Image: An ionic liquid tested for the application in lithium-ion batteries at Chalmers University of Technology Gothenburg and Uppsala University (Sweden).
Ionic liquids are basically salts with low melting points of under 100 degrees Celsius. Some might pause now because in school we are normally taught that salts have very high melting points. This is true for most – let us say – ”common” salts as for example table salt with the scientific name sodium chloride (NaCl). Sodium chloride has a melting point of 801 degrees Celsius which I guess can be considered a high melting point.
Salts consist of positive and negative ions that attract each other electrostatically which results in the formation of ionic bonds. This is how they form the compounds that we know as salts. Sodium chloride contains positive sodium ions and negative chloride ions. The strong attraction between the positive and negative charges causes the high melting points we observe in most salts.
Nevertheless, salts with low melting points and ionic liquids do exist. Why is that? The melting points of a salt are generally lower, the larger the ions are that it is made up from. For example, when changing the positive ion ( also called cation) in sodium chloride from sodium to the bigger rubidium ion, the salt rubidium chloride is obtained which has a melting point of 715 degrees Celsius. The melting point of rubidium chloride is 86 degrees Celsius lower than that of sodium chloride because the rubidium ion is bigger. When choosing even larger postive ions, for example the organic cation 1-ethyl-3-methyl-immidazolium, a salt called 1-ethyl-3-methyl-immidazolium chloride (a real tongue breaker) is obtained which has a melting point of only 89 degrees Celsius. This salt is an ionic liquid which melts under 100 degrees Celsius. We arrived at this ionic liquid simply by exchanging the positive ion (cation) with larger ones.
Also, the negative ions (also called anions) – the chloride ion in our case – can be exchanged with larger ones. When switching from the chloride ion to the organic anion bis(trifluoro methane sulfonyl) imide (another tongue breaker), the melting point can be lowered even more. When putting together the anion bis(trifluoro methane sulfonyl) imide with the cation 1-ethyl-3-methyl-immidazolium a salt with a melting point of -15 degrees Celsius is obtained. This salt (I will spare you the name) is a room temperature ionic liquid (RTIL). This is indeed a salt which is liquid at room temperature.
So, why do chemists make liquid salts? I admit a salt that melts at a lower temperature than water is pretty cool, but are they good for anything? To answer this question, we have to first look at the properties. The most important properties of ionic liquids are that they can conduct electricity due to their mobile ions and that they are stable even at high temperature which also means that they are non-flammable and cannot easily catch fire. Especially, their good high-temperature staibility makes ionic liquids interesting for many applications, for example as high-temperature lubricants in machines or as solvents for chemical reactions in the industry. Due to the conduction of electricity, ionic liquids are also tested in solar cells and batteries.
I have talked to Manfred Kerner, a PhD student in the Applied Physics Group at Chalmers University of Technology in Gothenburg (Sweden) who is working on the application of ionic liquids in batteries. Kerner is trying to use ionic liquids as the electrolyte (= the liquid placed between two battery electrodes) in lithium-ion batteries. One big problem lithium-ion batteries face is the high flammability of commonly used electrolytes which is a part of the reason for many youtoube videos showing laptops on fire. Ionic liquids can be a solution to this problem because they are stable at high temperatures and do not catch fire, while they are able to conduct electricity. These properties are especially useful for high-temperature lithium-ion batteries which run at temperatures higher than room temperature and are popular in hybrid electric vehicles.
Nevertheless, Kerner admits that there are still some hurdles to overcome until ionic liquids can be broadly applied in batteries. One example is their high cost. Another is that ionic liquids strongly attract water from air which damages the electrode materials of lithium-ion batteries. For this reason the use of controlled water-free environments such as gloveboxes is necessary. In addition, ionic liquids are not stable in contact with all the possible battery electrode materials which limits their use.
Despite these drawbacks some ionic liquids are already on the market as electrolytes for batteries according to Kerner. However, they are so far mainly limited to research applications. Nevertheless, some day in the future we might find ionic liquids in our own laptop or car batteries.