Phosphonium ionic liquids have great promise because previous studies have shown that they have high electrochemical stability. Phosphonium ionic liquids, with the generic formula [PR4]+, have four substituents on the phosphonium cation. Variations in the substituents and available anions represent a large number of possible salts (Figure.1).
Figure 1. The structure of phosphonium ionic liquids
Phosphonium ionic liquids possess high thermal stability, while their C2 protons tend to make them slightly acidic, which can lead to carbene formation. Alkylphosphine salts are generally less dense than water, which may be beneficial for product post-treatment steps involving aqueous layers containing inorganic salt by-products. For these reasons, phosphonium ionic liquids are often used as phase transfer catalysts, electrochemical media, epoxy resin cured strippers for montmorillonite clay catalysts, and they are also used in high temperature polymer carbonate reactions.
Electrolyte: Phosphonium ionic liquids can be applied as battery electrolyte, solar cell electrolytes, and super-capacitor electrolytes. For example, a binary ionic liquid system consisting of P222(2O1)-TFSI and lithium salt exhibits favorable transport properties and high thermal stability as an electrolyte of a lithium secondary battery. A lithium battery cell containing a P222(2O1)-TFSI electrolyte was found to exhibit higher charge-discharge performance than a lithium battery containing other ammonium ionic liquid electrolyte due to the low viscosity and high conductivity of the phosphonium ionic liquid electrolyte.
Figure 2. The structure of P222(2O1) cation
Figure 3. Suzuki cross-coupling reaction using phosphonium ionic liquids as solvent
1. Tsunashima, K.; et al. (2008). “A lithium battery electrolyte based on a room-temperature phosphonium ionic liquid.” Chemistry letters 37(3), 314-315.
2. McNulty, J.; et al. (2002). “Suzuki cross-coupling reactions of aryl halides in phosphonium salt ionic liquid under mild conditions.” Chemical Communications (17), 1986-1987.