Image credit: Ronnie Mogensen, 2017. The materials ”Prussian white” (left) and ”Prussian blue” (right) are studied for the use in new battery technologies. CC BY-SA IGO 3.0.
With the dawn of electric vehicles and plummeting costs for renewable energies, battery technology is playing an ever more important role in our lives. Today lithium-ion batteries (LIBs) are being used in electric vehicles and for stationary power storage of renewable energy, for example in the Tesla powerwall. They can also be found in portable electronics such as laptops and mobile phones.
The problem is that lithium is a rather rare element with an abundance of only 0.0017 % in the Earth crust, most of which is found in South America. This inherent shortage combined with the high demand have lead to an increase of the lithium price by 14 % in 2016 with the price surges most likely continuing in the future. It is even doubtful if Earth has enough lithium resources to support the transformation from the fossil fuel-based society we are today to one depending mainly on renewable energy.
I have talked to two scientists who are trying to tackle this problem, Assistant Professor Dr. Reza Younesi and PhD student Ronnie Mogensen from the Ångström Advanced Battery Center (Uppsala University, Sweden). They are working on new kinds of batteries called sodium-ion batteries (SIBs) and magnesium-ion batteries (MIBs). These batteries rely on the use of sodium or magnesium instead of lithium, but work based on the same ”rocking-chair-principle”. In these batteries, metal-ions travel between two different electrodes during charge and discharge. The main difference is the nature of the travelling metal-ions, i.e. sodium-ions are used in SIBs, magnesium-ions in MIBs and lithium-ions in LIBs.
The huge advantage of these new batteries is, according to Younesi, the high abundance of sodium (2.3 % in Earth crust, atomic number 11) and magnesium (2.9 % in Earth crust, atomic number 12) as compared to lithium (0.0017 % in Earth crust, atomic number 3). In addition, large amounts of sodium can be found dissolved in sea water. Their good abundance enables the production of cheaper batteries from sodium and magnesium.
Nevertheless, Younesi admits that these battery technologies also have some drawbacks. Both sodium and magnesium are for example heavier than lithium, which is indicated by their higher atomic numbers. This leads to an overall lower energy density of these materials in batteries. This means that sodium-ion and magnesium-ion batteries need to be much heavier than lithium-ion batteries in order to store the same amount of energy.
Another challenge is the search for new electrode materials as not all state-of-the-art-materials used in lithium-ion batteries work for sodium- or magnesium-based systems. In addition, electrodes in lithium-ion batteries usually contain other scarce, toxic and expensive elements like cobalt, nickel or manganese. But, Younesi says, in order to truly produce cheap batteries, even the electrodess have to be made from abundant and inexpensive raw materials. For this reason one focus of Younesis and Mogensens research are iron- and carbon-based electrodes for sodium-ion batteries.
The most promising material they are studying at the moment together with researcher Dr. William Brant is called ”Prussian white”. It is derived from ”Prussian blue”, a compound that you might remember from high school chemistry lessons for its bright blue colour. It can be seen to the right in the image above. Prussian blue is a salt containing potassium, iron, carbon and nitrogen. In Prussian white the potassium has been replaced with sodium making it suitable for the use in sodium-ion batteries. It also gives the material a more ”whitish” colour as seen to the left in the photograph above. According to Mogensen, the beauty of this compound is that it only consists of very abundant elements (i.e. sodium, iron, carbon and nitrogen) and that it is non-toxic.
In the future, Younesi sees great potential for sodium- and magnesium-ion batteries to replace lithium-ion batteries in stationary applications such as the energy storage in solar and wind power plants or ”home wall” batteries. The reason for this is that the higher weight of sodium and magnesium is less important in these applications than in portable electronics or cars.
In fact, sodium-ion batteries have already been commercialized in other fields. The battery company Faradion based in Sheffield (UK) produces sodium-ion batteries, e.g. for electric bicycles. Other steps have been taken in France by the Centre Nationale de la Recherche Scientifique (CNRS) where a sodium-ion battery prototype has been built that can fit and run in a conventional laptop. Also, Younesi and Mogensen in Sweden are not missing out on this rapid development. They are working towards the commercialization of their Prussian white material in sodium-ion batteries. With such a high innovation speed, it seems only a matter of time until we will see a wide-spread utilization of these new battery technologies.