## 3 Results and Discussion  
The morphology of the synthesized SP/C was analyzed using TEM and HR-TEM. From the TEM image (Figure 2a), it was confirmed that the particles of SP/C were ball-shaped, similar to carbon black, and were connected to each other. The SEM image is also shown in Figure S1. This structure revealed that SP/C had channels, through which Na ions could easily move between particles. Previous studies revealed that this structure facilitated fast electrochemical reactions, resulting in efficient Na ion exchange, even at large current densities [18,19]. In addition, the EDS results confirmed that C, S, and P were evenly distributed in the synthesized material. The elemental composition of SP/C was C, S, and P at 75.1, 18.6, and 6.3 wt.%, respectively, according to the EDS results. Figure 3a shows the XRD pattern of SP/C. The peaks observed in the (002) and (100) planes represent typical graphite peaks. These peaks are widespread, indicating that SP/C has an amorphous structure. To investigate the changes in the XRD pattern caused by S and P doping, an XRD pattern of an undoped carbon material synthesized by the same  
Figure 2: Morphology of SP/C (**a**) TEM image, (**b**) HR-TEM image and (**c**) EDS mapping. synthesis method, using a xylene precursor, was obtained (Figure S2). However, the SP/C and undoped carbon are both amorphous carbon structures; therefore, there is no noticeable peak resulting from heteroatom doping. The d-spacing value of SP/C was calculated to be \(\sim\)0.40 nm using Bragg's equation. Furthermore, based on the Scherrer equation, the c-axis length was calculated to be 1.58 nm, indicating that the number of stacked graphitic sheets was approximately four layers. This nanocrystalline carbon structure can improve specific surface areas with sufficient pore volumes to facilitate rapid Na ion exchange [20]. In addition, the pore structure of SP/C was analyzed using the N\({}_{2}\) adsorption-desorption isotherms (Figure 3b). The specific part of SP/C was measured at 92.06 m\({}^{2}\) g\({}^{-1}\) using the BET measurements, and this large specific surface area indicated that the contact surface between the electrode and electrolyte interface is wide, thereby facilitating Na ion exchange. The data related to the BET plot and the fitted line used to calculate the specific surface area are shown in Figure S3 and Table S1. The pore size distributions measured by the BJH and MP methods are shown in Figure 3c,d, respectively. The pore size was distributed over 12 nm, which indicated the abundant formation of macropores. However, micropores hardly existed. These large and small pores facilitated fast sodium ion exchange, and the large specific surface area significantly increased the storage volume by the adsorption of sodium ions. Furthermore, the macropore and total pore volume was 1.0317 cm\({}^{3}\) g\({}^{-1}\), and the average pore diameter was 33.046 nm. Figure 4a shows the first-cycle charge/discharge profile at a current density of 1 A g\({}^{-1}\). Although it was a nanomaterial, SP/C exhibited a high first-cycle Coulombic efficiency of approximately 84.6%, owing to the use of the plasma-in-liquid process. Previous studies confirmed that a solid electrolyte interface (SEI) layer is not formed in an ether-based electrolyte [21,22]. Therefore, the absence of an SEI layer may affect the high initial Coulombic  
Figure 3: (**a**) XRD patterns of SP/C; (**b**) N\({}_{2}\) adsorption–desorption isotherm; (**c**) BJH pore size distribution; (**d**) micropore size distribution. efficiency. The oxidation/reduction of SP/C, in the potential window, was investigated using the CV curve, as shown in Figure 4b. In the first cycle, a cathodic peak was observed at approximately 1.2 V. This could be attributed to the Na ions trapped inside the carbon material in the anode. When the Na ions intercalated into the carbon material, the occurrence of local and large expansion inside the structure trapped some Na ions in the intercalated position [23].