Electrolytes Based on Mixed Ionic Liquid-Carbonate Solvents for Silicon Anodes in Li-Ion Batteries

In this work, mixtures of ionic liquid pyrrolidinium bis(fluorosulfonyl)imide (PYR 13 FSI) and the carbonate solvents dimethyl carbonate (DMC) and fluoroethylene carbonate (FEC) with lithium bis(fluorosulfonyl)imide (LiFSI) salt have been evaluated as electrolytes for anodes made from micron-sized metallurgical grade silicon. The electrolytes were characterized with respect to thermal and transport properties. Electrochemical performance was assessed in pseudo-full cells with a silicon anode and a commercial oversized LiFePO 4 cathode. They were then compared to electrolytes made with only PYR 13 FSI or carbonates as solvents. The addition of DMC and FEC to the PYR 13 FSI resulted in a minor increase in the glass transition temperature, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> </mml:math> . No change of state was observed for the mixed electrolytes in the differential scanning calorimetry traces up to 100 °C, showing that a wide temperature window was maintained. The conductivity of the mixed electrolytes was slightly improved compared to that of the pure LiFSI:PYR 13 FSI. The best rate performance was obtained with the electrolyte containing both DMC and FEC, which was attributed to the superior solid electrolyte interphase-forming properties of this electrolyte. Cycling within a limited voltage window of 2.4–3.4 V with a limited initial capacity was necessary to attain satisfactory cycling stability.