Phosphonium Ionic Liquids for Electrochemical Energy Storage: Structure–Property–Interphase Relationships and Design Rule

ABSTRACT

Phosphonium ionic liquids are emerging electrolyte platforms for safer and more durable electrochemical energy‐storage systems because they combine low volatility, reduced flammability, thermal robustness, broad electrochemical stability windows, and highly tunable cation–anion chemistry. Their performance emerges from coupled cation–anion structure, salt concentration, electrolyte formulation, solvation environment, and electrode chemistry. This Review develops a structure–property–interphase framework for phosphonium‐based electrolytes in lithium, sodium, zinc, magnesium, and aluminum batteries, as well as supercapacitors and quasi‐solid‐state systems. For phosphonium‐based electrolytes, we analyze how alkyl chain length, branching, ether functionalization, and anion identity govern viscosity, conductivity, ion coordination, metal plating/stripping, corrosion, gas evolution, and solid–electrolyte and cathode–electrolyte interphase formation. The analysis shows that smaller, asymmetric, branched, or ether‐functionalized cations generally promote faster ion transport, whereas long‐chain hydrophobic cations are better suited for thermally robust, low‐volatility, gel, or deep‐eutectic‐type formulations. Finally, we propose practical design rules emphasizing low‐viscosity cation design, fluorine‐reduced anion chemistry, standardized benchmarking, full‐cell validation, and surface‐sensitive/operando interphase characterization.