Fig. 1. (Color online) (a) Schematic illustration of a Li–S cell configuration with a N-CNFs interlayer inserting between cathode and separator. (b) Digital image of a N-CNFs interlayer. (c, d) SEM images of the as-spun PAN/PVP nanofibers and the final N-CNFs (inset is the SEM image of the final PAN/PVP nanofibers).

Fig. 2. (Color online) (a–c) TEM images of a single N-CNFs fiber. (d–f) HAADF STEM image and the element mappings for C and N, respectively.

Fig. 3. (Color online) (a) XPS general spectrum of N-CNFs and the corresponding high resolution spectra of (b) C 1s and (c) N 1s.

Fig. 4. (Color online) (a) XRD pattern of N-CNFs. (b) Raman spectra of N-CNFs. (c) N₂ adsorption/desorption isotherms of N-CNFs and PAN/PVP nanofibers. (d) Thermogravimetric analysis of N-CNFs in the air.

Fig. 5. (Color online) Electrochemical performance of Li–S batteries with N-CNFs an interlayer. (a) Galvanostatic charge/discharge profiles at various cycles of the Li–S cells with a N-CNFs interlayer. (b) CV curves of the initial two cycles from 3.0 V to 1.0 V vs Li⁺/Li at a scan rate of 0.05 mV/s. (c) Cycling performance of the Li–S batteries using different interlayers and (d) rate capabilities at various current rates, from 0.1 C to 5 C and back to 0.2 C.