The objective of this project is to study the mechanisms of the insertion of Na+ and Mg2+ ions on a nanoscopic scale, and to compare it with the model ion Li+. The emphasis of the research at TUM is also on the materials system V2O5 that can be nanostructured in multiple ways, and has already demonstrated good Mg insertion capabilities. In the end of the project suitable materials including nanostructuring methods shall be identified and the mechanisms and kinetics of the insertion of Li+, Na+ and Mg2+ in these materials shall be fully characterized.
The cyclic voltammogram of a LixV2O5 cathode in a Li+-ion battery (Figure 1) shows many highly reversible phase transitions due to a restructuring of the crystal lattice occurring at different stages of lithiation / delithiation.
Li+-ion batteries are the state of the art battery technology, however the battery community tries to substitute it by other technologies in order to increase the energy density (LiO or LiS batteries), or to reduce the battery costs. The second can be achieved by using Na+-ions instead of Li+-ions. Vanadium oxides and derivatives from it, like V2O5 blended with 10 % TiO2 (Figure 2), show a reversible Na+-ion intercalation and hence can be used as cathodes in Na+-ion batteries.
Another way how to drop the costs and increase the energy density of a battery at the same time is to use Mg2+-ions. The benefit of magnesium is its large abundance in the earths crust and the double valency of its ions, which allows to store double the energy per ion compared to Li+- or Na+-ions. Again, V2O5 is a suitable cathode material to host Mg2+-ions. Figure 3, for instance, shows the reversible Mg2+-ion intercalation into the cathode.
Many different electrode materials with a widespread of morphologies can be synthesized. The left SEM micrograph (Figure 4) for instance shows hollow V2O5 micro beads. The right SEM image shows a nanostructered α-phase MnO2. Both materials show excellent performance in Li+- and Na+-ion batteries.
Commercial Li+-ion batteries use graphitic anodes, which can not only intercalate Li+-ions at very low potentials, but also forms a few nanometer thick surface layer by reducing the electrolyte. This surface layer is known as the Solid Electrolyte Interphase (SEI) and improves the performance, safety and lifetime. However, the formation mechanism, its composition and its appearance is still under debate. For that purpose, we investigate the SEI-formation on a molecular level, making use of a Scanning Tunneling Microscope (STM), operated in electrolytic environments. This allows to follow the SEI-formation in real time and in dependence of the electrode potential (Figure 5). On the other hand, it also gives insight into the morphology and the appearance of the SEI (Figure 6 and Figure 7), showing that there is an internal as well as an external component.