What are electrolyte solvents?

Electrolyte solvents must enable high ionic conductivities of the resulting electrolyte. Therefore, they need to combine a high relative permittivity to dissociate proper amounts of Li-salt with a low viscosity. At the same time, electrolyte solvents should be inert to all cell components over a wide potential range, especially towards both composite electrodes. To fulfill safety requirements in commercial lithium-ion batteries, electrolyte solvents should be non-toxic and should have a wide temperature range of operation, in terms of the melting point, the boiling point and the flash point. Because a high relative permittivity and low viscosity commonly cannot be integrated into a single molecule, solvent mixtures are used. In the so called “mixed solvent approach”, lithium-ion battery state-of-the-art electrolytes usually comprise a mixture of cyclic carbonates, owing to their high relative permittivity and linear carbonates due to their low viscosity.