Engineering Materials for Electrochemical Energy Storage via Ionic Liquid and Deep Eutectic Solvent Synthesis Media

The development of high-performance electrode and solid electrolyte materials is crucial for the advancement of next-generation electrochemical energy storage systems. Among emerging synthesis strategies, ionic liquids (ILs) and deep eutectic solvents (DESs) have gained increasing attention as alternative reaction media due to their unique physicochemical properties, including high thermal stability, a wide electrochemical stability window, low vapor pressure, and tunable composition and polarity. These features offer unprecedented control over particle morphology, composition, and surface chemistry, enabling the formation of novel or metastable phases, as well as <i>in situ</i> surface functionalization or generation of homogeneous carbon coatings through postannealing treatments. Despite these promising attributes, the implementation of ILs and DESs at an industrial scale remains to date limited. Major challenges include high viscosity, recycling difficulties, high costs, and a lack of large-scale proofs of concept. After introducing ILs and DESs, and their specific properties, this review critically evaluates the potential and limitations of IL- and DES-based synthesis methods in comparison to conventional techniques such as solid-state and hydrothermal approaches. The benefits and impacts of these uncommon solvents on material morphology and functional properties are discussed along with a systematic comparison with the electrochemical performance of similar materials synthesized via classical methods. This review further discusses the prospects for industrial integration and highlights key areas where further research is essential. Finally, this review provides some perspectives that would allow for mastering these synthesis approaches and developing optimized materials for electrochemical energy storage.