## 3 Results and Discussion
Figure 6: Charge/discharge profiles of the CaC-S and CaC/TU-S cells: (a) 1 st cycle at 0.1 C rate; (b) 10 th and 100 th cycles at 1 C rate. However, the CaC/TU-S cell exhibited higher discharge capacities than the CaC-S cell, as compared in the cycling response in Figure 7a. The cells were cycled at 0.1 C for the first five cycles, followed by an additional 100 cycles at 1 C. Both samples showed fast capacity fading during the initial five cycles at 0.1 C, possibly due to the dissolution of lithium polysulfides mainly present on the surface of mesoporous carbon [4]. During the 1 C cycling, the CaC/TU-S cell delivered an initial capacity of 667 mAh g\({}^{-1}\) and a capacity of 520 mAh g\({}^{-1}\) after 100 cycles, giving a capacity retention of 78.0%. In contrast, the CaC-S cell showed an initial capacity of 513 mAh g\({}^{-1}\) and a capacity of 310 mAh g\({}^{-1}\) after 100 cycles, giving a capacity retention of only 60.5%. The CaC/TU-S also showed a stable coulombic efficiency as high as 98.7% after 100 cycles at 1 C, while the coulombic efficiency of the CaC-S cell fluctuated at lower levels than that of the CaC/TU-S cell, indicating electrochemical inefficiencies in the CaC-S cell over repeated cycles. To provide further insights into the Li-S storage properties, electrochemical impedance spectroscopy was conducted to compare the electrical conductivities of the CaC-S and CaC/TU-S cells after 105 cycles. Impedance is a powerful diagnostic tool to investigate physical and chemical processes that occur on electrode/electrolyte interfaces during cycling and to identify the causes of degradation. The equivalent circuit is shown in the inset of Figure 7b, where R\({}_{8}\) and R\({}_{ct}\) represent the electrolyte resistance and charge transfer resistance between the solid electrolyte interphase layer and electrode interface, respectively, and W is the Warburg impedance for Li\({}^{+}\) diffusion in the electrode. The R\({}_{ct}\) of the CaC/TU-S cell was much smaller than that of the CaC-S cell, supporting the hypothesis that nitrogen and sulfur co-doped mesoporous carbon leads to a stronger bonding strength and better  
Figure 7: (**a**) Cycling performance of the CaC-S and CaC/TU-S cells; (**b**) Nyquist plots of cycled cells; and (**c**) Photographs of electrode, polypropylene (PP) separator, and Li metal of cycled cells disassembled in fully charged state after the initial 100 cycles. encapsulation of insulating sulfur in the mesoporous carbon pores than that of the sample without any doping state. As can be clearly seen in Figure 7c, the black surface of the CaC-S electrode faded partially to grey, and the original white color of the PP separator changed to yellow after 105 cycles. It is evident that the amount of sulfur residues dissolved out of the CaC-S electrode was much larger than that out of the CaC/TU-S, which showed much smaller capacity drop as compared in Figure 7a. We compared the data reported in several preceding papers (Table 2) and found that the data for CaC/TU was either slightly better than or similar to those for reported materials. Sulfur remains on the surface of porous carbon, leading to a reduced reactivity of the sulfur as well as facile dissolution of polysulfides into the electrolyte. Calcium citrate is prepared by adding only the thiourea without activation--a simpler process than for other materials, which is a great advantage. Performance was also observed to remain stable. Later, we will use the advantages of the CaC/TU-S composite to increase the cycle life and conduct experiments, and investigate what will happen with high loading mass.