**3. Results and Discussion**

Figure 2a shows the digital photo of a hierarchical porous and Si/N co-doped carbon nanofiber cloth, which is flexible and foldable. It can be used directly as a flexible lithiumion anode without the addition of binders. The microstructure of the carbon nanofiber cloth is further tested, and the SEM images of CNFs-PMMA1 and CNFs-PMMA3 are shown in Figure 2b,c. The diameter of the carbon nanofibers is about 400 nm and the fibers are crosslinked. With the increase in the PMMA content from CNFs-PMMA1 to CNFs-PMMA3, many macropores of different sizes emerge in the fibers. It is speculated that micropores and mesopores may also exist in the fibers. However, it is difficult to judge whether there are smaller holes through SEM images. Therefore, the pore size distribution is further explored by a nitrogen adsorption–desorption test in the follow-up study.

**Figure 2.** (**a**) A digital photo of the hierarchical porous and Si/N co-doped carbon nanofiber cloth, SEM images of (**b**) CNFs-PMMA1 and (**c**) CNFs-PMMA3.

To obtain the optimal addition content of PMMA, the cycle and rate performances of CNFs-PMMA1, CNFs-PMMA2, CNFs-PMMA3, and CNFs-PMMA4 anodes in lithiumion batteries are tested, and the results are used as the basis for selecting the content of PMMA. Figure 3a shows cycle performance curves of the four anodes at a current density of 0.05 C, from 0.01 to 3 V. The CNFs-PMMA3 anode has the highest initial capacity of 1437.2 mAh g−<sup>1</sup> among the four anodes. After 100 cycles, the CNFs-PMMA3 anode can still reach 985.3 mAh g−1. Figure 3b shows the rate performance curves of four anodes. Compared with CNFs-PMMA1, CNFs-PMMA2, and CNFs-PMMA4 anodes, the CNFs-PMMA3 anode shows excellent rate performance. The specific capacity of the CNFs-PMMA3 anode is 1469.9, 1307.6, 1213.1, 956.3 and 686.3 mAh g−<sup>1</sup> at the current density of 0.05, 0.1, 0.2, 0.5 and 1 C, respectively. Therefore, the amount of PMMA added in CNFs-PMMA3 is selected as the optimal result.

**Figure 3.** (**a**) Cycle performance and (**b**) rate performance curves of CNFs-PMMA1, CNFs-PMMA2, CNFs-PMMA3, and CNFs-PMMA4 anodes in lithium-ion batteries.

After determining the amount of PMMA, the effect of the silicon content on the hierarchical porous carbon nanofiber anode is further studied. The microscopic morphologies of CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15% materials are shown in Figure 4a–d. These carbon nanofibers have a good fiber shape with a diameter of about 1 μm, which are randomly intertwined to form a fiber cloth. With the increase in the Si content from CNFs-Si0%, CNFs-Si5%, CNFs-Si10% to CNFs-Si15%, the white particles increase, due to the agglomeration of nano silicon particles. The SEM image of CNFs-Si10% and the corresponding EDS mapping of elements are shown in Figure 4e. Some holes exist in the fiber, and the nanofiber is cross-linked. In addition, Si, C and N elements are evenly distributed, indicating that Si and N are effectively doped in nanofibers. It is worth noting that the N element is doped through ammonia activation during the experiment. It was reported that N doping can improve the conductivity of carbon materials [28–30], which is not discussed further in this work.

**Figure 4.** SEM images of (**a**) CNFs-Si0%, (**b**) CNFs-Si5%, (**c**) CNFs-Si10% and (**d**) CNFs-Si15%. (**e**) SEM image and the corresponding EDS mapping of Si, C, and N elements in partial CNFs-Si10%.

Nitrogen adsorption–desorption curves of CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15% are shown in Figure 5a. When P/P0 > 0.4, a hysteresis loop appears in the nitrogen adsorption–desorption curves of all samples, which belong to the IV isotherm. The corresponding pore size distribution curves are shown in Figure 5b, which show that all samples have micropores (pore size less than 2 nm) and mesopores (pore size between 2 and 50 nm), which aligns with the experimental expectation. Combined with the SEM images and EDS mapping in Figure 4, this result indicates that a Si/N co-doped carbon nanofiber anode with a hierarchical porous structure has been synthesized.

**Figure 5.** (**a**) Nitrogen adsorption–desorption curves, and (**b**) the corresponding pore size distribution curves of CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15%.

Figure 6a shows cycle performance curves of CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15% anodes at 0.05 C, from 0.01 to 3 V, in lithium-ion batteries. The initial capacity of CNFs-Si10% is 1737.2 mAh g−1, and still maintains a high reversible capacity of 985.3 mAh g−<sup>1</sup> after 100 cycles, which is the highest capacity among the four anodes. CNFs-Si10% and CNFs-Si15% anodes have a high initial capacity, which can mainly be due to the addition of Si with high capacity, compared with CNFs-Si0%. The capacity of CNFs-Si15% anode decreases significantly after 20 cycles, because the nano-silicon in the materials experiences large volume expansion with the increase in silicon content, destroying its structure. The initial Coulombic efficiency of the CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15% anodes is only 49.72%, 50.24%, 77.82% and 50.74%, respectively. The low initial coulombic efficiency is due to the fact that with the addition of the pore-forming agent PMMA, the number of micropores increases, providing many reactive sites, thus forming a high irreversible capacity with the formation of an SEI film with a large area during the first charge–discharge process. After the first charge–discharge, the Coulombic efficiency of each anode is close to stable, basically around 90%, which can be attributed to meso/macropores, which can offer more active sites for Li storage and enable the electrolyte to penetrate the inner part of carbon nanofibers, improving the electrolyte/electrode contacting area.

