A patent-pending material developed at the Technical University of Munich (TUM; www.tum.de) has achieved the fastest-ever conductivity of lithium ions, and is now poised to significantly improve the efficiency of future solid-state batteries. Solid-state batteries, often seen as a more energy-dense and safer alternative to those that depend on liquid electrolytes, usually are comprised of mobile lithium cations and a rigid crystal lattice of complex anions. For all-solid-state batteries, faster lithium conductivity enables rapid charging capabilities and higher power densities.
Prof. Fässler in the lab (Source: TUM)
“Prevailing approaches for good lithium-ion conductors at room temperature looked to modify the complex anions using sulfur-based materials, and the material holding the last record [for fastest conduction] contains sulfur, lithium and five additional elements. Our design concept went back to exploring more simple compounds using only lithium and phosphorus or lithium and antimony,” explains Thomas Fässler, from TUM’s Chair of Inorganic Chemistry with a Focus on Novel Materials. Fässler’s research team found that the addition of a few scandium cations (Sc 3+) to replace selected lithium ions provided the conductivity breakthrough they were seeking, due to scandium’s three-fold positive charge superseding lithium’s one-fold positive charge — one replacement scandium ion essentially “kicks out” two more “bonus” lithium ions, better promoting mobility through the solid.
“Each introduced scandium ion forms two vacancies, or empty lattice sites, for lithium ions. These vacancies are used by the remaining lithium ions to migrate through the solids. The introduction of these vacancies significantly facilitates lithium-ion transport, thereby enhancing ionic conductivity. In summary, the material is more efficient while only containing one additional element (scandium) instead of five,” comments Fässler. The simplicity of the three-element material could also better facilitate battery recycling compared to those that contain more elements.
In the laboratory, 3-g synthetic batches of the new material have been produced using a conventional solid-state route via ball milling, followed by high-temperature treatment and rapid quenching. “For mass production, the process could be scaled up by increasing the capacity of the ball-milling equipment to produce larger quantities of the reactive mixture, which can then be thermally treated to obtain the desired material,” notes Fässler.
The next steps will be to build various cell configurations to validate cycling performance and electrochemical stability windows. From this, larger-format cells, such as battery pouches, will be created to better evaluate battery performance under conditions more analogous to real-world applications.