A review of imidazolium ionic liquid-based phase change materials for low and medium temperatures thermal energy storage and their applications

The application of ionic liquids (ILs) in the field of thermal energy storage is attracting increasing attention owing to their thermophysical properties, such as an adjustable phase change temperature, low flammability/volatility, and good thermal and chemical stability. A recent utilization was provided by the National Aeronautics and Space Administration (NASA), which employed eutectic phase change materials (PCMs) composed of functional ILs to manage the extreme space environment (solar radiation and extreme cold/hot) of crewed spacecraft for future deep exploration. While the concept of storing latent heat during the ILs' phase transition is not new, large-scale applications employing this concept have not yet realized their full potential. In addition, although a considerable amount of review has been published for traditional PCMs, the information on ILs and their application remain unsystematic; thus, benefits such as structural alterations to cations and anions for tunable chemical and phase properties are long-term neglected in the field of thermal energy storage. This review aims to provide the necessary information on the choice of well-studied ILs and promote further research in this field. This review first discusses the defects of traditional PCMs, followed by reviewing and summarizing the commonly used ILs in terms of their chemical structure, phase transition mechanisms, and thermophysical properties. Finally, the applications of ILs-based PCMs are introduced in detail, and existing problems, solutions, and future research directions are proposed.