## 3 Results and Discussion
### Morphological and Structural Characterization  
Scanning electron microscope (SEM) analysis was conducted to feature the micro-morphology differences of the coal gangue during the fabrication process. As shown in Figure 2a, the crushed coal gangue after simple ball milling delivered the coexisting structure of the bulk and layered particles, and the particle sizes ranged between 2-10 microns. The high-resolution SEM in Figure 2b further indicated that the surface of the coal gangue particles had rich folds and porous skeleton structures. After the acid and alkali activation, the activated coal gangue showed a loose and porous colloidal structure with a nanometer scale (Figure 2c). Figure 2d demonstrated that the surface of activated coal gangue contained abundant microporous and macroporous structures. The macropores were conducive to the full infiltration of electrolyte and provided sufficient space for the accumulation of discharge products, while the micropores could promote the ion diffusion rate in the electrode reaction during the discharge-charge process in the LOBs [14]. In addition, the unique colloidal nano-morphology could ensure sufficient exposure of the active sites on the surface of the activated coal gangue. In short, the acid and alkali activation reaction effectively optimized the surface morphologies of the coal gangue, leading to the improvement of the electrocatalytic activity in the LOBs. Figure 1: Schematic illustration of the preparation of activated coal gangue. To excavate the differences in-depth in the surface area for the as-prepared samples, the nitrogen adsorption/desorption isotherms and pore size distributions of the samples were examined. As shown in Figure S2a, the total BET surface area of the activated coal gangue is significantly larger than that of the crushed coal gangue, which was due to the formation of loose and porous colloidal structures with a nanometer scale. In addition, the pore sizes of both samples were mainly distributed mainly in the range of 5-20 nm (Figure S2b), which could supply fast diffusion channels for O\({}_{2}\) and sufficient storage space for the discharge product Li\({}_{2}\)O\({}_{2}\). The crystal structure of the crushed and activated coal gangues was characterized by X-ray diffraction (XRD) in Figure 3. Apparently, the strong diffraction peaks at 21.09\({}^{\circ}\) and 26.91\({}^{\circ}\) corresponded to the crystalline planes of quartz, the peaks at 11.61\({}^{\circ}\) and 29.75\({}^{\circ}\) related to the crystalline planes of kaolinite, and the peak at 35.15\({}^{\circ}\) was attributed to the crystalline planes of hematite, respectively. The natural coal gangue contained a variety of mineral components, leading to low electrochemical activity [28]. However, there were no evident diffraction peaks in the XRD patterns of the activated coal gangue, indicating that the activated coal gangue had an amorphous structure. It should be emphasized that the only strong diffraction peak at 23.70\({}^{\circ}\) corresponded to the sample stage. Figure 2: (**a**) SEM image of crushed coal gangue; (**b**) High-resolution SEM image of crushed coal gangue; (**c**) SEM image of activated coal gangue; (**d**) High-resolution SEM image of activated coal gangue. X-ray photoelectron spectroscopy (XPS) was performed to further understand the elements' compositions, chemical states, molecular structures, and other aspects of the activated coal gangue. From the full spectra in Figure 4a, there are various peaks corresponding to Si, Al, C, O, and many other impure elements on the surface of the crushed coal gangue. However, the activated coal gangue contains only Si, C, and O peaks. As shown in Table S1, the Si/C and Si/O ratio in the activated coal gangue is higher than those in crushed coal gangue, suggesting the acid-alkali activation treatment could remove the impurities of the gangue and retain the silica-based compounds.