Development of Organic Ionic Plastic Crystals As Solid-State Electrolytes

Abstract

The advantages of ionic liquids as electrolytes for electrochemical applications such as lithium batteries and dye-sensitized solar cells are now well recognized. Organic Ionic Plastic Crystals (OIPCs) are crystalline phases found in many of the same organic salt families as ILs but these materials have elevated melting points and exhibit various forms of disorder, which is the origin of their plastic mechanical properties. These plastic crystal materials are showing increasing promise as solid state electrolytes for various electrochemical applications.[1] We have also recently discovered that this disorder allows the use of OIPC-based membranes for light gas separation.[2] OIPCs display one or more solid-solid phase transition before melting, which is associated with the onset of rotational or translational motions of the ions and thus a progressive transformation from an ordered crystalline phase to an increasingly disordered structure. This disorder is fundamentally responsible for the fast ion conduction of target ions, such as lithium or sodium, and core to their efficient performance as solid-state electrolytes. Key to the development of OIPCs is expanding the range of cations and anions available, and understanding the resulting structure and dynamics. Here we discuss our development of new OIPC-based materials for energy applications, in particular lithium metal batteries, and insights into the mechanism of ion transport using techniques such as variable temperature solid state NMR. The recent development of new OIPCs using the N,N-diethylpyrrolidinium ([C2epyr]) cation will be discussed, in addition to the use of very high lithium content [C2epyr][FSI] / Li[FSI] ionic liquids and OIPCs as battery electrolytes.