Lithium is a very reactive material and therefore generates a relatively high voltage. Thus, the alkali metal is well suited for batteries. On the other hand, this very reactivity also poses a danger: The batteries must be completely airtight so that the material does not catch fire. In the past, various incidents involving Tesla electric vehicles have shown how easily this can happen. And some airlines do not allow certain mobile phones to be taken along because of fire hazards. Nevertheless, for small portable applications, lithium-ion batteries are still the first choice today,” says chemistry professor Peter Strasser from the Berlin Technical University, “but the safety risks posed by lithium-ion batteries in large battery storage units, as we need them for a shift towards renewable energies, make their long-term use an enormous challenge.”
For this reason, scientists have been working for some time on alternatives based on magnesium or aluminium. These metals can be stored in the air more safely – but this greater safety is paid for with lower voltage. In return, these ions provide not just one, but two or three positive charges like lithium, and therefore allow much denser storage of electrical charge, which is beneficial for large, compact battery storage devices.
The problem: Up to now, the two- and trivalent charged ions could not be stored in a host material (electrode material) in such a way that they could be reversibly exchanged between the electrodes. Strasser’s team colleague Dr. Toshinari Koketsu has now succeeded in reversibly storing these ions in a chemically modified form of the white color pigment titanium oxide. The titanium oxide was doped before with fluoride ions by cooperation partners at Sorbonne University in Paris. As a result, fluoride ions in the grid structure of the titanium oxide replace part of the oxygen ions, emitting some of the positively charged titanium ions and thus producing a kind of ‘hole’ or defect in the grid. “It is evident that these defects are ideal storage sites for positively charged magnesium or aluminum ions,” Strasser explains.
In several test series, the scientists have been able to prove for the first time that the reversible storage of aluminum and magnesium ions functions stable over several hundred cycles and shows high charge capacities. “This shows that fluoride-doped oxide materials with special defects can indeed provide a fundamentally new battery chemistry with magnesium and aluminum ions, which will be of fundamental and practical importance”, Strasser said.
The findings of the Berlin research team could eventually yield a battery technology not for tomorrow, but for the day after tomorrow: “We will continue to use different types of batteries in the future. At the moment, lithium-ion batteries are the cheapest and best method for many applications. At the same time, scientists are working on lithium-sulphur batteries, which are being followed with interest by the automotive industry. The aluminum/magnesium ion battery is more of a technology of the day after tomorrow, for applications that focus on safety, for example”, Strasser explains his perspective.
Published in: Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO2. Toshinari Koketsu, Jiwei Ma, Benjamin J. Morgan, Monique Body, Christophe Legein, Walid Dachraoui, Mattia Giannini, Arnaud Demortière, Mathieu Salanne, François Dardoize, Henri Groult, Olaf J. Borkiewicz, Karena W. Chapman, Peter Strasser & Damien Dambournet; Nature Materials (2017), DOI: 10.1038/nmat4976