Ternary mixtures of N-butyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and carbonate as advanced electrolytes for lithium-ion batteries

The electrochemical energy storage plays a crucial role towards the decarbonisation of the electricity sector by improving the integration and exploitation of renewable energy sources.

Commercially available electrolytes for lithium-ion batteries are based on organic carbonates with lithium salts, offering good electrical conductivities although, as organic solvents are volatile and highly flammable, limiting the safety of these devices. During the last years smart new electrolytes based on mixtures of ionic liquids (ILs) with inorganic salts have been proposed with regards to their good electrical properties, good thermal and chemical stabilities and low toxicity.

In this work, thermophysical properties of the ternary liquid mixtures IL + lithium salt + carbonate were studied in terms of thermal transition temperatures, thermal stability and electrical conductivity of mixtures against pure compounds. The selected ionic liquid is the N-butyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide; the lithium salt with common anion, lithium bis(trifluoromethanesulfonyl)imide salt and two different carbonates (dimethyl carbonate or diethyl carbonate) were used to prepare the ternary mixtures.

The main results observed were the cryoscopic decrease in IL with the salt addition, a reduction of crystallinity of ternary mixtures with regards to pure IL and IL+salt samples and the electric conductivity of mixtures, which increase with increasing temperature, presented a maximum for x_carbonate=0.6 and decrease with the increasing chain length of carbonate.