Figure 6b shows the rate performance curves of CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15% anodes in lithium-ion batteries at different current densities. The CNFs-Si10% anode still exhibits excellent specific capacities of 1719.9, 1593.1, 1484.3, 1255.7 and 995.8 mAh g−<sup>1</sup> at 0.05, 0.1, 0.2, 0.5 and 1 C, respectively. It can be seen that when the current density changes to 0.05 C again, the specific capacity of the CNFs-Si10% anode is greater than 1518.1 mAh g−1, indicating good rate performance. This is because the synergistic effect of N doping and the hierarchical porous structure can improve the conductivity, increase the reactive sites, and improve the Li-ion transport rate. The results of cycling and rate performances indicate that proper silicon/nitrogen co-doping and hierarchical porous structure regulation are important to improve the electrochemical performance of carbon nanofiber anodes. To the best of our knowledge, the outstanding electrochemical performances of the CNFs-Si10% anode are much higher than other reported flexible carbon fiber anodes for lithium-ion batteries (Table 1).

**Figure 6.** (**a**) Cycle performance and (**b**) rate performance curves of CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15% anodes in lithium-ion batteries. (**c**) The charge–discharge curves of the CNFs-Si10% anode, (**d**) CV curves of the CNFs-Si10% anode, and (**e**) Nyquist plots of CNFs-Si0% and CNFs-Si10% anodes in lithium-ion batteries.

**Table 1.** Electrochemical performance of flexible carbon fiber anodes for lithium-ion batteries reported in this work.


Figure 6c shows galvanostatic charge–discharge curves of the CNFs-Si10% anode during the 1st, 10th and 50th cycle at 0.05 C in the voltage range of 0.01–3 V. During the first cycle, the charge–discharge capacity is 2232.5 and 1737.2 mAh g−1, respectively, corresponding to an initial Coulombic efficiency of 77.82%. Irreversible capacity loss is mainly due to the formation of the SEI film and the decomposition of the electrolyte during the initial charge–discharge process [48]. From the 10th to 50th cycle, the discharge capacity dramatically reduces from 1640 to 1421.7 mAh g−1, which may be due to the production of a residual irreversible side reactant after charging and discharging multiple times. It is

noteworthy that the Coulombic efficiency value increases to 95.7% and 96.6% during the 10th and 50th cycle, respectively.

To further appraise the lithium storage performance of the CNFs-Si10% anode, CV curves of five initial cycles were obtained from 0.01 to 1.5 V at a scan rate of 0.1 mV s−1, as shown in Figure 6d. During the first lithiation, generation of the SEI film and the decomposition of the electrolyte could lead to a broad irreversible peak at 1.3 V, which disappears in the following cycles. Subsequently, a sharp anodic peak below 0.1 V corresponds to the phase transformation from Si to LixSi and lithium-ion insertion with carbon nanofiber materials in the CNFs-Si10% anode [48]. Upon delithiation, two cathodic peaks at around 0.31 and 0.5 V are attributed to the dealloying process of LixSi into Si, which is consistent with the reported results [49]. In the second cycle, a broad anodic peak located at around 0.19 V appears, which is associated with the lithiation of Si. In the following cycles, the cathodic and anodic peak positions are consistent, indicating good concurrency and reversibility. For the anodic peak at 0.19 V and the two cathodic peaks at 0.31 and 0.5 V, the current intensities gradually enlarge in subsequent cycles, corresponding to the activation process as a result of the reaction of more active sites with lithium-ion [50]. This phenomenon is typical for the Si-based anode for lithium-ion batteries [51,52]. The results show that the CNFs-Si10% anode has good stability.

To further explore the effect of Si doping on the performance of lithium-ion batteries, the EIS of CNFs-Si0% and CNFs-Si10% anodes were tested. Figure 6e shows the Nyquist plots of CNFs-Si0% and CNFs-Si10% anodes in lithium-ion batteries. The curve consists of an overlapping semicircle in the high-frequency region and an oblique line in the lowfrequency region. The semicircle at high frequency refers to the diffusion and migration process of Li<sup>+</sup> in the SEI, and the oblique line at low frequency represents the migration impedance of Li+ in the active substance [53]. The semicircle of the CNFs-Si0% anode is almost the same as that of the CNFs-Si10% anode, meaning that the small amount of silicon doping does not reduce the impedance of CNFs. Compared with the CNFs-Si0% anode, the CNFs-Si10% anode has a larger slope at low frequency. The larger the slope, the lower the Li-ion diffusion resistance. Therefore, the CNFs-Si10% anode has a higher capacity and lower lithium-ion diffusion resistance than the CNFs-Si0% anode. Meanwhile, the excellent cycle and rate capability results of the CNFs-Si10% anode also show that the nano silicon is dispersed in the porous of carbon fibers, and the porous structure can provide a buffer space for the Si volume change during the Li deintercalation process [53,54].
