Around a quarter of greenhouse gas emissions in the EU are caused by transport. In Germany, passenger cars and motorized two-wheelers account for around 76 % of individual traffic. Low- and zero-emission vehicles can therefore make a major contribution to decarbonizing the mobility system. At the current state-of-the-art, lithium-ion cells are still too cost-intensive and their available energy densities are too low to be used across the board in the mobility sector. The establishment of next-generation lithium-ion batteries with gravimetric energy densities of more than 400 Wh/kg and volumetric energy densities of more than 750 Wh/L is therefore a main goal in the Strategic Energy Technology Plan of the European Union to be achieved by 2030. For this purpose, new materials and production strategies play a major role in the development of advanced lithium-ion cells. With novel electrode materials, which have higher specific capacities compared to state-of-the-art electrode materials like graphite or NMC 111, higher energy densities on the electrode level can be achieved. Additionally, the establishment of electrodes with high mass loading and good rate capability increases volumetric and gravimetric energy densities of the batteries.
A promising candidate as active material on the anode side is silicon. It has a theoretical capacity of 3579 mAh/g at room temperature, which is one order of magnitude higher compared to the commonly used graphite (372 mAh/g). However, silicon undergoes a huge volume change of about 280 % during lithiation, which reduces the mechanical integrity of the electrodes. A reduced capacity retention and a strong decrease in cell lifetime are the consequence.
In this work, a multi-scale approach is implemented to realize reliable and long-lasting silicon-graphite composite electrodes for lithium-ion cells. Silicon is incorporated in the electrode as finely dispersed nanomaterial, while graphite buffers part of the volume expansion. Electrode thicknesses of about 80 µm with areal capacities of about 3.7 mAh/cm² are realized. Microscale groove structures with widths up to 80 µm are generated by laser ablation applying a high power, high repetition rate ultra-short pulsed laser source. The electrode patterning creates an artificial porosity which provides essential free space for the volume expansion of the silicides formed. Additionally, it significantly improves the wetting of the electrode with liquid electrolyte. The mixing process of the electrode material slurry is optimized to enable a large batch processing in a roll-to-roll environment (TRL 4-5). The electrode material is characterized in half cells with coin cell design as well as in laboratory-scaled pouch cells (full cells). Rate capability measurements, cyclic voltammetry, and thermal characterizations are performed and evaluated with regard to the impact of the 3D battery concept.