Improving the cyclability of sodium-ion cathodes by selection of electrolyte solvent

A composite material containing orthorhombic Na1.8FePO4F and carbon is prepared by mechanical activation and ceramic procedures. The material is studied in sodium test cells as a potential candidate for sodium-ion battery cathodes. The effect of the solvents in the electrolyte on the electrochemical performance is analysed by X-ray absorption spectroscopy. The structural changes on cycling are small, while the changes in the oxidation state of iron agree with the sodium insertion–extraction processes. The oxidation state is especially affected by the upper limit of the voltage window, and the discharge capacity is strongly affected when using propylene carbonate solvent. Capacity and capacity retention are higher for sodium cells using mixtures of ethylene carbonate and diethyl carbonate as the solvent of NaPF6 electrolytes.