Entwicklung von elektronenleitenden Schutzschichten gegen die anodische Auflösung von Stromsammlern in neuartigen „Dual-Ionen“-Energiespeichern

In times of the energy revolution, electrochemical storage systems gain in importance to balance the fluctuating electricity generation from renewable energies. Dual-ion batteries represent a cost-efficient and environmentally friendly concept for the application as stationary and local energy storage system, because graphite is used as the only material for both electrodes. In these batteries cations and anions of a novel electrolyte intercalate simultaneously into the graphite electrodes. This reversible principle provides a cell voltage above 5 V. However the electrolyte anions (bis(trifluoromethanesulfonyl)imide – TFSI-) do not form a passivation layer in contact with the aluminum current collector. Therefore further oxidation of the metal surface is not prevented and leads to a degradation of the current collector. Since even noble metals like gold are not sufficiently electrochemically stable at such high potentials, the aim is to develop an electronically conductive ceramic layer to prevent the anodic dissolution of the aluminum current collector. The applied sol-gel process is easily scalable and enables the deposition of the selected oxide ceramics using a few coating steps. Necessarily, the deposited thin films need to be thermally treated at a temperature below the melting temperature of aluminum. Therefore aluminum doped zinc oxide (ZnO:Al) and lanthanum doped strontium titanate (SLT) were selected as materials since they crystallize at low temperatures. The developed sol synthesis routes yielded appropriate sols with good long-term stability and film formation properties. Even on the rough surface of aluminum substrates homogenous coatings were possible, after optimizing the wettability of the surface by a pretreatment. Calcined thin films of both materials showed single phase crystal structures and a crack-free morphology of densely packed particles. However the conductivity of SLT was too low for the application as artificial protective coating and this material was not considered further on. Electrochemical measurements showed a significantly reduced anodic dissolution of aluminum for the protected current collector. Furthermore it was observed, that the protection effect increased with the homogeneity of the protection layers. In additional tests, aluminum plates were used to avoid typical cracking effects of the brittle ceramic protection layer at the edge of the samples due to mechanical stress. Using this setup, the electrochemical stability of ZnO:Al and the protection effect of the layer was demonstrated by a 120 times reduced anodic dissolution. Herein the occurrence of only few corrosion spots indicated that the corrosive attack of TFSI anions on the aluminum surface was mainly prevented. In conclusion, the developed protection layer will contribute to an improved dual-ion cell by maintaining the contact of the aluminum current collector to the positive electrode