Invited Presentation: New Aspects of Ionic Liquid Based Electrolytes for Battery Applications

Reliable, safe and high performance electrolytes are a critical step in the advancement of high energy density secondary batteries such as lithium or sodium based devices.  Ionic liquids (ILs) and their solid state cousins, organic ionic plastic crystals (OIPCs), offer unique and attractive properties (such as low volatility and flammability, high electrochemical stability, and high conductivity) that make them important candidates for a number of battery applications.  ILs and OIPCs based on the Bis(trifluoromethanesulfonyl)amide (NTf₂or TFSI) and the Bis(fluorosulfonyl)imide (FSI) anions  coupled with pyrrolidinium or phosphonium cations have shown particularly advantageous properties in lithium metal cells, with respect to stability and reversibility.

Recently, FSI based electrolytes containing a high Li salt concentration (up to 3.2 mol.kg^-1) were shown to successfully cycle Li | LiCoO₂cells, with fast charging and discharging rate capability (up to 5 C rate at 25 °C), in spite of the high viscosity and relatively low conductivity of these solutions. This suggests that an alternative transport mechanism may be responsible for more rapid lithium transport in these highly concentrated electrolytes and further investigation of the physicochemical properties of these materials has been undertaken.  On the other hand, OIPC solid state electrolytes based on FSI have also shown remarkably high stability and cyclability in lithium metal batteries.  Phase behavior, dynamics and transport in these lithium containing OIPCs and ILs will be presented in addition to cell performance.

In recent years concerns about the sustainability of lithium supplies, and therefore its cost, have prompted researchers to consider other anodic metals such as sodium.  This paper will also present temperature and concentration dependence of structure, dynamics and electrochemical behaviour in these sodium ion containing IL electrolytes based on NTF₂ and FSI anions. For example cyclic voltammetry indicated a stable sodium cycling behavior with the current of 5 mA.cm^-2 at 25 ^oC to 20 mA.cm^-2 at 100 ^oC, with high reversibility in such an IL electrolyte.