Recent progress in the development and use of organic ionic plastic crystal electrolytes

Significant progress has been made recently in the development of Organic Ionic Plastic Crystals (OIPCs), a unique family of solid state electrolytes with applications in electrochemical devices such as lithium batteries and dye-sensitised solar cells. The negligible volatility of OIPCs renders them more suitable than molecular species for long-term device use, while the high thermal and electrochemical stability of many OIPCs fulfils an essential requirement for solid state electrolytes for many device applications. However, the complex mechanisms of conduction through these materials, both in their pure state and in the presence of a small amount of a second component (such as lithium salts to enable their use in lithium batteries) are still not fully understood. At the same time, the range of anions and cations utilised in the synthesis of plastic crystal phases continues to increase. This perspective concentrates on recent research into both fundamental and device-oriented aspects of these materials. Important fundamental understanding of the physical properties and transport mechanisms of different OIPCs has been achieved through use of techniques including variable temperature solid-state NMR and crystallographic analysis, as well as detailed molecular dynamics simulations. In parallel, the applicability of these materials as electrolytes for dye-sensitised solar cells and lithium batteries is being more widely demonstrated. The possibility of using OIPCs as solid state electrolytes for fuel cells is also discussed.