## 3 Results and Discussion
It could be obviously observed that the PCNF anode showed a good invertible capacity of 317.7 mAh g\({}^{-1}\) at initial discharge process and remained at 278 mAh g\({}^{-1}\) for the 1000 th circulation, with a high capacity retention of 87.51% and a capacity decrease ratio as low as \(\sim\)0.013% for each time. In contrast, the CNF sample exhibited a relative poor discharge specific capacity of 109.7 mAh g\({}^{-1}\) at the initial discharge process and 88.3 mAh g\({}^{-1}\) after cycling for 1000 times, respectively. Based on these results, the PCNF electrode sample with hierarchical pore thin film structures delivers a remarkable rate capability as well as cycling ability which was better than the carbon materials reported in previous work (Table 1). The porous network structure derived from PVP precursor could speed up the conveying of both electron and lithium ion in order to enhance the electrochemical property and serve as a feasible electrode for LIBs. Finally, electrochemical impedance spectroscopic (EIS) experiments were employed to further understand the kinetics of the CNF and PCNF samples. As shown in Figure 5d, the EIS Nyquist plots consisted of a depressed semicircle located on the high-frequency region and an oblique line seated at the low-frequency region for both of the CNF and PCNF electrodes, which accorded with the electrode/electrolyte interphase interfacial charge-transfer resistance (R\({}_{\text{ct}}\)) [49; 50] and the Warburg impedance (W\({}_{\text{p}}\)). Obviously, the PCNF electrode (90 \(\Omega\)) showed a much lower value of R\({}_{\text{ct}}\) than that of the CNF (135 \(\Omega\)) because of its large BET and multi-level porous framework which could facilitated transportation of both electron and ion, bringing about an excellent rate performance and outstanding cycling ability. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & **Initial Discharge Capacity** & **Rate Performance** & **Cycling Performance** & **Ref.** \\ \hline Short Carbon Nanotubes & 1291 mAh g\({}^{-1}\) at 25 mA g\({}^{-1}\) & - & 410 mAh g\({}^{-1}\) at 25 mA g\({}^{-1}\) after 50 cycles & [21] \\ Porous carbon nanofibers & 1900 mAh g\({}^{-1}\) at 50 mA g\({}^{-1}\) & - & - & [51] \\ Carbon nanofiber & 885 mAh g\({}^{-1}\) at 50 mA g\({}^{-1}\) & 345 mAh g\({}^{-1}\) at 0.54 C & 454 mAh g\({}^{-1}\) at 0.13 C after 80 cycles & [52] \\ Free-standing carbon nanofibers & 826 mAh g\({}^{-1}\) at 200 mA g\({}^{-1}\) & 125 mAh g\({}^{-1}\) at 900 mA g\({}^{-1}\) & 143 mAh g\({}^{-1}\) at 200 mA g\({}^{-1}\) after 200 cycles & [53] \\ Porous carbon nanofibers & 950 mAh g\({}^{-1}\) at 150 mA g\({}^{-1}\) & 256 mAh g\({}^{-1}\) at 900 mA g\({}^{-1}\) & 354 mAh g\({}^{-1}\) at 200 mA g\({}^{-1}\) after 100 cycles & [54] \\ PVP-derived carbon nanofibers & 1770 mAh g\({}^{-1}\) at 100 mA g\({}^{-1}\) & 650 mAh g\({}^{-1}\) at 250 mA g\({}^{-1}\) & 766 mAh g\({}^{-1}\) at 100 mA g\({}^{-1}\) after 100 cycles & [55] \\ Nitrogen Doped Porous Carbon & 880 mAh g\({}^{-1}\) at 0.1 C & 190 mAh g\({}^{-1}\) at 10 C & 410 mAh g\({}^{-1}\) at 5 C after 500 cycles & [38] \\ Lignin-based fibrous nanocarbon & 1471 mAh g\({}^{-1}\) at 50 mA g\({}^{-1}\) & 75 mAh g\({}^{-1}\) at 1000 mA g\({}^{-1}\) & 110 mAh g\({}^{-1}\) at 300 mA g\({}^{-1}\) after 1000 cycles & [56] \\ Porous Carbon Nanofibers Film & 2070 mAh g\({}^{-1}\) at 0.1 C & 101 mAh g\({}^{-1}\) at 15 C & 278 mAh g\({}^{-1}\) at 5 C after 1000 cycles & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The comparison of electrochemical performance of various carbon materials.