3.1. Characterization of Textured Cu Foil
As illustrated in
Figure 1a, the laser parameters and beam path of the laser texturing process are controlled to regulate the width (
W), depth (
d), and spacing (
S) of the textured groove structure. The groove density (
D) can be calculated using the formula
.
Figure 1b shows the surface morphologies of the Cu foil in different regions. The pristine Cu foil surface exhibits a scratch-like structure at the micron scale. On the other hand, the laser-ablated area exhibits a uniform nanostructure, which is commonly observed in femtosecond laser ablation. During femtosecond laser ablation, the material experiences intense and ultrafast heating, resulting in rapid melting and vaporization. Upon cooling, a part of the vaporized material re-deposits on the surface, forming nanoparticles around the laser-ablated area [
15,
19]. The extremely short pulse duration of the femtosecond laser enables precise control over the energy deposition while minimizing thermal diffusion into the surrounding material. Consequently, no discernible thermal effect is observed at the boundary between the pristine and laser-ablated regions. In addition, EDS results reveal that the Cu element content on the surface of the Cu foil remains at 98.96 wt.% before and 99.21 wt.% after laser ablation, suggesting no significant oxidation. This indicates that the femtosecond laser ablation process primarily affects the surface morphology while preserving the elemental composition.
Surface structures with varying groove densities and depths are created on Cu foil with thicknesses of 4 μm, 9 μm, and 20 μm. The structure nomenclature and sizes are shown in
Table 1. The thickness of the current collector in the battery plays a significant role, and opting for an ultrathin current collector is an effective way to reduce the overall weight of the battery [
20]. However, achieving deeper ablation on a 4 μm-thick Cu foil is challenging. As a result of using scanning with adjacent scanning paths overlap**, an entire ablation of the Cu foil surface is achieved, resulting in the formation of only nanostructure (referred to as Cu4-nano, as shown in
Figure 2a). A Cu foil with a thickness of 9 μm represents one of the most often used current collectors on the market, so it is critical to investigate its texturing and its influence on Si anode performance. Furthermore, a Cu foil with a thickness of 20 μm possesses higher mechanical strength and proves useful for investigating the enhancement mechanism of Si anode performance using deep-textured current collectors.
Figure 2b,c show the typical surface morphology of the 9 μm-thick (Cu9-
D75-
d6) and 20 μm-thick (Cu20-
D75-
d15) current collectors, respectively. Both configurations exhibit a microgroove morphology, and the ablated surfaces feature nanostructures similar to those observed on the Cu4-nano current collector.
3.2. Electrochemical Performance of Si Anodes
The half-cell is assembled using Si nanoparticles as the anode active material. As shown in
Figure 3a, the Si anode with the Cu9-P current collector experiences rapid capacity fade during the initial cycling stage and gradually depletes in subsequent cycles. This is due to the formation of the solid electrolyte interface (SEI) and the electrical isolation caused by the detachment of the expanded anode coating from the current collector, both of which are major challenges that to be addressed in Si anodes and other energy storage devices [
21,
22]. In contrast,
Figure 3b demonstrates that the Si anode with the Cu4-nano current collector exhibits a significant increase in initial capacity from 563 mAh/g to 891 mAh/g, highlighting the positive impact of the textured nanostructure on Si anode performance. After 300 cycles at 1 C, the Cu4-nano current collector leads to a much higher discharge capacity of 463 mAh/g compared to the Si anode with the Cu9-P current collector, further proving the enhanced cycling stability provided by the textured nanostructure.
The cycling performance of the textured Cu foil with a thickness of 9 μm is presented in
Figure 3c. By comparing the effects of different groove structure characteristics on Si anode performance, it is found that the initial capacity increases with larger groove densities and depths. For instance, when the groove depth
d is 6 μm, the initial capacities of the Cu9-
D25-
d6, Cu9-
D50-
d6, and Cu9-
D75-
d6 anodes are 754 mAh/g, 1004 mAh/g, and 1023 mAh/g, respectively. Similarly, when the groove density
D is 75%, the corresponding initial capacities of the Cu9-
D75-
d2 and Cu9-
D75-
d4 anodes are 761 and 994 mAh/g, respectively. The capacity retention after 300 cycles also increases with the higher groove densities and depths. The Cu9-
D75-
d6 anode retains the highest capacity of 911 mAh/g after 300 cycles, surpassing that of the Cu4-nano anode (463 mAh/g). This improvement is attributed to the abundant and deep microgrooves formed through laser texturing, further enhancing the cycling stability of the Si anode compared to a current collector with only surface nanostructures.
Similarly, the cycling performance of the textured Cu foil with a thickness of 20 μm is shown in
Figure 3d. For a groove depth
d of 15 μm, the initial capacity of the Cu20-
D25-
d15, Cu20-
D50-
d15, and Cu20-
D75-
d15 anodes are improved to 1386 mAh/g, 1307 mAh/g, and 1478 mAh/g, respectively. For a groove density
D of 75%, the Cu20-
D75-
d5 and Cu20-
D75-
d10 anodes show initial capacities of 1229 mAh/g and 1429 mAh/g, respectively. The variation in capacity with groove density and depth follows a similar trend observed in the anodes with 9 μm- and 20 μm-thick current collectors. Among all the anodes, the Cu20-
D75-
d15 configuration, with the highest groove density and depth, exhibits the highest initial capacity and retained capacity after 300 cycles. The Cu20-
D75-
d15 anode shows an initial capacity of 1478 mAh/g with a retention of 80% (1182 mAh/g) after 300 cycles at 1 C, indicating that the more abundant and deeper ablated microgroove structure further enhances the cycling performance of the Si anode.
