Ionic liquids are substances that are completely composed of anions and cations and they are liquid state at room temperature. The non-volatilization and non-combustion of ionic liquids provide a new choice for improving the safety performance of batteries. In addition, ionic liquids are also known as designable green solvents, which can be synthesized by adjusting anions and cations with special properties. Furthermore, the environment-friendly characteristics of ionic liquids also promote the research and development of them as new materials for batteries. With the demand for high capacity, high power, high safety and green performance of new type batteries, the research and development of high-performance electrolyte materials has become the focus, and ionic liquids as a new generation of electrolyte materials have gradually attracted attention in the battery field.
- Lithium secondary battery: At present, ionic liquids used in lithium secondary batteries mainly include imidazole ionic liquids, quaternary ammonium ionic liquids, pyrazole ionic liquids, piperidine ionic liquids and others. Imidazole ionic liquids are the most studied, which has low viscosity and high conductivity and they can meet the requirements of lithium ion battery electrolytes. Currently, imidazole-hexafluorophosphates, 1-ethyl-3-methyl-imidazole-tetrafluoroborate salts and 1-ethyl-3-methyl-imidazole-bis(trifluoromethyl sulfonyl) imide salts are the mainstream. Quaternary ammonium ionic liquids usually need to be modified by introducing cyanide group into their chemical structure. Pyrazole ionic liquids have good electrochemical properties and can be used in lithium secondary batteries without modification. Moreover, piperidine ionic liquids have the advantages of low melting point, large electrical conductivity and wide electrochemical window, so they are suitable to be used as electrolytes for high potential lithium ion batteries.
Figure 1. An example of ionic liquid applied in lithium secondary battery.
- Electrochemical capacitors: Electrochemical capacitors have high specific power, excellent cycle life, and can usually be charged and discharged instantaneously with large current. In order to realize the application of ionic liquids in electrochemical capacitors, it is crucial to select suitable electrode materials to match ionic liquids with excellent physical and chemical properties. For example, a research team applied 1-methyl-3-butyl-imidazole-tetrafluoroborate ionic liquid to an electrochemical capacitor composed of porous activated carbon, whose specific capacity reached to 111F/g and specific energy were 4.1Wh/Kg, achieving the standard of industrial super-capacitors.
Figure 2. An example of ionic liquid applied in electrochemical capacitor.
- Proton exchange membrane fuel cell: Proton exchange membrane fuel cell not only has the general characteristics of fuel cell, but also has the outstanding characteristics of long life, high specific power and energy. Generally, the electrolyte of proton exchange membrane fuel cell is fluoresulfonic acid solid polymer, which is unstable at high temperature, which restricts the application scale of proton exchange membrane fuel cell. Ionic liquids have conductivity comparable to that of aqueous solution. This type of ionic liquids is promising to solve the existing problems of proton exchange membrane fuel cells.
- Sudha J. Devaki and Renjith Sasi (February 22nd 2017). Ionic liquids/ionic liquid crystals for safe and sustainable energy storage systems, progress and developments in ionic liquids, scott handy, IntechOpen, DOI: 10.5772/65888.
- N Xu, Klein J M, Huang P, et al. Improved accessibility of porous carbon electrodes with surfactant ionic liquids for supercapacitors[J]. Journal of Applied Electrochemistry, 2019, 49(2):151-162.