Empa, a research group at ETH Technical University in Zürich within the German Fraunhofer ICON R&D organization, has developed an alternative electrolyte for solid state EV batteries.
The results are based on a new lithium lanthanum zirconium oxide (LLZO) material replacing the conventional electrolytes of lithium-ion batteries. The advantages of LLZO are high ionic conductivity and chemical stability, ideal for batteries in mobile applications with frequent charging cycles.
“With an anode made of pure metallic lithium instead of graphite, we could store many times more energy in a cell of the same size,” says Kostiantyn Kravchyk, researcher at Empa. “However, the lithium is not stripped and deposited evenly when charging and discharging. This results in the formation of so-called dendrites: branched structures of metallic lithium that can short-circuit the battery.”
Formation of dendrites can be prevented by changing the electrolyte form from liquid to solid. The bilayer LLZO membrane consists of a dense and a porous layer. If lithium is stored in the pores, a very large contact area is created between the lithium and the electrolyte. This method is also inexpensive and scalable.
The research group has also studied stationary batteries for storage of renewable energy. “The most important metric for stationary storage is the price,” Kravchyk explains. The lithium-ion batteries are comparatively expensive due to expensive cathode materials. The study led to one of the most common elements: iron.
The researchers combined the inexpensive metal with fluoride as iron (III) hydroxyfluoride. “Previous approaches to making a battery based on iron fluorides relied on chemical conversion. Fluorides have poor conductivity, both for electrons and for lithium ions. But Kravchyk’s team has the solution: Using a simple and inexpensive process, they have given their cathode a particular crystalline structure, which contains channels inside it that conduct lithium ions. “We are totally surprised that hardly anyone has explored to develop the low-cost synthesis of this material until now.”
