## 2 Results and Discussion
### Electrochemical Performance of Li-S Batteries with the Modified Separators
As for Co@g-C\({}_{3}\)N\({}_{4}\)/G (600 \({}^{\circ}\)C), highly reduced graphene with obvious large-size sheets can hinder the transport of Li ions across the functional separator [61], resulting in the continuous accumulation of polysulfides over long-term cycling and then undergoing a rapid capacity decay. Hence, these observations indicate that the Co@g-C\({}_{3}\)N\({}_{4}\)/G (550 \({}^{\circ}\)C) material with superior electronic conductivity and Li-ion transfer capability is well fitted in separator modification for the better reuse of active sulfur. Given this, Co@g-C\({}_{3}\)N\({}_{4}\)/G (550 \({}^{\circ}\)C) as a separator modification material was adopted to compare with other controlled samples in the following electrochemical performance discussions. In this section, the whole obtained materials calcined at 550 \({}^{\circ}\)C were used to modify the PP separator for comparison of electrochemical performances in Li-S cells. As shown in Figure 6a, it was clearly observed that the cell with the Co@g-C\({}_{3}\)N\({}_{4}\)/G separator exhibited a superior rate property, delivering specific capacities of 1213, 1020, 935, and 860 mAh g\({}^{-1}\) at 0.2, 0.5, 1.0, and 2.0 C, respectively, which are much higher than those of other cells with corresponding separators. As for the cell with the CoO/G-PP separator, it showed the worst rate performance, which is mostly due to the hindrance of the large-size sheet G coating layer to Li ion transfer and electrolyte penetration. This reason can be supported by other previously reported papers using graphene- and GO-functioned separators [16; 23; 62]. Figure 6b and Figure S5 show the representative charge-discharge profiles of cells with corresponding separators, all displaying two discharge plateaus and a long charging slope. Besides, we also calculated that the voltage hysteresis between charge and discharge in the four cells were 91.7, 153.9, 169.0, 152.6 mV, from the voltage curves in Figure 6b, revealing the smallest polarization in the cell with the Co@g-C\({}_{3}\)N\({}_{4}\)/G-PP separator. Further, this result can be confirmed from the CV curves in Figure 6c; two reduction peaks were  
Figure 5: (**a**) Rate performances, (**b**) change in the high-plateau capacity and low-plateau capacity, and (**c**) long cycling performances of Li-S cells with the corresponding Co@g-C\({}_{3}\)N\({}_{4}\)/G-PP separators. observed at 2.32 (A) and 2.02 V (B) and two oxidation peaks were observed at 2.31 (C) and 2.42 V (D) for the cell with the Co@g-C\({}_{3}\)N\({}_{4}\)/G-PP separator, which exhibited a higher CV current and smaller polarization between peaks A and C compared to the control cell, meaning more enhanced redox reaction kinetics for polysulfide conversion in the charge-discharge process [25; 59]. This is also supported by the EIS measurements (Figure 6g), the lowest charge transfer resistance (\(R_{cl}\): 49.4 \(\Omega\)) for the cell with the Co@g-C\({}_{3}\)N\({}_{4}\)/G-PP separator among the three fresh cells, which is possibly ascribed to the improved catalytic conversion of polysulfides and the high electrical conductivity of the Co@g-C\({}_{3}\)N\({}_{4}\)/G interlayer [63]. To further determine the catalytic activity of the Co@g-C\({}_{3}\)N\({}_{4}\)/G heterostructure, Figure 6d shows the cyclic voltammetry (CV) curves of symmetrical cells assembled using two Co@g-C\({}_{3}\)N\({}_{4}\)/G-coated electrodes tested in the Li\({}_{2}\)S\({}_{6}\) electrolyte. The CV curves of symmetric cells with respective electrodes all exhibited distinct peaks, which belonged to the reduction/oxidation of polysulfides. Since only Li\({}_{2}\)S\({}_{6}\) served as the active material in this case, it was first reduced to short-chain polysulfides (peak A) and then Li\({}_{2}\)S (peak B) during the discharge process. In the subsequent charge process, Li\({}_{2}\)S was reversibly oxidized to polysulfides (peaks C and D) and the final sulfur element [64; 65].