Green ionic liquids for next-generation thermal energy storage

Biomass-derived ionic liquids are promising compounds which can transform thermal energy storage (TES) by coupling high-performance phase change behaviour with renewable origins. Based on a comprehensive review of more than 120 key studies published between 2002 and 2026, this review targets Green Chemistry agenda by moving beyond generic claims of “green” ionic liquids and systematically linking precursor origin, synthetic routes, and atom economy to thermophysical performance in thermal, waste-heat recovery, and building energy storage systems. Recent advances in sugar-, lignin-, amino acid-, cholinium- and fatty acid-based ionic liquids are reported, showing how rational cation–anion design can tune melting point, latent heat, heat capacity, viscosity, and cycling stability to compete or outperform conventional molten salts and organic phase change materials. At the same time, it is discussed that sustainability is not intrinsic to the final ionic liquid: since life-cycle burdens, side contaminations and processes, fluorinated anions, and energy demands can fake environmental benefits if not quantified. By including cradle-to-grave considerations and bio-origin-driven classification with structure–property trends, this review provides design guidelines for ionic liquids that are not only advanced TES media but also aligned with circularity and climate mitigation goals.