Ionic liquid is also known as room temperature ionic liquid, which is an organic salt compound composed of cation and anion. Ionic liquids are characterized by inflammability, high thermal and chemical stability, low vapor pressure, high conductivity, wide electrochemical window, adjustability, excellent solubility and others. When the cation is phosphonium, this type ionic liquid is named as phosphonium salt ionic liquid. Compared with other types of ionic liquids, phosphonium salt ionic liquids have more advantages in toxicity and thermal stability. In addition, phosphonium salt ionic liquid is a functional substance that can be designed and modified by adding different functional groups on cation according to different requirements. Furthermore, phosphonium salt ionic liquids have great freedom of functionalization due to the large atomic radius of phosphorus atoms. For example, aromatic and heterocyclic substituents with large volume can be added into alkyl groups. Therefore, phosphonium salts ionic liquid is very suitable to be used as an alternative material in the field of energy storage and battery. The general molecular formula of phosphonium salt ionic liquid is shown in Figure 1.
Figure 1 The general molecular formula of phosphonium salt ionic liquid.
- Capacitor: Capacitor is a kind of energy storage device, and the insulating oil is an important constituent part. Capacitor insulation oil used in capacitor equipment plays the role of insulation and cooling, sometimes it also has the function of increasing capacitor capacitance. At present, the production of capacitor insulating oil is mainly achieved by catalytic Fourier alkylation of aromatic ring. For example, benzyl toluene capacitor insulating oil is obtained by Fourier alkylation of toluene with benzyl chlorine, with concentrated sulfuric acid as the catalyst. However, this method has to use a large amount of concentrated sulfuric acid and it cannot be recycled. In order to solve this problem, composite catalyst prepared by phosphonium salt ionic liquid can be used for Fouke alkylation reaction. In addition, this type composite catalyst could be recycled with simple phase separation, which avoids the environmental pollution of catalysts and can greatly reduce the treatment cost of waste liquid.
- Electrolytes of batteries: Lithium ion secondary battery has excellent battery characteristics such as high working voltage, high energy density and long cycle life. It is widely used in mobile phones, portable computers, video cameras and other portable electronic products. With the continuous expansion of the application field of lithium ion secondary battery, its safety performance is becoming more and more important. So far, the commercial electrolyte of lithium ion secondary battery mainly uses the mixture of organic solvent and lithium salt. At high temperatures, organic solvents tend to vaporize and cause safety problems. However, phosphonium salt ionic liquids can be used as the electrolyte of lithium ion secondary battery due to its high thermal and chemical stability, which can greatly improve the safety of the battery.