What types of materials did you use and why?
Tempel: Most problems with solid-state batteries pertain to the interfaces. Our idea was to use structurally similar materials to improve the interfaces and thus solve the problem. All components were made from phosphate compounds: lithium aluminum titanium phosphate acts as an electrolyte, lithium titanium phosphate as an anode and lithium vanadium phosphate as a cathode material. This clever choice of materials improved the fit.
How were the batteries made?
Tempel: The first step was to produce the individual materials. The morphology of the anode and cathode materials had to be tested, that means to examine their structure, alter it and subsequently study the impact on the battery characteristics. In the second step, we used the material to set up a liquid battery and evaluated the result: How can the materials be adapted to match the capacities and what is the range of voltage for this to actually happen? We then modified the solid-state electrolyte so that it could be densely sintered. When then printed the active materials onto the densely sintered solid electrolyte.
How long have you been working on these batteries?
Tempel: The project took about three years. Essentially, the team was made up of one person. Even though the responsible doctoral candidate had some assistance and support from various parties, the synthesis, and the entire development was handled by him.
What was the biggest challenge during the project?
Tempel: The biggest challenge has been and remains the fact that you try to make these layers of electrolyte as thin as possible, yet still maintain their mechanical stability and density. After all, this is ultimately the strongest resistance in the entire cell. If the layer ends up too thin, you can no longer print on it because it breaks apart. This area requires improvement and perhaps another production process that might be more efficient. The challenge is to still keep the layer almost gas-tight. You need an electrolyte that is dense enough to ensure that no paste is pushed through during printing since it is electrically conductive and would instantly cause a short circuit.