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The ability to store energy will be critical for achieving a functioning electrified society largely based on renewable energy sources. Batteries are anticipated to be a vital part of the infrastructure required to facilitate this energy storage. Electrolyte solutions are an essential component of most batteries, including sodium-ion batteries, which are emerging as a potentially more sustainable alternative to lithium-ion batteries.

This thesis critically assesses the use of sodium hexafluorophosphate as an electrolyte in sodium-ion batteries. Although widely used in lithium-ion batteries, the suitability of hexafluorophosphate for sodium-ion batteries needs re-evaluation. In this thesis, properties meriting the use of sodium hexafluorophosphate are explored, including its solubility in different organic solvents, conductivity, ability to prevent anodic aluminium dissolution, and cycling performance in battery cells. Sodium bis(oxalato)borate is investigated as an example of a fluorine-free alternative that may better align with the goal of increasing the sustainability of contemporary batteries. The main drawback of sodium bis(oxalato)borate is its significantly lower solubility compared to sodium hexafluorophosphate. However, at the same concentration in a given solvent, both electrolytes exhibit similar conductivities, challenging the notion that hexafluorophosphate enhances conductivity through low ion association.

Both electrolytes also prevent anodic aluminium dissolution. However, the use of sodium hexafluorophosphate does not consistently ensure adequate passivation of the negative electrode, suggesting that solvents or additives are more central for this process in these systems. In contrast, sodium bis(oxalato)borate appear to significantly contribute to the passivation of the negative electrode, even when used as an additive. As a sole electrolyte, sodium bis(oxalato)borate enable promising cycling performance in both lab-scale cells and in cells close to commercial standards. This research indicates that sodium hexafluorophosphate can be replaced with a fluorine-free electrolyte without compromising battery performance. The findings highlight the potential for more sustainable sodium-ion batteries and represent a step towards reducing the environmental impact of an electrified society.