Electrolytes in rechargeable batteries for cars, mobile phones, and other portable electric devices typically contain negatively charged counterions that together with lithium and a non-aqueous solvent form an ionic liquid. The bis(fluorosulfonyl)imide anion is a rather new ionic liquid constituent which thus need new quality control procedures.
This application uses a straightforward carbonate eluent to screen for contaminants and forced degradation products in a lithium bis(fluorosulfonyl)imide (LiFSI) electrolyte. The screening procedure did not include any sample preparation at this point, but a formalized quality control procedure would require either gradient elution or a sample preparation procedure to avoid that the FSI¯ anion is extensively retained on the analytical separation column.
Ion chromatographic analysis of lithium bis(fluorosulfonyl)imide (LiFSI) ionic liquid on a Shodex IC SI-50 4E column (250×4 mm) equipped with Shodex IC SI-50G (4.6×10 mm) guard column, using an eluent containing 3.2 mM Na₂CO₃ and 1.0 mM NaHCO₃ in water pumped at 0.75 mL/min at 24 °C. Background reduced to ~14 µS/cm by XAMS suppressor with ASUREX-A200 automatic regenerator. Eluent pumping and conductivity detection by Metrohm 761 Compact IC. Injection of 20 µL of LiFSI solid raw material dissolved in ultra-pure water and diluted 1:1000, giving signals up to 2.8 µS/cm. Qualitative identification of anions by separate standard injections of F¯, Cl¯, NO₂¯, HPO₄²¯, SO₃²¯, SO₄²¯ in water. HCO₃¯ eluted due to eluent ion displacement by the strongly retained FSI¯ ion.