Recent progress in ultrafast laser structuring of advanced silicon/graphite thick film anodes for Li-ion-cells

Lithium-ion batteries already play an important role in providing electrical energy, and the need for advanced Li-ion cells will continue to grow. Thus, the continuous engineering improvement and material development related to the already existing Li-ion cell concept will be essential. To increase the energy density on cell level, new materials for anodes and cathodes have to be developed. On anode-side, silicon is a promising material to partly or completely replace the commonly used graphite in SoA Li-ion cells. At full lithiation, the specific capacity of silicon and graphite are 3579 mAh/g and 372 mAh/g, respectively, so an almost one magnitude higher areal energy density on anode side might be possible. However, since silicon undergoes a huge volume change of up to 300 % during cycling, the usage of silicon as anode material is still quite a challenge. In this work, laser structuring is used to compensate the volume change of silicon during electrochemical cycling and to reduce diffusion overpotential at elevated C-rates. The amount of material which is removed during ultrafast laser ablation has to be kept at a minimum, while the channel width still needs to be large enough to compensate the volume change. When additionally high areal mass loadings are being used, the laser structuring process plays an even more important role. With laser structuring, the active surface of the anode is increased and the diffusion path of the Li ions is shortened, which leads to a lower ohmic cell resistance and overpotential. An improved wetting behaviour with liquid electrolyte can also be achieved by introducing capillary structures in the electrodes, which in turn leads to an increased battery lifetime and a more cost-efficient battery manufacturing process. Here, a water-based slurry with a high solid fraction was produced via ball milling and was tape casted on a copper current collector foil. The dried electrodes were calendared and subsequently laser structured with an ultra-short pulsed laser. The use of a roll-to-roll system to structure the electrodes was also implemented. To analyse the newly developed Si-C anode materials, CCCV half-cell measurements were carried out. Laser-induced breakdown spectroscopy was used to determine the binder and the lithium distribution in the anodes. The cycled anodes were also examined by SEM to investigate the impact of laser-generated structures on the film adhesion and crumpling of the current collector foil.