Ammonium salt ionic liquid is a salt completely composed of anions and cations that is liquid at or near room temperature, also known as a low temperature molten salt. Ammonium salt ionic liquid has a wide range of liquid temperature, good physical and chemical stability, excellent solubility, great polarity adjustment and near zero toxicity, so it has a broad application prospect. In the chemical structure, all four hydrogen atoms in the ammonium ion are replaced by hydrocarbon groups, whose general formula is R4NA, in which four hydrocarbon groups R can be the same or different. In addition, A is mostly halogen anion, including F-, Cl-, Br-, I-. Similar to inorganic salts, ammonium salt ionic liquids are easily soluble in water and can conduct electricity in aqueous solution.
Figure 1. The general formula of ammonium salt ionic liquid.
- Traditional synthesis methods: Traditional synthesis methods include one-step synthesis and two-step synthesis. The one-step synthesis method is to synthesize ammonium salt ionic liquid in one step through acid-base neutralization reaction or quaternary ammonium reaction without adding any solvent as the reaction medium. The separation and purification of the ionic liquid requires organic solvent, and the organic solvent is easy to recycle and use. Furthermore, water can be used as solvent for separation and purification of hydrophobic ionic liquids. For ammonium salt ionic liquids which are difficult to be prepared by one-step synthesis method, two-step synthesis method can be used.
Figure 2. An example of ammonium salt ionic liquid synthesis.
- Novel synthesis method: The novel synthesis methods include ultrasonic assisted synthesis and microwave assisted synthesis. Ultrasonic technology refers to the biochemical method of using sound waves with frequencies higher than the range of the human ear can hear. Ultrasonic wave can reduce the size of particles suspended in liquid and increase the rate of heterogeneous reaction. Ultrasonic radiation method has the advantages of simple operation, short reaction time, mild condition, less side reaction and high yield. Microwave is a kind of strong electromagnetic wave, and high energy atoms, molecules and ions can be produced under microwave irradiation, which cannot be obtained by thermodynamic methods. In addition, this method can rapidly increase the concentration of free radicals or carbon cations in the reaction system. From the perspective of energy, it is possible to increase the reaction rate and shorten the reaction time as long as the energy of reactant molecules can be instantly increased and the number of activated molecules in the system increased. For example, a novel perfluoroalkyl-containing ammonium salt ionic liquid was synthesized from N-methyldiethanolamine and perfluorinated hexyl ethyl iodide by microwave assisted synthesis. Moreover, under the optimal conditions, the reaction time was only 10.42% of the conventional heating time.
Ammonium salt ionic liquids, as alternative energy materials, have been widely used in batteries, such as lithium iron phosphate batteries, lithium iron manganese phosphate batteries, nickel cobalt manganese ternary batteries and others. For example, trimethylethyl ammonium ionic liquid can be used as an electrolyte in lithium iron phosphate batteries. The experimental results show that the migration rate of lithium ions can be significantly improved by controlling the battery operating temperature. Moreover, no obvious side reactions occur, so as to improve the cyclic properties of the battery and the compatibility of the ionic liquid with the electrode materials.
- Pernak, Juliusz & Łęgosz, Bartosz & Walkiewicz, Filip & Klejdysz, Tomasz & Borkowski, Andrzej & Chrzanowski, Lukasz. (2015). Ammonium Ionic Liquids with Anions of Natural Origin. RSC Adv.. 5. 10.1039/C5RA11710K.