## 3 Results  
The in situ observation of morphology variation of the two types of Si/C anodes (unstructured and 3D-line-structured) was performed during the lithiation and delithiation processes (Figure 2, Videos S1 and S2). Figure 2a,c compared some of the typical SEM images of unstructured and 3D-line-structured electrodes in the in situ experiments in which the thicknesses of the electrodes were marked. Figure 2b,d present the galvanostatic charge-discharge curves of unstructured and 3D-line-structured electrodes in the in situ experiments. The images in Figure 2a,c correspond to the red points marked Figure 2b and d, respectively. The images marked with I in Figure 2a,c show the initial state of unstructured and 3D-line-structured Si/C anodes, respectively. For the unstructured electrode, the electrode began to peel off from the current collector at 56 min, as shown in image II of Figure 2a, while for the 3D-line-structured ones the channels disappeared (Figure 2c (II)) and some void-like defects appeared after 125 min (see Video S2). The images marked with III in Figure 2a,c present the morphologies of unstructured and 3D-line-structured electrodes when lithiation was finished. It is obvious that for the unstructured electrode a lift-off from the current collectors occurred, while for the laser-structured electrodes some small voids at the bottom of the composite could be detected after lithiation. In the lithiation process, the thickness of the unstructured electrode increased from 39.7 \(\upmu\)m to 54.8 \(\upmu\)m, and that of 3D-line-structured electrodes increased from \(\sim\)46.6 (\(\sim\)46.1) \(\upmu\)m to \(\sim\)157.0 (\(\sim\)146.0) \(\upmu\)m. Accordingly, the charge capacity of the unstructured electrode was 350 mAh/g, while that of laser-structured one was 890 mAh/g (see Figure S1). The comparison revealed that the 3D-line-structured electrodes produced a significantly improved electrochemical performance. After lithiation, a delithiation process was performed. The images marked with IV in Figure 2a,c exhibit the morphologies of electrodes after delithiation, showing that the contraction in the delithiation process was very small for both the unstructured and laser-structured electrodes, which was attributed to the short delithiation time. Figure 2b,d show the galvanostatic charge-discharge curves of unstructured and 3D-line-structured electrodes, respectively, revealing both super-low coulombic efficiency (\(\approx\) 1.7\(\%\)) and abnormal plateaus (0.5 V), which are due to the use of the ionic liquid as an electrolyte. The coulombic efficiency might be attributed to an irreversible EMIM cations decomposed and the plateaus can be ascribed to an irreversible EMIM cations intercalation [21,22]. Figure 1: (**a**) The photograph of the SEM-electrochemical system. (**b**) Schematic drawing of in situ SEM observation for the cross-section of the Li-Si/C battery. (**c**) Low-magnification SEM image of the Li-Si/C battery for the in situ experiment. SEM images of (**d**) unstructured and (**e**) 3D-line-structured Si/C composite material. (**f**) The thickness of Si/C composites and the width of the channel. Figure 3 illustrates the lithiation process of two types of electrode materials based on in situ SEM observation. As the Figure 3a depicts, the volume of unstructured Si/C composites increased gradually during the lithiation process, resulting in most of the electrode materials peeling off from the collector at the later stage of lithiation. Figure 3b shows that the laser-generated channels were completely filled with active material due to the material expansion. As a result, the electrode materials might peel off from the collector because of the internal stress caused by the volume expansion. This is because, compared with the unstructured electrode (Figure 3a), the laser generated grooves (Figure 3b) that enlarged the contact area of electrodes and electrolyte, which allowed more lithium ions to be inserted into the electrode material and thus greatly improved its capacity.