Ionic liquids refer to salts composed of organic cations and inorganic or organic anions in liquid state at or near room temperature, usually also known as room temperature ionic liquids. According to the type of cations, ionic liquids can be divided into quaternary ammonium salts, phosphonium salts, piperidinium salt, imidazole salts ionic liquids and others. Piperidinium salt ionic liquid is a kind of important environmental protection solvent, which has attracted extensive attention of scientists in recent years. There are many kinds of anions that can be chosen to constitute piperidinium salt ionic liquid, including F-, Cl-, Br-, I-, N(CN)2-, C(CN)3-, (CnF2n+1)3PF3-and others. The general formula of piperidinium salt ionic liquid is shown in Figure 1.
Figure 1. The general formula of piperidinium salt ionic liquid.
In the field of new energy, piperidinium salt ionic liquid is mainly used in the design of electrolyte. For example, the electrolyte of lithium ion battery widely used in the market is mainly composed of lithium hexafluorophosphate (LiPF6) and organic carbonate. However, LiPF6 is very sensitive to water. Even if there is trace water in the electrolyte, it can decompose and produce highly corrosive HF, which destroys the electrode structure and shortens the service life of the battery. Organic carbonate solvents are flammable, explosive and volatile, easy to cause battery safety accidents. Compared with other ionic liquids, piperidinium salt ionic liquids have lower viscosity and higher conductivity, which is the focus of the application of ionic liquids in lithium ion batteries. In addition, the performance of pure piperidinium salt ionic liquid is poor, so it is usually necessary to prepare a mixed electrolyte by combining it with other compounds. Depending on the application requirements, there are also different types of combinations. Furthermore, the practical application of piperidinium salt ionic liquid requires a comprehensive consideration of other components in lithium ion batteries, including anode materials, anode materials, diaphragm and electrolyte.
Figure 2. An example of piperidinium salt ionic liquid used as electrolyte.
At present, the synthesis of piperidinium salt ionic liquids mainly can be divided into direct synthesis and two-step synthesis. The direct synthesis method is one step synthesis of piperidinium salt ionic liquid through acid-base neutralization reaction or quaternary ammonium reaction. Although the method is simple to synthesize, the types of ionic liquids prepared by this method are limited. Therefore, most piperidinium salt ionic liquids must be synthesized by two-step method. In addition to the above two basic synthesis methods, other synthesis methods such as microwave technology, ultrasonic radiation technology and solvothermal technology are also used in the preparation of piperidinium salt ionic liquids, and the diversification of methods is conducive to shortening reaction time and improving efficiency.