*2.2. Preparation of Materials*

The flow chart of preparation of the hierarchical porous and Si/N co-doped carbon nanofiber anode materials is shown in Figure 1. Firstly, 0.1 g graphene, 2 g polyacrylonitrile (PAN) and a certain amount of silicon were dissolved in 50 mL N,N-dimethylformamide (DMF) and vibrated with ultrasound for 20 min. Then, the pore-making agent polymethylmethacrylate (PMMA) was added to this solution, heated at 70 °C using a water bath and stirred for 10 h. The obtained precursor solution was continuously spun on the flat plate

for about 1 h to obtain a fiber cloth by the gas–electric co-spinning method. The feed speed was 4 mL h−1, the voltage was 5 kV, and the air flow was 10 Psi. Subsequently, the fiber cloth was placed in a blast drying oven for pre-oxidation, in which the temperature was raised from room temperature to 280 ◦C with a heating rate of 2 ◦C min−<sup>1</sup> for 6 h. The purpose of pre-oxidation was to make PAN undergo three chemical reactions: cyclization, dehydrogenation and oxidation in this process, so as to make the carbon fiber cloth more stable before carbonization. Finally, the fiber cloth was calcined at 950 °C under the protection of argon for 1 h, and then activated in ammonia for 30 min. High-temperature tubular furnace was used to increase the temperature from room temperature to 200 °C with a heating rate of 2 ◦C min<sup>−</sup>1, and then the temperature was reduced to room temperature to obtain the hierarchical porous and Si/N co-doped carbon nanofiber anode materials.

**Figure 1.** The flow chart of preparation of the hierarchical porous and Si/N co-doped carbon nanofiber anode materials.

In the process of preparing the materials above, by changing the mass ratio of PAN:PMMA = 2:1, 2:2, 2:3 and 2:4, carbon nanofibers with different proportions of PMMA were obtained (named CNFs-PMMA1, CNFs-PMMA2, CNFs-PMMA3 and CNFs-PMMA4, respectively, according to the added PMMA masses of 1 g, 2 g, 3 g and 4 g). After determining the optimal amount of PMMA, according to the added Si mass (0%, 5%, 10% and 15%, the mass ratio of Si/PAN), the hierarchical porous Si/N co-doped carbon nanofiber anode materials were named CNFs-Si0%, CNFs-Si5%, CNFs-Si10% and CNFs-Si15%, respectively.
