## 3 Results and Discussion
### Morphological Change of Si Micropillars during Electrochemical Cycling  
To clarify the reasons for the rapid capacity decay in the early stage and the insignificant discrepancy in the capacities of three samples at the later period of cycling, as shown in Figure 4b, the morphological change of Si micropillars during electrochemical cycling was further examined. Micropillars growing from the 13-\(\upmu\)m-thick slurry layer were used to study the morphological change  
Figure 4: (**a**) Rate capabilities of Si micropillars measured at the varying current rates. (**b**) Discharge capacities of Si micropillars and corresponding coulombic efficiencies as a function of cycle number. during electrochemical cycling. The anode surface was characterized using SEM in terms of top view (Figure 5a), 45\({}^{\circ}\) tilted view (Figure 5b), and side view (Figure 5c) after 1, 5, 35, and 100 cycles, respectively. After give cycles, the individual pillar structure could be observed, suggesting that volume expansion of Si had not occurred significantly. SEI products may form on the surface of micropillars due to the reductive reaction of electrodes and electrolytes [47]. After 35 cycles, the pillar structure turned into a separate island-like structure of irregular shapes. This may be because Si micropillars expanded during the charge/discharge process, and adjacent pillars aggregated to form islands with irregular shapes [48]. After 100 cycles, the spaces between the islands were further reduced, and the anode surface became flat. Some cracks can be seen on the anode surface, which could be associated with the contraction of Si in the delithiation process. The similar surface morphology of Si nanowire electrodes after 100 cycles was reported by [PERSON] et al. [49]. It should be pointed out that, even though there were cracks on the anode surface, the Si layer was attached firmly to the Cu foil after 100 cycles, as seen from Figure 5c. The strong bond between the Si pillars and Cu foil could be due to two aspects: one is the remaining electrically conductive binder existing between them, and the other is the direct interfacial diffusion occurring between Si and Cu at a high temperature [50]. This demonstrates that the pillar structure significantly absorbed the volume expansion like the Si NW structure and prevented the destructive delamination of the Si layer from the Cu foil, which had been a serious problem for Si NP anodes. Figure 5d compares the thickness of the Si layer on the Cu foil after a varying number of cycles. It can be seen that the volume expansion was negligible in the first cycle and the Si layer thickness remained about 8 \(\upmu\)m. The thickness of the Si layer increased slightly to 11 \(\upmu\)m after five cycles. However, a huge growth in Si layer thickness to about 30 \(\upmu\)m can be observed after 100 cycles. This means that the  
Figure 5: SEM images of Si micropillars after a varying number of cycles from (**a**) top view; (**b**) 45\({}^{\circ}\) tilted view; (**c**) side view. (**d**) Thickness of Si anode measured from (**c**). Si layer thickness increased by a factor of four compared with the pristine anode sample. The volume expansion of Si could be one of the causes of this change. Another cause could be the continuous formation of SEI products. According to the study of SEI growth on Si nanocones by [PERSON] et al. [47], the SEI should have grown on the surface of micropillars continuously. After filling the space among the micropillars, SEI would further form and grow on the top layer of the micropillars. The thickness of the extra SEI layer on top of the micropillars could be several micrometers after dozens of cycles [47]. The EDX spectrum of a Si micropillar anode after 100 cycles and the mapping analysis of the detected elements are shown in Figure 6. The detected elements are O, C, Si, F, and P. The presence of O, C, Si, and F is consistent with the possible SEI products such as Li\({}_{2}\)CO\({}_{3}\), (CH\({}_{2}\)CO\({}_{2}\)Li)\({}_{2}\), and LiF [51].