## 3 Results and Discussion  
As shown in Figure 2a, the nitrogen adsorption-desorption isotherm of the CaC and CaC/TU is typical type IV behavior with hysteresis loops according to the IUPAC classification [21], with a sharp capillary condensation step at relative pressures (P/P0) in the range of 0.45-0.99 evincing a mesoporous structure as well as a narrow mesopores size distribution. In addition, a steep increase of nitrogen uptake above 0.99 and a slow increase of nitrogen uptake below 0.45 can also be observed, indicating that both samples also have macropores and micropores in their structure The pore size distribution (Figure 2b) implies that the CaC/TU has smaller pores than CaC, but it is difficult to find any significant difference in their average pore sizes, as shown in Table 1. The pore size is estimated to be about 7.20 nm and 7.30 nm for the CaC and CaC/TU, and BET-specific surface area of the CaC and CaC/TU were 816 m\({}^{2}\) g\({}^{-1}\) and 929 m\({}^{2}\) g\({}^{-1}\). The TEM image was measured to support BET data in Figure 2c. The images also show the formation of the pores, with both the CaC and CaC/TU samples showing porosity. Figure 2: (**a**) Nitrogen adsorption-desorption isotherms and (**b**) BJH pore size distributions of the mesoporous carbon CaC (carbonized calcium citrate) and CaC/TU (with thiourea). (**c**) Transmission electron micrographs of the CaC and CaC/TU powders at high magnification. In order to confirm the existence of the sulfur and nitrogen elements in the CaC and CaC/TU samples, scanning transmission electron microscopy was carried out as shown in Figure 3. The dispersion of carbon, sulfur, and nitrogen in the carbon matrix was confirmed by the corresponding elemental mapping images. While the sulfur, nitrogen, and carbon elements were found in the CaC/TU sample, only carbon elements could be found in the CaC sample; there were no nitrogen or sulfur elements. In addition, the distribution of carbon elements looks like porous carbon, even though the dark-field image is not clear. This is good evidence to support that thiourea could make N/S-doping states in a porous carbon matrix. XPS was used to analyze the chemical constituents and functional groups of CaC/TU as shown in Figure 4a. As calculated from the XPS survey result (data not shown in this paper), the atomic percentages of C, O, N, and S in CaC/TU were 85.9, 5.12, 4.91, and 4.07 wt %, respectively. However, the XPS spectrum of CaC reveals the existence of C and O elements, and the absence of N and S elements. The high-resolution N 1s spectrum of the CaC/TU depicts three individual configurations attributed to pyridinic-N (398.1 eV), pyrrolic-N (400.1 eV), and graphitic-N (401.3 eV), which is very common in N-doped carbon materials [22; 23]. The pyridinic-N atom is located at the edges of the graphitic carbon layer by substituting a carbon atom on the C\({}_{6}\) ring (Figure 4b) [24], which contributes one pair of lone electrons and imparts basic properties to the carbon surface. Thus, the adsorption ability (with lithium polysulfides, Li\({}_{2}\)Sx-N bonding) of the carbons should be improved due to the strong interatomic attractions between the electrophilic Li\({}^{+}\) ion and the above-mentioned electronegative N atoms [25], thus enhancing the cycling performance of Li-S batteries. As shown in Figure 4b, the pyrrolic-N is a nitrogen atom on a five-membered ring and contributes two p-electrons to the \(\pi\) system, which may also favor surface adsorption, but its interaction is not as strong as that of pyridinic-N. The S 2p spectrum of CaC/TU can be deconvoluted into three peaks, corresponding to the S 2p\({}_{3/2}\) (at 163.9 eV), S 2p\({}_{1/2}\) (at