Sodium Ion Batteries (SIBs) are expected to be promising alternatives to the well established but cost-intensive Lithium Ion Batteries (LIBs), which eventually have to be replaced due to depleting lithium reserves. However, various extensively examined SIB systems still suffer from poor cycling stability, which is mainly due to unwanted decomposition of the conventionally applied organic carbonate based electrolytes. Furthermore, these electrolytes exhibit high volatility and flammability and therefore pose a serious safety issue. Featuring negligible vapor pressure combined with non-flammability on the one hand and wide electrochemical stability windows (ESWs) on the other hand, Ionic Liquids (ILs) represent promising alternatives to overcome these drawbacks. To comparably study the effect of the nature of the IL (cationic core structure, anion, presence of functional groups) on important parameters like thermal stability, viscosity, conductivity and ESW, a broad library of various ILs was synthesized and uniformly characterized. All ILs show significantly higher thermal stability compared to conventional organic electrolytes and enhanced ESWs. Viscosity and conductivity were found to be very susceptible towards structural changes in the ILs, covering broad ranges in both cases. Finally, the electrochemical performance of selected ILs tested in sodium half cells with Na0.6Co0.1Mn0.9O2+z as cathode material show clearly enhanced cycling stabilities compared to organic carbonates. This uniform investigation enables the comparison of different IL based systems and a rational design of ILs, being suitable alternatives for conventional electrolytes, paving the way to powerful SIBs.

Next generation electrolytes for safer sodium ion batteries

256th National Meeting (2018)

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