## 3 Results and Discussion
The electrochemical properties of poly(S-MVT) were investigated in detail through Li-S cells assembled with these copolymer-based cathodes. As is exhibited in Figure 6a, for all the copolymers, the specific capacities of the batteries, which are higher than that of elemental sulfur, decline rapidly in the first few cycles, but then tend to be stable. Among the six copolymers, poly(S-MVT)s with MVT in 15-20% presented better cyclic performance. The poly(S-MVT) (15% MVT) showed the best performance which exhibited a high initial discharge capacity of 872 mAh\(\cdot\)g\({}^{-1}\) (based on the mass of poly(S-MVT)) and a reversible  
Figure 4: SEM images of poly(S-MVT)s with different MVT contents of (**a**) 50%, (**b**) 40%, (**c**) 30%, (**d**) 20%, (**e**) 15%, (**f**) 10%. Figure 5: TEM images of poly(S-MVT) (15% MVT): (**a**) low magnification TEM image, (**b**) high magnification TEM image, (**c**) selected area image for mapping and corresponding elemental mappings of (**d**) carbon, (**e**) nitrogen, and (**f**) sulfur for poly(S-MVT) (15% MVT). capacity of 597 mAh\(\cdot\)g\({}^{-1}\) at 2 nd cycle at 0.1 C (1 C = 167.5 mA\(\cdot\)g\({}^{-1}\)). After 100 cycles, the maintained capacity is up to 514 mAh\(\cdot\)g\({}^{-1}\), which is comparable and even superior to the recently reported sulfur-containing polymers (Table S1). This monomer content-dependent electrochemical performance change rule is similar to that of the previously reported sulfur copolymers [36]. To further investigate the electrochemical mechanism, a cyclic voltammetry (CV) test was conducted for the poly(S-MVT) (15% MVT)-based cathode. The CV result of poly(S-MVT)s presented similar electrochemical behavior to that of elemental sulfur. It can be seen from Figure 6c that at the first scan cycle, two reduction peaks appear at around 2.25 and 2.00 V, which can be related to the formation of higher order polysulfides and lower order sulfides, respectively. One oxidation peak at 2.47 V was observed, which can be attributed to the conversion of low-order sulfides to high-order polysulfides and eventually to sulfur. The oxidation peak at 2.47 V varies little throughout the three cycles and indicates the stability of electrochemical oxidation reactions. The well-maintained oxidation and reduction peaks in three cycles indicates that the electrochemical reaction is of good reversibility. Thus, the possible electrochemical reactions of the poly(S-MVT) based cathode are mainly polysulfide-based reactions, as is illustrated in Figure S8. Charge-discharge profiles further revealed the electrochemical reaction mechanism of the poly(S-MVT)-based Li-S batteries (Figure 6d). Two distinct plateaus were observed during the first discharge process, which correspond well to the CV profiles. The high plateau at 2.3 V was assigned to the formation of higher order polysulfides (C-S\({}_{x}\)-Li and Li\({}_{2}\)S\({}_{x}\), x = 4-8), while the low plateau at 2.0 V was assigned to the formation of lower order sulfides (C-S\({}_{x}\)-Li and Li\({}_{2}\)S\({}_{x}\), x = 1-3). It is worth noting that the initial coulombic efficiency of poly(S-MVT) (15% MVT) based Li-S cell is 96.57%, and after the second cycle, the coulombic efficiencies are all  
Figure 6: **(a)** Galvanostatic charge–discharge cycling of different poly(S-MVT)s at 0.1 C (capacity calculated based on the mass of poly(S-MVT)) and elemental sulfur (S). **(b)** Rate capacity of the poly(S-MVT) (15% MVT) copolymer and elemental sulfur S based electrodes at varied rates. **(c)** Cyclic voltammograms of the poly(S-MVT) (15% MVT) copolymer cathode at a scan rate of 0.1 mV\({}^{-1}\). **(d)** Charge–discharge profiles of Li-S batteries with poly(S-MVT) (15% MVT) copolymer cathode at 0.1 C.  
around 100%. Rate capability test showed that at C-rate of 0.1, 0.2, 0.5, 1, and 2, the poly(S-MVT) (15% MVT) delivered capacities of 609, 465, 376, 321, and 288 mAh\(\cdot\)g\({}^{-1}\) (Figure 6b), respectively, indicating a promising rate performance (far better than that of elemental sulfur).