Lithium is considered an ideal electrode material which can help achieve extremely high energy densities but is very reactive. Coating an lithium anode in two layers of a new polymer protects the ceramic electrolyte and prevent destructive metal deposits called dendrites that cause short circuits and fires.
“The goal was to expand our concept for a solid state battery to enable stable operation with a lithium anode, and we’ve done that,” said Dr Hermann Tempel from the Jülich Institute for Energy and Climate Research.
Using lithium for the electrode created a solid-state battery with an energy density of 460 Wh/kg, almost twice that of current liquid lithium ion cells. The solid-state batteries are also significantly less sensitive to temperature than conventional lithium-ion batteries and so do not need temperature management devices such as those used in electric cars so far, which should also save weight.
“The polymer functions like a protective layer, which makes the use of a lithium anode possible in the first place,” said Tempel. “It prevents the ceramic electrolyte from coming into direct contact with the metallic lithium at the anode. This prevents damaging processes such as dendrite formation and chemical changes in the ceramic electrolyte, which impair the function of the battery. Initial tests in the laboratory have already been very successful. There were hardly any performance losses over 500 charge and discharge cycles.”
“The special thing about the cell is that it works despite the moderately conductive polymers; In some ways even better than without,” said Professor Hans-Dieter Wiemhöfer of the Helmholtz Institute Münster (HI MS), who has developed the special polymer that belongs to the class of polyphosphazenes. He coordinates the MEET-HiEnD II project, from which the new battery emerged.
The polymer layer in the preparation is applied as a liquid and penetrates deep into the porous ceramic electrolyte. This improves the contact between the solid electrolyte and the solid electrode, a common problem with solid state batteries, so no heavy housing is needed to mechanically compresses the various components to get a good connection. This also saves weight and helps to increase the energy density.
As an additional barrier between the individual components, however, the polymer layers also have an adverse effect on the performance of the battery, in particular on the current flow. Last year, Jülich scientists presented a well-functioning, fast-charging solid-state battery that can be charged and discharged within half an hour. Using lithium anode and hybrid electrolyte, they have succeeded in doubling the theoretical energy density. However, the loading time was extended to two hours. For solid state batteries this is still a good value.
The battery is still at an early stage of development, as it has to be kept above 50 ºC to keep the electrolyte permeable. “For low-cost applications, the manufacturing process is still too expensive. However, the functioning cell shows that the hybrid electrolyte makes it possible to avoid typical problems at the interfaces of solid-state batteries,” said Professor Rüdiger-A. Eichel, Director of the Research Center Jülich (IEK-9). For niche applications where cost is less of a concern, the inherently safe high energy density battery may already be interesting.