## 3 Results and Discussion
### Width, Removal Energy, and Cut Surface of the Uncompressed Cathode
However, LiFePO4 and conductive carbon powder in the electrode do not undergo chemical modifications or phase change during laser cutting. Cutting regions with the SEM images are shown in Figure 12. The observed SEM images are classified by five regions: (1) Proper ablation region, (2) Excessive ablation region, (3) Defective cutting region, (4) Proper cutting region, (5) Excessive cutting region. First, the proper ablation region (Figure 12a) is assigned when the active electrode material is completely removed from the target region without melting the current collector. Second, the excessive ablation region (Figure 12b) is assigned when the active electrode material is removed and the melting of the current collector is observed. Third, the defective cutting region (Figure 12c) is assigned when the active electrode material is completely removed and the current collector is partially penetrated or cut in the target region. Forth, the proper cutting region (Figure 12d) is assigned when the active electrode material and the current collector are uniformly removed. Finally, the excessive cutting region (Figure 12e) is assigned when the current collector is melted and re-solidified, so that the kerf width is not uniform. Moreover, delamination can be observed due to the high recoil pressure. Figure 12: Category and volume energy of cutting region depending on five physical phenomena. (**a**) Proper ablation region; (**b**) Excessive ablation region; (**c**) Defective cutting region; (**d**) Proper cutting region; (**e**) Excessive cutting region. The classification and comparison in terms of the volume energy are shown in Figure 12 (top). The proper ablation region and the proper cutting region of the compressed cathode are wider than the uncompressed cathode. However, the excessive ablation region and the excessive cutting region of the uncompressed cathodes is wider than the compressed cathodes. According to this comparison, we can insist that the proper laser cutting of the compressed cathode can be obtained with wider volume energy. In addition, since the compressed cathode provides a wider range of volume energy for the proper ablation, the laser ablation of the compressed cathode is more easily applicable for the mass production to selectively remove or pattern [43; 44] the active electrodes. Figure 13 shows the comparison of absorption rate of the compressed and uncompressed cathodes. If the compressed and uncompressed cathodes had a difference in absorption, interaction characteristics and material removal mechanism may be affected by this and the difference should be considered for the analysis. Hence, the absorptivity of compressed and uncompressed cathodes is compared with the wavelength of 1070 nm. The compressed cathode has an absorptivity of 97.22% while the uncompressed cathode has an absorptivity of 97.30%. As a result, the difference of the absorption rate between compressed and uncompressed cathodes is 0.08%. The compression of cathode does not significantly affect the absorptivity of the cathode. Therefore, we can verify that the compression, or density change, of the active electrode material is the only factor to affect laser cutting characteristics. The EDX analysis of the cathode material is shown in Figure 14. Figure 14 clearly shows the aluminum composition in the cathode cross-section. The boundary line refers to the thickness (20 \(\upmu\)m) of the current collector. The composition is measured from the bottom of the current collector to the top of active electrode material. Figure 14a shows the cross-section of compressed cathodes. The cathodes are cut by laser (top) and by mechanical cutting (bottom). It is cut by two methods, or laser and mechanical cutting, not only to investigate the effect of compression, but also to compare traditional mechanical cutting. For the compressed cathodes, the mechanically cut cathode gradually decreases its aluminum content from the current collector area, but the aluminum content decreases dramatically at the boundary line in the laser-cut cathode.