## 4 Discussion  
Laser structuring of advanced NMC 622 cathode materials using ultrafast laser ablation was investigated, regarding an improvement of cycling performances, such as capacity retention as a function of C-rate [7; 8; 9]. With ultrafast laser radiation, almost an ideal cold ablation process can be realized due to the well-known time-delay coupling of electron and phonon system [11; 12]. Table 1 shows that structured 91 \(\upmu\)m cathodes with a pitch distance of 200 \(\upmu\)m provided the highest mass loss of about 14%, while electrodes with higher film thickness showed a mass loss of about 8%. This is due to the wide crater opening of the established line structures which could be stated from the cross-section analyses (Figure 2). It was observed that within a length of about 50 \(\upmu\)m beneath the electrode surface, the side-walls of the generated groove structures had a slight inclination of about 5\({}^{\circ}\), while for length larger than 50 \(\upmu\)m, the channel geometry tended to become rectangularly shaped. This means that for thin film electrodes (\(\leq\) 50 \(\upmu\)m), the mass loss could increase rapidly, while for higher film thicknesses, the slope of increase of active mass loss is decreased and converges to a linear rise as a function of thickness. In addition, it was found that the groove walls were not always ideally smoothly contoured, which might be due to the different ablation rates for different types of materials, such as binder and NMC 622 particles. Figure 4 shows the rate capability of cells containing structured and unstructured NMC 622 cathodes. For the formation process at C/10, cells with structured cathodes have lower discharge capacity compared to unstructured cathodes. During formation, a larger solid electrolyte interface (SEI) was formed due to the increase of interface between electrode and electrolyte [13]. This procedure consumes more active materials and electrolyte. Therefore, fewer lithium-ions are available for the intercalation process (discharge). Cells with structured NMC 622 cathodes begin to show superior performance at discharge rates from C/2 to 3C. Especially for electrodes with thicknesses of 151 \(\upmu\)m and 250 \(\upmu\)m, laser-structured cathodes with a pitch distance of 200 \(\upmu\)m provide 40 mAh/g more discharge capacity at C/2 and 1C. The diffusion-controlled transport of lithium-ions in the cathode is considered a bottleneck, which strongly influences the discharge rate. The laser generated line structures lead to an increase in active surface area. With reduced pitch distance, the number of the line structures across  
Figure 6: Comparison of the maximum current in relation to the square root of the scan rate for cells containing unstructured and structured cathodes with different thicknesses. the cathode increases, which results in a shorter length of average diffusion path for lithium-ions from electrolyte to bulk material. Therefore, this leads to a reduced cell polarization and an improved high rate capability. It is verified that the lithium concentration along the contour of 3D architectures in electrodes is significantly increased using laser-induced breakdown spectroscopy, which indicates an improvement of lithium mobility by providing additional lithium diffusion pathways [14]. However, as the discharge rate increased from C/2 to 5C, all cells suffered from successive loss in discharge capacity. The decreasing of the discharge curve at higher C-rates corresponds to an enhanced cell polarization induced by the internal cell resistance [6]. When being cycled again at C/5, the cell with 250 \(\upmu\)m cathode shows an irreversible capacity loss of 5%, while other cells have 2%-3% loss. According to [PERSON] et al. [15], during electrochemical cycling, side reactions can be induced, such as electrolyte oxidation with metallic lithium, which can promote the growth of the resistive surface layer. Another factor is the mechanical stability.