*2.3. Materials Characterization*

Microscopic morphology of the samples was observed using scanning electron microscopy (SEM, HITACHI-SU8220, Tokyo, Japan), and the corresponding element mapping on the surface of materials was analyzed by an energy dispersive spectrometer (EDS). The hole size distribution of samples was measured using a nitrogen adsorption–desorption apparatus (BET, BELSORP-miniII, BEL Japan Inc., Osaka, Japan).

## *2.4. Electrochemical Measurements*

The prepared anode sheet of lithium-ion batteries was a flexible carbon fiber, which did not require binders to be added and did not have to be coated on the copper foil, compared with traditional electrodes. The prepared hierarchical porous and Si/N co-doped

carbon fiber, after being sliced, can be used directly as a battery anode. The weight of the anode was ~0.76 mg, the diameter of the electrode was 9 mm, and the mass loading was ~1.2 mg cm<sup>−</sup>2. A metal lithium sheet was used as the counter electrode, the electrolyte was added in the ratio EC:DEC:EMC = 1:1:1 (*v*/*v*) to the solvent containing 20% fluoroethylene carbonate (FEC) and lithium hexafluorophosphate (LiPF6), and the separator was porous polypropylene. The electrochemical experiments were carried out by using commercial CR2032 cells at room temperature. The blue electric system (Land CT2001A, Blue Power Company) was set to maintain constant current charge–discharge, and the voltage range was from 0.01 to 3 V. The rate performance of batteries was tested at different current densities (0.05, 0.1, 0.2, 0.5 and 1 C, 1 C = 1000 mAh g<sup>−</sup>1). Princeton (PMC1000A) electrochemical workstation was used to test the cyclic voltammetry (CV) in the voltage range of 0.01~3 V with a scanning rate of 0.01 mV s−1. The electrochemical impedance spectroscopy (EIS) tests were carried out in a frequency range between 0.1 Hz and 100 kHz with an amplitude of 5 mV.
