrochemical dilatometry was also used to examine electrode expansion during cycling; to our knowledge, this is the first time that this procedure has been extended to sodium ion storage system investigations in hard carbons. Combined experimental studies and theoretical calculations reveal that it is the Na-ion storage at defect sites and intercalation in expanded graphene layers that corresponds to the slope capacity, while pore filling is responsible for the low voltage plateau region. It shows that hierarchically structured porous materials with partially large closed (internal) pores could act as active site for electrochemical storage and lead to reversible redox capacity. Therefore, in this thesis, a simple pore forming technique known as soft templating method is applied to synthesize mesoporous carbon with large closed pores to investigate the effect of closed mesopores and the results are compared with the microporous hard carbons. Here, the electrochemical these
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findings are of interest to the battery community, as they are important for the design of future electrode materials with high capacity and therefore building better batteries.
In order to develop high performance anode electrodes, a more integrated and holistic approach to the cell components is required. It should be noted that the performance of anode materials is influenced by the application of electrolytes and counter electrode; therefore, all components need to be investigated simultaneously. It is of crucial importance to understand and eventually control electrode interface and electrolyte chemistry, because the choice of electrolyte and the chemistry of the electrode interface has an important role on the energy density, cycle life, storage performance and reversible capacity of active materials. The second focus topic of this thesis is to study the surface chemistry of the anode electrode and electrolyte interface, which is another key consideration for the development of anode materials. The most commonly used Na salts, namely NaClO4 and NaPF6, were compared in EC: DMC and diglyme solvents. The vital role of the salt anion in the process of electrolyte decomposition at the sodium metal were further demonstrated. Comparison of half-cell configuration with sodium metal and full cell with NVPF/C cathode showed that employing metallic sodium anodes has critical consequences for the long-term cycling performance of batteries.