Sodium Batteries: From Liquid to Polymer Electrolytes

Today lithium-ion batteries (LIBs) are dominating the field of rechargeable batteries for portable devices. More recently LIBs have also been considered for large-scale applications, such as in the areas of electromobility and grid storage. With the growing demand of lithium and its limited resources/production and price issues, it is advantageous to also consider other chemistries to complement/replace LIBs. Sodium, being abundant and similar to lithium, is a suitable alternative. In addition, batteries also need to meet stringent safety requirements. These requirements will influence the selection of suitable battery components (electrodes, electrolyte, etc.) with the overall aim to obtain more sustainable and safer battery, while guaranteeing adequate performance. The common electrolyte used in LIBs as well as the analogous sodium-ion batteries (SIBs) contains fluorinated salts dissolved in organic liquid solvents, with many intrinsic safety concerns. Appealing alternatives could be liquid electrolytes free of fluorine and organic liquids, or altogether solid electrolytes. In this thesis, an overview of liquid and solid electrolytes for sodium (-ion) batteries is given, outlining their differences. Aiming at safer batteries, mainly two broad classes of electrolytes are studied: fluorine-free electrolytes and solid polymer electrolytes. A known drawback of the latter is the low ionic conductivity at room temperature, which can be improved by e.g. adding an ionic liquid, a salt which is liquid at room temperature. All electrolytes are evaluated in terms of their ionic conductivities, ionic speciation and, for the polymer electrolytes, polymer dynamics and stability vs. sodium metal