## 3 Results and Discussion
### Electrochemical Properties and Characterization of NiCo\({}_{2}\)S\({}_{4}\)/rGO Composites  
As discussed above, the Co doping in MISR has successfully improved the cycling performance of NiS. However, the improvement in conductivity and rate performance of NiCo\({}_{2}\)S\({}_{4}\) is far from enough. Therefore, the corrugated rGO prepared above was used to modify NiCo\({}_{2}\)S\({}_{4}\) in MISR. The XRD pattern of the obtained NiCo\({}_{2}\)S\({}_{4}\)/rGO is displayed in Figure 6a. It demonstrates that the rGO modification did not change the original crystal structure of NiCo\({}_{2}\)S\({}_{4}\). The characteristic peaks of rGO were masked because of its low adding amount, resulting in low diffraction intensity [42]. The above results show that the introduction of rGO would not change the chemical composition of NiCo\({}_{2}\)S\({}_{4}\). Figure 5: **(a)** Raman spectra and (**b**) XRD spectra of graphite, GO, and rGO; (**c**) SEM and (**d**) TEM image of rGO. Figure 6: **(a)** XRD pattern and (**b**) Raman spectrum of NiCo\({}_{2}\)S\({}_{4}\)/rGO (D, G: Characteristic peaks of rGO). Figure 6b displays the Raman spectrum of NiCo\({}_{2}\)S\({}_{4}\)/rGO. Compared with pure rGO, the composites also had two prominent peaks D and G at 1340 and 1618 cm\({}^{-1}\), but the intensity was slightly lower. However, the peak-intensity ratio \(I_{\rm D}/I_{\rm G}\) still maintained a high value of 1.02, which means that the complex reaction did not change the highly disordered structure of rGO, probably because the deposition of NiCo\({}_{2}\)S\({}_{4}\) particles on the rGO surface could effectively inhibit the stacking of graphene. In addition, there were peaks at 509 cm\({}^{-1}\) and 661 cm\({}^{-1}\) belonging to metal sulfides [12], confirming that MISR has successfully loaded NiCo\({}_{2}\)S\({}_{4}\) onto rGO. Elemental composition and corresponding chemical states on the surface of the NiCo\({}_{2}\)S\({}_{4}\)/rGO sample were evaluated by XPS. Figure 7a shows an XPS survey spectrum of the NiCo\({}_{2}\)S\({}_{4}\)/rGO sample in the range 100-1100 eV. The peaks at 169.7, 284.1, 781.5, and 855.7 eV correspond to S 2p, C 1s, Co 2p, and Ni 2p, respectively [43], indicating the existence of S, C, Co, and Ni elements in the as-prepared sample. The deconvolution fitting of the Ni 2p high-resolution XPS spectrum (Figure 7b) confirmed the existence of the two spin orbitals of Ni\({}^{2+}\) and Ni\({}^{3+}\) corresponding to Ni 2p\({}_{3/2}\) and Ni 2p\({}_{1/2}\). Specifically, the Ni 2p\({}_{3/2}\) spectrum had a peak at 852.8 eV, corresponding to Ni\({}^{2+}\) ions at the octahedral position, and a peak at 855.9 eV, corresponding to Ni\({}^{3+}\) ions at the tetrahedral position. The Co 2p XPS spectrum exhibited similar behavior. The convex peaks of Co 2p\({}_{3/2}\) at 778.9 eV and Co 2p\({}_{1/2}\) at 796.3 eV are shown in Figure 8c, indicating that Co\({}^{3+}\) and Co\({}^{2+}\) coexisted. The XPS results for nickel and cobalt elements indicate that the prepared NiCo\({}_{2}\)S\({}_{4}\)/rGO hybrids contained Ni (Ni\({}^{2+}\), Ni\({}^{3+}\)) and Co (Co\({}^{2+}\), Co\({}^{3+}\)) with different valence states, which is consistent with the CV analysis. This Faraday behavior guaranteed the excellent electrochemical performance of the as-fabricated electrode materials [44,45]. Figure 7d shows the S 2p spectrum at high resolution, where the two main peaks at approximately 162.5 and 161.5 eV correspond to the metal-sulfur bond and the S\({}^{2}\) in low coordination on the surface, respectively. The C 1s XPS curve was fitted by peak splitting and four peaks at 284.6 eV, 285.1 eV, 286.1 eV, and 288.9 eV were obtained, corresponding to the four bonding modes of carbon elements in the NiCo\({}_{2}\)S\({}_{4}\)/rGO composites (carbon-carbon double bond C=C, carbon-oxygen single bond CO, carbonyl carbon C=O, and carboxylic acid carbon OC=O), respectively.