Furthermore, rate performance is compared to highlight the structural benefits of laser-textured current collectors. Four different current collectors, namely, the smooth Cu9-P, nanostructured Cu4-nano, and hierarchical micro/nanostructured Cu9-
D75-
d6 and Cu20-
D75-
d15, are selected for the evaluation, and the results are shown in
Figure 4. The rate performance of Si anodes exhibits evident improvement when using the nanostructured Cu4-nano compared to the smooth Cu9-P. Furthermore, this improvement is further enhanced by the hierarchical micro/nanostructure and increases with higher groove density and depth. Among the evaluated anodes, the Cu20-
D75-
d15 configuration with the largest groove density and depth shows the highest capacity of 684 mAh/g at 3 C.
3.3. Discussion
Three typical fading behaviors of Si anodes are caused by the volume change of Si during the lithiation and delithiation process: (i) unstable SEI layer, (ii) pulverization of Si material, and (iii) cracks and exfoliation of the anode coating [
23]. These behaviors not only result in an unstable anode structure, degrading cycling performance, but also lead to poor electric contact, reducing the rate capability. Based on the experimental results and the reasons for Si anode failure, the effect of current collector surface texturing on Si anode performance is discussed as follows.
Figure 5 illustrates the galvanostatic discharge/charge profiles of Si anodes with Cu9-P, Cu4-nano, Cu9-
D75-
d6, and Cu20-
D75-
d15 current collectors after the 2nd, 20th, and 40th cycles. In
Figure 5a, it is evident that the Cu9-P anode has no obvious voltage plateau during following cycles in the galvanostatic test, and the cell voltage increases/decreases rapidly as the charging/discharging proceeds, accompanied by a rapid decay in capacity. The main reason is the expansion-induced disintegration of the anode structure, leading to significant polarization and increased internal resistance. Consequently, there is a loss of electrical contact between the Si materials and the current collector, resulting in a shortened cycling life. In
Figure 5b (Cu4-nano) and 5c (Cu9-
D75-
d6), the anodes present a distinct voltage plateau, during which the cell voltage varies slowly and the capacity increases rapidly. The reversible and stable charging–discharging process indicates successful achievement of good electric contact, low internal resistance, and a stable structure. As shown in
Figure 5d, the slope of the Cu20-
D75-
d15 anode’s curve is the smallest, indicating the presence of the most conductive environment within the cell. This enhanced conductivity is attributed to the nanostructures and the abundant and deep microgrooves, which provide an effective pathway for electron collection and help reduce polarization [
24].
The electric contact is further validated through EIS measurement. In
Figure 6, the Nyquist plots of the Si anodes with the four types of current collectors are shown at the delithiation state after the 50th cycle. The semicircle at high-medium frequency (10
3–10
5 Hz) mainly represents the charge transfer resistance (
Rct). After laser texturing, there is a significant reduction in
Rct, which continues to decrease as the groove density and depth increase. This reduction in
Rct contributes to enhanced electric contact in the Si anode. The electric contact is primarily determined by the contact area between the anode coating and the current collector, as well as the structural stability [
25]. These factors are crucial in determining cycling life, as validated and analyzed below.
The roughness values of the Cu9-P, Cu4-nano, Cu9-
D75-
d6, and Cu20-
D75-
d15 current collectors are 0.275 μm, 0.569 μm, 2.052 μm, and 4.410 μm, respectively. The increased thickness of the Cu foil enables deeper microgrooves, leading to higher roughness. This larger roughness has the potential to enhance slurry wettability and facilitate the infiltration of slurry into the current collector [
26]. As a result, improved electric contact is achieved, resulting in the optimal cycling and rate performance of the Si anode when using the Cu20-
D75-
d15 current collector with the largest roughness.
The slurry wettability is evaluated using contact angle measurement. As shown in
Figure 7a, the contact angles for Cu9-P, Cu4-nano, Cu-
D75-
d6, and Cu-
D75-
d15 current collectors are 92.11°, 53.97°, 53.01°, and 46.95°, respectively. These results demonstrate that the Cu surface with nanostructures becomes more hydrophilic after laser texturing, and the hydrophilicity increases with an enlarged microgroove density and depth. The Cu-
D75-
d15 current collector, with the highest hydrophilicity, exhibits the most favorable slurry wettability, contributing to improved electric contact. Adhesive strength is further assessed using a scratch test. As shown in
Figure 7b, the anode coating visibly peels off from the Cu9-P current collector, particularly near the scraped grid. The integrity of anode coating increases after laser texturing, improving further with increased groove density and depth. This demonstrates the improvement in adhesive strength achieved by the hierarchical nanostructures and microgrooves [
17]. On the Cu20-
D75-
d15 current collector, the anode coating shows very little peeling.
Microscopic morphology observations are conducted to provide further insights into the aforementioned surface properties. Cross-sectional morphologies (
Figure 7c) reveal that on the current collector without microgrooves, Cu4-nano presents an intact adhesive interface, whereas Cu9-P shows an obvious interface gap. This discrepancy explains the improvement in electric contact and adhesive strength, which is attributed to the laser-textured nanostructures that increase the contact area and provide a cohesive interface. In the Cu9-
D75-
d6 and Cu20-
D75-
d15 anodes, the slurry coating predominantly infiltrates the microgrooves with an integrated interface [
27,
28]. The microgrooves not only improve the contact area but also provide protection by creating periodic separations within the coating to prevent extrusion between adjacent expanded Si materials [
29,
30]. As a result, the integrity of the anode surface morphology after 50 cycles (
Figure 7d), primarily due to the presence of laser-textured nanostructures (as observed in the comparison between Cu9-P and Cu4-nano), with further improvement achieved through the formation of abundant and deep microgrooves. Notably, no significant cracks or exfoliation are observed in the Si anode with the Cu20-
D75-
d15 current collector.