Interfacial properties between ionic-liquid-based electrolytes and lithium-ion-battery separator

For lithium salts, ionic liquids (ILs) are promising alternatives to conventional solvents in lithium-ion batteries (LIBs) due to a more favorable high-voltage operating window, and due to improved safety through reduction of flammability. Toward better understanding of wetting properties of IL-based electrolytes on a LIB separator, wetting properties affect electrochemical performance, experimental studies were made to determine the influence of solvent, lithium-salt type and salt concentration. Surface tensions and advancing contact angles were measured for two pure ILs ([C(4)C(1)im][BF4] and [C(4)C(1)im][OTf]) and for four IL/alkylcarbonate solvent blends (1:1 mass ratio, [C(4)C(1)im][BF4]/PC, [C(4)C(1)im][BF4]/DMC, [C(4)C(1)im][OTf]/PC, and [C(4)C(1)im][OTf]/ DMC) with several concentrations of a lithium salt (LiClO4, LiPF6, and LiTFSI). A significant improvement of wettability of pure ILs was observed by adding DMC, while adding PC with surface tension higher than that of pure ILs is detrimental to wetting behavior. Contact angles decrease by adding LiTFSI but show almost no change upon addition of LiPF6 or LiClO4. Surface tensions follow the same trend as that for contact angles. Incorporation of TFSI- anion gives favorable separator wettability. Estimates were made for interfacial properties of the separator (dispersive and polar components of the surface free energy for solid-vapor, for liquid-vapor, and for solid-liquid interfacial free energy).

Automated summary

Ionic liquids are potential alternatives to conventional solvents in lithium-ion batteries due to their high-voltage operating window and improved safety. Wetting properties of IL-based electrolytes on a LIB separator were studied, with findings showing improved wettability with the addition of DMC and detrimental effects with the addition of PC. The influence of different lithium salts on contact angles and surface tensions was also observed, with TFSI- anion contributing to favorable separator wettability.