## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c} \hline  & **CaC** & **CaC/TU** \\ \hline S.A., a (m\({}^{2}\) g\({}^{-1}\)) & 816 & 929 \\ VP, b (cm\({}^{3}\) g\({}^{-1}\)) & 1.39 & 1.76 \\ DP, c (nm) & 7.20 & 7.30 \\ \hline \end{tabular} a Brunauer–Emmett–Teller (BET) surface area; b Total pore volume; c Barrett–Joyner–Halenda (BJH) adsorption average pore diameter.  
\end{table} 
Table 1: Textural properties of the mesoporous carbon. Figure 3: Scanning transmission electron micrographs and elements mapping images of the (a) CaC and (b) CaC/TU powders. 165.1 eV), and C-SO\({}_{x}\) (oxidized S) bonds (at 168.8 eV). [PERSON] et al. reported that the hetero-doped nitrogen/sulfur sites were demonstrated to show strong binding energy (E\({}_{b,N/S}\) = 1.82 eV; this value is much higher than that of single doping of nitrogen or sulfur--E\({}_{b,N}\) = 1.43 eV and E\({}_{b,S}\) = 1.02 eV) and be capable of anchoring polysulfides based on first-principles calculations and experimental evidence through electrochemical performance [17]. Therefore, the N and S co-doping guarantee the strong affinity of polysulfides to the carbon surface. Figure 5a shows the XRD patterns of samples like elemental sulfur, CaC and CaC/TU mesoporous carbon powders, and the CaC-S and CaC/TU-S composites. The XRD patterns of both CaC and CaC/TU powders show two broad peaks at \(\sim\)23 and 44\({}^{\circ}\), attributed to the (002) and (100) reflections, respectively. The broad low-intensity diffraction peaks reflect the disordered carbon structures. After sulfur loading in the mesoporous carbon templates, both the CaC-S and CaC/TU-S composites show overlapped sharp sulfur peaks on mesoporous carbon diffraction, indicating the presence of well-crystallized sulfur (orthorhombic sulfur phase, JCPDS card no. 08-0247). Figure 4: (**a**) XPS spectra of the CaC and CaC/TU powders. (**b**) Schematic for the formation of N- and S-doped mesoporous carbon via reaction between calcium citrate and thiourea. The thermal decomposition characteristics of the CaC-S and CaC/TU-S composites in a nitrogen atmosphere were investigated using TGA. The TGA curves (Figure 5b) show that the weight losses of the CaC-S and CaC/TU-S composites ranged up to 71.2 wt % and 79.5 wt %, respectively, consistent with the proportions of the added amounts. Consequently, the sulfur content of the CaC/TU-S composite was higher than that of CaC-S, owing to the increased BET surface area and pore volume. Figure 6a shows the first charge-discharge profiles of the CaC-S and CaC/TU-S cells at 0.1 C (1 C = 1672 mA g\({}^{-1}\)). The specific capacities were measured and calculated by compensating loaded sulfur weight ratio. In the first cycles, both CaC-S and CaC/TU-S cells showed three plateaus in the discharge curve. The first plateau occurred nearly at 2.4 V, and then sloped down to a second plateau at 2.1 V and a third plateau was seen in a rapid drop to 1.8 V. These phenomena are represented by the formation of long-chain soluble polysulfides (Li\({}_{2}\)S\({}_{x}\): 6 \(<x<\) 8) in the first region and short-chain solid sulfides (Li\({}_{2}\)S\({}_{2}\) and Li\({}_{2}\)S) in the second, and loss of recursive ability in the third region [26, 27, 28]. This shows the greater coulombic efficiency in the CaC/TU-S cell. The coulombic efficiencies of CaC-S and CaC/TU-S cells were 92% and 99% for the first cycle, respectively. While the initial charge capacity was exactly the same in both cells, the initial discharge capacity of the CaC-S cell (1120 mAh g\({}^{-1}\)) was slightly higher than that of the CaC/TU-S cell (1052 mAh g\({}^{-1}\)). It is difficult to find significant differences between two first charge-discharge profiles. The CaC-S cell not only had a lower discharge capacity than the CaC/TU-S cell, but a lower retention as well. When increased from the 10 th to the 100 th cycle, the ratios of reduction in discharge capacity of CaC-S and CaC/TU-S cells were 71% and 86%, respectively. Figure 5: (a) XRD patterns of elemental sulfur, the CaC, CaC/TU, CaC-S, and CaC/TU-S powders.