Future products in the field of mobile electronic devices, the task of saving electrical energy and the progressively propagated electrical vehicles by the automobile industry are all dependent on the development and production of new efficient battery systems.
These systems have to fulfill diverse technological and physical demands: next to a reasonable lifetime measured by the amount of charging/discharging cycles, they need to be lightweight and small in their geometrical dimensions, material- and production costs need to be low and most of all their electrical performance (energy density and specific capacities) need to be high without compromising any safety issues during practical usage.
One encouraging solution for this task can be battery cells based on solid electrolytes. The usage of multivalent metal ions (e.g. Al3+) is of particular interest. The basic objective of the Fraunhofer IWS in this field is to design and produce a solid electrolyte thin-film-battery with the help of established PVD-coating technologies (e.g. MSD, PLD, IBSD) combined with an innovative cell design.
Within a broad research network the Fraunhofer IWS has developed a solid state battery system based on aluminum as electrode on the anode side. The total thickness of the whole battery cell is smaller than 10 µm. There are two main cell designs the Fraunhofer IWS is pursuing: one is the conventional layer design where anode, electrolyte and cathode are stacked on top of each other (1).
The other more inventive solution which is already applied for a patent is a structured cell design that enables higher ionic conductivity even at room temperature by introducing regular phase boundaries (2).
Designing this cell the Fraunhofer IWS is able to bring in its unique experience with laser structuring by generating micro-trenches into a solid multilayered electrolyte. These structured design than provides the foundation for inserting anode and cathode materials by PVD-methods. In contrast to two-dimensional thin-film-batteries, three-dimensional ones allow to yield a relatively high cell-capacity referred to their footprint.
Several coating series which involved distinct ionic conducting materials (Al3+ - as well as Li+-ion conducting) have been successfully prepared by PLD. The used materials originate from the group of NASICON-type materials or spinel groups. But also LIPON which is known for being ion conducting in its amorphous state has been deposited. One great advantage of PLD is that especially new materials can be easily pressed and sintered of powder material and then be used as targets.
Furthermore experiences made with the deposition of solid electrolytes could be already transferred to create new separator materials in battery cells using liquid electrolyte. One additional advantage of the PLD deposition is the opportunity to work under different gas-atmospheres and pressures what allows to generate very complex stoichiometric material compositions.
Next to conventional furnace tempering, the Fraunhofer IWS experiments with alternative methods of heat treatment, e.g. Flash Lamp Annealing (FLA). This relatively new technology allows a fast, local adjustment of the materials structure by recrystallization and can be furthermore fully integrated into the deposition routine.