Technology University in Graz develops new electrolyte for solid state batteries

An international team working with TU Graz presented a new solid electrolyte for batteries.

The electrolyte exhibits one of the fastest ever measured lithium migration processes in a lithium-ion conductor.

Solid state batteries are currently the most promising technology to help electromobility break through. Batteries with solid electrolytes, in which lithium ions move between the electrodes, are thus the “holy grail” of solid-state battery research. The advantages of such systems over current lithium-ion batteries with liquid electrolytes are obvious: they have a higher energy density and are much safer due to their non-flammable components.

However, what was missing so far were suitable materials with a similar ionic conductivity as compared to liquid electrolytes. Together with colleagues from the Technical University of Munich and the Belgian Université Catholique de Louvain, researchers from Graz University of Technology have now demonstrated a promising crystalline ionic conductor with remarkably high lithium-ion mobility; the measured diffusion coefficients of which exceed the current top candidates for solid state electrolytes.

The new ionic conductor with the molecular formula LiTi 2 (PS 4 ) 3 is a lithium titanium thiophosphate, hence the abbreviation LTPS. LTPS shows an unusual crystal structure, which is characterised by so-called “geometric frustration”. In contrast to other ionic conductors, the crystal structure of LTPS offers no energetically favored residence for the ions. They are therefore never satisfied with their current place and are thus subject to a frustration. Calculations by the group led by Geoffroy Hautier of the Belgian Université Catholique de Louvain show that this frustration of the ions leads to a very high lithium mobility.

“The lithium ions are more or less desperately seeking a suitable place, moving very rapidly through the crystallographic structure of LTPS. It is precisely this high mobility of ions that we want in electrolyte bodies for solid-state batteries, “explained Martin Wilkening from the Institute of Chemical Technology of Materials at Graz University of Technology and Head of the Christian Doppler Laboratory for Lithium Batteries.

Wilkening's team was able to confirm this calculated high mobility measure of the ions experimentally with nuclear magnetic resonance methods. Martin Wilkening added: “We found clear indications of two jump processes that fully support the results of the calculations. In the structure of LTPS, the lithium ions can jump back and forth on circular paths as well as from one ring to the next. The last step, the inter-ring process, enables long-range ion transport. “

Even under cryogenic conditions, having extremely low temperatures, the intra-ring hopping processes of lithium ions could not be completely eliminated. The lithium ions are still mobile at 20 Kelvin (-253 degrees Celsius) on the sensitivity scale of nuclear magnetic resonance spectroscopy and are looking for the right potential well in the very flat energy landscape of LTPS.

Such behavior is extremely rare, according to Wilkening: “When the temperature drops, the ions are deprived of thermal energy and their mobility decreases significantly. It is noteworthy that in LTPS, even at such low temperatures, we find ion mobility. This shows how strong the urge to move ions in LTPS is.”

LTPS is a representative of a new class of solid electrolytes with this super-fast diffusion process, the cause of which is energetic frustration. Although these are crystalline, they have motion properties that are more similar to those of liquid electrolytes. The discovery and experimental investigation of LTPS is now the starting point for the search for further compounds in which a similar mechanism to conduct predominates.

The study was created in collaboration with Toyota. UCLouvain has filed a patent for the discovery of LTPS.