Ionic liquids are substances consisting of single anions and cations that are liquid at or near room temperature. In general, the cation is larger in volume than the anion. When the cation is sulfonium, it is called sulfonium salt ionic liquid. As a kind of green material, sulfonium salt ionic liquid has many outstanding advantages. First, sulfonium salt ionic liquid is odorless and less irritating to the human body. Moreover, sulfonium salt ionic liquid is not flammable and it has the advantage of high safety. Finally, sulfonium salt ionic liquid has the characteristic of easy recycling and is very friendly to the environment. Therefore, in recent years, sulfonium salt ionic liquid has gradually attracted the attention of researchers at home and abroad.
Figure 1. An example of sulfonium salt ionic liquid synthesis process.
Sulfonium salt ionic liquid is mainly used as a new safe electrolyte material to replace the traditional electrolyte material in the field of energy storage and battery. It can be applied in lithium ion secondary battery, solar cell, capacitor and other energy storage or battery devices. In order for sulfonium salt ionic liquid to be used as electrolyte, the core requirement is to reduce the viscosity of the substance. To solve this problem, perfluoroalkyl borate anion is usually chosen as anion. This type sulfonium salt ionic liquid has the characteristics of low viscosity, low melting point and high electrochemical stability, so it is very suitable to be used as electrolyte. For example, a research team designed sixteen new sulfonium salt ionic liquids and a large part of them show properties that can be used as an electrolyte. In addition, these ionic liquids were synthesized in two steps. First, a series of trialkyl sulfonium salt were designed using sulfide as raw material and then the new sulfonium salt ionic liquids were prepared by ion exchange reaction of the synthesized trialkyl sulfonium salt and potassium salt of perfluoroalkyl borate.