Phosphonium Ionic Liquids for Electrochemical Energy Storage: Structure-Property-Interphase Relationships and Design...

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.