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In order to move away from fossil fuels, batteries are one of the most important technologies to store energy from renewable sources. The rapid demands of battery applications put pressure on supply chains of raw materials, such as lithium, nickel, copper, aluminium and cobalt. There is a concern about the availability of such elements in the future. Sodium-ion batteries based on naturally abundant elements have become an attractive alternative to lithium-ion batteries due to their potential to reduce the cost and to improve the sustainability of batteries. A low electrochemical cycling stability of these Na-ion batteries can hinder long-term implementation in large-scale applications. It is necessary to understand what can lead to ageing and electrochemical cycling failure in sodium-ion batteries and how such detrimental side-reactions can be prevented. Compared to lithium-ion batteries, the research on sodium-ion batteries is not as mature yet.

This thesis work sheds light on the ageing mechanisms at the electrode/electrolyte interfaces and in the bulk of electrode materials with the help of a variety of spectroscopic and electrochemical methods. The electrochemical properties at the anode/electrolyte interface have been carefully investigated with different galvanostatic cycling protocols and x-ray photoelectron spectroscopy (XPS). The solid electrolyte interphase (SEI) in sodium-ion batteries is known to be inferior to its Li-analogue and hence, its long-term stability needs to be thoroughly investigated in order to improve it. Fundamental properties of the SEI in regards to formation, growth and dissolution are investigated on platinum and carbon black electrodes in different electrolyte systems. As well as the use of unconventional additives have proven to saturate the electrolyte and to mitigate SEI dissolution. This work shows one of the few studies highlighting SEI dissolution using electrochemical cycling tests coupled with pauses, in order to detect SEI ageing in batteries. Ageing mechanisms in manganese-based cathodes have also been studied due to the abundance of manganese and their electrochemical performance at high voltages with synchrotron-based XPS, x-ray absorption spectroscopy (XAS), resonant inelastic x-ray scattering (RIXS) and muon spin relaxation measurements coupled with electrochemical techniques. Surface-sensitive studies revealed how capacity losses stem from electrolyte degradation which results in a redox gradient between surface and bulk electrode. The work also shows how anionic redox contributions and incomplete phase transitions are reasons of additional capacity losses observed in manganese-based cathodes. Furthermore, it shows how a low Na-mobility is also an indicator for inferior long-term cycling properties leading capacity losses.