## 3 Results and Discussion

Table 1: Some typical achievements for lithium oxygen cells in cathode materials, electrolytes, difference of charge and discharge voltage, and current density. Figure 3: Electrochmical properties of CNF electrodes. Voltage profiles of **(a)** CNF 1000, **(b)** CNF 1200 and (c) CNF 1400. Cycle performances of **(d)** CNF 1000, **(e)** CNF 1200, and **(f)** CNF 1400.  
tens of nanometer. However, it is observed that CNF 1200 and CNF 1400 electrodes have bigger discharging products (200-400 nm diameters) with a bead-like shape than that of CNF 1000 electrode. These results suggest that the formation mechanism of lithium peroxide could be greatly affected by the graphitization degree and surface defect of carbon cathode. After charging, the noticeable discharging product was not observed in all CNF electrodes, which indicates that discharge products Li\({}_{2}\)O\({}_{2}\) were totally decomposed. The morphological change of CNF electrodes after the fully discharging (cut-off voltage: 2.3 V) was further investigated (Figure S3). The toroidal shaped lithium peroxides formed on the surface of CNF electrodes without clogging even after full discharge, as shown in Figure S3. Interestingly, the CNF electrodes show the different Li\({}_{2}\)O\({}_{2}\) morphology as a function of the pyrolysis temperature. The CNF 1000 electrode with low graphitization degree and many defects shows film like Li\({}_{2}\)O\({}_{2}\) formation with small beads of \(\sim\)200 nm. In contrast, CNF 1200 and 1400 electrodes with higher graphitization and lower defect amount compared to CNF 1000 favor large toroidal Li\({}_{2}\)O\({}_{2}\) with 500 nm and 1 \(\upmu\)m, respectively. These results imply that the graphitization degree and functional groups on the surface play important roles for the formation shape of Li\({}_{2}\)O\({}_{2}\). Previous reports represented that the current density effects on the morphology evolution of the discharge product [21; 40; 41; 42]. Our results suggest further study is required to understand the dominant factors governing the shape of discharge product, which is critical to improve the electrochemical properties of Li-O\({}_{2}\) batteries including the rate capability, polarization and round-trip efficiency. Figure 5 shows X-ray diffraction (XRD) of the CNF electrodes after discharging and the Raman results of the CNF electrode after 1 cycle. The XRD results of CNF electrodes in Figure 5a reveal the formation of lithium peroxide (Li\({}_{2}\)O\({}_{2}\)) without Li\({}_{2}\)O and LiOH [7; 43]. The Li\({}_{2}\)Co\({}_{3}\) of byproducts peak observed from XRD pattern [44; 45]. In the Raman spectra (Figure 5b), the intensity ratios of the D-band to the G-band for CNF electrodes after 1 cycle increase significantly compared to those of pristine CNF electrodes. The I\({}_{\textrm{D}}\)/I\({}_{\textrm{G}}\) of CNF 1000 electrode after 1 cycle is 1.6, which is 64.9% and 55.3% higher than those of CNF 1200 (I\({}_{\textrm{D}}\)/I\({}_{\textrm{G}}\) -1.03) and 1400 (I\({}_{\textrm{D}}\)/I\({}_{\textrm{G}}\) -0.97) electrodes after 1 cycle. The intensity ratio of I\({}_{\textrm{D}}\)/I\({}_{\textrm{G}}\) of the CNF webs after 1 cycle show similar trend to those of the pristine CNF webs. And notably, the values of I\({}_{\textrm{D}}\)/I\({}_{\textrm{G}}\) of the CNF webs after 1 cycle are increased compared those of pristine CNF webs, which results from increasing defectivity after the electrochemical cycling [46]. Figure 4: Electron microscope images of (**a–d**) CNF 1000, (**e–h**) CNF 1200 and (**i–l**) CNF 1400 after the discharging and charging.