## 3 Results and Discussion  
Figure 2 show the microscopic morphology of carbon nanofibers before activation and after activation for 30 min by ammonia. It can be seen from the images that the carbon nanofiber has a good fiber shape, and its diameter is about 300 nm. The SEM images in Figure 2a show that some particulate structures emerge on the surface of the carbon nanofibers before activation, which is caused by pre-oxidation and carbonization. In Figure 2b, the surface of the carbon nanofibers after activation for 30 min by ammonia not only has no small agglomerations but also shows the appearance of abundant pores, including the microporous, mesoporous, and macroporous structures produced by ammonia activation. The hierarchical and heterogeneous porosity structure is instrumental in increasing the active sites and facilitates the transport of ions of flexible carbon anode materials [7; 8; 9]. To analyze the types and states of surface elements of the carbon nanofibers after different ammonia activation times, all NH\({}_{3}\)-activated carbon nanofibers were tested by XPS. The full spectrum in Figure 3a shows that these carbon nanofibers after ammonia activation contained three elements: C, N, and O; the corresponding three orbits C\({}_{18}\), N\({}_{18}\), and O\({}_{18}\), were located at 285, 400, and 533 eV, respectively. The C/N atomic ratios of CNFs-10 NH\({}_{3}\), CNFs-20 NH\({}_{3}\), CNFs-30 NH\({}_{3}\), and CNFs-40 NH\({}_{3}\) were 457%, 5.64%, 5.76%,  
Figure 2: SEM images of carbon nanofibers (**a**) before activation and (**b**) after activation for 30 min by ammonia. and 6.76%, respectively. The results show that ammonia activation can not only make pores but can also dope N, and with the increase in ammonia activation time, the N content in the carbon nanofibers also increased. Furthermore, Figure 3b-e show the N\({}_{\rm 1s}\) spectra of the carbon nanofibers after different ammonia activation times. It can clearly be seen that all samples have two peaks of pyridinic nitrogen (N-6) and pyrrolic/pyridone nitrogen (N-5), which are located near 398.5 and 400.1 eV, respectively. It is reported that the existence of pyridine nitrogen and pyrrole nitrogen in carbon nanofibers can increase the conductivity [12; 13]. Simultaneously, pyridine nitrogen and pyrrole nitrogen have a strong ability to adsorb lithium, which can improve the lithium storage performance of carbon nanofibers. The effect of the different ammonia activation times on the electrochemical properties of the carbon nanofiber anode was tested in lithium-ion batteries. Figure 4a show the cycle performance curves of the CNFs-0 NH\({}_{3}\), CNFs-10 NH\({}_{3}\), CNFs-20 NH\({}_{3}\), CNFs-30 NH\({}_{3}\), and CNFs-40 NH\({}_{3}\) anodes at the current density of 50 mA g\({}^{-1}\) from 0.01 to 3 V. The digital picture in Figure 4a shows the anode sheet activated by ammonia, which is flexible and foldable and can be used directly as a flexible lithium-ion anode without the addition of binders. It should be noted that the capacity vs. cycle number curves (and the CE curves as well) exhibit a lot of fluctuations (or wavy nature), especially at the slow rate of 50 mA g\({}^{-1}\). This is mainly because the temperature of the laboratory was affected by the ambient temperature, which led to the temperature fluctuation of the test battery near the room temperature. However, in the whole test process, this performance change trend was basically correct, and the data can explain the performance of the materials. With the increase in the ammonia activation time, the initial capacity of the carbon nanofiber anode gradually increased. After 80 cycles, the reversible capacity of the CNFs-0 NH\({}_{3}\), CNFs-10 NH\({}_{3}\), CNFs-20 NH\({}_{3}\), CNFs-30 NH\({}_{3}\), and CNFs-40 NH\({}_{3}\) anodes was 406.3, 424.4, 440.2, 728.1, and 689.2 mAh g\({}^{-1}\), respectively, indicating that the CNFs-30 NH\({}_{3}\) anode had the best cycle stability.