fig4

Figure 4. (A-C) Stable configurations and (D-F) charge density difference analysis of K⁺ adsorbed on phase-pure carbon, N-doped carbon, and N/S co-doped carbon surface. K, C, S, and N are purple, brown, yellow and gray, respectively. And yellow and cyan contours with isosurface value of 0.001 e Å^-3 in panels indicate the accelerated and decreased charge density, respectively; (G) Density of states. Fermi level is set to 0 eV; (H) Voltage plots of N/S-CNF-700 during the first charging and discharging processes (0.1 A g^-1) and corresponding contour image of in-situ XRD peak intensities. N/S-CNFs: N/S co-doped carbon nanofibers; XRD: X-ray diffraction.