“But in practice, the solid-state electrolytes available up to now, mostly oxidic ceramics or compounds based on sulphur, have proven unable to completely meet expectations,” said Thomas Fässler, whose team at TUM is dedicated to looking for more efficient electrolytes. “The problem is that lithium ions only diffuse slowly through solid materials. Our objective was to better understand ion transport and then to use this knowledge to increase conductivity.”
The result of their efforts is a crystalline powder which is an above-average conductor of lithium ions. It contains no sulphur, but rather phosphorus, aluminum and a comparatively high proportion of lithium. Laboratory measurements have shown that this previously overlooked substance class has a high level of conductivity.
Within a very short period of time, the chemists successfully created about a dozen new, related compounds that contain, for example, silicon or tin instead of aluminum. This broad new material basis makes it possible to quickly optimize material properties.
To figure out why these elements are such good ion conductors, Fässler and his colleagues resorted to neutron beams to observe the processes that take place inside their crystals.
“The neutrons we have from the research reactor make it possible to find even the lightest of atoms. This is because the neutrons interact with the nuclei of the atoms and not with the atomic shell, as is the case with X-ray radiation,” Anatoliy Senyshyn, who supervises the powder diffractometer at the Research Neutron Source Heinz Maier-Leibnitz, which was used to analyze the new electrolyte material, said. “In the past, we had already investigated a variety of members of the new and diverse family of solid lithium ion conductors. We can use neutron diffraction to visualize how the ions use free space in the crystal lattice to move.”
In the new substance class, these free spaces are arranged in such a way that the ions can move equally well in all directions. This is a result of the high degree of symmetry found in the crystals and is probably the cause of the superionic lithium conductivity, which the TUM team has now been able to observe.
The synthesized powders are, thus, highly promising electrolyte candidates for future solid-state batteries. “Our basic research has the potential to accelerate the development of higher-performance batteries,” Fässler said.