## 3 Results and Discussion
### Electrochemical Performance  
The rate performances of Si micropillars at a series of increased current rates of 0.1, 0.2, 0.5, and 1.0 C are presented in Figure 4a. The coulombic efficiencies in the initial cycle are 79.9, 79.4, and 75.7% for Si pillars growing from 8-, 13-, and 20-\(\upmu\)m-thick slurry layers, respectively. They all increase to over 97% in the subsequent cycles. The coulombic efficiency in the first cycle was naturally poor because the high surface-area-to-volume ratio caused more Li\({}^{+}\) to be trapped in the SEI layer [44]. It should be noted that the coulombic efficiency shows at slightly over 100% in cycle 26. The reason for this is likely to be that, in cycle 25 with a 1.0 C rate, the delithiation is not complete; some Li ions still remain in the anode. This causes the delithiation in cycle 26 to be a little higher than the lithiation, and therefore the coulombic efficiency is over 100%. The Si micropillars from 20-\(\upmu\)m-thick slurry layer exhibited the worst performance with a capacity of 1378 mAh/g at 0.1 C and 303 mAh/g at 1.0 C, respectively. As the slurry layer thickness decreased to 13 \(\upmu\)m, the specific capacity of micropillars increased to 1463 mAh/g at 0.1 C and 548 mAh/g at 1.0 C, respectively. The capacity was further improved when the slurry layer became much thinner. The highest capacity of 1647 mAh/g at 0.1 C was obtained for the thinnest slurry layer (8 \(\upmu\)m), with a capacity of 582 mAh/g at 1.0 C that was almost doubled compared with that of the 20-\(\upmu\)m-thick slurry layer. Figure 4b compares the cycling performances of Si micropillars at a current rate of 0.1 C. Significant differences in discharge capacity can be observed at the early period of cycling among the three samples. Si micropillars from the thinnest slurry layer (8 \(\upmu\)m) present the best electrochemical performance with an initial discharge capacity of 1780 mAh/g, followed by the 13-\(\upmu\)m-thick slurry layer with an initial discharge capacity of 1423 mAh/g. When the thickness of the slurry layer increased to 20 \(\upmu\)m, the initial discharge capacity dropped to 1387 mAh/g. However, the subsequent capacity decay was rapid; it dropped down to nearly half of the initial discharge capacity at the 50 th cycle for all the three samples. The discrepancy in discharge capacity became smaller as the cycle number increased. After 60 cycles, the discharge capacities of all three samples were very close to each other. The reversible capacities over 100 cycles of 8, 13, and 20 \(\upmu\)m slurry layers were approximately 545, 453, and 385 mAh/g, respectively.  
The enhanced electrochemical performance of Si micropillars at the early period of cycling has been achieved by reducing the slurry layer thickness. Such improvement can be attributed to the following reasons:  
1. Smaller micropillars had a higher surface-area-to-volume ratio, which could reduce the occurrence of cracks on the surface and maintain the mechanical integrity.
2. Increased surface area and denser distribution of smaller micropillars provided more contact area between electrolyte and Si for large Li\({}^{+}\) flux.  
Figure 3: Schematic drawing of the laser irradiation process on (**a**) a thin slurry layer and (**b**) a thick slurry layer.  
3. Shorter Li+ diffusion pathway in micropillars with a smaller diameter, which ensured a fast and homogeneous lithiation [35].  
It is well known that the high conductivity of the active material and short path length of ions are two important factors that improve the rate performance of electrode for Li-ion batteries [45]. The characteristic time constant \(t\) for ion diffusion is proportional to the square of the diffusion length \(L\) and inversely proportional to ion diffusivity (\(D\)) [22; 46], as shown in Equation (1). The reduction in the diameter of Si micropillars (equivalent to diffusion length \(L\)) is an effective way to shorten the diffusion time \(t\), and therefore improve the electrochemical properties of electrode material.  
\[t\approx L^{2}/D \tag{1}\]