Magnesium as Cathode Enhances Performance of Rechargeable Batteries
Rechargeable batteries developed from magnesium, in place of lithium, possess the ability to increase the driving range of the electric vehicle by loading more energy into small-sized batteries. However, unexpected chemical obstacles have decelerated the scientific advancement.
The areas where solid comes into contact with liquid, places in which oppositely charged battery electrodes react with the enclosing chemical mixture called electrolyte, are the challenging spots.
Scientists from the U.S. Department of Energy’s Joint Center for Energy Storage Research, headed by researchers at Lawrence Berkeley National Laboratory (Berkeley Lab), have found out an surprising set of chemical reactions that involve magnesium. These reactions cause the degredation in the performance of the battery even before the battery is charged.
The findings could be relevant to other battery materials, and could steer the design of next-generation batteries toward workarounds that avoid these newly identified pitfalls.
The researchers adopted theoretical modeling, X-ray experiments, and supercomputer simulations to have an in-depth knowledge of the chemical composition of a liquid electrolyte located just tens of nanometers from an electrode surface that affects the performance of the battery. The outcomes of the study have been reported in the online publication of the Chemistry of Materials journal.
The battery being test contained magnesium metal, used as the negative electrode, or anode, which was in contact with a liquid electrolyte called diglyme, and a dissolved salt, that is, Mg(TFSI)2.
The combination of materials were thought to be well-suited and nonreactive during the resting state of the battery. However, experiments performed by using a synchrotron (an X-ray source) at Berkeley Lab’s Advanced Light Source (ALS) found that this wasn’t the case and took the research in a different direction.
“At that point it got very interesting,” stated Prendergast. “What could possibly cause these reactions between substances that are supposed to be stable under these conditions?”
Scientists at the Molecular Foundry created elaborate simulations of the place at which the electrode and electrolyte come into contact (i.e. the interface), suggesting that spontaneous chemical reactions must not take place even under suitable conditions. However, the simulations did not explain all the chemical details.
“Prior to our investigations,” stated Ethan Crumlin, an ALS scientist who was the co-lead author of the study with Prendergast and who coordinated the X-ray experiments, “there were suspicions about the behavior of these materials and possible connections to poor battery performance, but they hadn’t been confirmed in a working battery.”
In the case of commercially popular lithium-ion batteries which power various portable electronic devices (e.g. laptops, mobile phones, and power tools) and a broad range of electric vehicles. Lithium ions (or lithium atoms that get charged after losing an electron) are made to shuttle to and fro between the two electrodes of the battery. Such electrode materials are porous at the atomic level and are alternatively loaded with or emptied of lithium ions when the battery gets charged or discharged, respectively.
The negative electrode in such batteries is usually formed of carbon, with restricted potential to store the lithium ions when compared to other materials.
Therefore, when the density of stored lithium is increased by adopting a different material, the resulting batteries would be smaller, light-weight, and more powerful. For instance, if lithium metal is used in the electrode, more lithium ions are loaded in the same space. However, it is highly reactive and gets ignited during exposure to air, and mandates further studies to learn the best way to load and protect it to ensure long-term stability.
